Rivers of Southern Chile and Patagonia: Context, Cascade Process, Geomorphic Evolution and Risk Management 3031266463, 9783031266461

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The Latin American Studies Book Series

Carlos Oyarzún Bruno Mazzorana Pablo Iribarren Anacona Andrés Iroumé   Editors

Rivers of Southern Chile and Patagonia Context, Cascade Process, Geomorphic Evolution and Risk Management

The Latin American Studies Book Series Series Editors Eustógio W. Correia Dantas, Departamento de Geografia, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, Ceará, Brazil Jorge Rabassa, Laboratorio de Geomorfología y Cuaternario, CADIC-CONICET, Ushuaia, Tierra del Fuego, Argentina

The Latin American Studies Book Series promotes quality scientific research focusing on Latin American countries. The series accepts disciplinary and interdisciplinary titles related to geographical, environmental, cultural, economic, political and urban research dedicated to Latin America. The series publishes comprehensive monographs, edited volumes and textbooks refereed by a region or country expert specialized in Latin American studies. The series aims to raise the profile of Latin American studies, showcasing important works developed focusing on the region. It is aimed at researchers, students, and everyone interested in Latin American topics. Submit a proposal: Proposals for the series will be considered by the Series Advisory Board. A book proposal form can be obtained from the Publisher, Juliana Pitanguy ([email protected]).

Carlos Oyarzún · Bruno Mazzorana · Pablo Iribarren Anacona · Andrés Iroumé Editors

Rivers of Southern Chile and Patagonia Context, Cascade Process, Geomorphic Evolution and Risk Management

Editors Carlos Oyarzún Instituto Ciencias de la Tierra Universidad Austral de Chile Valdivia, Chile Pablo Iribarren Anacona Instituto Ciencias de la Tierra Universidad Austral de Chile Valdivia, Chile

Bruno Mazzorana Instituto Ciencias de la Tierra Universidad Austral de Chile Valdivia, Chile Andrés Iroumé Instituto de Conservación, Biodiversidad y Territorio Universidad Austral de Chile Valdivia, Chile

ISSN 2366-3421 ISSN 2366-343X (electronic) The Latin American Studies Book Series ISBN 978-3-031-26646-1 ISBN 978-3-031-26647-8 (eBook) https://doi.org/10.1007/978-3-031-26647-8 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Foreword

Many years ago, I had the opportunity to meet Ken Bencala who is one of the scientists who led research on the interaction between rivers and aquifers. On his website and in many publications, Ken always uses the phrase “the stream is not a pipe”, combating the conceptualization that rivers are mere water transport systems (pipes). I believe that Ken’s phrase should be expanded by saying that rivers are not pipes but complex systems from the hydrological, ecosystem and social point of view. Around rivers important civilizations have emerged, as is the case of the Nile. The Egyptian civilization developed knowing the dynamics of the river, since it depended on its flood regime to maintain food production. In fact, the flows of the Nile River have been measured for thousands of years in the oldest fluviometric station that we know of, which is called “Nilometer”. However, it is also very interesting to think that the origin of the Nile River was a mystery for thousands of years and was only discovered during the nineteenth century and that the first navigation along the entire length of that river occurred only in 2004. Regarding navigations along rivers, I cannot fail to mention Lope de Aguirre who decided to explore the Marañon River in Peru, which turned out to be one of the tributaries of the Amazon and became the first European to explore that river. I imagine him every day saying, “the river is bigger, bigger and bigger.” Other rivers whose stories have captivated me are the Rhine and the Danube, which were the borders of the Roman Empire and were crossed and navigated by Julius Caesar and Trajan; there is also Mark Twain’s Mississippi that captivated me with the stories of Huckleberry Finn and Tom Sawyer. In Chile, we have rivers that are much smaller, but that also have great stories, as is the case of the Biobío, which was a border for many years and a means of communication, as described in the book A Veteran of Three Wars where the protagonist sailed from Angol to Concepción using a ship that navigated by that river, thing that now looks impossible due to embankment. However, I must regret that except in the case of the city of Valdivia, most of Chilean cities turn their backs on their rivers. For this reason, I celebrate this book, which is a tremendous contribution to the knowledge of our rivers through different perspectives that explain the ecosystem functions of the rivers of southern Chile and Patagonia, the relationship between v

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Foreword

the communities and those rivers and the functioning of the rivers and estuarine systems, which is something that we Chileans from the central zone do not know in our daily experience. Another important aspect is the relationship between our rivers and the volcanoes, which we know through the study of the formation processes of the territory, but which we were able to see and analyze after the recent eruptions of the Caulle, Chaiten and especially Calbuco volcanoes. The territory of the southern zone of Chile and Patagonia is marked by the glacial processes that formed that territory, and for this reason, the two chapters that refer to it are an interesting contribution to the knowledge of this subject. On the other hand, although rivers are beautiful places, the risk associated with flooding must also be understood, and I have seen with concern that the mega-drought has caused a kind of forgetfulness of the risk of flooding, and I particularly celebrate the chapters that refer to that topic. In short, I congratulate the authors of this book and I thank them for their work that contributes to the knowledge of our rivers that we love so much. August 2022

José Luis Arumí Professor Universidad de Concepción Concepción, Chile

Preface

This book provides a comprehensive analysis of the evolution of rivers affected by natural disturbances in Southern Chile and Patagonia (39–46° S). It presents a detailed description of the rivers affected by climatic extremes, volcanic eruptions, large wood dynamics and impacts, sediment-laden flows and Glacier Lake Outburst Floods (GLOFs) located in a territory undisturbed by large human settlements. Therefore, it analyzes hydrological and fluvial processes in one of the last places on the planet considered with low anthropogenic impact, since a great proportion of this territory is protected by national parks. However, due to the current crisis caused by the coronavirus pandemic, many people are moving toward these territories, which is why human settlements are growing. From this point of view, it proposes methods to analyze and strategies to mitigate the effects of large natural disturbances on river corridors and the human settlements expanding therein. In this context, this work aims to answer several research questions, such as: How do volcanic disturbances, the rapid retreat of glaciers, forest fires and climate change influence the geomorphological evolution of the rivers of southern Chile? What concatenations of primary and secondary processes can be observed? Can the affected rivers quickly recover their pre-disturbance states of equilibrium or do they evolve towards an unprecedented dynamic equilibrium? How can the specific hazards and risks be assessed and what sets of tools can be used to analyze the involved processes? And, finally, how can the impacts generated be mitigated, thereby contributing to an increased societal resilience? River managers, researchers and students of many interrelated disciplines such as geography, geology, environmental science and engineering but also policymakers concerned with the environment and its sustainability all over the world constitute the principal audience of this book. The topics covered in the book have a wide appeal also beyond the aforementioned disciplinary boundaries. Moreover, it contains an in-depth discussion of a variety of topics encompassing the ecosystem functions of

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Preface

Pacific Patagonia rivers, the geomorphic signatures of Glacier Lake Outburst Floods (GLOFs) and their impacts within river corridors, as well as the assessment of the associated natural hazards and risks. Further, it provides proposals for public and territorial policies that improve the management and sustainable strategies of the hazards and risks. This book was conceived by leading researchers of the Universidad Austral de Chile (UACh), closely collaborating in the FONDECYT Nr. 1200091 project titled “Unravelling the dynamics and impacts of sediment-laden flows in urban areas in southern Chile as a basis for innovative adaptation (SEDIMPACT),” funded by ANID-Agencia Nacional de Investigación y Desarrollo, and includes contributions by distinguished scholars from around the world. It will attract a wide range of readers, including the scientific community, researchers, undergraduate and graduate students, and policymakers from Chile and abroad. Valdivia, Chile August 2022

Carlos Oyarzún Bruno Mazzorana

Contents

1

An Introduction to the Rivers of Southern Chile and Patagonia . . . . Carlos Oyarzún

2

Landscape Disturbance and Ecosystem Function of Pacific Patagonia Rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brian Reid and Anna Astorga

3

San Pedro River: A Biological and Cultural Treasure in Northern Patagonia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nicole Colin, Konrad Górski, Juan José Ortiz, Pablo Iriarte, and Ana M. Abarzúa

4

Large Wood Research and Learning in Chile . . . . . . . . . . . . . . . . . . . . Héctor Ulloa and Andrés Iroumé

5

River Water Characteristics After Recent Volcanic Eruptions in Southern Chile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Eduardo Jaramillo, Alexandre Corgne, and Aldo Hernandez

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Deciphering the Morphologic Change in the Radial Drainage System of the Calbuco Volcano Caused by the 2015 Eruption . . . . . . Christopher Sepúlveda, Bruno Mazzorana, Héctor Ulloa, and Andrés Iroumé

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35

49

67

77

Investigating the Geomorphological Footprint of Moraine-Dammed Lake Failures in Patagonian Rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Diego Bahamondes, Bruno Mazzorana, Pablo Iribarren Anacona, and Héctor Ulloa

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Contents

8

Cascading Impacts of GLOFs in Fluvial Systems: The Laguna Espontánea GLOF in Patagonia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 Pablo Iribarren Anacona, Catalina Sepúlveda, Jorge Berkhoff, Ivan Rojas, Valeria Zingaretti, Luca Mao, Bruno Mazzorana, and Gonzalo Durán

9

Improving the Channel Network Management After a Large Infrequent Disturbance, Taking Advantage of Sediment Connectivity Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 Lorenzo Martini, Lorenzo Picco, Marco Cavalli, and Andrés Iroumé

10 Mitigating Complex Flood Risks in Southern Chile in a Particular Spatial Planning Context: Towards a Sustainable Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 Bruno Mazzorana and Francisco Maturana 11 Cascading Processes and Multiple Hazards and Risks in Chilean Rivers: Lessons Learnt and Remaining Challenges . . . . . 235 Virginia Ruiz-Villanueva, Bruno Mazzorana, Diego Bahamondes, and Iván Rojas Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251

About the Editors

Carlos Oyarzún has a Ph.D. in Environmental Sciences from the Universidad de Concepción, Chile. He has proven experience in the investigation of the water balance at a catchment level in both the Andean and Coastal mountain range in southern Chile. He also extensively investigated the sediment and nutrient fluxes in forest ecosystems at catchment level. He studied the sources of water for catchments with different vegetation covers in southern Chile. As an active member of the research center “Transdisciplinary Research Center for Socio-ecological Strategies for Forest Conservation” (TESES), he is developing transdisciplinary research lines for the use and conservation of forest ecosystems in southern Chile from a socio-ecological perspective. He is a professor in the Institute of Earth Science, Universidad Austral de Chile. Bruno Mazzorana contributed, as a specialist from 2002 to 2006 at the Department of Hydraulic Engineering (Autonomous Province of Bolzano, Italy), to the mitigation of debris flow risks in mountain streams by designing open check dams for sediment dosing and wood entrapment. During his Ph.D. at the University of Applied Life Sciences–BOKU–Vienna, Austria (2006–2009), he explored the role of wood transport in alpine rivers from a modeling perspective. From 2009 to 2015, he extended his research interests to river corridor management in several EU-financed research projects. From 2015 to 2018, he was involved as research partner in the Project P 27400-NBL “Physics-based flood risk vulnerability analysis of buildings” supported by the FWF (Austrian Science Fund) where distributary dynamics on alluvial fans and their impacts on the built environment have been analyzed following experimental and numerical approaches. Since 2015, he is a professor in the Institute of Earth Science, Universidad Austral de Chile. Currently, he leads the Fondecyt-ANIDfunded research project (Grant Nr. 1200091) “Unravelling the Dynamics and Impacts of Sediment-laden Flows in Urban Areas in Southern Chile as a Basis for Innovative Adaptation (SEDIMPACT)” which is devoted to many key topics discussed in this book.

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About the Editors

Pablo Iribarren Anacona is a geographer from Universidad de Chile, with a Ph.D. in Physical Geography from Victoria University of Wellington, New Zealand. He is currently a lecturer at the Institute of Earth Sciences, Universidad Austral de Chile. His research interests include the study of natural hazards in mountain ranges affected by glacier retreat and thinning and the study of fluvial systems affected by extreme events. He has studied glacier and permafrost related hazards and their geomorphological impacts in the North, Central and Patagonian Andes and his research has helped to understand the dynamics and geomorphic impact of Glacial Lake Outburst Floods. His research approaches include dynamic modeling of floods, the use of remote sensing techniques in big data analysis and the development of low cost dataloggers to monitor rivers, lakes and glaciers. Andrés Iroumé is a civil engineer from Universidad de Chile, with a Ph.D. in Forest Sciences from University of Gottingen (Germany). He is currently a full professor at the Faculty of Forest Sciences and Natural Resources, Universidad Austral de Chile. He has been involved since the early 1990’s studying morphological processes. Through FONDECYT projects Nº1080249 (2008–2011), 1110609 (2011–2014), 1141064 (2014–2017) and 1170413 (2017–2020), he investigated in-stream large wood and channel morphology, sediment and large wood transport evolution patterns following natural disturbance processes. He has been principal investigator of several research projects funded by national and international organizations, among others by CONICYT (Chile), Forestal Mininco (Chile) and BMBF (Germany), with over 1,400,000 USD$ research funds obtained in the last twelve years. Furthermore, he oversees monitoring programs including a network of more than 20 experimental catchments.

Chapter 1

An Introduction to the Rivers of Southern Chile and Patagonia Carlos Oyarzún

Abstract The rivers of southern Chile and northern Patagonia (39–47° S) are located in a region affected by natural disturbances, such as climatic extremes (high variability of rainfall), retreat of glaciers, large volcanic eruptions, glacial lake outbursts of floods, seismic activity, and frequent landslides. Annual precipitation records show great differences in the region. In general, there is a decrease in precipitation from north (39° S) to south (47° S), and Patagonia region shows a strong E-W gradient, with the driest areas in the Andes mountain areas and the wettest on the coast. In general terms, there are strong differences between the discharge of the rivers in southern Chile and Patagonia. The discharge of the last ten years (2011–2020) shows a slight downward trend in all rivers. Keywords Southern Chile · Patagonia · Climatology · Hydrological regimes

1.1 Introduction and Context Rivers are natural bridges between terrestrial and oceanic coastal systems, where water, nutrients, energy, sediments, contaminants, and organisms meet and are transferred through land-margin and margin-open ocean interactions. River-coastal estuaries are susceptible to changes in biodiversity, water quality, and productivity and have been increasingly perturbed by human activities (Smith 2003). The global production of agricultural fertilizers alone released ~ 10 million metric tons of nitrogen in 1950 but may exceed 135 million metric tons of N by the year 2030 (Vitousek et al. 1997). Depending on the delivery ratio or the fraction of the sediment delivered to the river system, global gross soil erosion by water may be 75 billion tons, of which 15–20 billion tons are transported by the rivers into the aquatic ecosystems and eventually into the ocean (Lal 2003). C. Oyarzún (B) Instituto Ciencias de la Tierra, Universidad Austral de Chile, Valdivia, Chile e-mail: [email protected]

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. Oyarzún et al. (eds.), Rivers of Southern Chile and Patagonia, The Latin American Studies Book Series, https://doi.org/10.1007/978-3-031-26647-8_1

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C. Oyarzún

The rivers of southern Chile and northern Patagonia (39–47° S) are located in a region affected by natural disturbances, such as climatic extremes (high variability of rainfall), retreat of glaciers, large volcanic eruptions, glacial lake outbursts of floods, seismic activity, and frequent landslides. The Patagonia region, due to its complex and steep topography, is characterized by steep slopes and materials affected by glaciers, these factors added to climatic conditions with intense rainfall, volcanic and seismic activity, and it increases the occurrence of landslide phenomena at different scales and of different types (Muñoz 2020). The eruption of the Chaitén volcano (42° 38' S, 72° 65' W) that occurred on May 2008 generated one of the largest eruptions of the last 30 years globally (Ulloa 2020). The mobilization of the material expelled by the eruption, followed by intense rainfall (around 120 mm during two days) caused volcanic alluviums (lahars) and floods in the valleys of the Rayas and Blanco rivers, severely damaging the vegetation and reaching the city of Chaitén, located about 10 km away to the SW, downstream of the Blanco River (Ulloa 2020). Chaiten River basin released one of the greatest modern annual sediment yields estimated following volcanic disturbance. Within a year of eruption, Chaiten River likely delivered 25–80 × 106 t, equivalent to 0.3–1 × 106 t km2 (Major et al. 2016). Chilean Patagonia is a nearly pristine environment, with several endemic but poorly characterized species and unique aquatic ecosystems, including a high diversity of lakes, rivers, and fjords. National parks protect a large proportion of this territory. However, some authors showed the paradox that 68% of the total regional surface is affected to some degree by direct human transformation or subjected to pressure by various economic activities (Inostroza et al. 2016). The “Red de Parques Nacionales en la Patagonia Chilena” brings together 17 national parks. These territories, which include Los Lagos, Aysén and Magallanes regions, have a high biodiversity value and connect 2800 km between Puerto Montt (41° S) and Cabo de Hornos (55° S) (Contreras and Daure 2020). Gradients in topography and climate are exceptionally high. The distance between the fjords and the highest peaks (> 3000 m altitude) is often less than 50 km. Precipitation may vary from 400 to 6000 mm per year over distances as short as 60 km (Dussaillant et al. 2012). Most rivers in the region are influenced by storage and release from lakes and/or glaciers, while geology is heterogeneous and topographic controls on streamflow vary strongly (HidroAysén 2008). Six large continental basins (Palena, Cisnes, Coyhaique, Baker, Bravo and Pascua) represent 29% of the water resources of the country. An important aspect of these waterways, in addition to their magnitude, is their high quality (Contreras and Daure 2020). Inorganic-N concentrations (NO3 -N + NH4 N) in streams in Antillanca, Puyehue National Park at the Cordillera de los Andes, southern Chile (40° 72' S, 1250 m a.s.l), were generally within the lowest range of those reported from other temperate forests in the world (Oyarzun et al. 2004). On the other hand, the chemistry of precipitation in southern Chile and Patagonia reflects one of the closest approximations of pre-industrial atmospheric conditions in the world (Weathers and Likens 1997; Godoy et al. 2001). The climate change projections of the Intergovernmental Panel on Climate Change (IPCC 2013) show, in general, in continental Chile, an increase in temperatures and significant decreases in rainfall. Some studies project for 2080 a decrease in rainfall

1 An Introduction to the Rivers of Southern Chile and Patagonia

3

of up to 40% and an increase of up to 2.8° C in southern Chile and Patagonia (39– 47° S) (Sarricolea and Meseguer 2020). These trends are part of a regional reduction in precipitation recorded in south-central Chile for the last 100 years (GonzalezReyes and Muñoz 2013). In fact, the rivers discharge in south-central Chile have been reduced in response to decreased rainfall. In the 2010–2014 period, the average deficit in the discharge of the rivers of Coquimbo (30° S) and Valparaíso (33° S) regions reached a maximum of 70%, reducing toward the south at values close to 25% (CR2 2015).

1.2 Climatology, Precipitation and Temperature Regimes In southern Chile, from 38°50' S up to approximately 43° S, the Cfb climate dominates, classified as temperate rainy (Marine West Coast) (Sarricolea and Meseguer 2020). On the coast, from 39° S up to 41° S, the temperate rainy climate with coastal influence Cfb(s) prevails. Toward the interior of this region, the temperate rainy cold climate Cfc dominates, with a slight summer dryness. In this area, the dismemberment and decrease in height of the Andes Mountains, the influence of the west winds, high rainfall and oceanic conditions determine the climatic configuration of this region. Patagonia has a great dominance of the ET tundra climate, while the glacial climate EF and its varieties are restricted to altitudes above 5000 m in the north and center of Chile. The Northern Ice Field areas (46–47°30' S) are considered temperate glaciers because the air temperature is close to 0 °C (Sarricolea and Meseguer 2020). Annual precipitation records show great differences in the region (Table 1.1). In general, there is a decrease in precipitation from north (39° S) to south (47° S), where Pirihueico and Puyehue stations have the highest rainfall with 3557 and 4100 mm yr−1 , respectively, while La Junta and Caleta Tortel have 2456 and 2355 mm yr−1 , respectively. The Patagonia region shows a strong E-W gradient, with the driest areas in Chile Chico (234 mm yr−1 ) and the wettest on the coast of Caleta Tortel (2355 mm yr−1 ). Precipitation records are scarce in the high mountains of southern Chile. However, amounts between 6000 and 8000 mm yr−1 have been registered in Antillanca, Puyehue National Park in the Cordillera de los Andes, southern Chile (40°72' S, 1250 m a.s.l.) (Godoy et al. 2001; Oyarzun et al. 2004). In general, the river basins have a monomodal precipitation pattern (Fig. 1.1a), with most of the precipitation occurring in the wintertime (May–August), except in Caleta Tortel in the coastal Patagonia with an irregular pattern (Fig. 1.1b). Figure 1.2 shows the maximum 24 h rainfall for selected sites in the region. In the period January 2011– December 2020, the maximum precipitations occurred in the stations of Coñaripe (182 mm day−1 ), Anticura (144 mm day−1 ), Pirihueico (147 mm day−1 ) and Lago Maihue (138 mm day−1 ). Temperature data are particularly scarce in the region. Monthly mean temperatures are maximum in January and minimum in July in all meteorological stations, from Lago Ranco (40° 19´ S, 100 m a.s.l.) to Chile Chico (46° 32' S, 215 m a.s.l.).

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Table 1.1 Precipitation at selected sites of southern Chile and Patagonia (period January 2011– December 2020, data source Dirección General de Aguas-DGA) Sites

Latitude

Longitude

Coñaripe

39° 15' S

72° 00' W

215

2765 ± 320

Pirihueico

39° 52' S

71° 53' W

600

3557 ± 590

Caunahue

40° 09' S

72° 15' W

100

1885 ± 248

100

2178 ± 415

13'

Lago Maihue

40°

S

72° 08' W

Anticura

40° 39' S

72° 11' W

Altitude (m a.s.l.)

Annual rainfall (mean ± sd)

350

2196 ± 321

Puyehue

40°

S

72° 12' W

420

4100 ± 620

Llanada

41° 52' S

71° 56' W

248

2434 ± 405

48'

47' 36'

Palena

43°

W

275

1425 ± 212

La Junta

43° 57' S

72° 24' W

45

2456 ± 491

Coyaique

45° 28' S

71° 36' W

730

374 ± 150

32'

S

71°

41'

46°

W

215

234 ± 57

47° 08' S

72° 43' W

160

767 ± 322

Caleta Tortel

47° 47' S

73° 31' W

10

2355 ± 245

450 400 350 300 250 200 150 100 50 0

S

71°

600

Coñaripe

Precipitation, mm

Precipitation, mm

Chile Chico Río Baker

500

Pirihueico

400 300 200 100 0

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Month

450 400 350 300 250 200 150 100 50 0

Anticura

Precipitation, mm

Precipitation, mm

Month 450 400 350 300 250 200 150 100 50 0

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV

Lago Maihue

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

450 400 350 300 250 200 150 100 50 0

Month

Caunahue Precipitation, mm

Precipitation, mm

Month

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Month

450 400 350 300 250 200 150 100 50 0

Llanada

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Month

Fig. 1.1 a, b Monthly precipitation at selected sites of southern Chile and Patagonia (period January 2011–December 2020, data source DGA)

450 400 350 300 250 200 150 100 50 0

Palena

Precipitation, mm

Preccipitation, mm

1 An Introduction to the Rivers of Southern Chile and Patagonia 450 400 350 300 250 200 150 100 50 0

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

La Junta

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Month

Coyaique alto Precipitation, mm

Precipitation, mm

Month 450 400 350 300 250 200 150 100 50 0

450 400 350 300 250 200 150 100 50 0

Chile chico

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Month

Río Baker

Precipitation, mm

Precipitation, mm

Month 450 400 350 300 250 200 150 100 50 0

5

450 400 350 300 250 200 150 100 50 0

Caleta Tortel

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Month

Month

Fig. 1.1 (continued)

Precipitation (mm day-1)

200

Coñaripe

180

Pirihueico

160

Caunahue

140

Anticura

120

Lago Maihue Palena

100

Llanada

80

Coyaique

60

La Junta

40

Río Baker Chile Chico

20

Caleta Tortel

0 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020

Year

Fig. 1.2 Maximum 24 h rainfall at selected sites of southern Chile and Patagonia (period January 2011–December 2020, data source DGA)

Monthly mean temperatures fluctuate between 12–17° C in January and 2–6.5° C in July (Fig. 1.3).

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16

Temperature (° C)

14 12

10 8 6 4 2

0

Anticura Lago Ranco Puyehue Chile chico Coyaique

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Month Fig. 1.3 Monthly mean temperatures at selected sites of southern Chile and Patagonia (period January 2011–December 2020, data source DGA)

1.3 Hydrological Regimes In the Patagonia region, the rivers are born on the eastern slope of the Andes, crossing the mountain range through gaps produced by fractures of the Andean massif to flows into fjords. This area presents mighty rivers through the narrowness of the Cordillera de Los Andes, being regulated by the presence of large lakes. The Baker River basin stands out with 20,945 km2 and 175 km in length, which is born in Lake Bertrand and flows into the Pacific Ocean in Caleta Tortel (47° 47' S) (DGA 2016). In general, glaciers in Chile have experienced strong retreats and thinnings (Rivera et al. 2002), changes that have been mainly associated with increases in air temperature, especially in the high mountain range. Also, the negative changes observed in the rainfall of many regions of Chile have led to the conclusion that snow is migrating to higher altitudes; therefore, the proportion of solid versus liquid precipitation in the high mountain range is decreasing (Rivera et al. 2020). In the Northern Ice Field (46°–47° 30' S), which had an area of 3593 km2 in 2001, significant area changes and thinnings were detected. The Colonia glacier, which has strongly regressed in the long term, has generated two cycles of sudden emptying of its proglacial lakes (GLOFs-Glacier Lake Outburst Flood) (Rivera et al. 2020). These emptyings have generated increases of up to 8 m in the level from Lake Colonia, discharges up to 3000 m3 s−1 and strong floods that have affected the Colonia River Valley and part of the Baker River (Dussaillant et al. 2010). Floods in the rivers of Patagonia usually

1 An Introduction to the Rivers of Southern Chile and Patagonia

7

involve Newtonian flows of great volume (tens of millions of cubic meters) whose dynamics are conditioned for the transport of wood debris (Iribarren-Anacona 2020). In general terms, there are strong differences between the discharge of the rivers in southern Chile and Patagonia. The Baker River has the highest mean annual discharge rate of all Chilean rivers (864 ± 132 m3 s−1 ) (Table 1.2). It flows out from Bertrand Lake, which in turn receives the draining waters from bi-national General Carrera Lake (Dussaillant et al. 2012). On the other hand, the Cochrane River has the lowest mean annual discharge in the Patagonia region (17 ± 5 m3 s−1 ) (Table 1.2). The discharge of the last ten years (2011–2020) shows a slight downward trend in all rivers (Fig. 1.4). The lowest flows occurred in 2016, which coincided with a decrease in rainfall in the region. For the Puelo River (41° S), using paleoclimate reconstruction techniques, a reduction in discharge associated with an increase in the South Annular Mode (SAM) has been described (Lara et al. 2008). On the other hand, the summer flows of Baker River show a pattern of sustained decrease since the 1980s (Lara et al. 2015). The trend of decreasing summer–autumn flows of this river as well as the regional precipitation could be explained by the increase in the SAM, which has been recognized as the main climatic force in South America and the cause of reduced precipitation in the region (Lara et al. 2015; Villalba et al. 2012). Figures 1.5a, b show monthly discharge at selected rivers of southern Chile and Patagonia. In the south region (39–41° 30' S), the mean monthly flows are highest from June to September and lowest from January to May. In northern Patagonia Table 1.2 Discharge at selected rivers of southern Chile and Patagonia (period January 2011– December 2020, data source Dirección General de Aguas-DGA) Discharge (m3 s−1 ) (mean ± SD)

Rivers

Latitude

Longitude

Altitude (m a.s.l.)

Liquiñe

39° 43' S

71°51' W

600

35.5 ± 7.1

San Pedro

39° 46' S

72° 27' W

115

313.4 ± 43.6

Fuy

39° 52' S

71° 53' W

600

57.2 ± 11.5

09'

Caunahue

40°

S

72° 15' W

90

30.8 ± 7.1

Calcurrupe

40° 13' S

72° 15' W

160

150.1 ± 23.5

Puelo

41° 45' S

72° 03' W

23

312.2 ± 48.6

Futaleufu

43°

S

71° 45' W

314

285.9 ± 56.6

Palena

43° 59' S

72° 29' W

40

771.9 ± 101.2

48'

10' 39'

Cisnes

44°

W

500

36.8 ± 8.6

Simpson

45° 33' S

72° 04' W

210

40.9 ± 12.2

Cochrane

47° 15' S

72° 33' W

140

17.0 ± 5.0

51'

105

863.7 ± 131.5

Baker

47°

21'

S

S

71°

72°

W

8

C. Oyarzún 1200 Liquiñe San Pedro

1000

Discharge (m3 s-1)

Fuy Caunahue

800 Calcurrupe Puelo

600 Futaleufu Palena

400 Cisnes Simpson

200 Cochrane

Baker

0 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020

Year

Fig. 1.4 Trends of annual discharge for selected rivers of southern Chile and Patagonia (period 2011–2020, data source Dirección General de Aguas-DGA)

(42–47° S), in some rivers, such as Puelo, Futaleufu, and Palena, it is possible to recognize a second peak during the months of November–January. While in other rivers, such as Baker River, the high flows tend to concentrate between October and January, the snowmelt period.

1.4 Concluding Remarks In this chapter, we presented a general overview of the main characteristics of the climatology (precipitation and temperature regimes) and hydrology regimes of southern Chile and northern Patagonia (39–47° S) a region affected by natural disturbances, such as climatic extremes (high variability of rainfall), retreat of glaciers, large volcanic eruptions, glacial lake outbursts of floods, seismic activity, and frequent landslides. Annual precipitation records show great differences in the region. In general, there is a decrease in precipitation from north (39° S) to south (47° S), and Patagonia region shows a strong E-W gradient, with the driest areas in the Andes mountain areas and the wettest on the coast. In general, the rivers basins have a monomodal precipitation pattern, with most of the precipitation occurring in the wintertime (May–August). The chemistry of precipitation in southern Chile and Patagonia reflects one of the closest approximations of pre-industrial atmospheric conditions in the world.

1 An Introduction to the Rivers of Southern Chile and Patagonia (a)

600

100

River Liquiñe

Discharge (m3 s -1)

Discharge (m3 s -1)

80 60 40

20

River San Pedro

500

400 300 200 100 0

0

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Month

Month 120

100

Discharge (m3 s -1)

Discharge (m3 s -1)

Rio Caunahue

80 60 40

20

80 60 40

20 0

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Month

Month 600

300

River Calcurrupe

Discharge (m3 s -1)

Discharge (m3 s -1)

River Fuy

100

0

250 200

150 100 50

500

River Puelo

400

300 200 100 0

0

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Month

Month

(b)

100

River Futaleufu Discharge (m3 s -1)

Discharge (m3 s -1)

600

500 400

300 200

100 0

80

River Cochrane

60 40 20

0 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Month

Month 100

100

River Cisnes

Discharge (m3 s -1)

Discharge (m3 s -1)

80 60 40

20

80

River Simpson

60 40

20 0

0

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Month

Month 1400

River Palena

1200

Discharge (m3 s -1)

Discharge (m3 s -1)

1200 1000

9

800 600 400 200

River Baker

1000

800 600 400

200

0

0 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Month

Month

Fig. 1.5 a, b Monthly discharge at selected rivers of southern Chile and Patagonia (period January 2011–December 2020, data source DGA)

10

C. Oyarzún

In general terms, there are strong differences between the discharge of the rivers in southern Chile and Patagonia. The Baker River has the highest mean annual discharge rate of all Chilean rivers (864 ± 132 m3 s−1 ). The discharge of the last ten years (2011–2020) shows a slight downward trend in all rivers. The lowest flows occurred in 2016, which coincided with a decrease in rainfall in the region. Some studies project for 2080 a decrease in rainfall of up to 40% and an increase of up to 2.8° C in southern Chile and Patagonia (39–47° S). Acknowledgements This research was funded by the project ANID/CONICYT FONDECYT N°1200091, “Unravelling the dynamics and impacts of sediment-laden flows in urban areas in southern Chile as a basis for innovative adaptation (SEDIMPACT)” led by Bruno Mazzorana.

References Centro de Ciencia del Clima y la Resilencia (CR)2 (2015) La megasequía 2010–2015: una lección para el futuro. Informe a la Nación Contreras P, Daure C (2020) Macro región Austral o Sur Grande. In: Borsdorf A, Marchant C, Rovira A, Sánchez R (eds) Chile cambiando. Revisitando la Geografía regional de Wolfgang Weischet. Instituto de Geografía, Pontificia Universidad Católica de Chile/Instituto de Ciencias Ambientales y Evolutivas, Facultad de Ciencias, Universidad Austral de Chile. Serie GEOlibros N° 36, Santiago de Chile, pp 145–165 Dirección General de Aguas-DGA (2016) Atlas del agua, Chile 2016. Ministerio de Obras Públicas, Santiago de Chile, Chile Dussaillant A, Benito G, Buytaert W, Carling P, Meier C, Espinoza F (2010) Repeated glacial-lake outburst floods in Patagonia: an increasing hazard? Nat Hazards 54(2):469–481 Dussaillant J, Buytaert W, Meier C, Espinoza F (2012) Hydrological regime of remote catchments with extreme gradients under accelerated change: the Baker basin in Patagonia. Hydrol Sci J 57(8):1530–1542. https://doi.org/10.1080/02626667.2012.726993 Godoy R, Oyarzún C, Gerding V (2001) Precipitation chemistry in deciduous and evergreen Nothofagus forests of southern Chile under a low-deposition climate. Basic Appl Ecol 2:65–72 Gonzalez-Reyes A, Muñoz A (2013) Cambios en la precipitación de la ciudad de Valdivia (Chile) durante los últimos 150 años. Bosque 34(2):191–200 HidroAysén (2008) Proyecto HidroAysén. Environmental impact study (in Spanish) [online]. Available at http://www.e-seia.cl Inostroza L, Zasada I, Konig H (2016) Last of the wild revisited: assessing spatial patterns of human impact on landscapes in Southern Patagonia, Chile. Reg Environ Change 16:2071–2085 Intergovernmental Panel on Climate Change-IPCC (2013) Climate Change 2013. The Physical Science Basis. Contribution of Working Group to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. In: Stocker TF, Qin D, Plattner GK, Tignor M, Allen SK, Boschung J, Nauels A, Xia Y, Bex V, Midgley PM (eds) Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. p 1535 Iribarren-Anacona P (2020) Vaciamiento de lagos de origen glaciar en Chile. In: Borsdorf A, Marchant C, Rovira A, Sánchez R (eds) Chile cambiando. Revisitando la Geografía regional de Wolfgang Weischet. Instituto de Geografía, Pontificia Universidad Católica de Chile / Instituto de Ciencias Ambientales y Evolutivas, Facultad de Ciencias, Universidad Austral de Chile. Serie GEOlibros N° 36, Santiago de Chile, pp 397–402 Lal R (2003) Soil erosion and the global carbon budget. Environ Int 29(4):437–450

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Lara A, Villalba R, Urrutia R (2008) A 400-year tree-ring record of the Puelo River streamflow in the Valdivian rainforest Eco-region Chile. Climatic Change 86(3–4):331–356 Lara A, Bahamondez A, González-Reyes A, Muñoz A, Cuq E, Ruiz-Gómez C (2015) Reconstructing streamflow variation of the Baker River from tree-rings in Northern Patagonia since 1765. J Hydrol 529:511–523 Major J, Bertin D, Pierson T, Amigo A, Iroume A, Ulloa H, Castro J (2016) Extraordinary sediment delivery and rapid geomorphic response following the 2008–2009 eruption of Chaiten Volcano, Chile. Water Resour Res 52:5075–5094. https://doi.org/10.1002/2015WR018250 Muñoz E (2020) Remociones en masa. Casos emblemáticos en la región de Aysén. In: Borsdorf A, Marchant C, Rovira A, Sánchez R (eds) Chile cambiando. Revisitando la Geografía regional de Wolfgang Weischet. Instituto de Geografía, Pontificia Universidad Católica de Chile / Instituto de Ciencias Ambientales y Evolutivas, Facultad de Ciencias, Universidad Austral de Chile. Serie GEOlibros N° 36, Santiago de Chile, pp 403–406 Oyarzun CE, Godoy R, De Schrijver A, Staelens J, Lust N (2004) Water chemistry and nutrient budgets in an undisturbed evergreen rainforest of southern Chile. Biogeochemistry 71:107–123 Rivera A, Acuña C, Casassa G, Bown F (2002) Use of remotely sensed and field data to estimate the contribution of Chilean glaciers to eustatic sea-level rise. Ann Glaciol 34:367–372 Rivera A, Bown F, Carrión D (2020) Nieves y hielos en la alta cordillera y la Antártica Chilena. In: Borsdorf A, Marchant C, Rovira A, Sánchez R (eds) Chile cambiando. Revisitando la Geografía regional de Wolfgang Weischet. Instituto de Geografía, Pontificia Universidad Católica de Chile / Instituto de Ciencias Ambientales y Evolutivas, Facultad de Ciencias, Universidad Austral de Chile. Serie GEOlibros N° 36, Santiago de Chile, pp 259–281 Sarricolea P, Meseguer O (2020) Climatología de Chile y sus escenarios futuros. In: Borsdorf A, Marchant C, Rovira A, Sánchez R (eds) Chile cambiando. Revisitando la Geografía regional de Wolfgang Weischet. Instituto de Geografía, Pontificia Universidad Católica de Chile/Instituto de Ciencias Ambientales y Evolutivas, Facultad de Ciencias, Universidad Austral de Chile. Serie GEOlibros N° 36, Santiago de Chile, pp 187–218 Smith VH (2003) Eutrophication of freshwater and coastal marine ecosystems a global problem. Environ Sci Pollut Res 10(2):126–139 Ulloa H (2020) La inundación de Chaitén producto de la erupción del volcán homónimo y de la crecida del río Blanco. In: Borsdorf A, Marchant C, Rovira A, Sánchez R (eds) Chile cambiando. Revisitando la Geografía regional de Wolfgang Weischet. Instituto de Geografía, Pontificia Universidad Católica de Chile/Instituto de Ciencias Ambientales y Evolutivas, Facultad de Ciencias, Universidad Austral de Chile. Serie GEOlibros N° 36, Santiago de Chile, pp 391–396 Villalba R, Lara A, Masiokas MH, Urrutia RB, Luckman BH, Marshall GJ, Mundo IA, Christie DA, Cook ER, Neukom R, Allen K, Fenwick P, Boninsegna JA, Srur AM, Morales MS, Araneo D, Palmer JG, Cuq E, Aravena JC, Holz A, Le Quesne C (2012) Unusual Southern hemisphere tree growth patterns induced by changes in the southern annular mode. Nature Geosci. https:// doi.org/10.1038/NGEO1613 Vitousek PM, Aber JD, Howarth RW, Likens GE, Matson PA, Schindler DW, Schlesinger WH, Tilman DG (1997) Human alteration of the global nitrogen cycle: sources and consequences. Ecol Appl 7(3):737–750 Weathers KC, Likens GE (1997) Clouds in Southern Chile: an important source of nitrogen to nitrogen-limited ecosystems? Environ Sci Technol 31:210–213

Chapter 2

Landscape Disturbance and Ecosystem Function of Pacific Patagonia Rivers Brian Reid and Anna Astorga

Abstract The rivers of southern Chile may be characterized by high specific discharge and extreme gradients in elevation, geography and climate over short trajectories. Major rivers are often transcordilleran, passing through cold-steppe to deciduous sub-Antarctic to evergreen temperate rainforest ecoregions, supplemented by glacial inflows or buffered by large mid-catchment lakes, all within relatively unimpaired or nearly pristine landscapes. As a consequence, instream disturbance regimes, a driver of ecosystem function in rivers, are expected to vary widely as a consequence of diversity of natural flow disturbance and sediment regimes over short distances and complex hydrologic networks. Superimposed on these base-line fluvial disturbance patterns are geologic disturbances such as landslides, deposition of volcanic ash, and floods triggered by extreme climatic events or glacial lake outburst events. Another characteristic of Patagonian southern Andes is that these otherwise discrete extreme events may nevertheless be concentrated or superimposed within confined areas of the hydrologic landscape. Discussed are some of the potential consequences of natural disturbance regimes, both base-line and extreme events. A conceptual model of ecosystem development and resilience across coupled terrestrial and aquatic systems affected by disturbance pulses is presented. Ecosystem function within hydrologic networks is expected to vary widely over short spatial scales, while the potential ecosystem consequences of extreme events is expected to significantly alter the export of energy, nutrients, materials and weathering substrates and to downstream systems such as lakes and extensive inland marine ecosystems of southern Patagonian fjords. Keywords Disturbance · Freshwater ecosystems · Glacial lake outburst flood · Explosive volcanic eruption · Shifting habitat mosaic · Biogeochemistry

B. Reid (B) · A. Astorga Centro de Investigación en Ecosistemas de la Patagonia (Centro CIEP), Coyhaique, Chile e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. Oyarzún et al. (eds.), Rivers of Southern Chile and Patagonia, The Latin American Studies Book Series, https://doi.org/10.1007/978-3-031-26647-8_2

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B. Reid and A. Astorga

2.1 Introduction This chapter focuses on an ecological and ecosystem perspective of natural disturbance in Pacific drainages of western Patagonia (Fig. 2.1.). These fluvial systems are seldom more than 100–200 km in length and all but invisible on global maps, yet they are some of the most significant discharges to the Southern Pacific Ocean, in terms of freshwater and also sediment delivery (Milliman and Farnsworth 2011). Complete glacier coverage of Pacific basins is prresumed during the Holocene (Davies et al. 2021), hence they are recent systems, with unstable valley walls characterized by post-glacial slope failures, frequent landslides, glacial lake outburst floods, within tectonically and volcanically active landscapes. Contemporary human colonization has been very recent, less than 150 years in the Aysén region of Chile and only slightly longer in adjacent regions. With large areas of intact forested watersheds (Astorga et al. 2018), this is perhaps among the most significant aspects of this region— that much of what might be characterized as ecosystem disturbance in the region is of natural origin, at least compared to other temperate zone systems (exceptions discussed in the last section). Disturbance is a defining characteristic of stream and river ecosystems, one that underlies function of systems and adaptation of organisms. The intensity, timing,

Fig. 2.1 General map of central and southern Patagonia, highlighting Pacific (westward) drainages discussed in this chapter, and Holocene active volcanos. a Expanded view of the Cisnes and Aysen basins, the focus for Figs. 2.2 and 2.3; b close-up of the Baker basin, the focus of Sect. 2.2 and Figs. 2.5 and 2.6. Sources: volcanoes from the PATICE dataset, Davies et al. (2020), while lake expansion in periglacial zones is from Wilson et al. (2018)

2 Landscape Disturbance and Ecosystem Function of Pacific Patagonia …

15

frequency, and duration of spates exert a direct physical stress on stream organisms or may also disrupt entire ecosystems via bed movement, resetting the primary productivity of the algal trophic base and adaptation of organisms, and is the subject of an extensive literature in stream ecology (Resh et al. 1988; Junk et al. 1989; Poff et al. 1989, 1992; Quinn and Hickey 1990; Townsend et al. 1997; Lake 2000; Death 2002, 2010). Compared to terrestrial ecosystems, where one may often observe an orderly and often predictable increase in living organisms (e.g., succession), biomass, and productivity, such post-disturbance progressions are not readily observed in aquatic ecosystems. As an approximate treatment midway between aquatic and terrestrial ecology and based on the patchiness of river floodplains, fluvial ecosystems may be conceptualized as mosaics of patches, a “shifting habitat mosaic” in an alleged equilibrium across larger spatial scales (Pringle et al. 1988; Stanford et al. 2005). Studies on disturbance in fluvial ecosystems also acknowledge the mixture of effects and drivers that characterize ecological systems. Ecological response to flow/substrate disturbance may interact with predation and competition at the organismal level (Peckarsky 1984; Death and Zimmerman 2005), while ecosystem response to flow-related disturbance may be modulated by nutrient supply (Biggs 2000). This chapter treats the topic of ecosystem disturbance from two somewhat distinct perspectives: (1) disturbance of fluvial systems from a stream ecology perspective; (2) landscape scale disturbance from a watershed perspective. The first is generally oriented toward instream habitat for aquatic organisms and their response to flow regime and substrate stability, treated here as a brief global summary of an ecological or ecosystem perspective on stream disturbance, and, in part given the very limited observations from the region, some limited discussion on disturbance regimes or patterns that might be distinctive to Patagonia. The second is focused on large-scale or otherwise dramatic events, from a largely physical or geomorphologic perspective. If knowledge of ecological disturbance in Patagonia streams might be characterized as limited, the second perspective of landscape scale geomorphic disturbance affecting fluvial ecosystems, effectively scarce in the general ecological literature, is essentially nonexistent in terms of regional observations. Despite these limitations, and in order to be consistent with other chapters in this book, we present two regional case studies of limited scope. One is based on the 1991 explosive volcanic eruption of the Hudson volcano, and the other from the repeated glacial lake outburst floods affecting the Río Baker river, discussing observed physical processes, hypothetical effects, and potential consequences for freshwater ecosystems in the region. Ultimately, the goal of this chapter is to explore ideas at the interface between ecosystem science and process geomorphology, within the Patagonia setting, based on shared insight on the fluvial disturbance from an ecological perspective, together with an ecologists’ take on the types of major landscape disturbance that attract much more attention from the physical sciences. The last section therefore presents a summary of ecosystem consequences of landscape disturbances, together with a visual conceptual model, mostly hypothetical, and perhaps only a starting point for future interdisciplinary research questions.

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2.2 Base-Line Disturbance Regimes in Patagonian River Networks Disturbance of flow and sediment delivery is a natural ecosystem function of rivers. In Patagonia these natural base-line disturbance regimes may vary widely over short distances, as a function of corresponding variability in climate and hydrology, and to an important degree their interaction with geomorphology and vegetation. As such, a discussion of disturbance in Patagonian fluvial systems from an ecological perspective begins with primer on local climate and hydrology. The region is noteworthy for its significant climatic gradients over short distances (Dussaillant et al. 2012; Lenaerts et al. 2014). For example, a transect at 44 deg. S (Aysén basin; Fig. 2.2) ranges from 400–-3000 mm/y (Fig. 2.2a, c) not unusual for a large cordilleran basin, nevertheless representing less than half of the precipitation range in the region (Fig. 2.1a). The corresponding hydrographs of Patagonian rivers show a wide range of variation corresponding with the climatic range. The snowmelt flood-pulse typical of mountain rivers is evident from these hydrographs, with range in peak flow from early (austral) spring snow melt (Fig. 2.2f, September to October) for rivers of pampas origin. Cordilleran streams generally have a late spring snowmelt flood pulse in November or December (Fig. 2.2e). Coastal streams and rivers show less pronounced snowmelt peak, instead characterized by nearly year-round spates and flashy hydrologic regime (not shown). Flashy coastal hydrology may be particularly pronounced during fall/winter storms, which may also propagate inland to produce secondary high flow events in more interior basins. Glacial meltwater streams (not shown) generally have a maximum discharge during the warmest months of January and February. Taken independently, the hydrologic regimes and their potentially corresponding disturbance regimes within fluvial ecosystems just described are probably comparable to systems elsewhere in the world. However, in Patagonia, these gradients are often manifested in close quarters along the river continuum, a typical westerly flowing river running counter to a climatic gradient (Fig. 2.3). Most of the major rivers in Patagonian are trans-cordilleran (Aysén, Cisnes and Baker rRivers; Table 2.1) are of arid pampas origin (precipitation between 250–400 mm/y), often forming in low gradient glacial outwash plains. Downstream, the major rivers accumulate smaller streams from lateral watersheds from different climate regimes, including moderate (1000–1500 mm/y in the temperate deciduous zone) to high levels of precipitation (up to 4000 mm/y in the coastal temperate rainforest; Fig. 2.3). This generalizable pattern, in some sense representing an unexplored scenario within the river continuum concept (Vannote et al. 1980), may be important in understanding the biology, ecosystem function, and biogeochemistry of rivers in the region, and the corresponding implications for productive marine systems in one of the world’s most extensive coastal fjord systems.

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Fig. 2.2 Extremes of the climatic and hydrologic gradient in Chilean Patagonia shown for three stations along the Rio Aysén basin: monthly mean climatograms for precipitation and temperature at a Puerto Aysén, temperate rain forest, precipitation 2500–3000 mm/y; b Coyhaique, 70 km upstream in temperate deciduous forest, precipitation 1000–1200 mm/y; c Balmaceda, 60 km upstream in cold-steppe/pampas, precipitation 400–500 mm/y; and corresponding flow regime based on monthly mean flows of nearby flow gauging stations d Rio Blanco, one of the principal tributaries to Rio Aysén originating in humid coastal climate; e Rio Simpson, tributary to Rio Aysén showing regime for temperate cordilleran and Pampas origin; f Rio Huemules, principal tributary to Rio Simpson, pampas origin and dry climate flow regime. Note that river flows are normalized by mean annual flow to compare seasonality across stations (inset shows uncorrected flows). Source Dirección General de Aguas, 2004)

A typical scenario for western Andean rivers, as in the previous example from Rio Cisnes (Fig. 2.3), is that source-to-sea transit time may be no more than 1– 3 days, based on a reasonable estimate of ∼ 1 m/s velocity over no more than 100– 200 km. With over 5350 lakes greater than 5 ha, and volume exceeding 1115 km3 , the distribution and connectivity (lake order) of individual mid-catchment lakes may alter this transit time, increasing residence time in the catchment from days, months, often over ten years, and exceeding 200 years for Lago Chelenko (South Americas second largest lake; 47 °S; Fig. 2.4). Central and southern Patagonia harbor the majority of lakes and ponds in Chile (Reid et al. 2021). Their role in buffering natural disturbance in the catchment may be significant, potentially offsetting the complex disturbance regimes. This may be all the more significant given the steep elevational gradients of the western cordillera (Fig. 2.3c), where otherwise a downstream decrease in slope and channel widening would establish limits on propagation of major landscape disturbances (Nakamura et al. 2000). Based on limited analysis of key benthic taxa such as mayflies and stoneflies, Patagonia is within the range of peak aquatic invertebrate diversity (Reid et al. 2021). How this relatively high ecological diversity, in terms of species or function, plays out against the potentially wide-ranging disturbance regimes (Fig. 2.2), or the combination and juxtaposition of wide-ranging disturbance regimes over relatively short flow distances (Fig. 2.3), or the potentially countering effects of mid-catchment lakes (Fig. 2.4), represents a potentially singular research opportunity for fluvial ecosystems. Within this relatively pristine landscape (Astorga et al. 2018), are there

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Fig. 2.3 Bioclimatic, elevational, and vegetation gradients for a counter-gradient watershed, example Rio Cisnes (44 °S); a precipitation varies between ~ 400 mm/y in the cold-steppe/pampas (red) up to ~ 4000 mm/y in the river mouth in Puerto Cisnes (blue); b representative trans-Andean transects based on river courses within the Cisnes (rose) Aysén (mandarin) and Senguerr (beige) basins; c elevational gradients along the five transects identified in b showing the significantly steeper western slope of the cordillera; d mean annual precipitation and annual coefficient of monthly precipitation variation for the Cisnes River transect, for both recent and 2070 climate change scenarios (climate raster from Hijmans et al. 2007, RCP 8.5 scenario; isopleths from Ministerio Obras Públicas 1987; elevation transects based on CGIAR-CSI version 4.1; Jarvis et al 2008; triangles indicate rainfall observation stations; vegetations zones based approximately on Leubert and Pliscoff 2018)

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Table 2.1 Summary of ecosystem processes and potential effects of natural disturbance from explosive volcanic events. A broad definition of ecosystem impacts is based on six categories of ecosystem processes proposed by Likens (1992), while potential consequences for near source and downstream ecosystems are proposed as confirmed (C), probable (Pr) or possible (Po) ecosystem impacts Ecosystem function

Indicator

Direct effects

Indirect/offsite effects

1. Biomass

(a) Primary producer biomass

Reduction due to reduced grain size or sediment stability (Pr) as a function of downstream distance from plume

Increase downstream following nutrient release (Po)

(b) Organic matter standing crop

Transfer from terrestrial or soil ecosystems to fluvial systems, surface, or burial (C)

Export to downstream ecosystems

(a) Chlorophyll

Reduction due to reduced grain size or sediment stability (Pr)

Increased productivity downstream from nutrient pulse and mineralization of buried organic matter

(b) Dissolved oxygen

Decrease from metabolism of buried organic matter and reduced primary production (Pr)

(c) pH

Increase or decrease depending on mineralogical composition

(a) Decomposition

Increased decomposition of terrestrial subsidized organic matter (Pr)

(b) Trophic structure

Increased importance of microbial production, microbial loop (Po), reduction in grazing functional feeding groups (Po)

(c) Organic matter drift

n/a

2. Productivity

3. Energy flow

Increased exportation of dissolved organic matter

Increased episodic organic matter subsidy downstream following flood events (Po) (continued)

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B. Reid and A. Astorga

Table 2.1 (continued) Ecosystem function

4. Nutrient cycling

5. Resilience/stability

6. Development

Indicator

Direct effects

(d) Secondary production

Increased dominance by small size class invertebrates (Pr), decreased invertebrate turnover time (Pr);

(a) Uptake/spiraling

Increased phosphorous sorption on sediments, biological uptake (phosphatase enzyme activity) of inorganic phosphorous (Pr) and organic nutrients (Po)

Indirect/offsite effects

(b) N fixation

Increased role of N-fixing cyanobacteria (Po)

(c) Nutrient limitation

Enhanced nitrogen limitation

(a) Sediment dynamics

Decreased sediment stability and decreased retention (Pr)

(b) Macroinvertebrate community stability

Decrease due to episodic movement and resuspension of sediment (Po)

(c) Periphyton community stability

Decrease due to episodic movement and resuspension of sediment (Po)

(a) Periphyton succession

Limited development or complexity of periphyton community

(b) Microbial succession

Increased sulfate and iron reducing bacteria, increased proportion of other microaerophilic or anaerobic microbes (Pr)

Increased long-term sediment supply, increased braiding index, expansion of lake deltas, alteration of surface wetlands (Pr)

(continued)

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Table 2.1 (continued) Ecosystem function

7. Hydrology

Indicator

Direct effects

(c) Invertebrate succession

Limited development or complexity of macroinvertebrate community

Indirect/offsite effects

(a) Transient storage

Increased water residence time from structural complexity and high transmissivity of volcanic sediments

Increase in weathering products and altered nutrient supply downstream

(b) Hyporheic exchange

Increase in bed permeability and local vertical hydraulic conductivity (Po)

n/a

Fig. 2.4 Lakes as disturbance buffers along the hydrologic continuum in Patagonia, Pacific westward drainages from 42–55 °S. Lake residence time in years (upper horizontal axis—log scale) and total volume (lower axis) are shown for over 5300 lakes (data from: Messager et al. 2016)

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corresponding spatial patterns in diversity and function of fluvial organisms and ecosystems? In terms of potential effects on periphyton or invertebrates (the usual subjects of investigation in streams and rivers) or ecosystem properties (an area of investigation as pristine as the Patagonia landscape), almost nothing is known. For both periphyton communities (Reid and Torres 2013) and invertebrate communities (Oyanedel et al. 2008; Moya et al. 2009; Astorga et al. 2021), existing regional studies are descriptive, based on broad geographical patterns, and limited to single observation periods. Moreover, the vast majority of studies have been inspired by potential response to perturbation of biological origin: newly emerging invasive species (Reid et al. 2021; Reid and Torres 2013). The few examples that are more aligned with biophysical gradients have focused on elevation as a primary driver (in part via effects on thermal regime), but more or less independent of disturbance regime (Contador et al. 2015; Miserendino and Pizzolon 2000). Hence, the practical implications of these disturbance patterns for Patagonian fluvial ecosystems, potential or real, remain at the level of the educated guesswork that follows. With nonexistent periphyton monitoring programs (e.g., comparable to NZ; Biggs 2000), extremely low background nutrient levels, visual evidence of periphyton biomass is usually limited to no more than a biofilm, evident to touch or when one unsuccessfully attempts to wade across a river. Disturbance effects on the benthic ecosystem, due to scouring effects of spates, may be frequent in the temperate rainforest (several events per year or more frequently), but only once per year in the deciduous belt or pampas, corresponding with snowmelt. Disturbance at the level of flow competence sufficient to mobilize the river bed has been observed approximately at 2 to 4 year intervals in reaches with flashy rainforest hydrographs, as well as larger trans-cordilleran rivers such as Rio Simpson (Fig. 2.2e), but remains unobserved elsewhere (noting that this is limited to casual observations in the course of no more than seven years of monthly monitoring of mostly small headwater streams). In moderate or larger rivers (> 4th order) and some headwaters streams of low slope, aquatic mosses indicate very limited disturbance of this type (Reid and Torres 2013). Finally, the massive landscape-level disturbances and disturbance cascades that are central to this book are only evident indirectly. Large wood is frequently embedded or even dominates the structure of channel banks of rainforest streams (A. Astorga, unpubl. data), while corresponding wood in temperate headwaters shows no evidence of having the urge for going. Aside from these observations, the examples presented elsewhere in this book, which attract the attention of physical sciences, no doubt result in complete obliteration of the exiting ecosystem from massive mobilization, mixing, reworking, displacement, and transfer across ecosystem components of sediment, organic matter resources and nutrients. This process, effectively one of primary succession and ecosystem development in aquatic ecosystems, is rarely investigated in general by aquatic sciences, nor are these terms used in such a manner (i.e., analogous to terrestrial systems). The following section is therefore an attempt to address these possibilities in more detail, based on local Patagonian examples comparable to other events characteristic of other subsequent chapters.

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2.3 Landscape-Scale Disturbances in Patagonia and Riverine Ecosystem Function We present two local case studies, volcanic tephra fall, and glacial lake outburst floods, as templates for discussing the potential ecological and ecosystem impacts, both local and downstream. The first example is based on the 1991 explosive eruption of Volcan Hudson. The second is focused on repeated glacial lake outburst floods (GLOFs) originating from the ice-dammed Cachet 2 Lake in the Baker River basin. Volcan Hudson is considered among the most globally active volcanoes in the Holocene (Weller et al. 2015). The 1991 Hudson eruption was among the most significant of the twentieth century, producing nearly 2.7 km3 of fallout (Scasso et al. 1994). Located in a sparsely populated region and poorly known at the time, the event was overshadowed by the Mt. Pinatubo eruption of that same year (ej. Hansen 1992; Fisher et al. 2016). Potential impacts of such events on aquatic ecosystems may be affected via lahars, mudflows, pyroclastic flows, rain of pyroclastic on aquatic systems, and/or creation of aquatic ecosystems on new substrate. Many such processes are described for the 1980 Mt. St. Helens Eruption (Swanson and Major 2005), perhaps the most extensively studied and monitored geologic event of its kind. Of these processes, pyroclastic rain and deposition of tephra over 10’s to 100’s of kilometers was the only significant consequence of the 1991 Hudson event. And in contrast to the attraction of St. Helens, published observations following the Hudson eruption are almost nonexistent. Nevertheless, a few general observations by the authors, which may be supported or confirmed by published observations from similar events in Patagonia or elsewhere, are presented in the following paragraphs. Soils based on volcanic tephra are globally uncommon; however, the Southern Andes are among one of the world’s hotspots, and depths ranging from a few centimeters to 5–10+ meters are common (Vandekerkhove et al. 2015). As a consequence of repeated eruptions and superimposed fallout plumes over the Holocene, soils are stratiufied, with frequent buried organic horizons inssterssoperssed with Andosols of varios ages of development. These sequences of tephra deposition, often the subject of paleo-chronology (e.g., Weller et al. 2015), also have implications for hydrology and biogeochemical cycling at the watershed level. For example, based on the 1991 Hudson eruption, tephra depths increased significantly over the Río Ibañez watershed, the area that was most closely aligned with the plume of the second eruption (Fig. 2.5). The addition of 1.05 km3 of new tephra in this watershed represented an average 14% increase, one that widely varied at the level of smaller nested watersheds between 0.02 and 83%. In terms of hydrology, adjusted for field capacity of old (57%) versus new (29%) tephra (Flores 2016), a 10% increase in potential water storage is implied: 0.3 km3 of new storage capacity for the (unrealistic) scenario that all soils were to become saturated. Empirical observations on potential hydrologic effects are limited at this site. Elsewhere, Swanson and Major (2005) describe an increase in specific runoff and variable duration of effect, by several 10’s of percent

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and from a few years to 2+ decades, depending on intensity of channel disturbance. This may be due to a variety of factors, such as the proportion of runoff to infiltration and decrease in evapotranspiration due to vegetation loss (compared to the 1980 St Helens eruption, the latter was not significant for the 1991 Hudson eruption). Another consequence is the sudden increase in new tephra, most notably the coarse lapilli deposited over 10’s of kilometers (from Fig. 2.5: observed at depths > 1 m in watershed 1, 40 cm in watershed 2, and washed downstream in watershed 3), while deposition of fine material may be evident over distances of 100’s of kilometers (Scasso et al. 1994). These deposits represent highly weatherable volcanic glass and

Fig. 2.5 Rio Ibañez watershed within the main axis of the ash plume from the second eruption of the Hudson Volcano (1991). Relative change in depth of unconsolidated pyroclastic material/tephra a, isopleths show depth distribution (cm) of 1991 ash plume (redrawn from Scasso et al. 1994), while color scheme represents percent increase in total depth of tephra deposits following the 1991 event (re-interpolated from Vandekerkhove et al. 2015). Numbers indicate three of six monitoring watersheds, inset shows typical deposition of lapilli within the main axis; b photo from above tree line in watershed 2, Rio El Alto drainage, showing persistent coverage of deep lapilli deposits, and stream generation; c small stream basin within the forested zone (watershed 2) and basin morphology, numbers indicate soil profile sampling locations, symbols correspond with local slope; d relationship between soil depth and local slope for recent coarse-grained pyroclastics (1991 eruption) and underlying finer volcanic paleosol. Dashed lines show approximate transition zone of slope-depth relationship for both horizons at approximately 20–30 degrees slope; e locations of transitions slopes (following panel d) for the Rio Ibanez watershed: slopes of 20–30° (red, with bold colors within a 100 m buffer zone of the stream network) and > 30° (green, bold colors within the 100 m buffer). Slopes were interpreted based on the ALOS 30 m digital elevation model (JAXA Global Palsar-2 Mosaic, http://www.eorc.jaxa.jp), soil core data is from Flores 2016

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associated trace elements, producing weathering products over short to indeterminately long timescales. Dissolved silica concentrations are strongly correlated with tephra depth (Vandekerkhove et al. 2015) and are 3–5 times greater in the Rio Ibanez watershed compared to the adjacent Rio Murta (immediately west of the plume in Fig. 2.5) by a factor of 3–5, despite similar bedrock parent material (B. Reid., unpubl. data). Orthosilicid acid, together with bioavailable forms of dissolved iron, is two of the most important contributions of continental runoff to the ecosystem productivity of Patagonian fjords and coastal ocean (Torres et al. 2014, 2020). If not for the intervening long residence time of Lago Chelenko ( ∼ 200 years; Fig. 2.4), the effects might have been directly observable, as was the case for the 2015 Calbuco eruption in northern Patagonia (Vergara-Jara et al. 2021). Other elemental cycles also seem to have been altered, most notably an increase in nitrate nitrogen and a 5 to tenfold increase in inorganic soluble phosphate (PO4 2− ). These observations have been confirmed by order of magnitude increases in inorganic phosphorous observed in lakes over short time scales (months) following the 1980 St Helens event (Dahm et al. 2005). More locally, similar increases have been observed four years following the 2011 Puyehue eruption in northern Patagonia (Carillo et al. 2018), in addition to significant increases in phosphatase enzyme activity and periphyton elemental composition (stoichiometry). What is perhaps most relevant with the respect to the 1991 Hudson eruption is that alteration of biogeochemical cycling is still apparent more than 25 years following the event (B. Reid, pers. obs.). A greater understanding of physical mechanisms for these patterns is needed: Release of volatile elements during active eruption phase (reviewed by Carrillo et al. 2018) is certainly one factor, but whose trajectory over medium to longer time scales is unknown. Burial of organic horizons may be another possibility that has not been adequately explored. A significant consequence of explosive volcanic eruptions is downstream alteration of fine sediment transport and corresponding disturbance regime. Physical effects may include changes in braiding index, islands, and channel morphology, such as observations from fluvial systems following the 2008 Volcan Chaiten eruption (Ulloa et al. 2015). From an ecological perspective, the newness of the substrate or stream morphology may be less important than substrate stability, especially in terms of mobile sand-sized grains, also larger grains of lower-density pumice. One of the founding assumptions of macroinvertebrates as biotic indicators of aquatic ecosystem integrity is based on the impact of fine sediment from erosion (Relyea et al. 2012). Several sites from within the 1991 Hudson plume were included in the study of regional macroinvertebrate diversity by Astorga et al. (2021; Fig. 2.5 watersheds 1, 2, 3 and vicinity). Many of the sites had among the lowest densities and diversity of macroinvertebrates, corresponding with among the lowest substrate index. However, a few sites were reasonably comparable to macroinvertebrate communities in the region. A study by Claeson et al. (2020), along streams of the 1980 St. Helens pumice plain, focused on longitudinal gradients in invertebrate and algal diversity. Downstream increase in diversity was observed for both groups although comparisons to non-pumice substrate were not possible. An increase in fine sediment transport follows tephra deposition: in the case of the 1991 Hudson eruption dominating the entirety of the Rio Ibañez floodplain up

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70 km downstream to the confluence with Lago Chelenco, significantly increasing delta deposition (Chavez 2016a, b). How long this transport process persists, and what are the patterns of movement and deposition of the ash and pumice plume following initial deposition, is perhaps one of the most interesting and relevant questions from a biological, biogeochemical, and physical perspectives, since all of the other processes mentioned above are dynamic at large spatial scales (i.e., weathering, mineralization, fertilization, and export downstream, in addition to habitat substrate). We expect that several processes of sediment movement downstream occur at distinct time scales: (1) rapid transport via material directly deposited along stream courses and floodplains; (2) moderately rapid transport from deposition and subsequent erosion on steep hillslopes; (3) and longer-term processes from slopes that are moderate enough to receive ash fall but steep enough to allow later remotion (a function of surface roughness/vegetation, infiltration capacity and soil water content). A potential illustration of this is shown in Fig. 2.5b, c, and d, implying that the biogeochemical processes associated with ash deposition is spatially patchy across the watershed and fluvial landscape. In summary, the massive deposition of new tephra at the landscape level may result in significant changes to aquatic ecosystems that may be summarized by the following physical processes: (1) change in unconsolidated material affecting storage, runoff, and hydrologic routing; (2) deposition of highly weatherable mineral substrate; (3) burial of vegetation and soils, mineralization and potential alteration of decomposition processes; (4) change in substrate stability in stream beds, impacting ecological productivity; (5) increase in export of nutrients, weathering products (especially silicic acid), organic matter (including large wood, Ulloa et al. 2016), and fine sediment. Proposed ecological, ecosystem, and biogeochemical consequences for both near-site and downstream lake and fjord systems are reviewed in Table 2.1 (either confirmed in isolated cases, or proposed as hypothetical, probable or possible effects). A second case study on glacial lake outburst floods (GLOFs), in contrast to the previous example, has no known comparable studies on the ecological or ecosystem consequences for fluvial or marine systems. Perhaps this is in part due to their short duration, unpredictability, remote locations lacking previous base-line, and potential risk. In the case of repeated GLOFs typical of ice-dam mechanism, time series observations on ecological impacts might be feasible. Other examples based on single event processes such as moraine failure (e.g., Iribarren et al. 2015) are often single event processes: Ecological studies might focus on long-term post-event succession patterns of recovery. However, such studies of succession in rivers from major disturbances are rare (Fisher et al. 1982), and the concept of succession itself is rarely used by stream ecologists, who favor base-line rates of frequent scour and bed stability disturbance from spates, as discussed in the first section. Despite these limitations, GLOFs are almost certainly important in terms of effects on downstream ecosystems, and the rapid increase in proglacial and ice contact lake area (Fig. 2.1c; Wilson et al. 2018) implies a corresponding rise in probability of occurrence in the region. In the case of repeated ice-dam failure from Lago Cachet 2 starting in 2008 (Dussaillant et al. 2010), the hydrograph pre- and post-GLOF events indicate two very distinct rivers (Fig. 2.6). Aside from this obvious threshold

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Fig. 2.6 Rio Baker hydrograph between 2004 and 2010 for three longitudinal stations, at the lake outlet (Lago Bertrand), and below the Rio Nef and Rio de la Colonia confluences, the first and second major tributaries, respectively, originating from the Northern Patagonia Icefield. Recurrent events starting in April 2008 indicate a state-change in flood intensity and frequency, representing pre- and post-glacial lake outburst events (GLOF) from the glacier-dammed Lago Cachet 2, Rio Colonia catchment. Note also the gentle hydrograph indicating the lake buffering effect of Lago Bertrand (part of the Lago Chelenko complex; Fig. 2.3), and superimposed flow regime of increasing downstream variance driven by glacial meltwater

change, the effects of the recent GLOFs in Rio Baker on downstream ecosystems are subtler than suggested by their dramatic origins. This may be due to the effect of lakes in the catchment (e.g., Fig. 2.4), where Lago Colonia receives the outburst flood from Cachet 2 as might a check dam, while the Rio Colonia joins with the discharge from South America’s second largest lake, Lago Chelenko (evident in the baseflow hydrograph of Fig. 2.6). Limited effects of GLOF disturbance on the river ecosystem are also suggested by observations of instream periphyton of the main Rio Baker, which registered the highest algal biomass observed in the region, driven by the invasive diatom Didymosphenia geminata (Reid and Torres 2013). Surprisingly, the principal ecological effects may be based on increased fine glacial sediment export, observed directly as suspended load (Quiroga et al. 2011, 2013, 2016), benthic accumulation within the algal mats of the invasive diatom (Reid and Torres 2013), indirectly in the historic register of GLOF fines accumulated within fjord and floodplain sediments (Vandekerkove et al. 2020a, 2020b, 2021), and finally, via the loss of glacial lakebed silts and clays from the Lago Cachet 2 source (Jacquet et al. 2017). Individual pulses to fjords may exceed 5% of annual load of fine sediment; however, the character of highly reactive glacial sediment should also be considered. Extremely high phosphorous loads 2–3 orders of magnitude greater than base-line concentrations, which are normally near analytical detection limits, have been consistently observed on the Rio Baker during GLOF events (B. Reid, unpubl. data). The significance of nutrient pulse effects on fjord ecosystems from GLOF events is unknown. Meanwhile, the fine sediments increase turbidity

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and reduce light available for photosynthesis, implying a trade-off of positive and negative effects on downstream aquatic ecosystems, one whose duration and spatial extent are unknown (e.g., until sediments settle from the water column). For sake of context, it should be taken into account that the Rio Baker is Chile’s largest and is also one of the largest freshwater discharges into the southern Pacific.

2.4 Synthesis and Recommendations for Geomorphology and Ecosystem Research As stated in the opening paragraphs, ecologists and geomorphologists may have very distinct ideas of what constitutes disturbance in fluvial systems. As a starting point, we present a conception model for the shifting habitat mosaic fluvial ecosystems and their flood plains (sensu Stanford et al. 2005), based on an ecological perspective (Fig. 2.7). The conceptual model considers standing stock of living and nonliving biomass. However, many of the ecosystem parameters of Table 2.1 could also be similarly incorporated, especially as they relate to successional stage and ecosystem development (i.e., Odum 1969). A few assumptions and variations of the general conceptual model are worth treating here. Most systems start with low stocks following disturbance and gradually accumulate to some steady state. Note that terrestrial systems maintain a very low level of nonliving organic matter, due to export to the soil subsystem and atmosphere. However, perhaps unique to fluvial and flood plain environments, an initial high deposition of organic matter from flood events (indicated by dashed line) may compensate. Note also that at finer time scales, river systems show high frequency of annual oscillations of low-level standing stocks—the response to low-level disturbance from spates. In contrast to other compartments, the aquifer (or more generally, any buried system) will begin the succession process with an initial input of organic matter which degrades over time. Finally, time scales for the trajectories may not be the same: River succession is very rapid and overwhelmed by disturbance, such that the trajectory is rarely manifested, time scales for forests or soils in a moderately active flood plain may be decades to centuries, while aquifers may slowly develop over millennia. This model based on equilibrium and cumulative dynamic effects of patches at the landscape level is reasonable in terms of the kind of base-line disturbance regimes within fluvial ecosystems considered by ecologists. However, compared to the major landscape disturbances and cascades treated in this book, the idea of equilibrium or resilience seems optimistic. It is likely that variations on Fig. 2.7 would consider thresholds in equilibrium, and permanent change in state, for standing stock, flows or any of the ecosystem process described in Table 2.1. The point is that with very rare exceptions, the landscape scale of disturbance is not adequately appreciated by aquatic ecologists, at least based on available empirical observations and published

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Fig. 2.7 Conceptual diagram of ecosystem patterns of the shifting habitat mosaic in fluvial ecosystems, together with an integrated response to disturbance across four subsystems: terrestrial vegetation, soil, aquatic, and hyporheic/aquifer. a Standing stock accumulation of living biomass and nonliving organic matter of a single patch; b disturbance probability, frequency, and/or intensity will vary depend on local geomorphology, watershed context, and climate, nevertheless the development of standing stocks is expected to provide an indicator or correlate of disturbance; c multiple patches within the landscape may have various successional trajectories as a response to disturbance events (shifting habitat mosaic); hence, distinct patches aggregated at the landscape level may compensate for overall losses or gains, resulting in lower net changes in ecosystem properties; d fluvial systems from the perspective of 4 distinct yet connected subsystems extend the concept of an ecosystem development mosaic at the watershed scale: Each patch within a subsystem likewise has a distinct trajectory over time (left panels); when organic matter stocks of all patches within each compartment are integrated over time (right panels), a pattern of net equilibrium response to disturbance is expected to emerge (assuming that downstream export is captured by the respective scale of observation), meanwhile patterns of resistance and resilience (inflections in the trajectories, right panel) correspond/compensate across subsystems

literature. Aquatic ecology might stand to benefit from observing the effects of landscape disturbance that are the focus of geomorphology. As an auto-criticism, it is fair enough that the reciprocal should also be considered: Landscape-level disturbance could be benefit from increased observations of geomorphology and processes both on site and downstream (as in Table 2.1) over medium to long time scales, with increased attention to the processes that have consequences for the biogeochemical cycling of carbon, nutrients, and other materials in their various states. For example, effects of major disturbance on the retention, burial, mineralization, and/or export of dissolved or fine particulate carbon may be far more significant than the more readily observable large wood. Ulloa et al. (2016) estimated the loss of large volumes of forest vegetation and inferred a pulsed contribution of 20–60% of annual load from Rio Rayas of carbon to “northern Patagonian fjords.” A perspective that “everything falls apart” (Kirchner and Ferrier 2013) reflecting landscape processes at their source, belies other generalizations: These same “parts” eventually re-assemble (marine sedimentation, diagenesis, and tectonic processes). However, the parts may take their time in getting to this final marine destination, meanwhile they are useful in the intervening space. The consequences of terrestrial wood loss for nearby fjords presumes rapid export based only on the coarsest size fraction. However, burial,

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fragmentation, mineralization, and export as other forms of carbon are more likely, and also a more ecologically relevant form of export, one that is yet unexplored in terms of the intermediate processes affecting river ecosystems and its various components (Fig. 2.7). The same logic applies to the roughly 25 × 106 m3 of fine sediment “lost” from the Cachet 2 GLOFs (Jacquet et al. 2017): What became of that sediment and what are the ecosystem consequences? Effectively, these are interdisciplinary research problems, which moreover should be observed over longer temporal scales in order to be more fully appreciated. This is the only chapter of this book to directly relate fluvial disturbance to ecosystems, which might appear out of place without the aforementioned research vision. The disturbances discussed in later chapters in this book, from the perspective of physical and process geomorphology, represent among the most intense stresses on natural ecosystems. The combined effect of these stressors, tephra fall, landslides, extreme climatic events, and GLOFs, over restricted geographic gradients, is perhaps singularly characteristic of Patagonia. Compared to common perspectives on disturbance typical of stream and river ecology, the major disturbances considered here are of an unusually high magnitude, with limited treatment in general by freshwater sciences. These epic-scale disturbances are more commonly appreciated in terrestrial ecology in terms of succession events and trajectories, however less so in terms of their transition to fluvial systems (terrestrial aquatic ecotone). Concerning the regional perspective on Patagonia treated in the following chapters, detailed observations on the consequences of major landscape disturbance on fluvial ecosystems are effectively nonexistent. Hence, this synthesis from an ecosystem perspective, preemptive to the detailed physical processes of the case studies discussed in the following chapters, is necessarily limited to potential or hypothetical consequences of major landscape disturbances in Patagonia, based on general observations or limited information from similar regions. Acknowledgements Research support from FONDECYT 11110293, 11075105, 1140385 and 1221049 to B.R., 11140495 to A.A., contributed to much of the observations and experiences in the Patagonian river ecosystems that are discussed in this chapter. The authors would also like to acknowledge the helpful suggestions of Dr. Fred Swanson. This chapter representa a contribution from projects ANID CHIC-FB210018. The authors would like to acknowledge current support from PATSER “Patagonia Socio-Ecological Research” (ANID Centros Regionales R20F0002) and Centro Internacional Cabo de Hornos (CHIC-ANID/BASAL PFB210018).

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Kirchner J, Ferrier K (2013) Mainly in the Plain. Nature 495:318–319 Lake S (2000) Disturbance, patchiness, and diversity in streams. J North American Benthological Soc 19(4). https://doi.org/10.2307/146818 Lenaerts J, van den Broeke M, van Wessem J, van de Berg W, van Meijgaard E, van Ulft L, Schaefer M (2014) Extreme precipitation and climate gradients in Patagonia revealed by high-resolution regional atmospheric climate modeling. J Clim 27:4607–4621 Leubert F, Pliscoff P (2018) Sinopsis bioclimática y vegetacional de Chile. Editorial Universitaria, U Chile, Santiago, Chile, p 384 Likens E (1992) The ecosystem approach: its use and abuse. Excellence Ecol Book 3. ECI Institute Messager M, Lehner B, Grill G, Nedeva I, Schmitt O (2016) Estimating the volume and age of water stored in global lakes using a geo-statistical approach. Nature Commun 7:13603. https:// doi.org/10.1038/ncomms13603 Milliman J, Farnsworth K (2011) River discharge to the coastal ocean—a global synthesis. Cambridge University Press. https://doi.org/10.1017/CBO9780511781247 Ministerio Obras Públicas (1987) Atlas hidrográfico de Chile. Santiago, Chile Miserendino L, Pizzolon L (2000) Macroinvertebrates of a fluvial system in Patagonia: altitudinal zonation and functional structure. Arch Hydrobiol 150:55–83 Moya C, Valdovinos C, Moraga A, Romero F, Debels P, Oyanedel A (2009) Patrones de distribución espacial de ensambles de macroinvertebrados bentonicos de un sistema fluvial Andino Patagónico. Rev Chil Hist Nat 82:425–442 Nakamura F, Swanson F, Wondzell S (2000) Disturbance regimes of stream and riparian systems a disturbance-cascade perspective. Hydrol Proc 14:2849–2860 Odum E (1969) The strategy of ecosystem development. Science 164:262–270 Oyanedel A, Valdovinos C, Azocar M, Moya C, Mancilla G, Pedreros P, Figueroa R (2008) Patrones de distribución espacial de los macroinvertebrados bentónicos de la cuenca del río Aysén (Patagonia chilena). Gayana 72:241–257 Peckarsky B (1984) Biotic interactions or abiotic limitations? A model of lotic community structure. In: Fontaine and Bartell (eds) Dyn Lotic Ecosyst Ann Arbor Sci Ann Arbor MI, USA Poff L (1992) Why disturbances can be predictable. J N Am Benthol Soc 11:86–92 Poff L, Allen J, Bain M, Karr J, Prestegaard K, Richter B, Sparks R, Stromberg J (1989) The natural flow regime. Biosci 47(11):769–784 Pringle C, Naiman R, Bretschko G, Karr J, Oswood M, Webster J, Welcomme R, Winterbourn L (1988) Patch dynamics inn lotic ecosystems: the stream as a mosaic. J N Am Benthol Soc 7:503–544 Quinn JM, Hickey CW (1990) Magnitude of effects of substrate particle size, recent flooding, and catchment development on benthic invertebrates in 88 New Zealand rivers. New Zealand J Mar Freshw Res 24: 411–427. https://doi.org/10.1080/00288330.1990.9516433. Quiroga E, Ortiz P, Reid B, Villagren S, Gerdes D, Quiñones R (2011) Organic enrichment and structure of macrobenthic communities in the Baker Fiord (Chile, Northern Patagonia) and their relationships with environmental factors. J Mar Biol Assoc United Kingdom. https://doi.org/10. 1017/S0025315411000385 Quiroga E, Ortiz P, Reid B, Gerdes D (2013) Classification of the ecological quality of the Aysén and Baker Fjords (Patagonia, Chile) using biotic indices. Mar Pollut Bull. https://doi.org/10.1016/j. marpolbul.2012.11.041 Quiroga E, Ortiz P, González-Saldías R, Reid B, Tapia F, Pérez-Santos I, Rebolledo L, Mansilla R, Pineda C, Cari I, Salinas N, Montiel A, Gerdes D (2016) Seasonal patterns in the benthic realm of a glacial fjord (Martinez Channel, Chilean Patagonia): the role of suspended sediment and terrestrial organic matter. Marine Ecology Progress Series. https://doi.org/10.3354/meps11903 Reid B, Torres R (2013) Didymosphenia geminata invasion in South America: ecosystem impacts and potential biogeochemical state change in Patagonian rivers. Acta Oecologica. https://doi.org/ 10.1016/j.actao.2013.05.003 Reid B, Astorga A, Madriz I, Correa C (2021) Estado de Conservación de Sistemas Dulce-acuícolas en Patagonia. Capítulo 14. In: Castilla J, Armesto J, Martínez-Harms M (eds) Conservación en la

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Patagonia chilena: evaluación del conocimiento, oportunidades y desafíos. Ediciones Universidad Católica, Santiago, Chile, p 598 Relyea C, Minshall G, Dannehy R (2012) Development and validation of an aquatic fine sediment biotic index. Environ Manag 49:242–252 Resh V, Brown A, Covich A, Gurtz M, Li H, Minshall G, Reice S, Sheldon A, Wallace J, Wissmar R (1988) The role of disturbance in stream ecology. J N Am Benthol Soc 7:433–455 Scasso R, Corbella H, Tiberi P (1994) Sedimentological analysis of the tephra from the 12–15 August 1991 eruption of Hudson volcano. Bull Volcanol 56:121–132 Stanford J, Lorang M, Hauer F (2005) The shifting habitat mosaic of river ecosystems. Verhandlungen Internationale Vereinigung Fur Theoretische Und Angewandte Limnologie 29:123–136 Swanson F, Major J (2005) Physical events, environments and geological-ecological interactions at Mounnt St, Helens: March 1980–2004. In: Dale V, Swanson F, Crisafulli C (eds) Ecological responses to the 1980 Eruption of Mount St. Helens. Springer Torres R, Silva N, Reid B, Frangopulos M (2014) Silicic acid enrichment of subantarctic surface water from continental inputs along the Patagonian archipelago interior sea (41°–56°S). Prog Oceanogr. https://doi.org/10.1080/0269249X.2014.890957 Torres R, Reid B, Frangopulos M, Alarcon E, Marquez M, Hausermann V, Forsterra G, Pizarro G, Iriarte J, Gonzalez H (2020) Freshwater runoff effects on the production of biogenic silicate and chlorophyll-a in western Patagonia archipelago (50–51°S). Estuar Coast Cont Shelf Res. https:// doi.org/10.1016/j.ecss.2020.106597 Townsend C, Scarsbroom M, Doledec S (1997) Quantifying disturbance in streams: alternative measures of disturbance in relation to macroinvertebrate species traits and species richness. J N Am Benthol Soc 16:531–544 Ulloa H, Iroume A, Pico J, Mohr C, Mazzorana B, Lenzi L, Mao L (2016) Spatial analysis of the impacts of the Chaiten volcano eruption (Chile) in three fluvial systems. Lat Am J Earth Sci 69:213–225 Ulloa H, Iroume A, Pico J, Korup O, Lenzi M, Mao L, Ravazzolo D (2015) Massive biomass flushing despite modest channel response in the Rayas River following the 2008 eruption of Chaitén volcano, Chile. Geomorphology 250:397–406 Vandekerkhove E, Bertrand S, Reid B, Bartels A, Charlier C (2015) Sources of dissolved silica to the fiords of Northern Patagonia (44–48S): the importance of volcanic ash soil distribution and weathering. Earth Surf Proc Landforms. https://doi.org/10.1002/esp.3840 Vanderkerkhove E, Bertrand S, Lanna E, Reid B, Pantoja S (2020a) Modern sedimentary processes at the heads of Martínez Channel and Steffen Fjord, Chilean Patagonia. Mar Geol. https://doi. org/10.1016/j.margeo.2019.106076 Vandekerkhove E, Bertrand S, Mauquoy D, McWethy D, Reid B, Stammen S, Saunders K, Torrejon F, (2020b) Neoglacial increase in high-magnitude glacial lake outburst flood frequency, upper Baker River, Chilean Patagonia (47°S). Quat Sci Rev 248. https://doi.org/10.1016/j.quascirev. 2020.106572 Vandekerkove E, Bertrand S, Torrejón F, Kylander M, Reid B, Saunders K (2021) Signature of modern glacial lake outburst floods in fjord sediments (Baker River, southern Chile). Sedimentology https://doi.org/10.1111/sed.12874 Vannote R, Minshall G, Cummins K, Sedell J, Cushing C (1980) The river continuum concept. Can J Fish Aquat Sci 37:130–137 Vergara-Jara M, Hopwood M, Browning T, Rapp I, Torre R, Reid B, Achterberg E, Iriarte J (2021) A mosaic of phytoplankton responses across Patagonia, the SE Pacific and SW Atlantic Ocean to ash deposition and trace metal release from the Calbuco 2015 volcanic eruption. Ocean Sci. https://doi.org/10.5194/os-2020-65. Weller G, Moranda C, Moreno P, Villa-Martínez R, Stern C (2015) Tephrochronology of the southernmost Andean Southern Volcanic Zone, Chile. Bull Volcanol 77:107 Wilson R, Glasser N, Reynolds J, Harrison S, Iribarren P, Schaefer M, Shannon S (2018) Glacial lakes of the central and Patagonian Andes. Glob Planet Change 162:275–291

Chapter 3

San Pedro River: A Biological and Cultural Treasure in Northern Patagonia Nicole Colin, Konrad Górski, Juan José Ortiz, Pablo Iriarte, and Ana M. Abarzúa Abstract Northern Patagonian River systems located between the Araucanía and the Los Lagos Regions of Chile are characterized by lake regulation and seasonal predictable flow regime. As these systems originate in large glacial lakes, their flow velocities are lower compared to other Andean River systems in Chile. The San Pedro River (Valdivia River basin) is an iconic largely pristine northern Patagonian River system characterized by rich hydrogeomorphology and biodiversity as well as unique paleontological and human histories. It originates from a chain of eight lakes that generate lacustrine influence in upper zone, followed by a middle section with high slope and flow velocities, lower reaches with developed floodplains, to discharge to the Pacific Ocean in Valdivia Estuary. Consequently, this river system in a stretch of just 100 km accommodates the highest diversity of freshwater fish species in Chile. The paleontological record along San Pedro River system is also highly relevant with fossil deposits found in sedimentary rocks originating between Triassic and Quaternary periods that have allowed historical climate and vegetation reconstructions. Besides, the strongest earthquake registered worldwide in 1960 in this region directly affects the river and human population which has generated respect for nature. In this way, geological history, fossils, and present biodiversity are input to protect the territory from potential threats. N. Colin (B) Instituto de Ciencias Ambientales y Evolutivas, Facultad de Ciencias, Universidad Austral de Chile, Valdivia, Chile e-mail: [email protected] K. Górski Instituto de Ciencias Marinas y Limnológicas, Facultad de Ciencias, Universidad Austral de Chile, Valdivia, Chile J. J. Ortiz Leufu, Consultoría e Investigación Ambiental, Concepción, Chile P. Iriarte Instituto de Historia y Ciencias Sociales, Facultad de Filosofía y Humanidades, Universidad Austral de Chile, Valdivia, Chile A. M. Abarzúa Instituto de Ciencias de la Tierra, Facultad de Ciencias, Universidad Austral de Chile, Valdivia, Chile © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. Oyarzún et al. (eds.), Rivers of Southern Chile and Patagonia, The Latin American Studies Book Series, https://doi.org/10.1007/978-3-031-26647-8_3

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Keywords San Pedro River · Free-flowing · Biodiversity · Fossil deposits · El Riñihuazo

3.1 Introduction The San Pedro River is a river system that drains the chain of eight Andean lakes in the Valdivia River basin (Los Ríos Region, Chile). This system covers 40 km between the mouth of Riñihue Lake and confluence with the Collileufu River, where it is renamed to Calle-Calle River (Fig. 3.1 and 3.2, Habit and Parra 2012) until confluence with Cruces River, where it is again renamed to the Valdivia River (Fig. 3.1 and 3.2). Regardless of its names, it is a continuous river system between the Riñihue Lake and the estuary structured according to the geomorphology of the channel. The Valdivia River basin is located in northern Patagonia between 39° and 41° latitude south (Fig. 3.1). This zone is characterized by high Andes Mountain range that gradually decreases toward the south. The climate is characterized by high rainfall with annual values reaching 2400 mm. Precipitation is present throughout the year with the highest concentration in winter (DMC 2017). However, within its natural dynamics, periods without precipitation may occur for up to 30 consecutive days. Such events are increasingly recurrent in the last decade and cause droughts and low river flows (González-Reyes and Muñoz 2013). San Pedro–Calle-Calle–Valdivia river system belongs to the northern Patagonian rivers (Araucanía and Los Ríos Regions) described as calm rivers with lacustrine

Calafquén Lake

Pullinque Lake

Cruces River

Pellaifa Lake

Panguipulli Lake San Pedro River Calle-Calle River Valdivia River

Neltume Lake Riñihue Lake Pirihueico Lake

Fig. 3.1 Location of Valdivia River basin in Chile. In the right box San Pedro River location inside Valdivia River basin

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Fig. 3.2 San Pedro River from Malihue bridge. Credit Nicole Colin

regulation (Niemeyer and Cereceda 1984). Precisely, the regulating effect of the flow produced by headwater lakes is a distinctive attribute of these rivers. High precipitation in the region strongly influences vegetation patters and gives origin to the Valdivian temperate rainforest characterized by tree species belonging to Myrtaceae, Lauraceae, and Nothofagaceae families Villagrán and Hinojosa (1997) (Luebert and Pliscoff 2006). As a consequence of the presence of large lakes and chains of lakes, abundant native vegetation river systems of the region are characterized by oligotrophic and ultra-oligotrophic conditions (Tomason 1963; Campos 1987). Furthermore, lakes located in the intermediate zone of the basin produce a discontinuity in the flow of majority of the rivers in the region. As such, streams and rivers located upstream of these lakes have higher flow velocities with only some degree of regulation by the volcanic soil and vegetation (Niemeyer and Cereceda 1984). In contrast, rivers downstream of large lakes such as San Pedro River are characterized by lake-regulated low flow velocities different from the majority of Chilean Andean rivers (Habit et al. 2012). In lower reaches of northern Patagonian rivers, significant accumulation of sediment has been observed as result of anthropogenic land-use changes and reduction of forest cover in the Central Valley and coastal mountain ranges. Indeed, Valdivia River (downstream of San Pedro and Calle-Calle rivers) requires continuous sediment dragging to maintain its function as expedited transport waterway.

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3.2 Characteristics, Origin and Morphometry Large lakes that characterize the Valdivia River basin (Fig. 3.3) originated in processes of erosion and glacial deposition during the last glaciation that occurred between 70,000 and 19,000 years BP (Clapperton 1993; Denton et al. 1999). Many of these lakes are characterized by elongated shape of a typical inland fjord with fingershaped shorelines inherent to their glacial origin (Clapperton 1993; Laugenie 1984). Downstream of the lake complex, rithral zone with high flow velocity is restricted to a short stretch and low altitude followed by wider channels and flood plains before reaching the estuary (Fig. 3.3). Northern Patagonian rivers, in comparison with rivers located on more northern and southern areas of Chile, are characterized by marked flow seasonality principally driven by high rainfall between July and September, and low rainfall between December and May (Fig. 3.4). The abundant rainfall in the area that could produce flows with high daily variability is regulated by the headwater lakes, giving rise to an abundant volume of water with little daily variability, but with marked seasonality. In the middle and lower part of the systems, a daily fluctuation governed due to local rainfall contributions. This strong seasonality of the flow also produces predictable floods in these rivers, which are essential for the ecological functioning of these ecosystems (De Los Ríos-Escalante et al. 2015; Ramírez-Álvarez et al. 2022). Next to flow regulation, large headwater lakes also directly contribute to the food-web of the river by abundant production of food items (zooplankton) consumed by fish along the river including its lower reaches (Manosalva et al. 2021). 641 60 Lacar 277 m

Pirihueico 145 m

221 Pellaifa 9m

38 msnm

Pullinque Neltume 25 m 86 m Calafquén 212 m

San Pedro River Panguipulli 268 m

Riñihue 117 m

Calle Calle River

Valdivia River

Fig. 3.3 Diagram of Valdivia River basin structure, characterized by presence of a chain of eight lakes of glacial origin (altitude is indicated on the y axis, depth below each lake’s name). The first lake in the chain, located at 641 m above sea level, is located in Argentina (red line represents the border)

Fig. 3.4 Comparison of the average daily discharge between the Riñihue lake outflow (San Pedro River, blue line) and the lower part of the Valdivia River (Calle-Calle River)

Daily discharge (m3s-1)

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Valdivia (San Pedro - Calle-Calle)

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F

M

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3.3 Physicochemical Characteristics of Water and Sediments In northern Patagonian rivers, the water temperature presents a marked seasonal variability with minimum temperatures in winter, an increase in spring, and maximum temperatures in summer and autumn (Fig. 3.5). Minimum temperatures oscillate around 10 °C, while maximum around 16 °C. This thermal pattern is consistent with the interannual air temperature variability characteristic of the temperate climate with Mediterranean influence. The concentration of dissolved oxygen is negatively correlated with water temperatures and is characterized by significant differences among seasons with maximum values in winter and spring and lower values in summer and autumn (Fig. 3.5). However, the differences between the minimum and maximum concentrations are around 1 mg/L, varying between 9 mg/L and 10 mg/L so the river is characterized by highly oxygenated waters in all seasons. Electric conductivity in Calle-Calle River is characterized by high variability with some seasonal differences (Fig. 3.5). The dilution effect due to seasonal flow differences (greater dilution in winter) drives these variations. Nitrogen (nitrates) and phosphorus (Kjeldahl phosphorus) concentrations in the Calle-Calle River are presently low; however, increasing intensity of anthropogenic activities (e.g., sewage discharge, aquaculture, industrial effluents, and agriculture-livestock) may cause increase of nutrient concentration over time (Colin et al. 2022).

3.4 Biological Characteristics The composition of biological communities, exemplified by fish fauna present in the main northern Patagonian rivers, shows a similar species composition in the Toltén, Bueno, and Valdivia River basins. However, fish fauna richness in Valdivia is the

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Fig. 3.5 Interannual variability of temperature, dissolved oxygen, and electric conductivity in waters of the Calle-Calle River (monitoring data recorded by Chilean General Directorate of Waters for the period between 1984 and 2013)

highest in northern Patagonian rivers and Chilean rivers systems in general with 22 species of which 17 are native and 5 introduced (Table 3.1). The most represented species are the galaxids with 6 species, 4 of them present in the four basins, and one registered only in Valdivia currently. The low representativeness of introduced species grants a high degree of naturalness to the present communities. In turn, low intensity of human-induced stressors in most of the sections, due to the absence of large industries and hydropower plants, prevents greater biological invasions and allows the Valdivia River basin, especially San Pedro River to maintain a good ecological status.

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Table 3.1 Species composition of the fish communities in northern Patagonian rivers Family Species Toltén Valdivia Bueno Maullín River Geotriidae

Geotria australis

Petromyzontidae

Mordacia lapicida

Diplomystidae

Diplomystes camposensis

Trichomycteridae

Hatcheria macraei

River

River

River

Trichomycterus areolatus Aplochiton taeniatus Aplochiton marinus Galaxidae

Aplochiton zebra Brachygalaxas bullocki Galaxias maculatus Galaxias platei

Atherinidae Atherinopsidae

Basilichthys microlepidotus Odontesthes mauleanum Odontesthes brevianalis

Percichthydae

Percichthys trucha

Perciliidae

Percilia gillissi

Characidae

Cheirodon australe

Cyprinidae

Cyprinus carpius

Poecilidae

Gambusia holbrooki

Salmonidae

Oncorhynchus tshawytscha

Salmonidae

Onchorhynchus mykiss

Salmonidae

Salmo trutta

Valdivia River basin registers the greatest richness (Arratia and Quezada-Romegialli 2017; Colin et al. 2012; Habit et al. 2010, 2015; Unmack et al. 2009)

The outstanding units of these rivers are the large lakes in the headwaters of the basins that not only influence the river flows, but also the ecological functioning of the whole system (e.g., trophic interactions). The influence of large headwater lakes causes a minimal sediment load and a very low contribution of nutrients to the system (Niemeyer and Cereceda 1984). Lake-driven flow regime, water quality as well as invertebrate and fish recruitment lacustrine habitats result in unique biological communities and ecological interactions in the San Pedro–Calle-Calle–Valdivia river system. These interactions are reflected in the diet of the most common fish species that inhabits the system puye Galaxias maculatus. Low diet diversity may be observed in the San Pedro River just below the lake outflow; the diet of the puye is composed exclusively of zooplankton. In the middle zone (Calle-Calle River), the

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Fig. 3.6 Diet (stomach content) of puye Galaxias maculatus in San Pedro–Calle-Calle–Valdivia river system

diet diversifies and also includes benthic prey (dipteran and mayfly larvae) more abundant due to increase of organic matter from tributaries and floodplains sources. In the estuary, the diet of puye still includes zooplankton and diptera larvae but in lesser proportion and becomes dominated by amphipods (Fig. 3.6). Furthermore, Gorski et al. (2018) suggest from sulphur isotope analysis that G. maculatus could ingest marine organisms as part of its diet in the Valdivia estuary, but less frequently than southern Patagonian rivers. Figure 3.7 summarizes eco-hydrogeomorphological functioning of the San Pedro–Calle-Calle–Valdivia river system considering three dimensions: longitudinal, lateral, and temporal that strongly influence structuring of biological communities. The longitudinal axis shows three important reaches along the river, upstream, middle, and estuarine zone. The lateral axis characterized by a central channel which widens as the slope of the river decreases. Strong intra- and interannual temporal changes influenced by rainfall and snow melting that drive floodplain development in middle and lower reaches. Floodplains strongly influence biological communities in these river zones as they drive high aquatic productivity of diverse food items for the river food-web and provide spawning and refuge habitats for fish (e.g., Galaxias maculatus, Basilychthys microlepidotus). The longitudinal dimension of the river is mainly determined by the slope and strong lacustrine influence.

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Fig. 3.7 Eco-hydrogeomorphological functioning of the San Pedro–Calle-Calle–Valdivia river system

3.5 Paleontological Heritage In the Los Ríos region, specifically along the San Pedro River, there are fossil deposits corresponding to Neogene sedimentary strata. These fossil records are found on the banks of the river, in greater abundance in the Malihue place and correspond to plant species. Specifically, these strata represent a continental sequence containing several botanical fossils, such as wood and foliar imprints, which age corresponds to the Oligocene–Miocene boundary (Sandoval et al. 2018). According to Hinojosa and Villagrán (1997), the forests of Chile between 38 and 50° S present three successions of Paleofloras during the Cenozoic: neotropical, subtropical, and mixed. The strata of San Pedro, therefore, according to their temporal dating, should contain representative elements of the so-called Mixed Paleoflora, which stands out for its wide temporal and spatial distribution. Records from San Pedro are characterized by Neotropical taxonomic genera such as Beilshmiedia, Pantropical such as Lomatia, and Austral-Antarctic as Nothofagus

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(Sandoval et al. 2018). These findings have also allowed the reconstruction of the past vegetation and its climate. Therefore, the climate that prevailed in the San Pedro River during the Oligocene–Miocene would have been warmer than the current one, and with less rainfall. In addition, the paleontological heritage has a socio-cultural meaning for the inhabitants of the rural areas surrounding these sites. Although, there is no marked connection with the scientific field, given the incipient development of the dissemination of the findings. In any case, they see fossils as an input to protect the territory from potential threats such as the construction of a hydroelectric plant. Being part of the river and being located below the remaining forests in some sectors, they are valued in order to protect the fossil deposits, the territory and the river itself and its associated biodiversity. Furthermore, it is obtained that the appreciation of the fossils by the inhabitants adds a greater temporal depth to the territory, allowing them to recognize the geology of the place and imagine the remote past of the place they inhabit, and relate the presence of the old, extinct, and current native forest, which it could not be generated or transmitted without the foliar imprints that can be observed in the territory today (Campos-Medina et al. 2018).

3.6 The Culture of Nature in the San Pedro River Like the river, those who inhabit it have an agitated history. The valleys of the San Pedro River have been an area of exchange and interaction of great intensity and dynamism for at least 500 years. The millenary indigenous settlement crashed with the colonization carried out during the sixteenth and nineteenth centuries. Different modes of inhabiting the fluvial territory have generated diverse cultural identities strongly shaped by the relationship with water that “appears as a natural matrix that shapes self-perception and the relationship with other groups” (Skewes et al. 2012).1 As in any fluvial society, the interaction between human groups and the river enables and defines traditional agricultural and livestock activities. Furthermore, the San Pedro was an important route for exchange used by logging industry since its beginnings in the twentieth century. Its use included its connection with railway roads that defined the development and importance of the city of Los Lagos and towns such as Huellelhue and Antilhue (Contreras et al. 2016; Moya and Vásquez 2014; Almonacid 1995). Nevertheless, the recent history of the basin is, above all, defined by big ecological changes. One of the events that has been more strongly imprinted in the memory of its inhabitants is the great earthquake of May 22, 1960. Known for being the largest earthquake ever measured (9.5 MW), it violently shook the south of Chile producing a 1000 km break between the Gulf of Arauco and the Taitao Peninsula (Rojas 2018; Araya-Cornejo 2014; Davis and Karzulovic 1963). Followed by a tsunami and the eruption of Cordón Caulle volcanic complex, the earthquake produced numerous 1

The translation is ours.

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human losses: at least 2300 people died and 3000 were injured throughout the country. Also, the vast destruction left more than 2,000,000 people homeless (Contreras et al. 2016; Hernández 1975). The earthquake produced large landslides among the hills that flank the river, blocking its flow a few kilometers downstream from its origin in Lake Riñihue. In response to the blockage of the river caused by the earthquake, one of the largest and most remembered collective efforts in socio-environmental history of Chile was organized: “Riñihue Operation”. Engineers, topographers, and 450 “paleros”2 incessantly worked for two months in order to establish a new channel that would allow a controlled drainage of the stagnant waters of the San Pedro. The objective was to avoid a violent rupture of the blocks or “tacos” that dammed the river, a rupture that would have resulted in an uncontrolled flooding downstream where the population in those years already numbered more than 100,000 inhabitants. In fact, old colonial chronicles recount the occurrence of a similar event in 1575 which resulted in great material destruction and the loss of hundreds of lives downriver (Mariño de Lobera 1865). But in 1960, history changed. After two months of working untiringly, the “paleros” made possible the controlled drainage of the river. Otherwise, 2.5 billion cubic meters of water stored up to that point would have been abruptly emptied. Both direct observers of the events and later compilers of local tales had recorded the dramatic moments experienced by tens of thousands of peoples who followed day by day, in tense waiting, the news about the progress of the operation until its succesful ending (Haefele et al. 2018; Castedo 2005; Hernandez 1975; Castedo 1961). The workers and professionals who participated in the operation were condecorated and the episode, known since then as “El Riñihuazo”, is still commemorated every year as a collective feat and an example of social resilience.3 But this event is also a reminder of the fragility of social life in the area and how strongly its culture is influenced, through experience and memory, by the changes in nature.

3.7 Nature’s Contributions to People and Human-Induced Stressors River systems provide multiple contributions to people, many of which are essential for our subsistence. Most are directly related to the natural functioning of the river system, such as biodiversity, nutrient cycling, and anthropocentric properties such as flood control and recreation (Gilvear et al. 2016; Thorp et al. 2010). In the case of the San Pedro and Calle-Calle rivers, these provide important contributions such as the supply of drinking water for important cities in the region. In addition, the increasingly intensive use of lowland areas for productive activities such as agriculture, livestock, and industrial processes (e.g., dairies, cellulose) creates an increasingly 2

A word used to name workers who use shovels (‘palas’). An example of an organization that carries out memory activation each year around this event is Fundación Proyecta Memoria: http://proyectamemoria.cl/.

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intensive use of water extraction for irrigation and processes. Valdivia River basin is also of great tourist interest, mainly due to the presence of large lakes and forests. Large headwater lakes and rivers are also used for recreational navigation. San Pedro–Calle-Calle–Valdivia river system is characterized by high hydroelectric potential. The river is still free flowing but hydroelectric projects have been proposed (Palma 2019). Construction of dams within the river would cause significant alterations in ecological functioning of the river and biological communities (García et al. 2012; Díaz et al. 2019). The most affected species would be those that are migratory or characterized by high gene flow within the basin. Microendemic and highly mobile species, such as Diplomystes camposensis, could be the most affected (Oyanedel et al. 2018; Habit et al. 2009). Climate change is expected to cause significant changes in ecological functioning of the San Pedro–Calle-Calle–Valdivia river system in coming decades. Significant temperature increases and rainfall decreases (~30%, Araya-Osses et al. 2020) are expected that will affect river flow and water quality.

References Almonacid F (1995) Valdivia, 1870–1935. Valdivia, Instituto de Ciencias Históricas, Universidad Austral de Chile, Imágenes e historias Araya-Cornejo C, Cisternas M, González F (2014) Evolución morfológica del principal deslizamiento del “Riñihuazo”, generado por el terremoto de 1960. https://doi.org/10.13140/RG.2.1. 4403.9520 Araya-Osses D, Casanueva A, Román-Figueroa C, Uribe JM, Paneque M (2020) Climate change projections of temperature and precipitation in Chile based on statistical downscaling. Abstr Clim Dyn 54(9–10):4309–4330. https://doi.org/10.1007/s00382-020-05231-4 Arratia G, Quezada-Romegialli C (2017) Understanding morphological variability in a taxonomic context in Chilean diplomystids (Teleostei: Siluriformes), including the description of a new species. PeerJ 5: e2991 Campos H, Steffen W, Aguero G, Parra O, Zúñiga L (1987) Limnology of lake Riñihue. Limnologica (Berlin) 18(2):339–357 Campos-Medina J, Vergara-Pinto F, Parra AE, Fuentes PC, Abarzúa AM (2018) Resignificación del patrimonio paleontológico presente en el río San Pedro (Cuenca del río Valdivia, Chile). PASOS Revista De Turismo y Patrimonio Cultural 16(3):655–670 Castedo L (Director) (1961) La Respuestas. (Video documental). Universidad de Chile, Santiago Castedo L (2005) Hazaña del Riñihue. El terremoto de 1960 y la resurrección de Valdivia. Crónica de un episodio ejemplar en la historia de Chile Clapperton CM (1993) Quaternary geology and geomorphology of South America. Elsevier, Amsterdam, The Netherlands Colin N, Piedra P, Habit E (2012) Variaciones espaciales y temporales de las comunidades ribereñas de peces en un sistema fluvial no intervenido: río San Pedro, cuenca del río Valdivia (Chile). Gayana 76:01–09 Colin N, Habit E, Manosalva A, Maceda-Veiga A, Górski K (2022) Taxonomic and functional responses of species-poor riverine fish assemblages to the interplay of human-induced stressors. Water 14(3):355 Contreras P, Concha R, Correa M, Guerrero I, Vergara F (2016) Relatos de paisaje y toponimia en el valle de los ríos San Pedro y Calle Calle. CEIBO: Santiago

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Davis S, Karzulovic J (1963) Landslides at Lago Riñihue, Chile. Bull Seismol Soc Am 53(6):1403– 1414 De Los R-E, Górski K, Habit E, Manosalva AJ (2015) First observations of crustacean zooplankton abundances in northern Patagonian rivers. Crustaceana 88(5):617–623 Denton GH, Lowell TV, Heusser CJ, Schlüchter C, Andersen BG, Heusser LE, Marchant DR (1999) Geomorphology, stratigraphy, and radiocarbon chronology of Llanquihue Drift in the area of the Southern Lake District, Seno Reloncaví, and Isla Grande de Chiloé, Chile. Geogr Ann Ser B 81(2):167–229 Dirección Metereológica de Chile (DCM) (2017) Informe Anual de Agua Caída, https://climatolo gia.meteochile.gob.cl/application/anual/aguaCaidaAnual/390006/2017 Díaz G, Arriagada P, Górski K, Link O, Karelovic B, Gonzalez J, Habit E (2019) Fragmentation of Chilean Andean rivers: expected effects of hydropower development. Revista Chilena de Historia Natural 92(1). https://doi.org/10.1186/s40693-019-0081-5 García A, González J, Habit E (2012) Caracterización del hábitat de peces nativos en el río San Pedro (cuenca del río Valdivia, Chile). Gayana 76:36–44 Gilvear DJ, Greenwood MT, Thoms MC, Wood PJ (eds) (2016) River science: research and management for the 21st century. Wiley Gonzalez-Reyes A, Munoz AA (2013) Precipitation changes of Valdivia city (Chile) during the past 150 years. Bosque 34(2):191–200 Górski K, Habit E, Pingram MA, Manosalva AJ (2018) Variation of the use of marine resources by Galaxias maculatus in large Chilean rivers. Hydrobiologia 1–13 Habit E, Parra O (2012) Fundamento y aproximación metodológica del estudio de peces del Río San Pedro. Gayana (Concepción) 76:01–09 Habit E, Jara A, Colin N, Oyanedel A, Victoriano P, Gonzalez J, Solis-Lufí K (2009) Threatened fishes of the world: Diplomystes camposensis Arratia, 1987 (Diplomystidae). Environ Biol Fishes 84(4):393–394 Habit E, Piedra P, Ruzzante DE, Walde S, Belk M, Cussac V, Gonzalez J, Colin N (2010) Changes in the distribution of native fishes in response to introduced species and other anthropogenic effects. Glob Ecol Biogeogr 19:697–710 Habit E, González J, Ortiz-Sandoval J, Elgueta A, Sobenes C (2015) Effects of salmonid invasion in rivers and lakes of Chile. Ecosistemas 24(1):43–51 Haefele V, Olivares H, Contreras C, Herrera N, Flores C (2018) Riñihuazo: Memorias de un desastre, Correa K, Muñoz J (eds). ISBN: 978–956–393–753–4 Hernández L (1975) Catástrofe en el paraíso. La Discusión, Chillán, Chile Laugenie C (1984) Le dernier cycle glaciaire quaternaire et la construction des nappes fluviatiles d’avant-pays dans les Andes chiliennes (38–42° de latitude Sud). Quaternaire 21(1):139–145 Luebert F, Pliscoff P (2006) Sinopsis bioclimática y vegetacional de Chile. Editorial Universitaria, Santiago de Chile Manosalva AJ, Pérez S, Toledo B, Colin N, Habit EM, Górski K (2021) Variation of stomach content and isotopic niche of puye Galaxias maculatus (Jenyns, 1842) in large river systems of Southern Chile. Freshw Biol 66(6):1110–1122 Mariño de Lobera P (1865) Crónica del Reyno de Chile (1528–1594). Colección de Historiadores de Chile, Tomo VI. Santiago, Chile Moya L, Vásquez N (2014) Relatos de balseros de los ríos San Pedro y Calle-Calle (1930–1960). Serifa, Santiago Niemeyer H, Cereceda P (1984) Geografía de Chile. Hidrografía. Instituto Geográfico Militar. Santiago, p 320 Oyanedel A, Habit E, Belck M, Solis-Lufi K, Colin N, González J, Jara A, Muñoz-Ramírez (2018) Movement patterns and home range in Diplomystes camposensis Arratia 1987 (Siluriformes: Diplomystidae), an endemic and threatened species from Chile. Neotropical Icthiology 18(1):1– 14. Palma L (2019) Historia de la central hidroeléctrica San Pedro. Kultrún, Valdivia, Chile

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Ramírez-Álvarez R, Contreras S, Vivanco A, Reid M, López-Rodríguez R, Górski K (2022) Unpacking the complexity of longitudinal movement and recruitment patterns of facultative amphidromous fish. Sci Rep 12(1):1–12 Rojas C (2018) Valdivia 1960. Universidad Austral de Chile, Valdivia, Entre aguas y escombros Sandoval C (2018) Análisis florístico y reconstrucción climática en base a ensambles foliares de los estratos de San Pedro, Región de Los Ríos. Universidad Austral de Chile, Chile. Tesis Magister en Paleontología Skewes J, Solari M, Guerra D, Jalabert D (2012) Los paisajes del agua: naturaleza e identidad en la cuenca del río Valdivia. Chungara Revista De Antropología Chilena 44(2):299–312 Tomason K (1963) Araucanian lakes: plankton studies in North Patagonia with notes in terrestrial vegetation. Acta Phytogeographyca Suecica 47:139 Thorp JH, Flotemersch JE, Delong MD, Casper AF, Thoms MC, Ballantyne F, Williams BS, O’Neill J, Haase CS (2010) Linking ecosystem services, rehabilitation, and river hydrogeomorphology. Bioscience 60(1):67–74 Unmack PJ, Habit EM, Johnson JB (2009) Nuevos registros de Hatcheria macraei (Siluriformes, Trichomycteridae) en la Provincia Chilena. Gayana 73(1):102–110 Villagrán C, Hinojosa LF (1997) Historia de los bosques del sur de Sudamérica. II: Análisis fitogeográfico. Revista Chilena de Historia Natural 70(2): 1–267 Wilkes MA, Maddock I, Link O, Habit E (2016) A community-level, mesoscale analysis of fish assemblage structure in shoreline habitats of a large river using multivariate regression trees. River Res Appl 32(4):652–665

Chapter 4

Large Wood Research and Learning in Chile Héctor Ulloa and Andrés Iroumé

Abstract Large wood (LW, wood pieces with diameter ≥ 10 cm and length ≥ 1 m) is present in rivers as individual elements or forming accumulations. During the 1970s, the study of LW in rivers began to be massified in the USA, mainly in the assessment of its effect on fish (Swanson et al., Earth Surface Processes and Landforms 46:55– 66, 2021). It is essential for the protection and restoration of aquatic ecosystems and geomorphological stability. In Chile, the first studies began in 2005 in mountain basins. Subsequently, studies are carried out on the contribution of LW and its impact after volcanic eruptions (Chaitén and Calbuco volcanoes). Among the results in Chile stand out: in Andean basins, LW volumes are very abundant, comparable to data from the Pacific northwest of North America; the amounts of LW are very variable between basins, because of the characteristics of the riverine forest and the level of alteration in forest basins; between 0 and 28% of annual mobility have been determined. In addition, volcanic eruptions can generate large LW inputs and mobility between 42 and 94%. Monitoring can be done from fieldwork, satellite imagery and lately using UAV flights. These learnings have made it possible to understand the role of LW and the processes involved. However, there is still a lack of studies for a better understanding of the processes, and on the other hand, disseminating the benefits and risks associated with LW. Keywords Large wood · Mountain streams · Long term learning · Chile

4.1 Introduction One of the greatest interactions that occur between terrestrial and aquatic ecosystems is the contribution of organic material from riparian vegetation to the channels H. Ulloa (B) · A. Iroumé Faculty of Forest Sciences and Natural Resources, Universidad Austral de Chile, Valdivia, Chile e-mail: [email protected] H. Ulloa Instituto Forestal, Forest Ecosystems and Water, Valdivia, Chile © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. Oyarzún et al. (eds.), Rivers of Southern Chile and Patagonia, The Latin American Studies Book Series, https://doi.org/10.1007/978-3-031-26647-8_4

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(Swanson and Lienkaemper 1978), while vegetation cover on the banks of channels provides an environment conducive to the development of aquatic communities. The contribution of leaves from trees and fine organic debris is an important source of nutrients for aquatic biota. The fall of trees and larger organic debris define the physical and biological characteristics of mountain channels. In this sense, wood from logs and trees plays an integral role in the production and maintenance of the morphology of the channel and the aquatic and riparian habitats of a basin. In addition, the amount and type of woody material existing in the main channel depend on the characteristics of the forest that feeds it (e.g., density, age and physiological conditions) and the processes that regulate its incorporation (Comiti et al. 2006). Large wood (LW) refers to any piece of wood that is associated with rivers or streams, whose dimensions exceed 10 cm in diameter and 1 m in length (Lenzi et al. 2006). This wood can be found as individual elements or accumulate (Wood Jams; WJ). In the 1970s, the study of LW in rivers began to become widespread in the USA, mainly in the evaluation of its effect on fish (Swanson et al. 2021). Currently, the study of wood is fundamental to understanding its role in the protection and restoration of aquatic ecosystems (Gregory et al. 2003; Roni 2019), the geomorphological stability of river channels (Gurnell et al. 2002) and the associated hazards in mountainous environments (Mazzorana et al. 2018). In Chile, the study of large woody material began around 2005, first with work carried out in the Andes Mountain range and then in the Coastal Range (Andreoli et al. 2007; Iroumé et al. 2010). In addition, large woody material has been extensively studied, for example, associated with morphological changes after volcanic eruptions (Ulloa et al. 2015; Tonon et al. 2017; Iroumé et al. 2019). Based on studies carried out in Chile in Andean basins, the volumes of wood are very abundant; in fact, data reported by Andreoli et al. (2007) are only comparable with values recorded in the mature native forests of North America, near the coast of the Pacific Ocean. The woody material is highly variable between basins, and this variability is mainly due to the characteristics of the riparian forest, with a potential supply of wood to the riverbed. However, the forest characteristics of each basin are mainly determined by the degree of natural or anthropic alteration and the intensity of land use to which it has been subjected over time (Andreoli et al. 2007; Ulloa et al. 2011). Wood can present an annual mobility between 0 and 28% in low-order mountain basins, with a tendency for greater transport in years where the maximum water level exceeds the level of the full channel (Bankfull level; Iroumé et al. 2015). Volcanic eruptions generate one of the greatest geomorphological modifications in the channels (Major et al. 2016) and, with them, large contributions of woody material. The number and volume of wood can exceed several tens of times the pre-eruption values (Ulloa et al. 2015). For example, the N of LW/km increased from 1.6 to 74.3 between pre- and post-eruption in one of the rivers affected by the eruption of the Chaitén Volcano. In these rivers, the mobility of wood during periods of flooding is considerably high, with values between 48 and 78%, according to reports by Ulloa et al. (2015), while Tonon et al. (2017) reported values between 42 and 94%.

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Methods for monitoring wood in rivers are extensive; the most basic procedure corresponds to the work carried out directly in the field. However, this is slow since it is only possible to cover small sections of rivers or streams of low order. Therefore, it is too expensive to monitor larger spaces (Marcus et al. 2002). However, technology has made the application of multiple tools possible, making monitoring faster and able to cover larger areas at a lower final cost. Some examples include the application of Geographic Information Systems (GIS) for the identification of woody material using high-resolution satellite images (Marcus et al. 2002; Leckie et al. 2005). Recently, the use of drones has been incorporated in a very satisfactory way, both to monitor the woody material and to obtain estimates of the available volume, through the structure from motion photogrammetric technique (Sanhueza et al. 2018a, b, 2019; Spreitzer et al. 2020). The knowledge gained from different investigations carried out in the last 1.5 decades has demonstrated that riparian systems in forested basins are not only water and mineral sediment but also contain woody material of different proportions and dimensions. In addition, they have shown the high complexity of this material and how to prevent potential damage associated with wood transport processes in mountain basins or in areas of human or natural alterations, such as fires or volcanic eruptions. Despite these significant advances, there is still work to be done for a full understanding of these processes and to ensure that those who work directly in these environments, whether carrying out civil works, planning the territory or protecting ecological restoration of riverside spaces, can consider wood and its implications.

4.2 Vegetation Chile is characterised by a great variety of landscapes, from the desert zone in the north to the cold or polar zone in the extreme south. It is in the central/southern zone where warm temperate climates are found with the presence of greater tree cover. From the geographical point of view, this area is divided into three large relief units: Coastal Range (to the west), Andes Mountain range (to the east) and Central Depression between these two mountain ranges. Historically, the landscapes of the Central Depression and part of the Coastal Range have been strongly transformed, mainly because of agricultural and forestry activities. For example, in the late nineteenth and early twentieth centuries in the Coastal Range (approximate latitudes between 36° and 38°), extensive areas of wheat replaced large areas of native forest (Millán and Carrasco 1993). In the 1970s, forest expansion intensified into fast-growing plantations with species, such as Pinus radiata (D. Don) (Pellet et al. 2005). Figure 4.1 shows the changes and degradation of vegetation around the studied basins, as reported by Lara et al. (2012), and the location of the basins. Today, many of these Coastal Range basins have been covered with plantations of E. globulus (Labill). The forests of this geographical area can be characterised as being in a transition between the Mediterranean and temperate rainy climates, with the presence of both evergreen and deciduous species. Further south, the history of

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Fig. 4.1 Temporal variation of land use cover in the geographic area where catchments are located. Additionally, catchments are indicated in blue circles; (1) Pichún, (2) Vuelta de Zorra, (3) El Toro, (4) Tres Arroyos and (5) Chaitén. Source Lara et al. (2012)

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land use in the Coastal Range is different; however, these regions share a history of degradation. This area began to intervene in a more systematic way from the midnineteenth century, with German colonisation. Since that time, the lumber industry has developed, based on the exploitation, manufacture and sale of Alerce wood (Fitzroya cupressoides (Mol.) Johnston) and other high-value native species. The Cordillera de los Andes presents vegetation with less degraded forests, dominated by Nothofagus species that correspond mainly to the Roble-Raulí-Coihue forest types (N. obliqua, N. alpina and N. dombeyi, respectively) and the Araucaria forest type (Araucaria araucana (Mol.) K. Koch), at altitudes greater than 1200–1300 m. Its southernmost part is dominated by the evergreen forest type (Donoso 1981; Schlegel 2001). In general, they correspond to adult forests, with trees that can reach 1–2 m in diameter and a maximum height between 40–50 m. The dynamics of these forests are highly characterised by the occurrence of natural events, such as landslides, fires and volcanic eruptions.

4.3 Basins Studied and Their Characteristics In Chile, there have been five basins where studies linking LW with respect to abundance and morphological processes have been carried out. The first four (Pichún, Vuelta de Zorra, El Toro and Tres Arroyos) are smaller basins with third-order channels. Pichún and Vuelta de Zorra are located in the Coastal Range, while the remaining two are in the Andes Mountain range. A fifth basin corresponds to the Chaitén River, which is also located in the Andes Mountain range. This last basin drains an important part of the slopes of the Chaitén volcano and was therefore strongly affected during the last eruption that occurred in 2008 (more details and main studies are shown in Table 4.1). From the watersheds of the Coastal Range, Pichún, which was originally covered by native forests, has underwent heavy intervention through logging and extensive burning since the early 1600s. The land was used in agriculture for the production and export of wheat to North America and Australia during the late nineteenth and early twentieth centuries (Cisternas et al. 1999). From the early 1950s to the present, this basin has been reforested with multiple rotations, with plantations of Pinus radiata and later with Eucalyptus globulus. The current native forest corresponds to smaller trees that are marginalised in riparian zones, which are mixed with trees left behind from old P. radiata plantations (for more details, see Ulloa et al. 2011; Iroume et al. 2020). Vuelta de Zorra is located in an area with native forests of second growth, originally covered by forests of the Evergreen and Alerce forest types. Since the mid-nineteenth century, there has been a timber industry based on the exploitation of Alerce (Fitzroya cupressoides (Mol.) Johnston) and other high-quality native woods. Between the 1990s and the early 2000s, this basin was intervened in by a new lumber industry that sought to harvest the native forest and replace it with plantations of Eucalyptus spp. Currently, the area is within a reserve created in 2003 (Valdivian Coastal Reserve) and managed by TNC (The Nature Conservancy; Farías and Tecklin

5.87 2300

150–200 year old second-growth evergreen native rainforest (75%) and 24% by Eucalyptus nitens plantations

4.31

1150

Eucalyptus globulus plantations (84%) and native riparian vegetation (16%)

Ulloa et al. (2011), Iroumé et al. (2011, 2014, 2015, 2018 and 2020)

Total catchment area (km2 )

Mean precipitation (mm)

Principal land cover and recent disturbances

Main articles

Ulloa et al. (2011), Iroumé et al. (2010, 2011, 2014, 2015, 2018 and 2020)

Coastal mountain range 39˚ 58' 12'' S; 73˚ 34' 13'' W

Coastal mountain range

Location

37˚ 30' 12'' S; 72˚ 45' 54'' W

Vuelta de Zorra

Pichún

Andreoli et al. (2007), Comiti et al. (2008), Mao et al. (2013), Iroumé et al. (2014, 2018 and 2015), Picco et al. (2019)

Covered by Coigüe-Raulí-Tepa, an evergreen type of forest. In 2000, a catastrophic fire affected nearly 98% of the forest cover

3000

17.83

38˚ 09' 11'' S; 71˚ 48' 12'' W

Andes range

El Toro

Old growth evergreen native forests (44%) and shrub forests (40%). About 60% of the vegetation of the basin was damaged by the Chaitén volcano eruption (2008)

3200

70

42° 50' 1'' S; 72° 39' 7'' W

Andes range

Chaitén

Andreoli et al. (2007 and Ulloa et al. 2008), Comiti et al. (2008), (2015), Tonon Mao et al. (2013), Iroumé et al. (2017) et al. (2014, 2018 and 2015)

Old growth native forests (64%), belonging to the Araucaria and Roble-Raulí-Coigüe forest types; herbs and shrubs near the tree line (23%)

2500

9.1

38˚ 27' 57'' S; 71˚ 33' 44'' W

Andes range

Tres Arroyos

Table 4.1 Main characteristics of the basins, current land cover and recent disturbances and main articles related to the study of large wood

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2003). Greater descriptions of the vegetation and basin can be found in Gallo (2009), Ulloa et al. (2011) and Iroumé et al. (2020). The El Toro and Tres Arroyos basins are relatively close, located in an area of forests of the Roble-Rauli-Coihue and Araucaria forest types. In the El Toro basin, part of the Malleco National Reserve, cattle grazing and logging were allowed inside this area between 1907 and 1960, leading to frequent and devastating fires (Gonzalez et al. 2020) and causing, among other things, the degradation of the forest and the elimination of large trees. The last fires recorded in this basin date back to 2002, burning 98% of the vegetated basin with different intensities. Currently, 98% of the basin is covered with old forest and second-growth forest (Andreoli et al. 2008). The Tres Arroyos basin is 72% covered by vegetation, corresponding to native forests belonging to the Araucaria and Roble-Raulí-Coigüe forest types. Another part of the basin is covered by experimental exotic plantations (6%) of Pinus contorta, Pinus ponderosa and Pseudotsuga menziesii, among others, established in the 1970s, after the last forest fires that affected part of the basin. Native old growth is characterised by trees with > 40–50 m tall, > 1–2 m diameter and > 500 years old (Andreoli et al. 2008, Mao et al. 2013). Until before the eruption of the volcano of the same name in 2008, the Chaitén basin was covered by native forests of the evergreen forest type. This type of forest has different strata, where dominant trees can reach heights above 35 m and diameters greater than 1.5 m. It has a great diversity of species; among the most characteristic are Nothofagus dombeyi, N. nitida, N. betuloides, Luma apiculata, Drimys winteri, Eucryphia cordifolia and Aextoxicon punctatum. The volcanic eruption affected 60% of the vegetation, causing death, defoliation and falling branches. Additionally, the material generated by the eruption caused strong changes in the river, and channel deposits of volcanic material reached 8 m in thickness (Swanson et al. 2010, Ulloa et al. 2015).

4.4 Large Wood Volumes The Pichún and Vuelta de Zorra basins have the lowest volumes of LW per ha, probably because they have smaller elements compared to the other basins (Table 4.2). In addition, this may respond to the historical use of these basins, where the forest has been mostly degraded by different activities, both agricultural and timber, especially the Pichún basin, which presents a low basal area in riparian forests (Ulloa et al. 2011). In addition, in terms of the mean volume per element, the Vuelta de Zorra basin has smaller dimensions than the Pichún basin. This could be explained by the presence and fall in the riverbed of remnant P. radiata trees from old plantations; however, these trees rapidly degrade compared to LW belonging to native woods. In contrast, the basins with the greatest volumes of wood are found in the Cordillera de los Andes. Extraordinary volumes have been recorded in the Tres Arroyos basin, according to Andreoli et al. (2008) and Mao et al. (2013). These data are only comparable to records from North American Pacific basins, where values close to

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Table 4.2 Volume and other characteristics of large wood in different basins Catchment

Volume (m3 /ha)

Pichún

55.5

Vuelta de Zorra El Toro

LW Mean diameter (cm)

LW Mean volume (m3 )

Articles

111

24

0.4

Iroumé et al. (2014)

109.4

311

22

0.2

Iroumé et al. (2014)

202

215

33

0.6

Iroumé et al. (2014), Mao et al. (2013)

786

50

0.7

Iroumé et al. (2014), Andreoli et al. (2007)

5730

25

0.7

Tonon et al. (2017)

Tres Arroyos 1057

Chaitén

144

Number of elements (N/km)

1000 m3 /ha have been recorded, for example, in a third-order basin located in ancient coniferous forests of about 500 years (Gurnell et al. 2002). However, the variability in terms of the volume of wood in the Andean basins is high; in fact, in El Toro, it is more comparable with LW storage in basins located in the Coastal Range. Andreoli et al. (2008) make comparisons of the LW found at El Toro, for example, with broadleaf catchments in New Zealand and other parts of the world. Among the different studies carried out in Chilean basins, such as in Iroumé et al. (2014), the volume stored in the channels would have a direct relationship with the potential area of wood supply to the canal. Based on these results, it is possible to interpret that watersheds with more degraded forests present a lower volume of LW. However, because of steeper slopes, their instability also plays an important role. Finally, the Chaitén basin also has a low volume of LW per hectare. However, when noting the number of elements (N/km), this is considerably high, at least seven times higher than in Tres Arroyos, which is the basin with the largest volume. Studies carried out in this river, as pointed out by Ulloa et al. (2015) and Tonon et al. (2017), showed that the recent eruption in this basin generated an extraordinary influx of LW into the channel. Additionally, the important floods in this river continue with the incorporation of high volumes of LW from the banks and upstream of the river. The eruption also generated an increase in the width of the active channel, which averaged 36 m before the eruption and later reached 107 m. Therefore, this increase in the width of the canal entails an increase in the area of the active canal, which would finally reflect a low volume in terms of m3 /ha. LW inventories in the different low-scale mountain basins have been carried out manually, covering the entire segment and identifying most or all of the LW present. However, this procedure requires a great deal of effort and is difficult to apply to rivers and segments of greater magnitude. To search for new alternatives, Sanhueza et al. (2018a, b, 2019) utilised a new method to characterise and measure woody

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material in large rivers. Therefore, the use of unmanned aerial vehicles (UAVs) has been proposed as a technological alternative that replaces traditional field methods. It has been demonstrated that the use of UAVs, in combination with structure from motion (SfM) photogrammetry, obtains accurate LW data in considerably shorter times and with the advantage of making different temporal repetitions. In this way, interesting results can be obtained on LW budgets and LW dynamics.

4.5 Origin of Large Wood in the Channels Much of the woody material present in the riverbeds originates from the fall and incorporation of trees from the banks of streams or rivers. At the same time, the fall of trees to the riverbed can be classified into two categories: (a) physical processes, where we can find falls due to wind, landslides, snow, riverbed erosion and fires and (b) biological processes, such as natural mortality, insect and disease occurrence. However, the forest type that sustains the contributions of LW will determine the amount and type of woody material found in a channel in addition to the processes that regulate its incorporation. In the studied basins, different causes have triggered the incorporation of LW in the channels; however, they correspond to physical processes that occur in the basin or on the margins. For each basin, there are main factors present in the origin and incorporation of LW into the channel. In Pichún, the largest proportion of LW has a residual origin, probably the product of previous harvests carried out in this basin; to a lesser extent, LW carried from upstream was recorded, as well as LW caused by the fall from the margin, mainly due to the action of the wind. In this basin, a large part of the LW is pine trees and trunks from previous rotations, many of them deposited perpendicular to the channel and elevated in the form of a bridge because the length of these trees exceeds the width of the channel. These trees, being elevated, have a low impact on the flow of the channel (Ulloa et al. 2011). In Vuelta de Zorra, over 60% of LW had an indication of having originated upstream; secondly, the origin came from the natural fall of trees from the shore, for example, because of the wind (Ulloa et al. 2011). The largest number of trees felled by the action of the wind in a segment where part of the riverbank was planted. Therefore, since the residual trees were left on the banks of the riverbed without the protection of the original forest, they were easily knocked down by the wind. Furthermore, this segment is flat land with a phreatic layer very close to the surface; therefore, the trees have mainly superficial roots (Nothofagus dombeyi; Donoso 1981). In El Toro, both the number and volume of registered LW are relatively low, considering the dimensions of the trees in Andean basins and that this basin was almost entirely affected by a forest fire (2002). In this sense, the background provided by Andreoli et al. (2008) and Mao et al. (2013) indicates that the short time elapsed

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since the fire has not been reflected in large landslides and the contributions of woody debris to the channel. Therefore, the woody material in the channel originates mainly from current drag from upstream (> 80%) and to a lesser extent from natural fall from the banks of the channel. A more recent study by Picco et al. (2021) on the longterm fluctuation of woody material in El Toro indicates that wood recruitment has increased in the highest part of the basin, probably because of the fire that occurred in 2002. In Tres Arroyos, extraordinary precipitation events occurred in 1972 and 1992, generating flow floods where LW played a crucial role. Large amounts of woody material were deposited both in the channel and in the basin’s alluvial fan. Andreoli et al. (2007) highlighted LW accumulations formed by 100–150 pieces of LW with a mean diameter of 0.5 m and between 4 and 5 m long. Regarding the origin of the wood deposited in the canal, according to Andreoli et al. (2007), more than 80% had evidence of being swept away by the current. The rest of the LW was associated with falls due to natural causes, such as landslides and slope erosion. Near the Chaitén Rivers, in a vast segment, the river valley was exposed to pyroclastic flows and the forest was removed, with the felled trees remaining on the ground or being displaced, a product of the eruption of the volcano of the same name. Ulloa et al. (2015) and Tonon et al. (2017) reported a strong entry of wood into the river. Over 90% of LW had indications of having entered through river transport (Tonon et al. 2017). The use and type of cover play an important role in the amount of LW that can be incorporated into the channel, while the dominant processes of each zone or basin will also be preponderant in the incorporation of woody material in each channel. For example, in basins where the forests have been more degraded, the forest reaches smaller dimensions, and it is expected that the volume of LW present in the channel will be lower compared to basins with larger mature forests. Processes such as volcanic eruptions and large landslides will have a high potential to carry large volumes of wood into the rivers and with this, woody material will contribute to generating strong morphological changes in the rivers. Additionally, it increases the potential to generate damage to infrastructure, such as bridges and road routes.

4.6 Large Wood Mobility Different works regarding mobility have been carried out in each of these basins; however, Iroume et al. (2015) present a study considering the Pichún, Vuelta de Zorra, El Toro and Tres Arroyos basins and utilise data from several annual periods. In Chaitén, Tonon et al. (2017) provide data on LW variation and mobility (Table 4.3). Among the main results that have been obtained from these studies, Iroumé et al. (2015) showed that for low-order basins, mobility is lower in periods where the maximum floods do not exceed the bankfull height of the channel, compared to

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Table 4.3 Mobility rates and parameters of the material mobilised in the study basins Basins

Active channel width (m)

Mobility (%)

Range of mobility (%)

LW mean diameter (cm)

LW mean length (m)

Source

Pichún

4.8

8.6

0–25.3

16

3.1

Iroumé et al. (2015)

Vuelta de Zorra

10.6

7.8

2.4–11.6

22

3.4

Iroumé et al. (2015)

El Toro

12.9

11.6

1.5–28.2

36

6

Iroumé et al. (2015)

Tres Arroyos

9.8

4.9

0.6–17

31

2.1

Iroumé et al. (2015)

Chaitén

94

65

42–94

24

3.9

Tonon et al. (2017)

periods in which they are exceeded. However, this difference is not statistically significant; mobility has also been correlated with unit stream power, while, in general, the LWs have greater mobility when the length is less than the bankfull width and when the diameter does not exceed a dimension greater than 0.5 times the bankfull height. The pieces that are most likely to move are those with an orientation parallel or oblique to the direction of the flow, and they are found alone or forming groups of no more than 5 pieces. These results consider floods with an IR ≤5 years. These findings are further explained in Iroumé et al. (2015). In the Chaitén basin, Ulloa et al. (2015) first made estimates based on satellite images of the mobility of LW in a 10.2 km-long segment. Mobility rates of 48 and 78% were reported for grouped elements (Wood jams; WJ) and single LW, respectively. Subsequently, Tonon et al. (2017) carried out a study, considering different periods and sections of length less than 100 m, and reported mobility rates ranging between 42 and 94%. Unlike the other four smaller basins in Chaitén, no relationship was observed between the rate of mobility and characteristics of the woody material (diameter, length, volume, abundance) or with the width of the active channel. However, Tonon et al. (2017) explained that the high mobility that occurred in this river was related to the characteristics of the channel, such as width and depth. In fact, due to the increase in width that occurred after the eruption (Ulloa et al. 2015), this river can be classified as a large river (Gurnell et al. 2002). In the Chaitén River, the mobilised LW is deposited in bars where the floodable width is greater, and due to the volcanic deposits in the Chaitén River valley, there is a high availability of LW in the channel margins that can easily be eroded and mobilised during large floods. More recent studies consider the mobility of LW, considering long-term monitoring in low-order basins (Pichún, Vuelta de Zorra, El Toro and Tres Arroyos). For example, Iroumé et al. (2018) monitored the LW dynamics for more than 8 years in these low-order basins and found that the path of the mobilised LW was greater when the depth of the flow was greater than the bankfull height compared to when it was less, mainly for smaller pieces. The mobility of LW was mainly determined by

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the length of the LW and its relationship between the length of the LW with respect to the width of the channel, while other factors that favoured mobility were the position compared to the bankfull level and the absence of roots in the channel’s LW. More details can be found in Iroumé et al. (2018). Finally, greater dynamics of the wood (extensive entrainment, deposition or repositioning of LW) can be observed in significantly wider sections and with a lower gradient. In contrast, Picco et al. (2021) showed that there is a high correlation, both in the number and volume of LW, with the roughness index proposed for El Toro. In this way, using a roughness index, sections that tend to have a greater capture and deposit of LW can be characterised. In short, in low-order basins, the mobility of LW has an inverse relationship with the stability of the pieces of wood in the channel, while the stability is determined mainly by the length of the pieces and then by the diameter and their relationship with the width of the active channel, the water level during floods and the unit stream power. From another angle, in a larger channel, the dimensions of the pieces and the width of the channel have no relation to the mobility rates of the wood.

4.7 Morphological Impact The LW in the channel, among other things, has a great influence on the morphology and hydraulics of the river (Gurnell et al. 2002). Mainly, the elements that form structures help dissipate the energy of the current and trap sediments upstream, reducing the slope of the channel and therefore reducing its erosive capacity. Andreoli et al. (2008) and Ulloa et al. (2011) provided data on the volume of sediment trapped due to LW and WJ and other objects that create obstructions in the channel. In basins such as Pichún and Vuelta de Zorra, the volume of trapped sediment is 30 and 82 m3 /km, respectively (Ulloa et al. 2011). In this case, the sediment trapped by the large rocks present in the channel is included. Andreoli et al. (2008) indicated that in El Toro, no wooden structures generated dams, unlike in Tres Arroyos, where dams were found with dimensions much larger than the aforementioned basins; for example, the largest of them measured around 3.5 m. high, with an influence of up to 60 m in length upstream and an approximate sediment deposit of 1700 m3 . In total, the amount of sediment trapped by wooden structures was about 1270 m3 /km. In general, the effect of these dams in Tres Arroyos translates into a dissipation of the total potential energy by 27%, a deposit of sediments upstream that generates losses in the slope of the channel. This length of the total sediment deposits is equivalent to 24% of the length of the channel (Andreoli et al. 2008). Another impact generated by LW is on buildings and infrastructure when large flood events occur. Andreoli et al. (2008) in Tres Arroyos and Ravazzolo et al. (2017), which document a wood-laden flow in an estuary in the central zone of Chile, provided background information indicating that these extraordinary floods generate debris flow-type flows, where an important part of the flow is made up of high volumes of wood. In addition, LW plays a crucial role in the dynamics of these flows during large floods.

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Fig. 4.2 Removal of accumulated wood on the Chaitén river bridge during a flood in 2015. Photo provided by El Llanquihue newspaper

The morphological impacts on a channel are related to the amount of wood, its orientation with respect to the flow of the current and the relationship between the dimensions of the LW and the channel, for example, the relationship between the average length and width of the channel and the relationship between the diameter medium and bankfull height. Abbe and Montgomery (2003) suggested that stems tend to be more stable if their length is greater than 1.5 times the width of the river under bankfull channel conditions. As observed in Tres Arroyos, LW with a diameter similar to bankfull height (0.5 m) is relatively common and significantly larger than that found in the El Toro River. Following Andreoli et al. (2008), these variables allow for the high presence of stable wooden structures in Tres Arroyos, which can dissipate more than a quarter of the potential energy equivalent to the total loss of elevation of the channel and a substantial increase in resistance to flow and sediment storage. In contrast, in the El Toro riverbed, the lack of structures of woody elements generates a low impact on the retention of sediments and on the increase in resistance to the flow of the current. In the Chaitén basin, the high contribution of wood also generates certain morphological impacts, mainly by increasing the risk of damage to the town of Chaitén in the face of large floods. For example, the accumulation of wood on the pillars of the bridge joins both banks of the river and is also part of the Carretera Austral (Fig. 4.2).

4.8 Final Comments The presence of woody material in mountain channels is mainly determined by complex processes that occur on the banks and slopes of a river, which involve falling trees, landslides and contributions of debris flows from tributaries. These processes, in general, are responsible for the contribution of large pieces of wood and whole trees to the canal, which will become key pieces where smaller LW coming from

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upstream will be recruited and sediments will be stored. At the level of sections, there will be a greater tendency to deposit and capture LW in sections with high roughness compared to those characterised by low roughness. Therefore, and in general terms, the estimates of wood, either in number or volume of LW, are mainly related to the availability of stable pieces of wood that are provided by the riverbanks and slopes, the roughness of certain sections and elements from upstream sections. In larger rivers, the disposition of wood in the channel and its effects on geomorphology are determined by hydraulic factors of the channel, such as the presence of bars and changes in the gradient of the channel. However, land use in the basin and natural impacts can influence the amount of wood within a riverbed; among them, a volcanic eruption has the capacity to generate a strong contribution of woody material to the river, maintaining a high supply of LW that can last for several years after the eruption, both from upstream and from the banks of the channel. In mountainous environments, the risk presented by the LW is related to the displacement that these elements present within the channel, mainly during extreme floods. The collapse of accumulations of woody material can cause debris flows, which are characterised by their high speed, erosive capacity and impact violence. This displacement has great potential to cause direct damage to human settlements, buildings and infrastructure, such as roads and bridges. Faced with these scenarios, it is important to consider these flows when designing infrastructure, such as bridges or dams, to avoid collapses and diversions of the flow towards more vulnerable areas. Woody material in rivers has also been recognised as an important component of the river ecosystem (Gurnell et al. 2002). LW accumulations generate favourable environments for the development of organisms, providing refuge for different species and capturing finer particles that can be used as food for local fauna. A channel in natural conditions is characterised by a great variety of habitats; that is, the greater the environmental diversity, the greater the diversity of plant and animal species. In Chile, studies focused on the ecology of the channels and their relationship with LW have been scarce compared to the works regarding inventories of LW, mobility and morphological impact. However, Vera et al. (2014) carried out a study on the Vuelta de Zorra Riverbed, where they studied ecological influence by comparing sections with a high volume of LW with sections with a lower volume. Stretches with a higher volume of LW were positively correlated with a higher retention of organic matter and greater abundance and diversity of macroinvertebrates, and the richness of species was double compared to sections with a lower volume of LW. Finally, knowledge gained regarding the functionality and impacts of LW in channels has been positive, achieving long-term studies that have contributed to the understanding of the dynamics of woody material at the level of basins and subsections within each basin. However, there are still challenges to achieving a better understanding of the possible impacts in different territories, in addition to the need to continue with long-term studies. Undoubtedly, a better understanding will facilitate better management of rivers and a reduction in the risks associated with woody materials. Additionally, there is a need to continue or deepen studies that link woody material with ecological characteristics within riparian environments. Although the benefit of wood has already been shown to be important in maintaining these healthy

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and diverse environments, it would be advisable to have more precise information to guide intervention processes or restoration of riverbeds. In addition, people give value to the woody material in the riverbeds from a landscape point of view. Determining the extent to which the wood in the riverbeds can help improve the quality of water resources, mainly in scenarios of water scarcity and droughts.

References Abbe TB, Montgomery DR (2003) Patterns and processes of wood debris accumulation in the Queets river basin Washington. Geomorphology 51(1–3):81–107 Andreoli A, Carlig G, Comiti F, Iroumé A (2007) Residuos leñosos de gran tamaño en un torrente de la Cordillera de los Andes, Chile: su funcionalidad e importancia. Bosque 28(2):83–96 Andreoli A, Comiti F, Mao L, Iroumé A, Lenzi MA (2008) Evaluación de los volúmenes y de los efectos hidro-morfológicos del material leñoso en dos torrentes andinos (Chile). Ingeniería del agua 15(3):189–204 Cisternas M, Martínez P, Oyarzun C, Debels P (1999) Caracterización del proceso de reemplazo de vegetación nativa por plantaciones forestales en una cuenca lacustre de la Cordillera de Nahuelbuta, VIII Región, Chile. Revista Chilena de Historia Natural 72:661–676 Comiti F, Andreoli A, Lenzi MA, Mao L (2006) Spatial density and characteristics of woody debris in five mountain rivers of the Dolomites (Italian Alps). Geomorphology 78(1–2):44–63 Comiti F, Andreoli A, Mao L, Lenzi MA (2008) Wood storage in three mountain streams of the Southern Andes and its hydro-morphological effects. Earth Surf Process Landforms: J Br Geomorphol Res Group 33(2):244–262 Donoso C (1981) Tipos forestales de los bosques nativos de Chile. Investigación y desarrollo forestal. CONAF, FAO, FO: DP/CHI/76/003. Corporación Nacional Forestal. (Documento de Trabajo Nº 38). Santiago, Chile, p 83 Farías A, Tecklin D (2003) Caracterización preliminar de los predios Chaihuín-Venecia, Cordillera de La Costa Décima Región. Documento N°6, Serie de Publicaciones WWF Chile, Programa Ecoregión Valdiviana. Gallo C (2009) Estudio de la movilidad y reclutamiento de material leñoso en la cuenca del estero Vuelta de la Zorra, Chaihuín, Cordillera de la Costa, sur de Chile. Ingeniería Forestal, Universidad Austral de Chile, Tesis González ME, Muñoz AA, González-Reyes Á, Christie DA, Sibold J (2020) Fire history in Andean Araucaria-Nothofagus forests: coupled influences of past human land-use and climate on fire regimes in North–West Patagonia. Int J Wildland Fire 29(8):649–660 Gregory S, Boyer KL, Gurnell AM (2003) Ecology and management of wood in world rivers. In: International Conference of Wood in World Rivers (2000: Corvallis, Or.). American Fisheries Society Gurnell AM, Piégay H, Swanson FJ, Gregory SV (2002) Large wood and fluvial processes. Freshwater Biol 47(4):601–619 Iroumé A, Andreoli A, Comiti F, Ulloa H, Huber A (2010) Large wood abundance, distribution and mobilization in a third order Coastal mountain range river system, southern Chile. Forest Ecol Manage 260(4):480–490 Iroumé A, Ulloa H, Lenzi MA, Andreoli A, Gallo C (2011) Movilidad y reclutamiento de material leñoso de gran tamaño en dos cauces de la Cordillera de la Costa de Chile. Bosque 32(3):247–254 Iroumé A, Mao L, Ulloa H, Ruz C, Andreoli A (2014) Large wood volume and longitudinal distribution in channel segments draining catchments with different land use Chile. Open J Modern Hydrol 4(2):57–66 Iroumé A, Mao L, Andreoli A, Ulloa H, Ardiles MP (2015) Large wood mobility processes in low-order Chilean river channels. Geomorphology 228:681–693

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Iroumé A, Ruiz-Villanueva V, Mao L, Barrientos G, Stoffel M, Vergara G (2018) Geomorphic and stream flow influences on large wood dynamics and displacement lengths in high gradient mountain streams (Chile). Hydrol Process 32(17):2636–2653 Iroumé A, Zingaretti V, Vericat D, Tenny J, Llena M, Batalla RJ (2019) Fluvial responses following volcanic eruptions: the Blanco-Este River Southern Chile. Trans Ecol Environ 234:21–29 Iroumé A, Cartagena M, Villablanca L, Sanhueza D, Mazzorana B, Picco L (2020) Long-term large wood load fluctuations in two low-order streams in Southern Chile. Earth Surf Process Landforms 45(9):1959–1973 Lara A, Solari ME, Prieto MDR, Peña MP (2012) Reconstrucción de la cobertura de la vegetación y uso del suelo hacia 1550 y sus cambios a 2007 en la ecorregión de los bosques valdivianos lluviosos de Chile (35º–43º 30 S). Bosque (Valdivia), 33(1):13–23 Leckie DG, Cloney E, Jay C, Paradine D (2005) Automated mapping of stream features with high-resolution multispectral imagery. Photogram Eng Remote Sens 71(2):145–155 Major JJ, Bertin D, Pierson TC, Amigo Á, Iroumé A, Ulloa H, Castro J (2016) Extraordinary sediment delivery and rapid geomorphic response following the 2008–2009 eruption of Chaitén Volcano Chile. Water Resour Res 52(7):5075–5094 Mao L, Andreoli A, Iroumé A, Comitid F, Lenzi MA (2013) Dynamics and management alternatives of in-channel large wood in mountain basins of the Southern Andes. Bosque 34(3):319–330 Marcus WA, Marston RA, Colvard CR Jr, Gray RD (2002) Mapping the spatial and temporal distributions of woody debris in streams of the Greater Yellowstone Ecosystem, USA. Geomorphology 44(3–4):323–335 Mazzorana B, Ruiz-Villanueva V, Marchi L, Cavalli M, Gems B, Gschnitzer T, Valdebenito G (2018) Assessing and mitigating large wood-related hazards in mountain streams: recent approaches. J Flood Risk Manag 11(2):207–222 Millán J, Carrasco P (1993) La forestación en la VIII Región Serie EULA Elementos cognoscitivos sobre el recurso suelo y consideraciones generales sobre el ordenamiento agroforestal Editorial Universidad de Concepción. Concepción, Chile, p 105 Pellet P, Ugarte E, Osorio E, Herrera F (2005) Conservación de la biodiversidad en Chile ¿legalmente suficiente?: la necesidad de cartografiar la ley antes de decidir. Revista Chilena de Historia Natural 78:125–141 Picco L, Scalari C, Iroumé A, Mazzorana B, Andreoli A (2021) Large wood load fluctuations in an Andean Basin. Earth Surf Process Land 46(2):371–384 Picco L, Scalari C, Faes L, Iroumé A (2019) Subsequent wildfires affecting the Rio Toro basin (Chile): large wood recruitment dynamics and budgeting. In: Geophysical research abstracts, vol 21 Ravazzolo D, Mao L, Mazzorana B, Ruiz-Villanueva V (2017) Brief communication: the curious case of the large wood-laden flow event in the Pocuro stream (Chile). Nat Hazards Earth Syst Sci 17(11):2053–2058 Roni P (2019) Does river restoration increase fish abundance and survival or concentrate fish? The effects of project scale location and fish life history. Fisheries 44(1):7–19 Sanhueza D, Picco L, Ruiz-Villanueva V, Iroumé A, Ulloa H, Barrientos G (2019) Quantification of fluvial wood using UAVs and structure from motion. Geomorphology 345:106837 Sanhueza D, Iroumé A, Ulloa H, Picco L, Ruiz-Villanueva V (2018a) Measurement and quantification of fluvial wood deposits using UAVs and structure from motion in the Blanco River (Chile). In: 5th IAHR Europe Congress—New Challenges in Hydraulic Research and Engineering, Proc. of the 5th IAHR Europe Congress Sanhueza D, Iroumé A, Ulloa H, Picco L, Ruiz-Villanueva V (2018b) Measurement and quantification of fluvial wood deposits using UAVs and structure from motion in the Blanco River (Chile). In: Armanini A, Nucci E (eds) 5th IAHR Europe congress: new challenges in hydraulic research and engineering. https://doi.org/10.3850/978-981-11-2731-1_216-cd Schlegel B (2001) Estimación de la biomasa y carbono en bosques del tipo forestal siempreverde. Simposio internacional medición y monitoreo de la captura de carbono en ecosistemas forestales 18:1–13

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Sedell RJ, Bisson PA, Swanson FJ, Gregory SV 1988 What we know about large trees that fall into streams and rivers. In: Maser C, Tarrant RF, Trappe JM, Franklin JF (eds) From the forest to the sea: a story of fallen trees, USDA For. Serv Gen Tech Rep PNW-GTR 229:47–81 Spreitzer G, Tunnicliffe J, Friedrich H (2020) Large wood (LW) 3D accumulation mapping and assessment using structure from Motion photogrammetry in the laboratory. J Hydrol 581:124430 Swanson F, Crisafulli C, Jones J, Lara A (2010) Volcano ecology at Chaitén Chile: geophysical processes interact with forest ecosystems. American geophysical union fall meeting 2010 abstract #V34B-06 Swanson FJ, Gregory SV, Iroumé A, Ruiz-Villanueva V, Wohl E (2021) Reflections on the history of research on large wood in rivers. Earth Surf Process Land 46(1):55–66 Swanson FJ, Lienkaemper GW (1978) Physical consequences of large organic debris in Pacific Northwest streams. USDA Gen Tech Rep, PNW-69. Pacific Northwest forest and range experiment station, Portland, OR Tonon A, Iroumé A, Picco L, Oss-Cazzador D, Lenzi MA (2017) Temporal variations of large wood abundance and mobility in the Blanco River affected by the Chaitén volcanic eruption southern Chile. Catena 156:149–160 Ulloa H, Iroumé A, Lenzi MA, Andreoli A, Álvarez C, Barrera V (2011) Material leñoso de gran tamaño en dos cuencas de la Cordillera de la Costa de Chile con diferente historia de uso del suelo. Bosque 32(3):235–245 Ulloa H, Iroumé A, Mao L, Andreoli A, Diez S, Lara LE (2015) Use of remote imagery to analyse changes in morphology and longitudinal large wood distribution in the Blanco River after the 2008 Chaitén volcanic eruption southern Chile. Geografiska Annaler: Ser Phys Geogr 97(3):523–541 Vera M, Jara C, Iroume A, Ulloa H, Andreoli A, Barrientos S (2014) Reach scale ecologic influence of in-stream large wood in a Coastal Mountain range channel, Southern Chile/Influencia ecológica a nivel de tramo de la madera en el cauce en un canal de la Cordillera de la Costa, sur de Chile. Gayana, 78(2):85

Chapter 5

River Water Characteristics After Recent Volcanic Eruptions in Southern Chile Eduardo Jaramillo, Alexandre Corgne, and Aldo Hernandez

Abstract Within 9 days after recent volcanic eruptions in southern Chile (Cordón Caulle, Villarrica and Calbuco), we analysed the physico-chemical quality of riverine waters of nearby rivers. Sulphates, total suspended solids, fluorides and conductivity were higher at the turbid waters of affected rivers, while pH and silicates showed no significant differences between affected and non-affected rivers. The results of principal component analysis demonstrate a similar overall impact in all affected riversheds, independently of the volcanological characteristics of the erupting centres. Keywords Volcanic eruptions · Riverine waters · Chile

5.1 Introduction Volcanic ash and pumice are the most widely distributed products of explosive eruptions (Del Moral and Grishin 1999; Bertrand et al. 2014), affecting water quality and benthic biota of nearby rivers (e.g., Del Moral 1981; Miserendino et al. 2012; Lallement et al. 2014; Lallement et al. 2016). This is of concern for communities located in the influence zone of dispersion and fallout of those volcanic products. In such cases, authorities are generally unable to diminish public fears because of a lack of water quality information collected before volcanic eruptions.

E. Jaramillo (B) · A. Corgne Facultad de Ciencias, Instituto de Ciencias de la Tierra, Universidad Austral de Chile, Valdivia, Chile e-mail: [email protected] A. Hernandez Holon SpA., Concepción, Chile © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. Oyarzún et al. (eds.), Rivers of Southern Chile and Patagonia, The Latin American Studies Book Series, https://doi.org/10.1007/978-3-031-26647-8_5

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We provide here basic information on water characteristics of rivers affected by three recent volcanic eruptions in southern Chile: that of the Cordón Caulle (from 3 June 2011) and those of volcanoes Villarrica and Calbuco (from 3 March and 22 April 2015, respectively). We primarily focus on the short-term effects (4–9 days) of those large natural disturbances on the water quality of river systems located around the volcanoes. On the 4th of June of 2011, after five decades of inactivity and 2 months of elevated seismicity, a new vent formed on the northern side of the fissural range of Cordón Caulle producing an ash and gas plume up to 14 km above sea level (Bertin et al. 2012). The Plinian-subplinian phases of the eruption (Volcanic Explosivity Index, VEI = 4) lasted until April 2012 and released during the first 27 h of the eruption about 0.5–1.0 km3 of rhyodacitic tephra (0.2–0.4 km3 Dense Rock Equivalent, DRE) (Bertin et al. 2012). In addition to tephra, the eruption produced later on an estimated 0.45 km3 of rhyodacitic lava (70% SiO2 ), which covered a surface nearly 7 km2 (Bertin et al. 2012). Part of the lava flow extended north east of the vent in the drainage of Nilahue river (Tuffen et al. 2013). Due to the predominant westerly winds, tephra dispersion mainly occurred towards the east (Collini et al. 2013). The Gol Gol river, which drains from the southern flank of Cordón Caulle and discharges in Puyehue lake, and the Nilahue river, which drains from the northern flank of Cordón Caulle and discharges in Ranco lake, were the most affected by ash and pumice contamination (Bertrand et al. 2014). Villarrica, Chile’s most active volcano, is characterized by regular eruptive cycles associated with moderate explosive activity (VEI = 2) that occur periodically every few decades. 22 lahar-forming eruptions have been documented during the last 600 years (Van Daele et al. 2014). The eruptive cycle that culminated in March 2015 was initiated about three months earlier in December 2014 with increased anomalies in thermal activity and volcanic gas emission (Global Volcanism Program 2016; Aiuppa et al. 2017). In February 2015, sporadic Strombolian explosions sent tephra up to 5 m in size down the flanks and within 1 km from the crater. The increased activity climaxed on the morning of March 3 with a vigorous Strombolian activity and subsequent lava fountaining that rose 1500 m above the crater rim and lasted only about half an hour. This paroxysmal explosion (VEI = 2) produced an ash plume that reached an altitude of 9 km and drifted to the east. Explosive activity with ash plumes and lava spattering continued at decreasing levels throughout 2015 (Global Volcanism Program 2016). The explosion of March 3 produced heavy tephra fallout to the east and incandescent scoria flows mostly on the northern and eastern flanks of the volcano (Johnson and Palma 2015). Glacier melting generated moderate lahar activity in Zanjón Seco river drainage to the north west but a lahar more than 20 km long in the Pedregoso and Turbio river drainages to the north east (Flores and Amigo

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2015; Johnson and Palma 2015). Estimates of the bulk non-DRE volume of emitted tephra vary between about 4 and 7 million of m3 (Bertin et al. 2015a; Romero et al. 2016a). The erupted materials were mostly medium-K basaltic andesite in composition (Bertin et al. 2015a; Romero et al. 2016a). The 2015 Calbuco volcano eruption cycle, the first in four decades, initiated with increasing seismic activity months prior to the April eruption (Sernageomin 2015a). Nonetheless, the intense subplinian explosive eruption of the 22nd of April 2015 (VEI = 4) came as a surprise in the absence of precursors in the days or hours preceding the event. A thermal anomaly and stronger seismic events were only detected minutes prior to the eruption (Sernageomin 2015a; Valderrama et al. 2015). The first eruptive pulse on April 22 lasted about 90 min and produced an ash plume that reached an altitude of nearly 17 km (Sernageomin 2015b). A second and more powerful pulse occurred the following day and lasted about 6 h, generating an ash plume of about 20 km that drifted initially towards the north east (Sernageomin 2015c). Collapse of the eruptive column produced pyroclastic density currents that travelled up to 6 km from the vent (Castruccio et al. 2016). A third and smaller pulse on April 30 generated a 4 km high ash plume, which deposited tephra towards the south east (Romero et al. 2016a, b) and only produced mm to sub-mm thick fall deposits near Lake Chapo and Ralún (Bertin et al. 2015b). The total bulk tephra deposit volume was estimated to be between 0.27 and 0.38 km3 , i.e. 0.12–0.15 km3 DRE (Romero et al. 2016b; Castruccio et al. 2016). The erupted materials correspond mostly to porphyritic basaltic andesite (about 55 wt% SiO2 ) (Romero et al. 2016b; Castruccio et al. 2016).

5.2 Methods Four days after the start of the Cordón Caulle eruption, we collected water samples from six rivers on its northern flank and from two rivers on its southern flank (Fig. 5.1a). As for the eruptions of the Villarrica and Calbuco volcanoes, water samples were collected from four and five nearby rivers, respectively, nine and six days after the start of the eruptions, respectively (Fig. 5.1b, c). The following water parameters were recorded in situ with a multi-parameter probe: water temperature, potential hydrogen (pH) and electrical conductivity. We also collected water samples to measure levels of sulphates, silicates, fluorides and total suspended solids (TSS), which are expected by-products of volcanic eruptions (e.g., Stewart et al. 2006; Witham et al. 2005). To improve statistics, samples for TSS were collected as triplicates, while samples for the other chemicals were collected as duplicates. Results presented in Table 5.1 are average values of the triplicates and duplicates. All water samples were collected with Nalgene xp 1 L plastic bottles. The analysis for TSS, sulphates, silicates and fluorides were carried out according to the standard methods for the examination of water and waste water (APHA 2005).

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Fig. 5.1 Location map of sampled rivers in the vicinity of the Cordón Caulle (a), Villarrica (b) and Calbuco (c) volcanoes in southern Chile. River names are as follows: a Liquiñe (Li), Reyehueico (Re), Curringue (Cu), Hueinahue (Hu), Los Venados (LV), Nilahue (Ni), Gol Gol (GG) and Anticura (An) close to Cordón Caulle; b Turbio (Tu), Pedregoso (Pe), Trancura (Tra) and Liucura (Liu) near Villarrica; c Blanco Este (BE), Hueño Hueño (HH), Blanco Sur (BS), Tronador (Tro) and Lauca (La) near Calbuco. Turbid and clear water rivers labelled in brown and green, respectively (see text)

Clear waters

Cordón Caulle

Turbid waters

Calbuco

Villarrica

Cordón Caulle

Calbuco

Villarrica

Volcano

Category

38.8

Liquiñe

34.2 67.8 43.9 12.6

Tronador

Lauca

Mean

Standard deviation

66.0

36.6

Reyehueico 35.4

34.6

Hueinahue

Liucura

40.4

Curringue

Pedregoso

39.0

38.6

Standard deviation 46.0

62.0

Mean

Anticura

26.0

Blanco Sur

Los Venados

53.2

Hueño Hueño

57.3 132.6

Trancura

Blanco Este

21.5

91.1

Turbio

52.4

Gol Gol

Conductivity (µS/cm)

Nilahue

Rivers

0.8

7.4

7.5

7.3

8.1

7.9

6.7

7.2

6.0

7.5

8.9

6.9

0.9

7.8

7.8

7.7

7.7

8.2

6.9

9.5

7.0

pH

14.8

12.5

3.2

4.4

4.3

5.4

2.1

2.7

40.8

13.1

10.2

38.6

2066.8

1981.1

1591.3

108.6

1399.2

33.0

1246.1

3738.7

5750.5

TSS (mg/L)

9.0

6.6

0.5

0.5

2.2

0.5

3.0

3.3

11.8

4.2

10.7

29.6

9.5

12.9

0.5

10.6

29.1

5.3

8.9

17.3

18.4

Sulphates (mg/L)

Table 5.1 Water characteristics of the rivers sampled near Cordón Caulle, Villarrica and Calbuco volcanoes

9.1

12.7

20.6

15.3

28.9

24.6

5.9

6.4

3.5

5.0

7.2

9.5

6.4

13.1

6.8

12.6

13.3

25.2

15.1

5.9

12.5

Silicates (mg/L)

0.03

0.06

0.08

0.08

0.08

0.08

0.03

0.03

0.07

0.03

0.04

0.12

0.24

0.3

0.18

0.13

0.76

0.08

0.2

0.28

0.47

Fluoride (mg/L)

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Based on direct field observations, the rivers were preliminarily categorized in terms of the levels of turbidity as turbid and clear rivers (cf. Fig. 5.2). This visual classification is generally correlated with TSS measurements, which indicated that turbid rivers had high TSS loads (Table 5.1). Turbid rivers included the Nilahue and Gol Gol rivers near Cordón Caulle, the Turbio and Trancura rivers near Villarrica and Blanco Este, Hueño Hueño and Blanco Sur near Calbuco (Figs. 5.1 and 5.2). On the other hand, clear water rivers had green clean waters without noticeable load of TSS including tephra (Los Venados, Hueinahue, Curringue, Liquiñe and Reyehueico near Cordón Caulle, Pedregoso and Liucura close to Villarrica and Tronador and Lauca near Calbuco) (Fig. 5.2). To graphically explore the water characteristics variation amongst the set of studied rivers, we performed principal component analyses (PCA) on log-transformed data of the above-mentioned variables. Eventual statistically significant differences between turbid and clear rivers and source volcanoes were analysed with PERMANOVA by using the scores of the first two principal components that resulted from PCA. This procedure is particularly suited to study groups characterized by a large number of variables, since it runs a multivariate analysis with permutations to avoid possible biases. All the analyses were made with R (http://www.r-project.org/) by using the routines vegan and ggbiplot.

5.3 Results and Discussion The results of the PCA show that less than ten days after the volcanic eruptions, the first two principal components explained nearly 70% of the variability of the data set (Fig. 5.3). The spatial segregation of turbid and clear rivers was statistically significant (PERMANOVA R2 = 19%; p < 0.01) and occurred along the first principal component, which explains 42% of the variance (Fig. 5.3a). That segregation was explained by the higher concentrations of sulphates, TSS, fluorides and higher conductivity for the turbid water rivers, being TSS and fluorides the main sources of differentiation (Fig. 5.3a and Table 5.1). The results of the statistical analyses do not show significant differences considering the overall data from each volcano (PERMANOVA R2 = 22%; p < 0.135) (Fig. 5.3b and Table 5.1), which may indicate that the differences found between rivers of turbid waters and clear waters constitutes a common pattern for all the volcanic eruptions analysed here. It follows that the response of the freshwater aquatic fauna could be quite different between nearby rivers on a given volcano (depending on turbidity). However it should be relatively similar for all clear rivers on one side and all turbid rivers on the other side, independently of their location.

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Fig. 5.2 Representative images of turbid and clear water rivers at the study area. The rivers Nilahue, Gol Gol, Trancura and Blanco Sur were classified as turbid water rivers, while Hueinahue, Anticura, Liucura and Tronador were categorized as clear water rivers

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Fig. 5.3 Bi-plot ordination for the first two principal components of the PCA. In the upper plot a sites are separated according to river category, while in the lower plot b rivers are differentiated according to the volcanic emission centres (river abbreviations as in Fig. 5.1). The ellipses represent the reclassification of the scores with a 90% confidence level

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Acknowledgements We thank J.P. Molina, C. Cárdenas, M. Manzano and C. Velasquez for assistance in the field. This study was funded by Vicerrectoría de Investigación, Desarrollo y Creación Artística de la Universidad Austral de Chile.

References Aiuppa A, Bitetto M, Francofonte V, Velasquez G, Bucarey Parra C, Giudice G, Liuzzo M, Moretti R, Moussalam Y, Peters N, Tamburello G, Valderrama OA, Curtis A (2017) A CO2 -gas precursor to the March 2015 Villarrica volcano eruption. Geochem Geophys Geosyst 18:2120–2132 APHA (2005) Standard methods for the examination of water and wastewater, 21st edn. American Public Health Association/American Water Works Association/Water Environment Federation, Washington DC Bertin D, Amigo A, Lara L, Orozco G, Silva Parejas C (2012) Erupción del cordón Caulle 2011– 2012: Evolución fase efusiva. XIII Congreso Geológico Chileno, Antofagasta, Chile Bertin D, Amigo A, Bertin L (2015a) Erupción del volcán Villarrica 2015: Productos emitidos y volumen involucrado. XIV Congreso Geológico Chileno, La Serena, Chile Bertin D, Amigo A, Mella M, Astudillo V, Bertin L, Bucchi F (2015b) Erupción del volcán Calbuco 2015: Estratigrafía eruptiva y volumen involucrado. XIV Congreso Geológico Chileno, La Serena, Chile Bertrand S, Daga R, Bedert R, Fontijn K (2014) Deposition of the 2011–2012 Cordón Caulle tephra (Chile, 40° S) in lake sediments: implications for tephrochronology and volcanology. https://doi. org/10.1002/2014JF003321 Castruccio A, Clavero J, Segura A, Samaniego P, Roche O, Le Pennec J-L, Droguett B (2016) Eruptive parameters and dynamics of the April 2015 sub-Plinian eruptions of Calbuco volcano (southern Chile). Bull Volcanol 78:62 Collini E, Osores MS, Folch A, Viramonte JG, Villarosa G, Salmuni G (2013) Volcanic ash forecast during the June 2011 Cordon Caulle eruption. Nat Hazards 66:389–412 Del Moral R (1981) Life returns to Mount St. Helens. Nat Hist 90:36–49 Del Moral R, Grishin SY (1999) Volcanic disturbances and ecosystem recovery. In: Walker IR (ed) Ecosystems of disturbed ground. Ecosystems of the world 16. Elsevier, New York, USA, pp 137–160 Flores F, Amigo A (2015) Dinámica de flujos laháricos asociados a la erupción del 3 de marzo del volcán Villarrica. XIV Congreso Geológico Chileno, La Serena, Chile Global Volcanism Program (2016) Report on Villarrica (Chile). In: Venzke E (ed) Bulletin of the global volcanism network, vol 41. Smithsonian Institution, Washington, D.C., p 11 Johnson JB, Palma JL (2015) Lahar infrasound associated with Volcán Villarrica’s 3 March 2015 eruption. Geophys Res Lett 42:6324–6331 Lallement ME, Juarez SM, Machi PJ, Vigliano PH (2014) Puyehue Cordón–Caulle: post-eruption analysis of changes in stream benthic fauna of Patagonia. Ecol Austral 24:64–74 Lallement ME, Machi PJ, Vigliano PH, Juarez SM, Rechencq M, Baker M, Bouwes N, Crowl T (2016) Rising from the ashes: changes in salmonid fish assemblages after 30 months of the Puyehue–Cordón Caulle volcanic eruption. Sci Total Environ 541:1041–1051 Miserendino ML, Archangelsky M, Brand C, Beltrán L (2012) Environmental changes and macroinvertebrate response in Patagonian streams (Argentina) to ashfall from the Chaitén Volcano (May 2008). Sci Total Environ 424:2012–2212 Romero JE, Keller W, Díaz-Alvarado J, Polacci E, Inostroza M (2016a) The 3 March 2015 eruption of Villarrica volcano, Southern Andes of Chile: overview of deposits and impacts. 11º Encuentro Internacional de Ciencias de la Tierra, Malargüe, Argentina

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Romero JE, Morgavi D, Arzillic F, Daga R, Caselli A, Reckziegele F, Viramonte J, Díaz-Alvarado J, Polacci M, Burton M, Perugini D (2016b) Eruption dynamics of the 22–23 April 2015 Calbuco Volcano (Southern Chile): analyses of tephra fall deposits. J Volcanol Geotherm Res 317:15–29 Sernageomin (2015a) Reporte Especial de Actividad Volcánica (REAV) Región de los Lagos. Año 2015 Abril 22 (20:45 HL) Sernageomin (2015b) Reporte Especial de Actividad Volcánica (REAV) Región de los Lagos. Año 2015 Abril 22 (22:30 HL) Sernageomin (2015c) Reporte Especial de Actividad Volcánica (REAV) Región de los Lagos. Año 2015 Abril 23 (10:30 HL) Stewart C, Johnston DM, Leonard GS, Horwell CJ, Thordarson T, Cronin SJ (2006) Contamination of water supplies by volcanic ashfall: a literature review and simple impact modelling. J Volcanol Geoth Res 158:296–306 Tuffen H, James MR, Castro JM, Schipper CI (2013) Exceptional mobility of an advancing rhyolitic obsidian flow at Cordon Caulle volcano in Chile. Nat Commun 4:2709 Valderrama O, Franco L, Gil-Cruz F (2015) Erupción intempestiva del volcán Calbuco, abril 2015. XIV Congreso Geológico Chileno, La Serena, Chile Van Daele M, Moernaut J, Silversmit G, Schmidt S, Fontijn K, Heirman K, Vandoorne W, De Clercq M, Van Acker J, Wolff C, Pino M, Urrutia R, Roberts SJ, Vincze L, De Batist M (2014) The 600 yr eruptive history of Villarrica Volcano (Chile) revealed by annually laminated lake sediments. GSA Bull 126:481–498 Witham CS, Oppenheimer C, Horwell CJ (2005) Volcanic ashleachates: a review and recommendations for sampling methods. J Volcanol Geoth Res 141:299–326

Chapter 6

Deciphering the Morphologic Change in the Radial Drainage System of the Calbuco Volcano Caused by the 2015 Eruption Christopher Sepúlveda, Bruno Mazzorana, Héctor Ulloa, and Andrés Iroumé Abstract The eruption of the Calbuco volcano on April 22, 2015 presented three eruptive pulses, the second eruptive pulse being the main responsible for the most considerable impacts both in the natural and anthropic environment. This pulse generated several lahars due to the interaction of pyroclastic flows with glaciers and snow close to the summit of the volcano, causing alterations in the morphology of the volcano’s radial drainage system. The most affected basins were those of the Blanco Este, Tepu, Blanco Sur y Este rivers, altered by the lahars´ geomorphic work resulting in a remarkable river widening on alluvial deposits and producing geomorphic changes in the typology and configuration of several rivers reaches. To retrace the geomorphic changes occurred in the Blanco Este, Tepu and Blanco Sur rivers, the IDRAIM method was used, which allows for a thorough hydro-morphological analysis encompassing the rivers past geomorphic evolution, the characterization of their current dynamics and the exploration of their possible future trajectories. Following this method, river basins were characterized according to their physiography, then the rivers were segmented into river reaches based on their confinement, their geomorphic units and hydro-morphological typologies were identified by calculating a set of geomorphic indices and morphometric parameters. With this information we assessed the geomorphic signatures left in the affected rivers by comparing the elaborated cartographies referring to the geomorphic configurations before and after the eruption, respectively. The affectation of the basins occurred mainly in the headwaters, generating remarkable changes, thus altering the geography of the place. C. Sepúlveda (B) · B. Mazzorana Instituto Ciencias de la Tierra, Universidad Austral de Chile, Valdivia, Chile e-mail: [email protected] A. Iroumé Facultad de Ciencias Forestales y Recursos Naturales, Universidad Austral de Chile, Valdivia, Chile H. Ulloa Instituto Forestal, Forest Ecosystems and Water, Valdivia, Chile

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. Oyarzún et al. (eds.), Rivers of Southern Chile and Patagonia, The Latin American Studies Book Series, https://doi.org/10.1007/978-3-031-26647-8_6

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The basins of the Blanco Este, Tepu, Blanco Sur, and Este rivers were the ones that suffered the greatest degree of alteration featuring on average wider active channels and an increased braiding tendency in the lower water courses. These rivers, affected by large sediment injections, will most probably continue to exhibit major adjustments in the years to come. Keywords Fluvial systems · Morphology · Geomorphological units · Photointerpretation · IDRAIM

6.1 Introduction Rivers shape the landscape through sediment erosion, transport and deposition processes. A variety of factors influence the morphological dynamics of many rivers, and these factors can be natural or human-induced and can act at different spatial and temporal scales. In this sense, fluvial geomorphology investigates the evolution of fluvial environments through qualitative and quantitative analyses (Delai et al. 2014). The landforms, which are fluvial in origin, are studied and interpreted by a branch of earth science called fluvial geomorphology, which can be defined as the study of fluvial processes that originate the production and accumulation of sediments in basins and riverbeds during short, medium and long periods of time, and of the resulting landforms, in basins and river corridors. Therefore, the main objective of fluvial geomorphology is the study of how fluvial processes generate new and modify existing landforms (Marchetti 2000) and how the resulting landforms influence the process propagation. Hydrological sciences and fluvial geomorphology are deeply intertwined. In fact, the theoretical and methodological development of the former boosted the conception and refinement of the analytic techniques of the latter and, conversely, the insights gained in the geomorphological domain broadened the scope and reach of the former. Fluvial erosion and deposition processes profoundly alter the sediment transport regimes and, hence, fluvial forms which, in turn, exert a tangible influence on hydrodynamics. These coupled processes ultimately determine solid and liquid fluxes leaving the basin at its outlet section and the associated hazards and risks for exposed inhabited areas (Sala 1984). Viewing the river basin through the lens of geomorphology also permits the study of the effects exerted by major external disturbances on the river corridors. Several river systems in Chile are affected by both primary and secondary disturbances caused by volcanic eruptions. The former are constituted by huge sediment injections and by complex process cascades of hot and cold flows causing direct geomorphic impacts on the affected rivers. Secondary disturbances exhibit a lagged onset and lead to major disruptions of the vegetation cover of forested river basins and consequently and to

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an increased landslide susceptibility, which can result into a renewed sediment inputs into the main channels even decades after the main event (Sala 1984). Therefore, in this chapter, apart from generating knowledge of fluvial dynamics, the focus is set on understanding the immediate response (i.e. within a time span of less than 5 years) of the fluvial courses affected by volcanic eruptions by comparing them through photointerpretation and morphometric analyses with their unaffected geomorphic condition prior to the primary volcanic disturbance. In this way, our study contributes to the knowledge of the gemorphic risks associated with alterations in the fluvial systems as a basis for an enhanced basin management and more accurate territorial planning with the purpose of effectively mitigating risks for the population. The Calbuco volcano is an active volcano as attested by its recent eruptive history (i.e. in geomorphologic terms). Its last eruption dates from April 22, 2015. This eruption was characterized by 3 eruptive pulses, the second of which caused the greatest impacts on the environment, altering the fluvial geomorphology of some rivers of its radial system, as well as causing considerable damage in the town of Ensenada, as a result of both lahars that were channeled through the bed of the Blanco Este River and the large ash deposits (Bertin et al. 2015). High volumes of sediments resulting from the eruption were injected into the radial drainage network and subsequently triggered lahars in steep relief, causing extensive channel incisions and the mobilization of large volumes of solid material (large wood, pyroclastic rocks, gravel, among others). However, not all rivers of the radial system were affected to the same degree, and therefore, a complete analysis of the radial system of the volcano was carried out in order to assess the level of impact on each river through a hydromorphological characterization. To carry out this characterization of the rivers of the radial system, the IDRAIM method was used (Rinaldi et al. 2014). This method allowed for the integration of relevant knowledge about the geological setting, the climate, and the hydrologic regime. Endowed with this integrated knowledge base, a hierarchical spatial subdivision of the analyzed river systems was carried out. Preliminarily, the physiographic characterization of the river basins was accomplished and, based on that, the geomorphological units were classified in order to obtain clear criteria for the identification of segments, subdivided into sections (Rinaldi et al. 2014). Each section was characterized according to its degree of confinement and channel typology, thus assessing the geomorphic condition of the rivers of the radial system prior and posterior to the volcanic eruption. We remind the reader that IDRAIM is a much more comprehensive analytic framework and we focused our attention only to those steps regarding specifically the hydromorphological multitemporal characterization. In the following section we provide a brief overview of the IDRAIM framework and describe the set of performed steps.

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6.2 Hydromorphological Evaluation, Analysis and Monitoring System of the River (IDRAIM) IDRAIM constitutes a general methodological framework for hydromorphological analysis and assessment, for the subsequent monitoring of the impacts and definition of mitigation measures, considering their effects adopting an integrated planning approach (Rinaldi et al. 2014). For this aim, the objectives related to environmental quality and the mitigation of risks related to fluvial dynamics are fundamental. Hence, it is a system to support river management and the assessment and control of geomorphological processes (Rinaldi et al. 2014). This methodological system involves a series of methodological steps, which are: STEP 1: Basin characterization. This methodological step provides an integral characterization of the hydrologic basins and the associated river systems, enables a coherent subdivision of the study areas into spatial units through the application of a hierarchical approach and provides analytic tools for assessment of the factors that control ongoing fluvial processes and the resulting morphologies and their distribution throughout the river systems. STEP 2: Past evolution and evaluation of existing conditions. The insights gained in step 1 are viewed in an evolutionary context considering the temporal dimension, in order to achieve an understanding of how the current morphological conditions of the river could stem from the previous ones. This step contemplates stages; the first analyzes the evolutionary past of the fluvial systems and morphologies of the studied basins; the second and third phases provide the methodological tools to evaluate, through indices, the aspects that characterize the fluvial system, through the evaluation and analysis of the morphological dynamics and morphological quality of the system. This is possible through the integrated evaluation of these two aspects, which makes it possible to identify the occurred adjustments (Rinaldi et al. 2014). STEP 3: Analysis of the evolution of the fluvial system. Phase where information is provided on the measurement of the main morphological parameters and their variations and the representation and classification of morphological modifications and trends. Evaluation of the morphological quality. Phase where information is provided on the morphological reference condition, the morphological quality index and the identification and critical analysis of the basin. Assessment of morphological changes. In this phase, some concepts and definitions that are useful for framing the dynamic morphological aspects within the hazard and flood risk zoning of the basin affected by the eruption.

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To reiterate, the research presented in this chapter describes how the rivers of the radial system of the Calbuco volcano altered in the short term their morphology in response to the large input of volcanoclastic sediments into the fluvial system, to the hydrological regime and surface runoff modifications and well as to instream changes of water and sediment fluxes (Pierson et al. 2013). Volcanically impacted rivers exhibit the highest sediment yields on record (Hayes et al. 2002a, b; Tagata et al. 2006; Pierson and Major 2014). Sediment surplus is generated from pyroclastic flows and lahars, as well as from the erosion of sediments from islands and bars present in the river corridor (Gob et al. 2016; Umazano et al. 2014). The geomorphic response was evaluated by analyzing the system, generating a classification and characterization of the affected rivers, through the calculation of a series of morphometric parameters and indexes and the identification of geomorphic units. The next section provides a detailed overview of the employed methodology.

6.3 Methodology 6.3.1 Data Requirements To carry out this research, information associated with the eruption of the Calbuco volcano in 2015 was taken into account, considering areas of imminent danger of lahar formation in the basins of the radial system of the Calbuco volcano (see Fig. 6.1 with information extracted from the volcanic hazard map of SERNAGEOMIN (2017)). With the information collected after the eruptive event and considering the fluvial morphological changes of the radial system of the Calbuco volcano, the analysis of the responses and characterization of the affected fluvial courses were carried out. The characterization of the river courses analyzed was carried out by means of the system for hydromorphological evaluation, analysis and monitoring of watercourses, called IDRAIM, which provides a general methodology for analysis and support for the management of geomorphological processes in watercourses (Fig. 6.2). The developed method implements a systematic and structured process on how to approach the geomorphology of the watersheds of the radial system of the Calbuco volcano (Rinaldi et al. 2014).

6.3.2 Study Area The Calbuco volcano is one of the most active volcanoes in Chile, and it is located in the southern part of the country, between the communes of Puerto Varas and Puerto Montt (41°20' S, 72°39' W) in the X region of Los Lagos. The volcano edifice has a height of 2003 m a.s.l. and an area of 39.25 km2 (SERNAGEOMIN 2017), where

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Fig. 6.1 Processes associated with eruptive activity of the Calbuco volcano according to Moreno, affecting basins of the radial system and pyroclastic flows of the 2015 eruption

Fig. 6.2 Methodological flow diagram detailing the conceptual cornerstones of the investigation, the accomplished basic operations and the achieved main results

initially the river basins of the radial system of the Calbuco volcano were delimited (Fig. 6.3). This preliminary basin delimitation was necessary for the subsequent geomorphological characterization according to the IDRAIM method. The climate of the study area is described in the literature as a rainy oceanic climate, with rainfall throughout most of the year, accompanied by strong winds,

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Fig. 6.3 Location of the study area in Chile and delimitation of the river basins of the radial system of the Calbuco volcano

which intensify during winter and spring. According to the data from the Chilean Meteorological Office, the area has annual rainfall figures of around 1764.8 mm, with a decrease during summer, and an average temperature of 10.3 °C. Figure 6.3 shows the delimitation of the study basins of the radial system.

6.3.3 Hydromorphological Evaluation Method, System Analysis and Monitoring of the Watercourse (IDRAIM) To characterize the changes that occurred at the geomorphological level in the fluvial system originating from the flanks of Calbuco volcano edifice affected by the 2015 eruption, the IDRAIM method was employed. The application of this method provided the basis for the understanding of the morphological processes occurring in the analyzed river basins. This method primarily focuses on aspects of morphological dynamics, but it contributes also to integral watershed management not exhaustively covering, however, all the details to be considered in water system management. To carry out the geomorphological characterization of the identified rivers, a hierarchical subdivision approach was adopted (Fig. 6.4), which consists of studying the river at different scales allowing for the identification of system characteristics with

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Fig. 6.4 Hierarchical subdivision in terms of considered spatial scales as proposed by the IDRAIM method (Rinaldi et al. 2014)

Basins/Sub- Basins Physiographic Units Segments

Control the characteristics and behavior of the river

Sections

Morphological Units Hydraulic Unit

Units that characterize the morphology th of the section

a tangible influence on the character and behavior of the system at the lower hierarchical order (i.e. a top-down approach) and vice versa (i.e. a bottom-up approach). For the geomorphological characterization of the basins, it is useful to resort to both satellite images and digital elevation models (DEMs). Following this approach, the IDRAIM system refers to the following spatial units arranged in a hierarchical order as shown in Fig. 6.4. Here we disclose that our research progressed to the geomorphic unit scale only neglecting an analysis at the hydraulic unit scale, which would require a thorough assessment of the flow patterns based on hydrodynamic analysis.

6.3.3.1

Physiographic Units

With the delimitation of the study basins and with the purpose of a first subdivision of macro-areas and segments, physiographic units were obtained considering the combination of several factors such as topographic elevations, geology, and vegetation cover of the study area, representing macro-areas with relatively homogeneous characteristics (Rinaldi et al. 2014). For the determination of the physiographic units, the topographic elevations were considered, which were obtained from the DEM loaded in the ArcGIS 10.2 software. By employing the tool ArcToolbox, in particular the capabilities of the Spatial Analyst Tools and its subroutines surface and contour, the isolines were generated every 25 m and represented conjointly with the digital terrain model. This allowed interpreting the topography of the study area, determining the zones exhibiting a high mountain morphology, a gentler hill morphology and plains areas (Kondolf 1994). According to these macro areas and their topography, the following physiographic units were identified: erosion zones, transport areas and sediment deposits. Therefore, the erosion zones were located predominantly in high mountain areas above 600 m a.s.l., the transport zones were prevailingly situated between 100 and 600 m a.s.l. and areas corresponding to hilly areas and areas below 100 m a.s.l. were considered as lowlands with a higher predisposition for sediment

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Table 6.1 Classification of fluvial morphology based on specific indexes (Rinaldi et al. 2014) Typology

Sinuosity index

Braiding index

Rectilinear (CS)

1 ≤ Is < 1.05

1–1.5 generally close to 1–1.5 generally close to 1 1

Anastomosing index

Sinuous (S)

1.05 ≤ Is < 1.5

1–1.5 generally close to 1–1.5 generally close to 1 1

Meander (M)

≥ 1.5

1–1.5 generally close to 1–1.5 generally close to 1 1

Sinuous with bar (SBA) < 1.5

Close to 1

Close to 1

Wandering (CI)

< 1.5

1 < Ii < 1.5

1 < Ia < 1.5

Braided (W)

Any (usually low)

≥ 1.5

< 1.5

Anastomosed (A)

Any (can be > 1.5) Any (usually low)

≥ 1.5

deposition. However, it should be noted that all the basins analyzed are high mountain basins, where erosion and fluvial deposit zones predominate. For this reason, the lower subdivision was defined below 100 m a.s.l.

6.3.3.2

Geomorphic Units

Using the photointerpretation technique, we identified and obtained the geomorphological units that correspond to islands, bars, alluvial deposits, and channels present in each section and segment of the river systems analyzed. This was done with the purpose of making a comparison of the morphological evolution of both river beds based on several factors such as the degree of confinement, the number of channels, planimetric shape and a series of geomorphological parameters.

6.3.3.3

Geomorphological Parameters

To obtain the geomorphological classification of the river courses, the sinuosity index, braiding index and anastomosing index were calculated (see Table 6.1).

6.3.3.4

Index and Degree of Confinement

The measurement of the degree of confinement for each of the selected reaches was performed using GIS tools, measuring the percentage of length of the analyzed watercourses, with banks not in contact with the floodplain, with the presence of slopes, landslide deposits, tributary cones, fluvial terraces and/or glacial deposits (Rinaldi et al. 2014). In summary, the degree of confinement explained the percentage of the active channel in contact with confining elements. On the other hand, the confinement index was defined as the ratio between the width of the floodplain

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Table 6.2 Classification of confinement classes based on degree and index of confinement (Rinaldi et al. 2014) Confinement Class

Description

Confined

Confinement degree > 90% or a confinement degree between 10 and 90% if its confinement index is ≤ 1.5

Semi-confined

Confinement degree between 10 and 90% if its confinement index is > 1.5 or Confinement degree < 10% and a confinement index ≤ k

Not confined

Confinement degree < 10% or a confinement index > k

The values of k are determined as follows k = 5 for sinuous channels, rectilinear, sinuous with the presence of bars k = 2 for stranded or wandering rivers

(including the active channels) and the width of the riverbeds of the active channels. Hence, it accounted for the confinement of the channel or riverbed in the cross section with respect to the floodplain. According to the confinement index, the following classes were defined: • High confinement (Confined), where the index is between 1 and 1.5; • Medium confinement (Semi-Confined), in which the index is between 1.5 and a value k; • Low confinement (Not Confined), where the index is greater than k. Therefore, by obtaining the degree and index of confinement, the sections of the river courses analyzed could be classified into (Table 6.2).

6.3.3.5

General Geomorphological Parameters of the Study Basins

Having determined the physiographic units, extracted the contour lines, delimitated the divides of the study basins and identified the drainage network, we calculated the following general geomorphological parameters of the considered river basins of the radial system by employing the GIS tool calculate geometry: • • • • • • • •

Basin area (A) Basin perimeter (P) Length of channel (L) Length of main channel (L p ) Initial elevation of the main channel (i) Final elevation of the main channel (f ) Total channel length of the main channel prolonged to the divide (L t ) Drainage density Dd =

Li A

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• Compactness index (K) P K = 0.28 √ A • Concentration time according to Kiprich ( Tc =

0.87L 3 Hms

)0.385

• Basin width (w) w=

A L

F=

A L2

• Form factor of the basin (F)

• Circularity ratio (Rc ) Rc =

4 Aπ P2

All these data obtained for each of the analyzed basins allowed us to know the shapes of the basins and their general morphometric settings after the eruption.

6.3.3.6

Hypsometric Curve and Elevation Frequency Curve

From the previously obtained contour lines of the study basins, the hypsometric curves were determined by calculating the percentage of area between the adjacent contour lines in the basins of the radial system. First, a TIN file was generated from the DEM and contour lines were obtained. Subsequently, a new TIN raster file was generated to reclassify the percentage of area between contour lines in homogeneous intervals every 200 m, thus obtaining the hypsometric curves and the elevation frequency curves.

6.3.3.7

Relief Parameters

To obtain the relief data, it was necessary to generate a slope map based on an analysis of the digital elevation model. The slope map was generated from the raster created from the TIN. This was done using the Raster Surface > Slope tool. We proceeded

88 Table 6.3 Classification according to slope (Van Zuidam 1986)

C. Sepúlveda et al. Percentage (%)

Classification

44

Extremely steep

with the choice of the unit of measurement which was set as percentage. Once the slope map was generated, we classified slopes into seven classes according to Table 6.3. Following these methodological steps, it was possible to carry out the geomorphological characterization of the basins of the rivers belonging to the radial system of the Calbuco volcano, referring to both pre- and post-eruption conditions.

6.4 Results In this section of the chapter, we report the results obtained by applying step by step the methodology outlined in the previous section. First, we show the assessed main characteristics of the study basins. Thereafter, the quantified geomorphological parameters of the basins, the relevant relief and drainage network data are presented. Subsequently, we exemplify how the rivers were segmented and subdivided into sections and homogeneous reaches. Based on this spatial representation, the presence or absence of a set of relevant geomorphic units for each section within each segment of the analyzed rivers are shown in tables for pre- and post-event conditions, respectively. Complementarily, we report the quantified hydromorphological parameters and indexes in tables. Finally, we show the determined morphological river typologies for pre- and post-event conditions in maps and provide a summary of the occurred morphological changes at a river section scale.

6.4.1 Main Characteristics of the Study Basins Table 6.4 shows the general characteristics of the study basins of the radial system of the Calbuco volcano, according to their location, impact, area, slope, and elevation. The basins of the radial system of the Calbuco volcano were affected in their entirety with varying degrees of intensity, with the headwaters being the most affected by pyroclastic flows.

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Table 6.4 General characteristics of the study basins General Volcanic Area characteristics impact (A) of the basins (laharic (km2 ) flows)

Average Min slope elevation (%) (i) (m a.s.l.)

Max Average Basin closure elevation elevation coordinates (f ) (m (m a.s.l.) a.s.l.)

Blanco Este

Yes

39.25 13.46

46

1928

680.13

41°15' 33.42'' S 72°28' 41.44'' O

Hueñu Hueñu

No

66.88 20.83

38

1281

599.79

41°15' 33.99'' S 72°28' 41.42'' O

Tepu

Yes

22.81 14.15

52

1834

457.13

41°13' 16.62'' S 72°35' 44.65'' O

Blanco Norte

No

45.30 11.07

56

1972

484.07

41°13' 25.56'' S 72°37' 31.35'' O

Sur

No

69.94 11.56

50

1972

440.47

41°15' 4.49'' S 72°47' 43.06'' O

Chamiza

No

8.12

11

1973

309.82

41°27' 10.59'' S 72°49' 34.42'' O

Blanco Sur

Yes

32.83 13.47

238

1862

520.38

41°26' 9.07'' S 72°34' 59.44'' O

Este

Yes

30.41 22.52

238

1928

775.30

41°24' 59.00'' S 72°32' 42.04'' O

309.19

Lahars caused by the interaction of blocks, ash, ice and snow flew out to the north (Tepu River), south (Blanco Sur River) and east (Blanco Este River), running out toward the Llanquihue and Chapo lakes. It is worth mentioning that the basins with a greater degree of affectation according to photointerpretation and subsequent morphometric analysis were the basins of the Tepu (i.e. also affected by hot flows charring trees along their way), Blanco Sur and Blanco Este rivers, which were affected mainly by laharic flows, while the Blanco Este River basin was the most affected by laharic and pyroclastic flows.

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6.4.2 Geomorphological Parameters and Relief Data of the Studied Basins After obtaining the geomorphological parameters, the area between contour lines was calculated for each of the study basins, and the hypsometric curves for all the basins of the radial system were derived from these data, which in short indicate the percentage of basin area between the contour line elevations (Fig. 6.5). Hypsometric curves can be used to infer whether the basins exhibit a high erosional, equilibrium or sedimentation potential based on the shape of the curve (Table 6.5). The analysis of the hypsometric curves may suggest that the basins of the Blanco Este, Hueñu Hueñu and Este rivers are in a phase of maturity or equilibrium, where the tendency to transport sediment predominates, while the basins of the Tepu, Blanco Norte, Sur, Chamiza and Blanco Sur rivers are in a more advanced evolutionary stage with a tendency toward sedimentation. It has to be considered, however, that a drainage system of an active volcano may be subjected to sudden changes and, thus, should be viewed as inherently transient. Next, other characteristics of the basins are presented in order to detect similarities between them, such as altitude characteristics, basin shape, relief and parameters related to the drainage network (Tables 6.6, 6.7, 6.8 and 6.9, respectively). The slope map of the radial system shown in Fig. 6.6 indicates that the slopes in the basins of the Blanco Este, Tepu and Blanco Sur rivers are wavy, while in the basins of the Blanco Norte, Sur and Chamiza, the slightly undulating type of relief predominates, and finally, in the basins of the Este and Hueñu Hueñu rivers, the steep type of relief predominates. 2000 1800

Altitude (masl)

1600 1400 1200 1000 800 600 400 200 0 0

10

20

30

40

50

60

70

80

Cumulated Area (%) C. Río Blanco Este C. Río Sur

C. Río Hueñu Hueñu C. Río Chamiza

C. Río Tepu C. Río Blanco Sur

C. Río Blanco Norte C. Río Este

Fig. 6.5 Hypsometric curves of the basins of the radial system of Calbuco volcano

90

100

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Table 6.5 Basic parameters of the study basins General characteristics of the basins

Basin length (km)

Total length of the channels (km)

Initial elevation of main channel (m a.s.l.)

Final elevation of main channel (m a.s.l.)

Basin area (km2 )

Perimeter (km)

Basin width (km) w = LA

Blanco Este

13.53

25.91

935

45

39.25

52.90

2.92

Hueñu Hueñu

13.56

35.24

571

45

66.88

66.89

4.93

Tepu

12.11

18.21

411

53

22.81

44.05

1.88

Blanco Norte

11.83

30.74

772

54

45.30

46.72

3.83

Sur

18.03

62.00

Chamiza

21.22

232.81

Blanco Sur

11.13

Este

10.79

1044

53

69.94

75.59

3.88

–

–

309.19

134.11

14.57

19.62

1136

261

32.83

43.66

2.95

20.04

1121

235

30.41

38.60

2.82

Table 6.6 Characteristic altitudes Basins

Weighted average altitude (m a.s.l.)

Simple mean altitude (m a.s.l.)

Frequency of partial areas (m a.s.l.)

Blanco Este

680.13

987.00

988

Hueñu Hueñu

599.79

659.50

815

Tepu

457.13

943.00

275

Blanco Norte

484.07

1014.00

296

Sur

440.47

1011.00

531

Chamiza

309.82

992.00

257

Blanco Sur

520.38

1050.00

442

Este

775.30

1083.00

872

Table 6.7 Shape of the basins

Basins

Gravelius index (K)

Form factor (F)

Blanco Este

2.38

0.0624

Hueñu Hueñu

2.16

0.0624

Tepu

2.61

0.0512

Blanco Norte

1.96

0.0987

Sur

2.55

0.0535

Chamiza

2.15

0.0789

Blanco Sur

2.15

0.0790

Este

1.97

0.0967

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Table 6.8 Drainage network parameters Basins

Drainage Density (Dd )

Average extent of surface runoff (E s )

Concentration time according to Kirpich (T c )

Blanco Este

0.66

0.38

2.15

Hueñu Hueñu

0.53

0.47

3.02

Tepu

0.80

0.31

1.80

Blanco Norte

0.68

0.34

1.78

Sur

0.89

0.28

3.25

Chamiza

0.75

0.33

6.09

Blanco Sur

0.60

0.42

1.79

Este

0.66

0.38

1.50

Table 6.9 Relief parameters of the radial system basins

6.4.2.1

Basins

Slope of the basin (%)

Average slope (%)

Blanco Este

7.51

13.48

Hueñu Hueñu

4.24

20.83

Tepu

8.44

14.15

Blanco Norte

8.94

11.08

Sur

5.32

11.56

Chamiza

3.13

8.16

Blanco Sur

7.96

13.47

Este

9.53

22.53

Physiographic Units

Erosion, sediment transport and deposition zones were identified (Fig. 6.7), previously described by Schumm in 1977, and thus tendentially associated with the confinement of the main channels of both basins. With the identification of the physiographic units, the landscape characteristics were obtained, which are homogeneous in the basins. Three large zones were defined: Firstly, Zone 1 was identified, which represents the highest portion of the basin, and this was characterized by the production of sediments through erosion, mass wasting among others; Zone 2 was characterized by transporting sediments to the distal part of the basin, while Zone 3 was characterized by having a gentler slope which is conducive to sediment deposition.

6.4.2.2

Segments and Sections

To identify the geomorphological units and subsequent characterization of the main river courses, it was necessary to segment them and thus obtain through a subdivision

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Fig. 6.6 Slope map of Calbuco volcano

Fig. 6.7 Physiographic units: (Z1) Erosion zone (from 600 to 1973 m a.s.l.), (Z2) Transport zone (from 100 to 600 m a.s.l.) and (Z3) Depositional zone (from 11 to 100 m a.s.l.)

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Fig. 6.8 Segmentation and subdivision by sections of the Blanco Este and Hueñu Hueñu rivers

of the segments the reaches for each of the rivers (Fig. 6.8). This was carried out by identifying physiographic and geomorphological units such as the size of significant tributaries and/or apparent discontinuities through their degree of confinement. It is worth mentioning that the following image shows, as an example, the segmentation and how the sections were obtained of the Blanco Este and Hueñu Hueñu rivers. This procedure was replicated in the rest of the fluvial courses of the radial system of the Calbuco volcano.

6.4.2.3

Geomorphic Units

Following the steps of hierarchical analysis proposed by the IDRAIM method, we proceeded to the identification of the geomorphological units by identifying bars, islands, alluvial deposits present in floodplains of the rivers before and after the eruption, respectively. Additionally, we surveyed the presence or absence of hydraulic works (i.e. levees, bridges and other structures) due to their capability to interfere with the morphological adjustment. The identification of geomorphic and physiographic units from the analysis of photointerpretation and digital elevation models allowed us to have the first results

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95

with respect to the differences in the alterations suffered after the eruption of the Calbuco volcano on April 22, 2015. In Tables 6.10, 6.11, 6.12, 6.13, 6.14, 6.15 and 6.16, we report the presence or absence of the abovementioned geomorphological units in all sections within every segment of the rivers of the radial system of the Calbuco volcano before and after the eruption, respectively. With the assessment of the presence and absence of the geomorphological units of the study basins, it was possible to qualitatively determine the level of affectation Table 6.10 Presence and absence of geomorphologic units in the East Blanco River S1 T1 T2 T3 Islands A A A Bars A A A Alluvial deposits P P P Hydraulic works A A A Before the eruption

S1 T1 T2 T3 Islands A A A Bars A A A Alluvial deposits P P P Hydraulic works A A A After the eruption

S4 S5 T4 T5 T1 T2 T1 T2 P P P P P P P P P P P P P P P P P P P A A A A A P=Present A= Absent S2 S3 S4 S5 T4 T5 T1 T2 T3 T4 T1 T2 T3 T4 T5 T1 T2 T1 T2 A A A A A A A A A A A P P P P A A A P A P P P P P P P P P P P P P P P P P P P P P P P P P A A A A A A A A A A A A A A A T4 A A P A

S2 T5 T1 T2 T3 A A A A A A P A P P P P A A A A

S3 T4 T1 T2 T3 A A A A A P P P P P P P A A A A

Table 6.11 Presence and absence of geomorphological units in the Tepu River

Before the eruption Islands Bars Alluvial deposits Hydraulic works

S1 S2 S3 S4 T1 T2 T1 T2 T1 T2 T3 T4 T1 T2 A A A A A P A A A A A A A A A A P P P P A

A

A

A

A

A

P

P

P

P

A

A

P

P

A

A

A

A

A

A

P= Present A= Absent S1 S2 S3 S4 After the eruption T1 T2 T1 T2 T1 T2 T3 T4 T1 T2 Islands A A A A A A A A A A Bars P A P P P P A P A P Alluvial deposit P A P P P P P P P P Hydraulic A A P P A A A A A A works

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Table 6.12 Presence and absence of geomorphologic units in the Blanco Norte River

Before the eruption Islands Bars Alluvial deposit Hydraulic works

After the eruption Islands Bars Alluvial deposit Hydraulic works

S1 T1 T2 T3 A A A A A A A A P A A A S1 T1 T2 T3 A A A A A A A A P A A A

S2

S3

S4

T1 T2 T1 T2 T1 T2 A A P P A A A A A P P P A A A P P P A A A P A A P= Present A= Absent S2 S3 S4 T1 T2 T1 T2 T1 T2 A A P P A A A A A P P P A A A P P P A A A P A A

Table 6.13 Presence and absence of geomorphological units in the Sur River

Before the eruption Islands Bars Alluvial deposit Hydraulic works

After the eruption Islands Bars Alluvial deposit Hydraulic works

S1 S2 T1 T2 T1 T2 T3 T4 A A A A P A A A A A A A P A A A A A A A A P P P S1 S2 T1 T2 T1 T2 T3 T4 A A A A P A A A A A P A P A P P P P A A A A P P

S3 T1 T2 T1 A P A A A A A A A A P A P= Present S3 T1 T2 T1 A P A A A A A A A A P A

S4 T2 T3 T4 A A P A A A A A A A A A A= Absent S4 T2 T3 T4 A A P A A A A A A A A A

in each section. In particular, we could detect an increase of the presence of alluvial deposits after the eruption when compared to the condition prior to the eruptive perturbance. The opposite could be observed in relation to the presence of levees or reinforced riverbanks which were largely destroyed by the impacts of the riverine processes.

6 Deciphering the Morphologic Change in the Radial Drainage System … Table 6.14 Presence and absence of geomorphological units in the Blanco Sur River

Before the eruption Islands Bars Alluvial deposit Hydraulic works

After the eruption Islands Bars Alluvial deposit Hydraulic works

T1 A A P A

S1 T2 A A P A

T1 A A P A

T1 A A P A

S1 T2 A A P A

T1 A A P A

S2 T2 T3 T1 A A A A A P A A P P A P P= Present S2 T2 T3 T1 A A A P P A P P P A A A

S3 T2 T3 T4 A A A P P P P P P A A A A= Absent S3 T2 T3 T4 A A A P P P P P P A A A

Table 6.15 Presence and absence of geomorphological units in the Este River

Before the eruption Islands Bars Alluvial deposit Hydraulic works P= Present After the eruption Islands Bars Alluvial deposit Hydraulic works

S1 S2 T1 T2 T1 T2 T3 A A A A A A A A A P P P P P P A A A A A A= Absent S1 S2 T1 T2 T1 T2 T3 A A A A A A P P P P P P P P P A A A A A

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Table 6.16 Presence and absence of geomorphological units in the Hueñu Hueñu River

S1 S2 S3 S4 T1 T2 T1 T1 T2 T1 T2 Islands A A A A A A A Bars A A A A P P P Alluvial deposit A A A A P P P Hydraulic works A A A A A A A P= Present A= Absent S1 S2 S3 S4 After the eruption T1 T2 T1 T1 T2 T1 T2 Islands A A A A A A A Bars A A A A P P P Alluvial deposit A A A A P P P Hydraulic works A A A A A A A Before the eruption

6.4.3 Morphological Characterization by River Sections of the Radial System, Before and After the Eruption of the Calbuco Volcano To finally characterize section of the rivers of the radial drainage system form a geomorphological viewpoint by determining their dominant typology and detecting changes in the post-event setting with respect to the geomorphic condition prior to the eruptive perturbation, we assessed the degree and index of confinement and thus obtaining a characterization of the fluvial systems and applying theoretical concepts of fluvial geomorphology through qualitative and quantitative methods for each of the sections. It is worth mentioning that these calculations were performed for the entire radial system of the volcano, except for the Chamiza river basin. In this case, a reliable photointerpretation at a section scale was rendered unreliable due to the high vegetation density present in that sector of the radial system. Through photointerpretation of satellite images Sentinel 2 from October 2014 and October 2016, we obtained data on the active width of the channel, bankfull and the flood plains. Based on these quantified parameters, we were able to calculate the index and degree of confinement of all studied rivers, expect, as aforementioned, the Chamiza River (see Table 6.17 where we report the mean values of the considered hydromorphological parameters).

2.8

88.0

5.2

88.0

Braiding index

Degree of (%) confinement

Confinement index

1.1

1.5

1.2

1.7

Sinuosity index

63.0

1.6

1.2

1.1

Pre

Post

Pre

64.7

2.4

1.4

1.1

Post

Tepu River

Blanco Este River

Eruption period

55.3

1.8

1.2

1.2

Pre

47.8

1.4

1.2

1.2

Post

Blanco norte River

Table 6.17 Mean values of the considered hydromorphological parameters

52.2

1.7

1.1

1.2

Pre

66.4

2.7

1.2

1.2

Post

Sur River

86.5

5.3

1.2

1.1

Pre

87.6

5.7

1.3

1.1

Post

Blanco Sur River

93.1

4.8

1.2

1.1

Pre

93.5

5.6

1.4

1.1

Post

Este River

49.7

1.5

1.0

1.2

Pre

47.2

1.6

1.1

1.2

Post

Hueñu Hueñu River

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6.4.4 Mapping of the Morphological Characterization of the Rivers of the Radial System Before and After the April 22, 2015, Eruption In Figs. 6.9, 6.10, 6.11, 6.12, 6.13, 6.14 and 6.15, we visualize for each considered river a map reporting for each section within each segment the leading morphological typology. Each map contains a comparison of the typologies present before and after the eruption. In Table 6.18, we report a summary of the alterations suffered by the rivers of the radial system, emphasizing changes in confinement and highlighting the river sections which underwent a change of typology. With respect to the former, a cautious interpretation is necessary since in principle the confinement shouldn’t suffer major changes. In this respect, it has to be remarked that at the adopted scale of analysis (i.e. extent of a section) the identification of confining elements was not always straightforward and the judgment of their stability in light of major disruptions could be somewhat unreliable.

Fig. 6.9 Characterization of the dominant morphology of each section within all segments of the East Blanco River before and after the Calbuco volcano eruption

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Fig. 6.10 Characterization of the dominant morphology of each section within all segments of the Hueñu Hueñu River before and after the Calbuco volcano eruption

Fig. 6.11 Characterization of the dominant morphology of each section within all segments of the Tepu River before and after the Calbuco volcano eruption

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Fig. 6.12 Characterization of the dominant morphology of each section within all segments of Blanco Norte before and after the eruption of Calbuco volcano

Fig. 6.13 Characterization of the dominant morphology of each section within all segments of the Sur River before and after the Calbuco volcano eruption

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Fig. 6.14 Characterization of the dominant morphology of each section within all segments of the Blanco Sur River before and after the Calbuco volcano eruption

Fig. 6.15 Characterization of the dominant morphology of each section within all segments of the Este River before and after the Calbuco volcano eruption

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Table 6.18 Summary of the adjustments observed in the studied rivers due to the impacts of the eruption of the Calbuco volcano based on the comparison of pre- and post-event conditions in terms of confinement and typology Legend C Confinement class

Channel typology

Segments Sections Pre confinement Post confinement Pre typology Post typology

S N CS S SBA M W

T1 C C S S

Confined river Semi confined river Unconfined river Rectilinear Sinuous Sinuous with bar Meandering Braided Change of typology S1 T2 T3 T4 C C C C C C S S M S S S

T5 S C S S

Blanco Este River S2 S3 S4 S5 T1 T2 T3 T4 T1 T2 T3 T4 T5 T1 T2 T1 T2 S S C C S S S S C N N S S S S C S S S S S S N N N S CS SBA S S S SBA SBA SBA SBA W W W W S W S SBA W SBA SBA W W W S S W

Tepu River S1 S2 T1 T2 T1 T2 T1 C C C S S S S C S S S S S S CS S S S S CS Blanco Norte River Segments S1 S2 Sections T1 T2 T3 T1 T2 Pre confinement C C C S S Post confinement C C C C S Pre typology S S S S S Post typology S S S S S Sur River Segments S1 S2 Sections T1 T2 T1 T2 T3 Pre confinement C C C S S Post confinement S S S S S Pre typology S S S S S Post typology S S S S S Blanco Sur River Segments S1 S2 Sections T1 T2 T1 T2 Pre confinement C C C C Post confinement C C C C Pre typology S S S S Post typology S S S SBA Este River Segments S1 S3 Sections T1 T2 T1 T2 Pre confinement C C C C Post confinement C C C C Pre tipology S S S S Post tipology S S CS S Segments Sections Pre confinement Post confinement Pre typology Post typology

S3 T2 S C S S

S4 T3 S C S S

T4 S S S S

T1 S C S S

T2 S C S S

T1 S S S S

T2 S S M M

T4 S S S S

T1 S S S S

T2 S S S S

T1 S S S S

S3

T1 C S CS S

T2 S S SBA W

S4

S3

T3 T1 C C C C S S SBA SBA

S4

S3 T2 S S S S

T3 S S S S

T2 S S S S

T3 S C S S

T4 S C SBA SBA

T4 S S S S

T3 S S S S

(continued)

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Table 6.18 (continued) Segments Sections Pre confinement Post confinement Pre typology Post typology

Hueñu Hueñu River S1 S2 T1 T2 T1 C C C C C C S S S S S S

S3 T1 C C S S

T2 C C S S

S4 T1 T2 S S S S SBA SBA SBA SBA

6.5 Discussion 6.5.1 Main Characteristics of the Study Basins The results of the morphometric analysis attest that the basins of the radial system (Table 6.7) are characterized by similar elongated shapes and exhibit similar average slopes, except for the Hueñu Hueñu, Chamiza and Este river basins. The former two feature an average slope > 20%, whereas the Chamiza River has an average slope less than 10%. The degree of affectation of the river morphology, rather than being related to these general characteristics, depended on the elevation range of the basins with respect to the edifice of the volcano. This explains why the Blanco Norte, Sur, Chamiza and Hueñu Hueñu rivers suffered only a minor affectation in contrast to Blanco Este River. It is worth mentioning that the headwaters of the Hueñu Hueñu River are at an altitude of 1281 m a.s.l. This basin drains the flanks of the volcano adjacent to those drained by the Blanco Este River, which is the most affected throughout the history of volcanic eruptions of the Calbuco volcano and the induced lahars and pyroclastic flows. Albeit the adjacency to the Blanco Este River, the Hueñu Hueñu River is protected by a mountain ridge. This may explain the comparatively low degree of affectation. The eruptive history of the Calbuco volcano during the Holocene is quite remarkable. In fact, as described by Castruccio et al. 2016 this stratovolcano (2003 m a.s.l.) appears to be one of the most active volcanoes of the southern Chilean Andes. They documented at least 37 eruptive periods during this geologic epoch. The first volcanic activity was dated by radiocarbon and tephrochronology methods to 8460 BCE, while the historical observations go back to 1893. From that date onward, 11 eruptive events were registered, being the first and the last, occurred in 1893 and 2015, respectively, cataclysmic (i.e. volcanic explosivity index = 4) and the intermediate ones less intense (Petit-Breuilh 1998; Zingaretti et al. 2023). Due to the surface morphology of the volcano’s edifice, the lava flows preferentially propagated along the channels of the northeast and southeast flanks of the volcano, thus affecting the morphology of the Este and Blanco Este rivers in the aftermath of the eruption. The edifice of the volcano presents higher elevations in the

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northwestern and western sectors providing a shield effect. Hence, the perturbation effects in the Chamiza, Sur and Blanco Norte rivers, draining those flanks, were lesser in magnitude.

6.5.2 Geomorphological Parameters of the River Basins The obtained parameter values reported in Table 6.5 signal, and the constructed hypsometric curves suggest that the basins of the Este and Blanco Este rivers are in an evolutive stage of equilibrium, whereas the basins of the Tepu Blanco Norte, Sur, Blanco Sur rivers show a tendency to sedimentation. Instead, the hypsometric curves of the Hueñu Hueñu River feature an evolutive tendency from equilibrium toward a mature stage. These data are interesting from a general morphometric appreciation standpoint but do not provide greater clues for the identification geomorphic signatures in the studied river corridors left by the last volcanic eruption occurred in 2015, because the most updated elevation model dates to the year 2011 and no elevation model representing the post-event morphology was available for the entire radial system. According to the quantified shape factor and Gravelius index, all studied basins of the radial system feature an oblong shape. The relief parameters (Fig. 6.6) indicated that the basins of the radial system exhibit, prevailingly, the slope classes sloping or moderately steep. In the basins of the Blanco Este, Tepu and Blanco Sur rivers, the sloping relief type predominates, while in the basins of the Blanco Norte, Sur and Chamiza rivers, the gently slope relief type preponderates, and finally in the basins of the Este and Hueñu Hueñu rivers, the moderately steep relief type predominates.

6.5.3 Physiographic Units The identification of the physiographic units (Fig. 6.7) showed that the basins of the radial system of the Calbuco volcano are mostly basins with sediment transport characteristics, and this is mainly due to the geographical location of the study area, as suggested by Schumm (1977) according to his subdivision of the fluvial systems into erosion, transport and sediment deposition zones.

6.5.4 Geomorphic Units The geomorphic units (fluvial channels, alluvial deposit, bars and anthropic features such as hydraulic works) present in the river corridors of the radial system were identified. In particular, Tables 6.10, 6.11, 6.12, 6.13, 6.14, 6.15 and 6.16 show the presence or absence of islands, bars, alluvial deposits and hydraulic works in

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all sections of the considered rivers. Overall, remarkable changes could be identified in the rivers affected by the eruption (foremostly in the Blanco Este, Este and Sur rivers), where alterations were observed with respect to the extent of alluvial deposits, to the formation of longitudinal and lateral bars, islands with vegetation. Moreover, constructions along the banks of the affected rivers suffered damages or were completely destroyed by the pyroclastic flows.

6.5.5 Section-Wise Morphological Characterization of the Rivers of the Radial System of the Calbuco Volcano Before and After the Eruption of the Calbuco Volcano By obtaining the values of the index and degree of confinement, it was possible to classify each of the sections of the studied rivers according to their class of confinement in the main riverbeds of the rivers analyzed. According to (Fig. 6.9), the most considerable alterations in terms of confinement were detected in the following rivers: Blanco Este River, in segments 2 and 3 and, specifically, between the sections T1 of S2 and T2 of S3, exhibiting alteration maxima according to the degree and index of confinement in sections T3 and T4 of S2 from 2014 to 2016; Tepu River, between sections T2 of S2 and T3 of S3; Blanco Sur River between sections T1 of S2 and T1 of S3; and Este River in the final segment between sections T2 and T3 of S2. These alterations were mainly imputable to the widening of the active channel and floodplain, product of the accumulation of sediments, injected by the volcanic eruption. On the other hand, the Hueñu Hueñu, Blanco Norte, Sur and Chamiza rivers did not suffer major alterations although some adjustments could be detected. These adjustments, however, are typical of any river, due to its normal fluvial dynamics (Bourrel and Pouilly 2004). In the case of the Chamiza river basin, photointerpretation did not allow a reliable classification according to typology and morphology. Based on the available spatial imagery, the narrow channel on the one hand and the dense vegetation on the other did not permit any discernment. Due to the eruptive event and the triggering of lahars channeled through the active channels of the investigated rivers of the radial system, remarkable typological changes took place. The most noticeable changes were: from a sinuous typology to a sinuous typology featuring the presence of bars, from straight channels to sinuous river planforms. Transitions from a sinuous planform to a braided typology were detected, except in segment 1, in individual sections of segments 2 and 4 of the Blanco Este River. Rather surprisingly in two sections of the last segment, a transition from a braided typology to a sinuous one (i.e. with alternating bars) was detected. We interpret this as a momentary relaxation of the alteration in this specific segment. Momentary, because the export of sediments is still ongoing and may happen in pulses. Based on the date of post-event assessment (i.e. in 2016), it is, however,

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still too soon to provide a definitive interpretation. Zingaretti et al. (in press) also underline that the post-eruption adjustment process is still very dynamic. Moreno, after mapping the hazards associated with eruptions of the Calbuco volcano and compiling the eruptive history of the Calbuco volcano and based on geologic and stratigraphic surveys, predicted that future eruptions would have had an impact on the landscape preferentially toward the NE, SE and S flanks of the Calbuco volcano. This prediction turned out to be rather accurate, as both the damages caused by the last eruption of April 22, 2015, and different studies carried out by SERNAGEOMIN (2017) indicated. The greatest level of affectation could be retraced on the northeastern and eastern flanks of the volcano. However, the volcano also generated eruptive pulses in western direction affecting to a lesser degree the headwaters of the Sur and Blanco Nord rivers. In fact, pyroclastic flows reached a variable distance from the crater between 1 km to the west and 7.5 km to the east (Mella et al. 2015) with this sector being the most affected by the eruption. In contrast, the fall of pyroclasts affected, preferentially, the NE sector of the volcano, generating a pyroclastic cone around the main crater of an approximate height of 15 m and a thickness in Ensenada of 28 cm (Bertin et al. 2015). Finally, the pyroclastic flows were channeled through the active channels of the Blanco Este, Tepu, Blanco Sur and Este rivers (to a lesser extent), while affecting only the headwaters of the Sur, Blanco Norte and Chamiza rivers. The longest runout distance was observed in the Blanco Este River, exceeding 7.5 km from the crater, burying a large number of trees and generating remarkable alterations in the geomorphology of the sector (Bertin et al. 2015). The changes in typology, which we detected in this study, corroborate previous findings.

6.6 Conclusion The focus of this chapter was to study the immediate geomorphic response to the volcanic eruption of the Calbuco volcano in 2015 of the rivers constituting its radial drainage system by applying the main characterization steps proposed by the IDRAIM method. Thereby we considered the following rivers: Blanco Este, Tepu, Blanco Norte, Sur, Sur, Este, Blanco Sur, Hueñu Hueñu and Chamiza rivers. First, the morphology of their basins was analyzed and different morphometric parameters were obtained allowing for a general description of their relief and form. Thereupon, in a second step, the physiography of the river basins was assessed. Subsequently, the rivers were spatially subdivided, in segments and homogeneous reaches. Based on this subdivision, their confinement was determined, their geomorphic units were identified and geomorphic typology was classified for pre- and post-event conditions, respectively. This structured approach allowed to analyze on a spatial scale relative to a river section but not beyond at finer scales, the extent of hydromorphological affectation of the river corridors and tentatively explain the link between the spatial deposition pattern of the main volcanic products, and the first post-event geomorphic adjustments retraced in all sections of the rivers of the radial system. Although geomorphic

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processes in the affected rivers are still ongoing, we could provide a first macroscopic assessment of the geomorphic signatures left in the immediate aftermath of the eruption in the rivers of the radial system. It’s worth reiterating that obtaining a comprehensive picture of the evolutive trajectories of these river systems and providing an in-depth causal explanation of the single process response relationships was beyond the scopes of this work and would require an assessment on a more detailed spatial scale and over much longer time period. Despite these limitations, this study provides a useful overview of the immediate hydromorphological response and serves as an starting point for further investigations aiming at comprehensively deciphering the post-event intermediate and long-term adjustment processes shaping the river landscapes composing the radial system of the Calbuco volcano. Funding Information This research was funded by the project ANID/CONICYT FONDECYT Regular, Folio 1200091, “Unravelling the dynamics and impacts of sediment-laden flows in urban areas in southern Chile as a basis for innovative adaptation (SEDIMPACT)” led by Bruno Mazzorana.

References Bertin D, Amigo Á, Mella M, Astudillo V, Bertin L, Bucchi F (2015) Erupción del volcán Calbuco 2015: Estratigrafía eruptiva y volumen involucrado. In: Congreso Geológico Chileno, No. XIV, La Serena, Chile Bourrel L, Pouilly M (2004) Hidrología y dinámica fluvial del Río Mamoré. Diversidad biológica en la llanura de inundación del Río Mamoré. Centro de Ecología Simón I. Patiño, Santa Cruz, Bolivia, pp 95–116 Castruccio A, Clavero J, Segura A, Samaniego P, Roche O, Le Pennec J-L, Droguett B (2016) Eruptive parameters and dynamics of the April 2015 sub-Plinian eruptions of Calbuco volcano (southern Chile). Bull Volcanol 78:62 Delai F, Moretto J, Picco L, Rigon E, Ravazzolo D, Lenzi MA (2014) Analysis of morphological processes in a disturbed gravel-bed river (Piave River): integration of LiDAR data and colour bathymetry. J Civil Eng Archit 8(5) Gob F, Gautier E, Virmoux C, Grancher D, Tamisier V, Primanda K, Wibowo S, Sarrazin C, de Belizal E, Ville A, Lavigne F (2016) River responses to the 2010 major eruption of the Merapi volcano, cenral Java, Indonesia. Geomorphology 273:244–257 Hayes S, Montgomery D, Newhall C (2002a) Fluvial sediment transport and deposition following the 1991 eruption of Mount Pinatubo. Geomorphology 45(3):211–224 Hayes SK, Montgomery DR, Newhall CG (2002b) Fluvial sediment transport and deposition following the 1991 eruption of Mount Pinatubo. Geomorphology 45:211–224 Kondolf GM (1994) Geomorphic and environmental effects of instream gravel mining. Landsc Urban Plan 28:225–243 Marchetti M (2000) Geomorfologia fluviale. Pitagora Editrice Bologna Mella M, Moreno H, Vergés A, Quiroz D, Bertin L, Basualto D, Garrido N (2015) Productos volcánicos, impactos y respuesta a la emergencia del ciclo eruptivo abril-mayo (2015) del volcán Calbuco. In: XIV Congreso Geológico Chileno, La Serena, Chile

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Petit-Breuilh S (1998) Cronología eruptiva histórica preliminar de los volcanes Osorno y Calbuco [41°-41°30' S] X región, Chile: efectos de las erupciones más documentadas. In: Estudio Geológico-económico de la X. región Norte, pp 1–72 Pierson T, Major J (2014) Hydrogeomorphic effects of explosive volcanic eruptions on drainage basins. Annu Rev Earth Planet Sci 42:469–507 Pierson TC, Major JJ, Amigo Á, Moreno H (2013) Acute sedimentation response to rainfall following the explosive phase of the 2008–2009 eruption of Chaitén volcano, Chile. Bull Volcanol 75(5):1–17 Rinaldi M, Surian N, Comiti F, Bussettini M (2014) IDRAIM: Sistema di valutazione idromorfologica, analisi e monitoraggio dei corsi d’acqua Sala M (1984) Geomorfología actual. Guía conceptual, temática y bibliográfica. Revista de Geografia 18(1):209–248 Schumm SA (1977) The fluvial system. Wiley, New York SERNAGEOMIN (2017) Calbuco. Disponible en: http://www.sernageomin.cl/volcan-calbuco/. Acceso: 8 Aug 2017 Tagata S, Yamakoshi T, Doi Y, Kurihara J, Terada H, Sakai N (2006) Post-eruption characteristics of rainfall runoff and sediment discharge at the Miyakejima Volcano, Japan. In: Proceedings of the INTERPRAEVENT international symposium, pp 291–301 Umazano A, Melchor R, Bedatou E, Bellosi E, Krause J (2014) Fluvial response to sudden input of pyroclastic sediments during the 2008–2009 eruption of the Chaiten Volcano (Chile): the role of logjams. J S Am Earth Sci 54:140–157 Van Zuidam RA (1986) Aerial photo-interpretation in terrain analysis and geomorphologic mapping. International Institute for Aerospace Survey and Earth Science, The Hague, ITC, 442 p Zingaretti, V., Iroumé, A., Llena, M., Mazzorana, B., Vericat, D., & Batalla, R. J. (2023). Geomorphological evolution of the Blanco Este River after recent eruptions of the Calbuco volcano, Chile. Geomorphology, 108570.

Chapter 7

Investigating the Geomorphological Footprint of Moraine-Dammed Lake Failures in Patagonian Rivers Diego Bahamondes, Bruno Mazzorana, Pablo Iribarren Anacona, and Héctor Ulloa Abstract Recent studies highlighted that Glacier Lake Outburst Floods (GLOFs) can profoundly alter the geomorphology of affected rivers. Attempting to quantify the geomorphic changes generated by recently occurred GLOF events, in this study we systematically compared a set of affected with another set of unaltered rivers in Chilean Patagonia. First, we performed a discriminant analysis on specific basin characteristics assuring that the two sets of river systems shared similar morphological characteristics. Once the basins of unaffected control rivers exhibiting a high degree of morphological similarity were selected, we carried out specific steps of the IDRAIM methodology based on multi-temporal areal images, thereby identifying the typology and dominant morphology of both groups of rivers, discerning among the affected ones the pre- and post-event conditions. This classification was based on the sinuosity (SI), braiding (BI), and confinement index (CI) parameters. Subsequently, an ANOVA analysis of the index values was performed. The obtained results showed that the affected rivers exhibited a higher BI than the unaffected ones. Conversely, no significant differences in the SI values could be detected between the two groups of rivers. Regarding confinement, the affected rivers featured on average larger flood plains and significantly greater active channel widths due to the “sweeping” effect that these high energy processes exerted on the different morphological units of the river corridor. Summing up, this study corroborated the role of a sudden drainage of glacier lakes as a potent geomorphologic agent. In the context of ongoing climatic change and intensifying human impacts, these transient river systems are particularly prone to acute flood risks. Targeted actions should be taken to reduce the adverse effects of GLOF events. Keywords Patagonian rivers · GLOFs · Fluvial geomorphology

D. Bahamondes (B) · B. Mazzorana · P. Iribarren Anacona · H. Ulloa Instituto de Ciencias de la Tierra, Universidad Austral de Chile, Valdivia, Chile e-mail: [email protected] H. Ulloa Instituto Forestal, Forest Ecosystems and Water, Valdivia, Chile © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. Oyarzún et al. (eds.), Rivers of Southern Chile and Patagonia, The Latin American Studies Book Series, https://doi.org/10.1007/978-3-031-26647-8_7

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7.1 Introduction Glacier Lake Outburst Floods (GLOFs) are processes that occur in mountain systems with glacial influence. These peculiar processes occurred in mountain regions located in various parts of the world (Osti and Egashira 2009). These events trigger suddenly increasing flows, which may lead to severe consequences in terms of human losses, infrastructural damages, and induce remarkable geomorphological changes (Carrivick 2007). The retreat of glaciers in the last century promoted the formation of new glacial lakes in several mountain regions of the world. These lakes are dammed either by ice or moraines (IPCC 2012; Worni et al. 2014). The drainage of these lakes has caused numerous disasters particularly in the Andes (Lliboutry et al. 1977; Worni et al. 2014), in the Caucasus and Central Asia (Aizen et al. 1997; Worni et al. 2014), and in the Himalayas (Vuichard and Zimmermann 1986; Worni et al. 2014), among many other GLOFs-prone mountainous areas of the world. Focusing on the Chilean Andes, the situation is quite similar and sudden drainage events of great magnitude occur frequently. The loss of ice mass in the Northern and Southern Patagonian Ice Fields is remarkable (Dyurgerov and Meier 2005; Benito et al. 2014). This retreat favors the thinning of glaciers that dam certain lakes and fosters the formation of new ones that are contained by moraines (Benito et al. 2014). Damming the Colonia Glacier and feeding the Colonia River, a major tributary of the Baker River, the Cachet 2 Lake stands out (Benito et al. 2014). Between 2008 and 2015, this glacier has generated at least 16 GLOFs, with an average drainage volume of 2.3 km3 and with maximum flows of around 3500 m3 /s (Iribarren-Anacona et al. 2015). More recently a noteworthy event occurred in the Río Exploradores (December 2015–February 2016). Thereby, the emptying of Lake Chileno produced several discharges and the largest of them occurred on December 25th with a maximum discharge of 408 m3 /s (Wilson et al. 2019).

7.1.1 Glacier Lake Outburst Flood’s Typology These phenomena can be triggered either by tunnel drainage or through the rupture of the dam, which can be made of ice or be constituted by a terminal moraine (Walder and Costa 1996). In the first case, they are unleashed by a substantial increase in the level of the lake, reaching the point at which the ice dam “floats” on water impounded under it (Walder and Costa 1996). In the second case, rupture occurs when the ice dam or terminal moraine retaining the glacier lakes partially or totally yields, suddenly releasing enormous volumes of water and resulting in a rapid discharge increase in downstream river reaches. GLOFs may differ in their onset and posterior dynamics. Moreover, the proposed typological classification (i.e. sudden drainage through tunnels and due to rupture) can be further refined by considering in detail the mechanisms by which these types of processes are generated. In glaciers that contain subglacial or intraglacial lakes,

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draining tunnels can be formed at the base or develop through the ice mass. When the water pressure under or within the glacier ice reaches a critical value, the ice mass of the glacier can be lifted, and flotation may set in and further tunnels may rapidly develop further accelerating the drainage of the retained water volume (Björnsson 2002). Conversely, in the case of ice- or moraine-dammed glacier lakes, the typological classification reflects the nature of the dam (Walder and Costa 1996). In the former case, the lake empties suddenly due to dam failure whose instability can be determined by external factors such as earthquakes or avalanches. In the latter case (i.e. moraine-dammed lakes), partial or total erosion of the dam is responsible for the release. These release processes usually require a trigger for their onset. Among those triggering processes, we mention avalanches or rockfalls (Vuichard and Zimmermann 1986; Westoby et al. 2014), “calving” or detachments of icebergs from the glacier that is in contact with the lake, generating waves of the “seiche” type which can surpass the crest of the moraine and cause its rupture immediately thereafter (Lliboutry et al. 1977; Westoby et al. 2014). In fact, the overtopping of the moraine crest due to one or more waves is one of the most common causes of the weakening and subsequent failure of the dam leading to the drainage of glacier lakes (Du et al. 2016). The stability and consolidation of the moraine material are pivotal factors in determining the extent of the erosion.

7.1.2 Geomorphological Significance of GLOFs Various authors agree on the notion that large-scale GLOF processes in mountain environments are potent geomorphological agents since they are capable of generating hydraulic forces necessary to move and transport sediments of large size (Fig. 7.1) which constitute an important fraction of the alluvial material (Stewart and LaMarche 1967; Cenderelli and Wohl 2003). The geomorphological change due GLOFs can be assessed according to the channel adjustments and the change of valley bottom morphology, and the time they take to return to the pre-event setting (Wolman and Gerson 1978; Cenderelli and Wohl 2003). Several factors control the morphological response of the affected river: (a) characteristics of the hydrographic basin such as morphology, lithology, type of soil, and vegetation; (b) characteristics of the channel and floodplain: foremostly slope, morphology, and sediment availability (Kochel 1988; Cenderelli and Wohl 2003). Factors such as event magnitude, intensity, duration (Costa and O’Connor 1995; Cenderelli and Wohl 2003) and the geomorphic legacy of previous floods (Beven 1981; Cenderelli and Wohl 2003), can control process dynamics and geomorphic response significantly (Bull 1979, 1988; Baker and Costa 1987; Cenderelli and Wohl 2003). The stream power is used as a useful indicator of how hydraulic forces can interact with the channel boundaries and determine the capacity of the flow to mobilize and transport solid material (Baker and Ritter 1975; Carling et al. 2009; Cenderelli and Wohl 2003).

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c

b

Fig. 7.1 Snapshots of areas affected by GLOFs. a Satellite image of Laguna El Duende postdrainage, in which the affected area produced downstream of the lake can be observed. b, c Images depicting the fluvial landscape of the valley below Laguna Los Leones, affected by a GLOF. Note the size of the transported material

There is an ongoing scientific debate regarding the role of large floods as geomorphological agents (Desloges and Church 1992). On the one hand, it is asserted that medium-magnitude but more frequent events feature the formative discharge responsible for most of the accomplished geomorphic work (i.e. erosion and deposition). Following this interpretation existing geoforms would reflect a river in dynamic equilibrium capable, if disturbed, of rapidly bouncing back toward an equilibrium (Wolman and Miller 1960; Desloges and Church 1992). However, this view is challenged by various studies indicating that extreme flood events can establish morphological conditions that persist over time. According to this perspective, smaller events would not be capable, at least temporarily, of redirecting the geomorphic evolution of the river toward equilibrium conditions (Stewart and LaMarche 1967). In fact, as various authors (Baker et al. 1979; Kochel and Baker 1982; Costa 1978) emphasized, large-magnitude events can reset the geomorphological setting of a river and returning to a state of dynamic equilibrium cannot be accomplished in a short period of time since the fabric of the previous geomorphic units has been disrupted and large amounts of finer sediment fractions have been exported (Desloges and Church 1992). Moreover, a substantial recovery of the system will depend on the reestablishment of the riparian vegetation in the floodplain and on hillslopes (Desloges and Church 1992). This entire process can last decades to centuries since such adjustment processes can be inherently slow (Desloges and Church 1992).

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The extreme floods erode enormous amounts of sediment of the river, releasing them downstream, which generates a sedimentary imbalance in the morphology of the river that can be maintained for decades (Lewin et al. 1988). However, magnitude does not always entail an imbalance in sedimentary dynamics. For example, the Knik River in Alaska has been the scene of sudden drainage every year for more than 40 years; however, the sediment sources are limited, which has generated a stable system braiding, although the drains were from 5000 to 10,000 m3 /s (Fahnestock and Bradley 1973). On the other hand, the Matanuska River (close to the Knik) presents maximum flows in summer of 450 m3 /s, with abundant sediment sources, generating an unstable braiding system (Fahnestock and Bradley 1973). The objective of this work is to study the geomorphological footprint of GLOFs through a comparative analysis between affected rivers and unaffected control rivers (selected with statistical methods based on the morphometric characteristics of the basin and river corridors) and by comparing the affected rivers with their morphology prior to the event. Therefore, by means of a well-established river morphology analysis, the geomorphological features of rivers, both affected and unaffected by GLOFs, were retraced aiming at identifying the morphological signature left by GLOF events.

7.2 Study Area The studied rivers are somewhat far apart. The Soler and Blanco Rivers are located in the Aysén Region in close association with the Northern Patagonian Icefield, whereas the Ultima Esperanza, P. Montaña, and Olvidado Rivers are situated in the Magallanes Region and constitute part of the drainage network associated with the Southern Patagonian Icefield (Fig. 7.2). It is not only the geographical distance that differentiates them (see Table 7.1), in addition to this, their basins and river corridors present very dissimilar morphological characteristics. The Soler River has a basin area 10 times larger than the rest of the rivers. The basin areas featured by the Blanco, Olvidado, and U. Esperanza Rivers are of the same order of magnitude ranging from 35 to 40 km2 . Finally, the P. Montaña River has the smallest basin of all.

7.3 Methodological Scheme In this work, three fundamental steps have been considered to perform the comparative analysis between the sets of affected and unaffected rivers. First, the control rivers (i.e. rivers not affected by GLOFs in the recent geomorphological evolutive stage) were selected. Thereupon, an IDRAIM analysis (Rinaldi et al. 2015) was performed on both river sets (i.e. affected and unaffected), followed by a systematic comparison between the rivers of both sets based on the quantification of several geomorphic indices (see Fig. 7.3 for a methodological overview). Quantitative differences were

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a

b

Fig. 7.2 Right panel: overview of the study areas in Chile, located in the Aysén and Magallanes Region, respectively. Left and middle panel: northern and southern study area with locations of the basins of the rivers affected by GLOFs in the recent past

finally subjected to a rigorous scrutiny by means statistical inference tests. Data used in this work were digital elevation models (ALOS PALSAR) and satellite images (Digital Globe) with a 1 × 1 m resolution. For the geomorphological characterization of the rivers prior to the GLOF, the aerial images of the GEOTEC flight (1997–1999) were used, with a scale of 1: 70,000 and a pixel resolution of 2.5 × 2.5 m.

7.3.1 Selection of Control Rivers The comparison of affected and unaffected rivers required, first of all, that river basins with a high degree of morphological similarity were selected to form two groups, the affected rivers of interest and the control group constituted by unaffected though “morphologically similar” river basins. The devised methodology foresaw a multivariate analysis of a specified set of morphological parameters of the basin of each affected river and its respective control river. This methodology consisted of four phases, which finally allowed for a rational selection of the control river basin exhibiting the highest degree of similarity to the affected one.

40

Olvidado (4362653 S, 624375 O)

P. Montaña (4245571 4.5 S, 614785 O)

44

35

Blanco (4876521 S, 663897 O)

11

33.5

Basin perimeter (km)

Basin area (km2 )

Rivers

Table 7.1 Studied rivers data

3

10.6

10.5

Basin length (km)

35

25

26.4

Mean slope (°)

0.649

1.04

1.2

Mean elevation (km)

2005–2006

2003

2000–2003

GLOF date

(continued)

The surface area of the lake was reduced from 0.07 to 0.06 km2 . The flow removed vegetation over a 2 km long stretch

The south face of the Olvidado glacier suffered a significant retreat. As a consequence, the pro-glacial lake grew considerably, reaching an area of 0.68 km2 and finally suffered a GLOF in 2003

The surface area of the lake was reduced from 0.12 to 0.09 km2 . The flow generated a significant reduction of vegetation along the river

GLOF description

7 Investigating the Geomorphological Footprint of Moraine-Dammed … 117

39.5

142

40.5

449

Ultima Esperanza (4300448 S, 637534 O)

Soler (4799455 S, 639494 O)

24

11.4

Basin length (km)

GLOF description was extracted from Iribarren-Anacona et al. (2015)

Basin perimeter (km)

Basin area (km2 )

Rivers

Table 7.1 (continued)

25.3

20.8

Mean slope (°)

1.023

0.6

Mean elevation (km)

1989

1999–2006

GLOF date

The lake surface area was reduced from 1.8 to 0.98 km2 . The volume drained was about 250–300 million m3 , with sediment transport of at least 13 × 106 m3

The drained lake was reduced from 0.09 to 0.08 km2 . The flow removed vegetation in a stretch of about 11 km long

GLOF description

118 D. Bahamondes et al.

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Fig. 7.3 Methodological scheme of this work, highlighting the objectives, the employed methods and the obtained results

7.3.1.1

Selection of Candidate Basins

The first phase consisted in the identification of the candidate basins. In this sense, rivers with basins of similar size to the affected river within a radius of approximately 100 km were identified. There are no significant climate differences in the study site. According to the Köppen climate classification, the Patagonia region has mainly a tundra climate. Also, Masiokas et al. (2009) established the South Patagonian Andes (50–55° S) as a big zone with similar glaciological and climatic fluctuations. Thus, it was reasonable to choose as a control river any basin in the vicinity of the affected river due to the climate similarity.

7.3.1.2

Basin Data

Once the set of candidate rivers was established, a series of basin parameters were calculated to determine the similarity between the control rivers and the affected rivers. For this, the QGIS 3.4 tool was used, with which the various parameters reported below were calculated and analyzed. These parameters are subdivided into (a) shape parameters, that is area, perimeter, length of the main channel, length of the basin, compactness index, Horton index, and elongation index; (b) relief parameters, such as mean elevation and mean slope.

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Principal Component Analysis (PCA)

Following the calculations of the basin parameters, a Principal Component Analysis (PCA) was performed in order to extract k components from the set of morphological variables of each basin. The PCA is a mathematical algorithm that reduces the dimensionality of the data while retaining most of the variation in the data set (Ringnér 2008). It accomplishes this reduction by identifying directions, called principal components that are defined as a group of linear orthogonal combinations that have the largest variance (Ringnér 2008). This method is used to reduce the size of a set of several predictor variables to a few main components, which can be used in further procedures (for example, cluster analysis). For this, we employed the Statgraphic Centurion © software, which was used for each affected basin in relation to the control basins. We defined three principal components, since with this number the explained variance was greater than 85%. The result of this was a score (a dimensional value) for each of the components that together correspond to a coordinate within a three-dimensional axis. These were used in the cluster analysis to select each of the control rivers.

7.3.1.4

Cluster Analysis and Selection

Based on the results obtained with the PCA, a cluster analysis was performed with the objective to determine which of the control rivers exhibited the highest similarity to the affected one. This analysis was used to group the principal components extracted from the morphological variables into clusters based on the similarities between them. We used the Statgraphic Centurion © and Python 3.9 software for the cluster analysis. In order to run this analysis, we set several options beforehand. The most important were the metric and hierarchical clustering procedures. The first one is used to measure the distance between the basins studied, and for this, we applied the Ward metric since it is the standard. The second is the clustering procedure to create the groups or cluster. Thus, we employed the Ward method, which reduces the loss of information in the analysis when variables are different from each other. This method was proven as one of the most efficient methods for cluster analysis (Kuiper and Fisher 1975).

7.3.2 Morphological Characterization of River Systems The IDRAIM methodology (i.e. system for stream hydromorphological assessment, analysis, and monitoring) consists of conducting an analysis of a particular river at different scales. The general structure includes the following four phases: (1) catchment-wide characterization of the fluvial system; (2) reconstruction of evolutionary trajectory and assessment of present river conditions; (3) prediction of future channel evolution; (4) identification of river management options (Rinaldi et al.

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2015). In this work, we only performed the first phase to compare affected and unaffected rivers, which provides the spatial context and is aimed to characterize the morphological typology of the river in its present conditions, including an initial characterization and segmentation of the fluvial system (Rinaldi et al. 2015). This is based on a catchment-wide, spatially hierarchical framework following the scheme proposed by Fryirs and Brierley (2005). The result of the segmentation was the delimitation of relatively homogeneous segments, defined as river stretches along which the boundary conditions are sufficiently uniform (Rinaldi et al. 2015). We also compared the affected river with its condition prior to the GLOF perturbation, and thus we have also an approximation of the phase 2 (temporal analysis). In this work, we only studied the channel pattern (based on sinuosity, braiding, or branching indices) and width based on a single image, and we did not reconstruct the evolutionary trajectory of the affected rivers. Based on the identified channel patterns we determined the dominant typology of each segment of the affected rivers.

7.3.2.1

Confinement Index

To assess the confinement of the river, IDRAIM proposes two indices to quantify it in each segment: the confinement index (CI) and the degree of confinement (DC) (Rinaldi et al. 2015). For the sake of simplicity, we only used the CI. The classification of the segment confinement depends on the CI value: (a) Less than 1.5 is a confined, (b) between 1.5 to k is classified as semiconfined, and (c) values higher than k is unconfined. The k value is taken as; k = 5, for single channel and sinuous rivers; k = 2, for braiding and wandering rivers (Rinaldi et al. 2015). The CI can be calculated as follows: Confinement index =

7.3.2.2

floodplain width (m) bankfull width (m)

(7.1)

Braiding Index

The braiding index (BI) is defined as the average number of flow channels within the floodplain of the considered segment. To determine the number of active channels, n section lines perpendicular to the direction of the river were drawn, and the number of channels intersecting each line was added and then the resulting sum is divided by n. (Σn Braiding index =

i=1

channels intsc. n

) (7.2)

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Sinuosity Index

It is the ratio between the length of the thalweg and the straight length between the upstream and downstream sections of the considered river segment.

7.3.2.4

Morphological Classification

With the values of confinement, braiding, and sinuosity indexes for each river, the dominant morphological typology was determined in order to detect how it was affected by the sudden drainage of glacial lakes. The dominant typology was established according to the classification scheme shown in Table 7.2. Herein the typology is determined based on values of the sinuosity and braiding indices, respectively. Since the analyzed rivers did not present anastomosed segments or sections, an associated index was not considered.

7.3.3 Statistical Analysis of Morphological Data Endowed with identified morphological typologies and with the determination of the morphological units for both affected and unaffected rivers, a thorough statistical analysis was performed. The purpose of this statistical analysis was to establish whether the morphological changes in the rivers affected by GLOFs presented significant differences in comparison to the unaffected rivers. For this, the ANOVA method was used as implemented in the Statgraphic Centurion © software. Thus, we compared the parameter values assessed for each river at a segment scale (sinuosity, braiding, and confinement). This analysis considers a null hypothesis, namely that the means are equal to each other and an alternative hypothesis that at least one mean is dissimilar. Also, as a complement, we used the Statgraphic Centurion © software to perform the nonparametric Kruskal–Wallis (KW) and Mood tests to enhance our analysis. The KW test (i.e. a nonparametric test) evaluates the null hypothesis that the medians of the indexes calculated within each of the considered levels are equal, Table 7.2 Typology of a river according to its shape

Typology

Sinuosity index Braiding index

Straight (ST)

1 ≤ SI < 1.05

1 ≤ BI < 1.15

Sinuous (S)

1.05 ≤ SI < 1.5 1 ≤ BI < 1.15

Meandering (M)

SI ≥ 1.5

1 ≤ BI < 1.5

Sinuous with alternating bars SI < 1.5 (SB)

1 ≤ BI < 1.15

Wandering (W)

SI < 1.5

1 ≤ BI < 1.5

Braiding (B)

SI < 1.5

BI ≥ 1.5

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whereas the Mood test is a nonparametric test that evaluates the null hypothesis that the medians of the calculated indexes are identical.

7.4 Results 7.4.1 Cluster Analysis and Choice of Control Rivers Once the morphological basin data were extracted for each river (both candidates to control and affected) and systematized, we applied the Principal Component Analysis (PCA). This method resulted in three-dimensional components for each river which we used as a parameter input of the cluster analysis. The final result of these steps was a dendrogram for each affected river and the associated control river candidates (see Fig. 7.4). Figure 7.4 shows which river has the greater similarity with the affected one. However, there are cases where the river with the shortest Euclidian distance (or higher likelihood) was not chosen as a control river. This was due to the fact that some of the candidate rivers presented footprints of a previous large flood with an unknown cause. Thus, we selected the river with the highest possible similarity but with no evidence of a prior disturbance by large floods. In this way, the selected control rivers were: Soler, S5; Blanco, B1; Olvidado, O2; Ultima Esperanza, U8; Peninsula de la

a

b

c

d

e

f

Fig. 7.4 Cluster analysis of the studied rivers to identify the “most similar” control river to each of the case study rivers. The panels correspond to: a Blanco River, b Soler River, c Olvidado River, d U. Esperanza River, e P. Montaña River. The last panel f is a cluster analysis between the affected rivers

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Fig. 7.5 Representation of affected rivers and the associated unaffected rivers (i.e. chosen control rivers)

Montaña, M1. The geographical position of each affected and unaffected river is shown in Fig. 7.5. The affected rivers exhibited important differences in terms of morphological parameters (see Fig. 7.4, panel f). Based on these differences, we grouped the studied basin in three different groups: rivers with small basins (P. Montaña), medium size basins (Blanco, U. Esperanza, Olvidado), and large basins (Soler). The main differences featured by the affected rivers were related to basin size, shape, mean elevation and mean slope. Nevertheless, we shall come back to this classification in the discussion.

7.4.2 Morphological Characterization of the Studied Rivers 7.4.2.1

Sinuosity Index

The sinuosity index (SI) was one of the studied parameters that delivered the smallest differences between affected and unaffected rivers. In Fig. 7.6 (panel a), the calculated SI for all the studied river’s segments did not exceed the value of 1.5, and therefore rivers were classified as straight rivers (SI values less then 1.05) or as sinuous (SI values between 1.05 and 1.5). The only different case was the control of the Soler River, which features SI values that reach 1.8 in some segments. However, this is not a homogeneous trend throughout the river, since in other segments the value of this index falls in the range of 1–1.5, similar to the rest of the rivers. The temporal difference of the SI of the affected rivers (pre- and post-GLOFs) showed slight changes. This is observed in Fig. 7.6 (panel b), in which the SI for each

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Fig. 7.6 Sinuosity index of affected and unaffected rivers quantified for each segment. The dotted line (LM) marks the border between a SI value (less than 1.5) typical of sinuous rivers and SI values (greater than 1.5) typical of meandering rivers. a SI values of the affected rivers and the control rivers; b SI values of the affected rivers in pre- and post-GLOF periods, respectively

affected river did not exceed the limit of a meander-like planform (i.e. 1.5). In this sense, the Soler River is separated from the rest of the rivers, featuring the highest values of this index, although the SI was not higher than 1.5.

7.4.2.2

Braiding Index

The braiding index (BI) values did not show significant differences between affected and unaffected rivers (see Fig. 7.7). The BI values of all rivers assessed at a segment scale vary between 1 and 1.5. In the same way as in the case of the SI, the Soler River is the case study river that presents BI values higher than the rest of the rivers. This is the case especially in the river segments that are part of the debris deposition zone caused by the massive drainage of Cerro Largo Lake. The BI value in these segments reaches 2.8 in the second segment; however, in the segments with

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milder slope and less affected by the debris flow, braiding is significantly reduced, which enables to classify them as wandering or straight. Other rivers such as the Blanco (and its control), the Peninsula de la Montaña, and U. Esperanza rivers reach BI values close to 1.5 in some of their segments. However, this condition is not homogeneous since the rest of the river segments present braiding index values very close to 1. The affected rivers underwent significant BI value changes from pre- to post-event conditions. In this sense, we observed the most important changes in the Soler River, which presented high BI values in the segments located most closely to the lake. This can be appreciated in Fig. 7.7, where the increase in the braiding index values in the first 2 segments in the Soler River is higher than 75% in both cases, decreasing the BI in the downstream segments. In the case of the other (smaller) rivers, a less marked trend is observed with respect to the Soler River. For example, the U. Esperanza, Olvidado, and P. de Montaña rivers show BI value reductions in the first segments to increase again in the downstream segments. The mean of all the rivers in each segment shows that, in most rivers, there is an increase in the BI value due to the sudden drainage event of glacial lakes.

Fig. 7.7 Braiding index of the studied rivers. a Comparison of BI values in the segments of both affected and unaffected rivers. b BI value difference between the segments of affected and unaffected rivers. The mean BI value per segment of all rivers is displayed in the same graph. c Temporal comparison of BI values in the segments of the affected rivers. d Temporal difference of BI values between the segments of affected rivers

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Fig. 7.8 Comparison of confinement index values of affected and unaffected rivers. a Comparison of the floodplain and active channel width of affected and unaffected rivers. b Confinement index values of the affected and unaffected rivers. c Temporal comparison of the floodplain and active channel width of affected rivers. d Temporal comparison of the confinement index values of the affected rivers

7.4.2.3

Confinement Index

Figure 7.8 shows the floodplain and the active channel width of the studied rivers. Generally speaking, all rivers that have been affected by GLOFs have a wider flood plain and a wider active channel than the unaffected ones. However, the results of the calculation of confinement index (CI) values were different. In some cases, the affected rivers have a higher CI than the control, and in other cases, they do not. The segments of the rivers P. Montaña, Olvidado, and Blanco were classified as confined orsemiconfined since the CI value was close to 1.5. The segments of the U. Esperanza River were classified as semiconfined and those of the Soler River as semiconfined/unconfined. It is worth mentioning that Fig. 7.8 shows the mean CI value of all the river’s segments, and therefore, the CI value could be significantly higher in specific segments. The temporal comparison between the CI values of the affected rivers showed a clear difference between the river’s segment previous and post-GLOF. The floodplain is the same for both conditions; since the rivers are embedded within mountain ridges, that is, the FP has a constant width at any location. Nevertheless, the active channel width presented significant differences. The post-GLOF condition was associated with wider active channels than the pre-GLOF condition, and this is also reflected in the CI value: The confinement index is lower in a river affected by GLOF, and

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therefore rivers classified as semiconfined or unconfined changed to confined due the widening of the active channel (see Fig. 7.8, panel d).

7.4.3 Morphological Classification In the majority of the segments of the studied rivers, the SI and CI values were quite similar between the affected and the unaffected rivers (see Table 7.3). The Olvidado, U. Esperanza, and Blanco rivers exhibited great typological similarities with the control rivers and also with the pre-event condition. The analysis of the BI values presented more conclusive results. The values of this parameter were decisive for the definition of the typology of the rivers since the other index values did not present important differences. The Soler and P. Montaña rivers showed higher BI values than the unaffected rivers, whereas the others affected rivers had quite similar or even lower BI values than the control rivers. The smaller rivers presented different typologies in comparison to the larger rivers. The differences of the morphological parameters (floodplain width, basin size, slope, and others) of the Soler River with respect to the rest of the rivers is reflected in the morphological classification. The other affected rivers showed changes in their fluvial geomorphology, although not as remarkable as in the case of the Soler River. It is worth mentioning that the magnitude of the GLOF unleashed by the drainage of the Cerro Largo Lake was really remarkable (Iribarren-Anacona et al. 2015), and thus there is a great difference not only in the morphological parameters but also in the magnitude of the event.

7.4.4 Statistical Analysis of the Morphological Parameter Values Table 7.4 shows the results of the statistical analyses conducted in this study, ANOVA, Kruskal–Wallis, and Mood tests, respectively. The purpose of these analyses was to detect significant differences in the average values of the assessed morphological parameters comparing two groups: (A) all the affected rivers and (B) all the control rivers. We found that both the Soler and Peninsula de la Montaña rivers presented significant differences with respect to the braiding index value and the active channel width (i.e. p-value was lower than 0.05). However, in relation to this result a subtle difference has to be considered. In the case of P. Montaña River, significant differences could be detected only in the median tests of the BI (with the control rivers) and active channel (with the rivers affected). Conversely, the Soler River presented significant differences in comparison to the groups of contrast of all rivers in the parameters of BI and active channel width. The other rivers did not present significant differences in the studied parameters in comparison to the respective control

Soler

P. Montaña

U. Esperanza

1.1

1.1

4

5

1.4

1.1

1.1

2

3

4

1.2

5

1.1

1.1

4

1

1.1

1.1

2

3

1.1

1.2

3

1

1.1

1.1

1

1.1

5

2

1.1

1.1

3

4

1.1

1.1

1

Olvidado

1.0

1.1

1.3

1.4

1.2

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.2

1.1

1.1

1.1

1.1

1.6

2.0

1.2

1.6

1.1

1.1

1.2

1.1

1.1

1.2

1.2

1.2

1.2

1.2

1.2

1.3

1.1

1.1

1.2

1.1

1.5

2.9

2.1

1.4

1.5

1.0

1.3

1.3

1.5

1.0

1.1

1.0

1.1

1.0

1.2

1.2

1.3

1.0

AG

1.0

1.2

1.4

1.2

1.0

1.0

1.0

1.3

1.5

1.2

1.0

1.1

1.1

1.1

1.2

1.0

1.0

1.2

1.3

BG

Braiding index UN

AG

BG

Sinuosity index

2

Segments

Rivers

Table 7.3 Morphological classification of the studied rivers

1.2

1.4

1.0

1.3

1.0

1.0

1.0

1.0

1.2

1.0

1.0

1.0

1.0

1.0

1.2

1.2

1.3

1.0

1.2

UN

SC

SC

SC

SC

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

AG

NC

SC

SC

SC

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

BG

Confinement index

SC

SC

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

UN

Typology

ST

B

B

B

B

B

ST

ST

ST

B

ST

ST

ST

ST

ST

ST

ST

ST

ST

AG

ST

S

B

S

ST

ST

ST

ST

ST

ST

ST

ST

ST

S

ST

ST

ST

ST

ST

BG

(continued)

M

M

S

M

ST

ST

S

ST

ST

S

S

S

S

S

S

S

ST

ST

S

UN

7 Investigating the Geomorphological Footprint of Moraine-Dammed … 129

1.2

1.0

1.2

1.0

2

3

4

5

1.1

1.1

5

AG

1.0

1.1

1.0

1.1

1.1

1.1

BG

Sinuosity index

1

Segments

1.0

1.1

1.1

1.1

1.0

1.2

UN

1.0

1.3

1.0

1.2

1.2

1.4

AG

1.0

1.3

1.2

1.0

1.2

1.1

BG

Braiding index

1.0

1.0

1.2

1.0

1.0

1.7

UN

C

C

C

C

C

SC

AG

C

C

C

C

C

SC

BG

Confinement index

C

C

C

C

C

SC

UN

ST

S

ST

ST

ST

B

AG

Typology

ST

ST

ST

ST

ST

ST

BG

ST

ST

ST

ST

ST

B

UN

The acronyms mean: AG after the GLOF event; BG before the GLOF event; UN unaffected control river; C confined; SC semiconfined; NC no-confined; ST straight; S sinuous; M meandering; B braiding

Blanco

Rivers

Table 7.3 (continued)

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groups. The Olvidado, Blanco, and U. Esperanza rivers did not exhibit even one p-value lower than 0.05. This means that the morphological characterization of these rivers did not show significant differences with the unaffected rivers.

7.5 Discussion The morphological dynamics in mountain rivers can be quite peculiar and exhibit characteristic features depending on whether the considered rivers were recently affected or not by a major disturbance. Patagonian rivers are, in this respect, no exception. The Northern and Southern Patagonian Icefields, represent an extraordinary source of potential energy (i.e. stored water in solid phase). When glaciers retreat (i.e. currently also under the influence of climatic change) and ice- or morainedammed glacier lakes form, the risk of a sudden release of huge water volumes drastically increases, and potent GLOFs can be triggered whenever the confining dams fail or extensive tunnels from as to drain the glacier masses from below. These sudden events are characterized by remarkable flow intensities potentially generating characteristic geomorphic signatures in the downstream river corridors. However, the interaction of various geomorphological processes within a basin is complex and cannot be exhaustively assessed by a simple quantitative analysis (Birot 1968; Wolman and Gerson 1978). It is widely acknowledged that fluvial systems indeed exhibit a remarkable complexity with respect to how fluvial processes interact in the aftermath of major disturbances (i.e. GLOFs, Cenderelli and Wohl 2003). For this reason, the analysis of geomorphological changes at different spatial scales may pose great challenges and require a structured procedure to be satisfactorily accomplished (Tomczyk et al. 2020). The IDRAIM methodology provides, in this respect, a useful set of tools to retrace the past evolution, to characterize the current geomorphic condition, to develop plausible trends of future change and manage river corridors accordingly (Rinaldi et al. 2015). In this study, we employed selected steps of this methodology to characterize two groups of rivers, one set affected by GLOFs in the recent past from a geomorphological perspective and a control group of unaffected rivers (i.e. with respect to the same time window). With the objective to detect and quantify salient differences in the geomorphological footprints among both groups of rivers, we analyzed the dominant morphology of the affected and unaffected rivers. We employed three-step analysis to fulfill this objective: (1) selection of control rivers, (2) characterization of the fluvial morphology and (3) statistical comparison of the values of specific morphological parameters describing the planform of the rivers. With respect to the selection of the control rivers, the obtained results indicated that even in the same geographical context, no equal basins or rivers to the affected rivers could be found. This result highlights an important conceptual challenge when comparing different rivers from a morphological viewpoint. In fact, the selection of the control rivers is not trivial. One has to make sure that rivers with a sufficient degree of similarity be compared. Moreover, the assessment of similarity depends on the selected parameters and on the

ANOVA (m)

Statistical tests

0

Soler (C)

0.003

0.019

0.876 0.123

0.109

0.493

0.688

0.124

0.109

0.493

0.161

0.491

0.161

Mood (M)

0.837

0.024

0.386

0.452

0.539

0.937

0.475

0.733

0.347

0.331

ANOVA (m)

Sinuosity

0.364

0.071

0.519

0.541

0.631

0.891

0.661

1

0.151

0.308

Kruskal–Wallis (M)

0.147

0.109

0.172

0.161

0.661

0.688

0.669

0.548

0.669

0.688

Mood (M)

0

0

0.851

0.516

0.501

0.141

0.594

0.187

0.833

0.313

ANOVA (m)

0.0006

0.0006

0.061

0.454

0.501

0.021

0.379

0.118

0.104

0.785

Kruskal–Wallis (M)

Active channel width

0.009

0.009

0.147

0.548

0.608

0.016

0.608

0.688

0.147

0.688

Mood (M)

The p-value fluctuates between 0 and 1, the limit being 0.05, with 95% confidence. Thus, the bold numbers note the significant differences in cases between the studied rivers. The terms m and M correspond to tests that analyze the mean and median, respectively

0.009

Soler (A)

0.371

0.317

0.961

Olvidado (A)

Olvidado (C)

0.0438

0.051

Montaña (C)

0.913

0.408

0.802

0.933

0.271

Esperanza (C)

0.389

Esperanza (A)

0.975

0.241

Kruskal–Wallis (M)

Montaña (A)

0.297

0.879

Blanco (A)

Blanco (C)

Rivers

Braiding

Index

Table 7.4 ANOVA, Kruskal–Wallis, and Mood tests of each one of the affected rivers in contrast with: (A) all the affected rivers except the one analyzed; (B) all unaffected and affected rivers prior to the event

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past evolutive trajectory of the rivers. The basin parameters always showed important differences between the studied rivers. These differences are reflected in the PCA and the cluster analysis (see Fig. 7.4). In some cases, the most similar river presented footprints of previous large floods within the active channel. Thus, we had to interpret how the affected rivers were related to the others in the cluster analysis in order to choose the river which was macroscopically similar with respect to the basin parameter values but also exhibited a morphology compatible with the concept of an unaffected river. The findings which emerged from the morphological characterization of the studied rivers showed that the footprints on the fluvial morphology were different. In Fig. 7.4 (panel f), the cluster of the affected rivers is presented highlighting remarkable differences among them. Macroscopically, we grouped the rivers into three groups: rivers with a small basin (P. Montaña River), rivers with a medium size basin (Olvidado, Blanco, and U. Esperanza rivers) and rivers with a large basin (Soler River). Differences could be detected in terms of fluvial morphologies between these groups with respect to the rivers of the respective control group. In particular, the Soler and P. Montaña presented significant differences with the control rivers, whereas the medium size rivers did not; the medium size rivers had quantitatively smaller differences than the P. Montaña and Soler rivers. The rivers with a medium size river basin exhibited smaller differences because the majority of their segments had sinuous or straight planform typology which did not differ greatly from the planform of their control rivers. On the other hand, morphological changes in the Soler and P. Montaña rivers occurred in a larger number of segments compared to the other rivers studied. In this case, our study corroborates the findings of Desloges and Church (1992). They observed that the movement of material due to extreme floods that accumulates in areas of milder slope generates a disequilibrium in the river, forming unstable braiding systems. This pattern can be clearly retraced in the Soler River, for which the different satellite images show a change in the braiding system over time. The ANOVA analysis of the morphological parameters corroborated statistically the insights gained by the application of the IDRAIM method: the rivers with a medium size river basin did not exhibit significant differences with their control rivers, whereas the P. Montaña and Soler rivers did. In Table 7.4, we highlighted the ANOVA, Kruskal–Wallis, and Mood tests of the studied rivers with: (A) all the rivers affected; (B) all the unaffected rivers. The results of this showed how the medium rivers did not exhibit significant differences with respect to the control rivers contrasted in any morphological parameter and statistical test. Whereas, the others river presented significant differences in the BI and the active channel width, although the Soler River showed more remarkable differences in the morphological parameters in all the tests applied. Differences in the geomorphological response of rivers have been discussed by different authors. Certain studies report that important changes (erosion of terraces, riparian forest reduction, among others) were observed in the floodplains that are affected by extreme flood events, such as lahars, GLOF, among others (Baker 1977; Thompson and Croke 2013). Other studies detected that large flood events did not

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exert significant changes in the morphology of rivers (Thompson and Croke 2013). Events similar in both magnitude and duration can generate different effects in the downstream river corridors, which makes the prediction of this type of changes in fluvial geomorphology difficult (Thompson and Croke 2013). The same could be observed in the rivers studied. It seems that events of great magnitude did not generate remarkable changes in the values of the assessed morphological parameters. On the other hand, the impact on river morphology cannot be considered solely by the magnitude of the flood, which is viewed by various authors as a “poor” indicator to predict the change in the morphological parameters of a river (Gardner 1977; Magilligan 1992; Kale 2008; Thompson and Croke 2013). In this sense, the difference between stream power or energy of the flow with their respective critical values (i.e. resistance to change) provides a better approximation of the GLOF’s capacity of doing geomorphological work (Baker and Costa 1987; Thompson and Croke 2013). Steam power thresholds for the onset of morphological work have been determined. For example, Magilligan (1992) proposed that, through an analysis in the field and various investigations of extreme floods, a value of 300 W/m2 is a minimum threshold for there to be an important geomorphological work on a river. This may explain the differences with respect to the change in the morphological parameter values assessed in the studied in the rivers.

7.5.1 Confinement Index In relation to this parameter, there are two interesting facts which merit to be addressed. First, the analysis of the confinement of the affected and unaffected rivers showed that there were no important differences among them. Since the similarity of the basin’s parameters, the confinement did not present great differences. The only river that exhibited a different confinement is the Soler, which presented certain unconfined or semiconfined sections. The rest of the rivers did not show differences in confinement in any of the studied segments, due to the valley form that restricts both their floodplain width and lateral mobility. The results obtained by applying the IDRAIM method showed that the type of confinement did not change. Nevertheless, negligible changes in type of confinement (i.e. the most stable elements located at the river corridor boundaries) do not imply that no geomorphic work has been done by the involved fluvial processes. The resulting geomorphic signatures could be observed in all affected rivers, mainly in form of remarkable channel incisions, loss of riparian forest in floodplain areas adjacent to the active channel, producing a sweeping effect on the river’s segments. Despite the persistence of the most stable elements at the river corridor boundaries, the calculated values of the confinement index showed a reduction, since not only vegetation was obliterated but also old terraces were partially removed and possibly boulder size rocks were mobilized. The latter is hypothetical and further investigation is needed to ascertain that effect. However, Desloges and Church (1992) described that GLOF events are capable of exerting huge hydrodynamic forces which generate

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extensive erosions both on active channel and floodplain areas. Conversely, in more distal river segments characterized by milder slopes, sediment loads exceeding the still remarkable transport capacity are partially deposited inducing river widening and active channel migrations and partially debouched into the downstream receiving waters (Krapesch et al. 2011; Magilligan et al. 2015).

7.5.2 Sinuosity Index In the majority of the rivers, the SI was lower than its control, and in the temporal comparison, the affected rivers presented a reduction of this parameter. This limited response of the affected rivers with respect to sinuosity of the main channels can be explained in two ways. First, in the affected small and medium size rivers the confinement is mainly determined by the mountains that form the valley. Therefore, the capacity of the river to “move laterally” within floodplain is limited. In other words, the affected rivers had a low SI and the GLOF event did not change this parameter greatly. For example, the Olvidado River had a higher sinuosity value prior to the sudden drainage that occurred in that river, but since it had previously been a very low value, the reduction was very small. On the other hand, the river with a larger basin (i.e. the Soler River) presented a more noticeable reduction of the sinuosity index value. In this sense, the trend is maintained that GLOFs generate that river’s segments reduce their SI and classify as sinuous or straight predominantly. However, this happens in some segments, while in others the braiding increased considerably.

7.5.3 Braiding Index The braiding index is the parameter that determined the morphology of the studied rivers. The great movement of material over some segments of the river creates conditions for an unstable braiding system (Desloges and Church 1992). This imbalance is observed in some of the studied rivers, such as the Soler River. In the rest of the rivers, this tendency is not clear. This is due to the fact that the small and medium size rivers presented a steeper slope than the Soler River. Also, the smaller rivers present narrower floodplains and thus there is not enough surface area to contain large amounts of sediment to be deposited. However, in both the U. Esperanza and P. Montaña rivers, there was a significant increase in BI, but only in the downstream segments. In other words, the flow deposited a large part of the sediment in the less sloping segments, thus forming a minor braiding system.

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7.6 Conclusions Glacial lake outburst floods are potent geomorphic agents that can transform the landscape and its footprints can persist for decades or centuries. This differentiates this kind of rivers form others. To highlight these differences, we compared the fluvial morphology of the rivers affected by GLOF with the unaffected rivers nearby. The results of this analysis showed how the same process have different consequences in different context. In some rivers, the morphological parameters presented significant differences with respect to its control, and in others, these changes were not noticeable. We restricted our analysis to only three parameters (i.e. CI, SI, and BI), which were measured by means of photointerpretation, and thus a lot of information of the morphological change remains hidden. In this sense, for a more comprehensive analysis of the morphological parameters of the rivers, it would be necessary to study in detail and, hence, also in situ the different parameters of the affected rivers (i.e. regarding sedimentology and the reconstruction of the GLOFs, among others). Nevertheless, this work highlighted the fluvial morphology of the rivers affected by GLOFs and how the morphological footprints change between unaffected and affected rivers. Acknowledgements This research was funded by the project ANID/CONICYT FONDECYT Regular, Folio 1200091, “Unravelling the dynamics and impacts of sediment-laden flows in urban areas in southern Chile as a basis for innovative adaptation (SEDIMPACT)” led by Bruno Mazzorana.

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Chapter 8

Cascading Impacts of GLOFs in Fluvial Systems: The Laguna Espontánea GLOF in Patagonia Pablo Iribarren Anacona, Catalina Sepúlveda, Jorge Berkhoff, Ivan Rojas, Valeria Zingaretti, Luca Mao, Bruno Mazzorana, and Gonzalo Durán Abstract The sudden drainage of glacial lakes (GLOFs) can result in high-energy flows with dramatic geomorphic consequences. GLOFs can erode consolidated river terraces and even bedrock and may generate thick valley aggradation changing rapidly the landscape of fluvial corridors. In Patagonia, GLOFs seem to be increasing in frequency since the 1980s and have affected dozens of rivers. Although a sound understanding of GLOF dynamics and geomorphic consequences is needed to better manage fluvial corridors and mitigate glacier hazards, the geomorphic consequences of GLOFs have been analysed in just a few cases in Patagonia. Here, we report an exceptional river blockage caused by a GLOF in October 2018 in one of the valleys most visited by tourists in the Chilean Patagonia. We describe the geomorphic consequences of the GLOF and reconstruct its dynamics through interpretation of drone images, digital surface models, and the analysis of meteorological and geomorphic data. The GLOF was triggered by a rock-avalanche that impacted a small moraine-dammed lake resulting in a rapid flow (≥ 10 m/s) with competence to transport boulders coarser than 20 m in diameter dozens of metres downstream. The sediment-laden flow impacted perpendicularly the Exploradores Valley damming the Norte River. This resulted in a new lake with a surface of 0.26 km2 and deeper than 6 m that flooded grass land, forest and the tourist route to Exploradores and San Rafael Glaciers. This processes cascade shows that even low magnitude GLOFs (60 × 103 m3 ) can cause extensive and long-term impacts in fluvial systems.

P. I. Anacona (B) · B. Mazzorana Instituto de Ciencias de la Tierra, Universidad Austral de Chile, Valdivia, Chile e-mail: [email protected] P. I. Anacona · C. Sepúlveda · J. Berkhoff · I. Rojas · G. Durán Laboratorio de Geoinformática, Instituto de Ciencias de la Tierra, Universidad Austral de Chile, Valdivia, Chile I. Rojas · V. Zingaretti Facultad de Ciencias Forestales y Recursos Naturales, Universidad Austral de Chile, Valdivia, Chile L. Mao School of Geography, University of Lincoln, Lincoln, UK © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. Oyarzún et al. (eds.), Rivers of Southern Chile and Patagonia, The Latin American Studies Book Series, https://doi.org/10.1007/978-3-031-26647-8_8

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Keywords GLOF · Patagonia · Rock-avalanche · Landslide-dammed lake

8.1 Introduction Glacial lake outburst floods (herein GLOFs) are one of the most common glacier hazards on Earth as glacier retreat and thinning frequently result in the formation of unstable dams. The sudden release of large volumes of water and sediment in high-relief catchments can trigger devastating floods capable of eroding alluvium and even bedrock, dramatically changing the fluvial landscape (Cenderelli and Wohl 2003; Carrivick 2007). Riparian forest can be obliterated, and valleys can be covered with thick (metres) deposits due to GLOFs. GLOFs are primary geomorphic controls on hillslope-channel coupling and are key erosive agents in large-boulder channels (Cook et al. 2018). The geomorphic work accomplished by large floods is determined by stream power, channel geometry, riverbank composition and persistent features such as bedrock and temporary obstacles such as logjams and boulders (Desloges and Church 1992; Cook et al. 2018; Tomczyk and Marek 2020). The geomorphic signature of GLOFs, especially on the first kilometres, is highly dependent on their triggering mechanism as fluvial corridors can feature the combined influence of cascading processes such as rock-avalanches, seiche waves, moraine-dam erosion and subsequent floods. Thus, knowing the triggering mechanism of GLOFs is key to understand its geomorphic and hydrologic development and consequences. Dozens of moraine-dammed lakes have failed since the twentieth century in Patagonia widening river channels and stripping vegetation patches along dozens of kilometres (Iribarren Anacona et al. 2014, 2015). Dozens of once densely forested river corridors have remained barren for decades after GLOFs. These common biogeomorphic changes occurred mostly in remote uninhabited valleys in Patagonia lacking public and scientific attention. However, on 27 October 2018, a small GLOF (in terms of volume of water released) created new remarkable features flooding one of the most frequented tourist routes in the northern Patagonia (Fig. 8.1). A rock-avalanche impacted a small lake unleashing a sediment-laden flow which dammed the Norte River originating a new lake (Fernandez and Coloma 2018). Despite of the unprecedented landscape changes generated by this GLOF and the relevant socioeconomical consequences, the dynamics and impacts of this event remain understudied. Here, we describe the geomorphic signature of the Laguna Espontánea GLOF in order to: (a) reconstruct the mechanisms that conditioned and originated the GLOF and (b) understand the flood dynamics and geomorphic impacts in the context of GLOF hazard assessment. With the detailed analysis of this exceptional river blockage caused by a GLOF, we hope to shed light on a complex new hazard that could emerge from the rapidly changing cryosphere in Patagonia and other glacierized regions on Earth.

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Fig. 8.1 Context maps of Laguna Espontánea GLOF and surrounding landscape. a Sentinel 2 false composite image taken after the GLOF showing the rock-avalanche source zone and the GLOF affected area. b Glaciers (light blue), lakes (blue), towns (yellow) and GLOFs affected lakes since the twentieth century (red). c Area covered by the 2021 UAV survey

8.2 Data and Methods 8.2.1 Remote Sensing Analysis and Retrospective Hazard Assessment We analysed satellite and drone images as well as digital surface models (DSM) to document geomorphic changes caused by the 2018 GLOF and to describe GLOF conditioning and triggering factors. These data allowed us to analyse retrospectively the ice and rock-avalanche hazard above the lake. A drone survey was undertaken on 19th February 2021 to generate an orthomosaic and a DSM of the area (Fig. 8.1c). We captured 1319 drone images using a Mavic 2 Pro drone. Drone images were georeferenced using nine ground control points surveyed using an Emlid Reach differential GPS. An orthomosaic and a DSM were obtained using the structure from motion (Sfm) technique and the software Agisoft Metashape (Smith et al. 2016). The

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resulting orthomosaic and DSM, covers part of the rock-avalanche deposits and the GLOF path and have a spatial resolution of 6.4 cm and a reprojection RMS error of 0.36 cm. To map zones of erosion and deposition along the GLOF path, we compared the WorldDEM DSM (9.8 m of spatial resolution and absolute vertical accuracy < 4 m dating from 2011 to 2015) with the DSM obtained with the 2021 drone flight. To do this, first we resampled the drone DSM to the WorldDEM DSM spatial resolution using the bilinear interpolation method. Then, we co-registered the DSMs using the method of Nuth and Kääb (2011) to reduce horizontal and vertical off-sets between datasets, and finally, we subtracted the post-GLOF to the pre-GLOF DSM. The vertical off-set between DSMs was reduced from − 7.1 to − 0.01 m after this procedure. To assess the lake exposure to rock falls, rock-avalanches and ice avalanches and figure out if it was possible to anticipate the GLOF hazard, we used the FlowPy V.1 model (D’Amboise et al. 2022a). This model can estimate the reach and intensity of gravitational mass flows based on the run-out angle concept (Heim 1932) and flow routines that allow to simulate flows propagation and spreading in flat terrain and uphill. We assumed a run-out angle of 25° and a divergence exponent of 8 for ice avalanches and a run-out angle of 32° and divergence exponent of 8 for rock falls and rock-avalanches (D’Amboise et al. 2022b). Glacier with slopes ≥ 25° were chosen as starting zones for ice avalanches (Alean 1985), while unvegetated slopes ≥ 45° were set as starting zones for rockfall and rock-avalanches (Mölk and Rieder 2017). We used the WorldDEM DSM resampled to 30 m in the simulations. To identify the possible triggering mechanism of the rock-avalanche, we analysed meteorological data (NOAA/CFSV2/FOR6H for temperature and TRMM/3B42 for precipitation), reviewed press reports and held semi-structured interviews with Exploradores Valley inhabitants in 2019. We also checked seismic data from the USGS earthquake catalogue. We estimated the lake volume using the empirical formula proposed by Muñoz et al. (2020). Lake Volume = area × (0.041 ∗ max. lake width + 2) Although this formula was developed with bathymetric data from glacier lakes of the Peruvian Andes, it used the largest dataset worldwide (121) and included small lakes as the one analysed here.

8.2.2 Reconstruction of Flow Dynamics and Granulometric Analysis In order to estimate both velocity and peak discharge, we used the critical-depth method which has been used successfully to reconstruct flow dynamics of large flows on steep channels (Jarrett 2016; Byers et al. 2019). The discharge (Q) and

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critical velocity (V c ) equations are calculated as follows: Q = AVc √ Vc = F Dg where F is the Froude number (equal to one as critical flow is assumed at the peak discharge), A is the cross-sectional area of the flow (m2 ) corrected for cosine of the channel inclination angle, D is the mean depth (m) of flow corrected for cosine of channel inclination angle, and g is acceleration due to gravity (9.8 m s−2 ). In order to calculate peak discharge and critical velocity, we extracted five cross sections from the DSM derived from UAV data. Most of the cross sections were drawn in the middle-channelized section of the flow since the rock-avalanche dynamics dominated the landscape changes upstream and the flow spread downstream. Reconstructed flow data helped us to document the attenuation of the flow along the channel. Both rock-avalanche and flow deposits were mapped and analysed using the drone orthomosaic. We manually mapped blocks (polygons) at scale 1:50 in order to document granulometric changes associated with different GLOF cascading processes (i.e., rock-avalanche, seiche waves and flood). Then, we measured the maximum polygon length to estimate the block diameter. We also measured the length of wood logs and toppled trees (lines) which helped us to interpret flow patterns.

8.3 Results 8.3.1 Retrospective Hazard Assessment The GLOF was sourced from a small (0.016 km2 ) unnamed lake, dammed by a sparsely vegetated moraine. The lake had an estimated volume of 60 × 103 m3 . The dam was low, and its height was not measurable with medium-resolution DEM data available before the GLOF. The lake had a well-defined outflow, and the lake surface was flanked by steep scree slopes (Fig. 8.2). Steep rock walls and highly crevassed glaciers are located less than a kilometre upstream the lake. Images predating the GLOF show a reconstituted glacier with a high debris cover indicating frequent snow/ice avalanches and rockfalls. Bing Maps images of spring also show snow and ice avalanche paths overrunning the reconstituted glacier and almost reaching the lake. Thus, the GLOF hazard of the lake was associated with mass movements, especially avalanches, and was not linked with the dam geometry. The analysis of satellite and drone images taken after the GLOF, however, shows that the GLOF originated from the impact of a rock-avalanche. Fresh landslide scars were visible in an almost vertical rock wall with a high density of fractures (Fig. 8.3). The slope face has light-grey colours and shows large reddish zones indicative of weathering

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Fig. 8.2 Geomorphological context and retrospective GLOF hazard assessment. a Shows an ice avalanche path close to the failed lake, and b depicts steep slopes surrounding the lake. Panels c and d show simulated rock fall and ice avalanche intensities, respectively, illustrating that the lake was highly exposed to both processes

processes inside the slope before the failure. The rock-avalanche travelled about 1100 m and descended 500 m leaving a blocky deposit. No seismic activity was recorded in the months before the rock-avalanche and subsequent GLOF. Thus, we focused our analysis on the meteorological conditions as potential triggers. The analysis of precipitation data shows a dry week before the rock-avalanche and average temperatures below extreme (i.e. ≤ 90th percentile on the 1980–2018 period) meteorological conditions (Fig. 8.4). The few inhabitants of the Exploradores Valley also recall “normal conditions” and a warm weather on day of the GLOF.

8.3.2 Flow Dynamics and Geomorphic Impact Flood features such as toppled trees in slopes located more than 20 m above the channel bed and freshly transported large boulders (maximum diameter ≥ 20 m) deposited along the GLOF path attest a very rapid and highly competent flow. The rock-avalanche completely covered the lake and dam with a field of large boulders

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Fig. 8.3 Steep-slope source of the rock-avalanche that impacted the lake (a) and erosion (b) and deposit (c) zones of the GLOF path. Note in b the forested slope (20 m above channel bed) overruned by the flow and the flooded forest patches in c

(maximum diameter of 21 m). The impact of the rock-avalanche into the lake probably generated an almost instantaneous lake emptying triggering a tsunami-like wave that overtopped the dam and travelled downstream at a high speed. The valley has a north–east orientation and 580 m from the dam the valley bends to the east. At this bend, the rapid flow overrun a slope and flowed downstream eroding a new channel

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b

Fig. 8.4 Meteorological conditions days before the GLOF. There was no extreme temperature (a) or precipitation (b) associated with the GLOF event

evidencing the large flow momentum. The analysis of the block diameters allows to clearly differentiate the rock-avalanche and the GLOF deposits. The rock-avalanche zone is composed by a cluster of angular blocks with a mean diameter of 2.7 m and a median diameter of 1.7 m. Large blocks (> 5 m) are less frequent in the GLOF deposits and are scattered along the flow path (Fig. 8.5). Wood logs are scarce along the GLOF path despite that 9.8 ha of forest was stripped by the flow. At 800 m from the moraine dam, the flow reached a less steep and unconfined area and spread the GLOF deposits. The sediment-laden flow reached the Norte River and formed a massive dam containing boulders of more than 9 m in diameter, although most of them were < 2 m in this area. The dam formed a new lake which flooded an area of 0.27 km2 of forest and grass land having depths larger than 6 m. The new lake was named Laguna Espontánea by the valley inhabitants due to it sudden appearance. Water flooded the X-728 route that links Puerto Tranquilo village to Exploradores and San Rafael Glaciers inhibiting the normal passage of tourists and local inhabitants for about 4 months. Our estimations of critical velocity and peak discharge agrees with the geomorphic evidence which indicates that a voluminous and fast flow affected the valley (Table 8.1). The flow could have reached velocities > 10 m/s and attained peak discharges surpassing 10,000 m3 /s (Fig. 8.5). However, our results also show that the flow peak discharge rapidly attenuated downstream. In the higher cross section, the flow reached a maximum depth of 21.5 m in a zone likely affected by a seiche wave (Fig. 8.6). Figure 8.7 features patterns of erosion and aggradation along the GLOF path revealing high rates of erosion of several meters.

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a

b

c

d

Fig. 8.5 Block diameter of the rock-avalanche and GLOF deposits. Note the high density of boulders on the rock-avalanche path (a) and that the block diameter on both areas show an unimodal distribution (b, c). a Depicts toppled trees on the flow margins and scarce wood logs on the flow deposit. d Shows that scarce logs surpass the 10 m

Table 8.1 Cross section hydraulic parameters and peak discharge and critical velocity estimations Cross Manning Channel Cross-sectional Hydraulic Water Mean Velocity Discharge section rugosity slope area (m2 ) radius surface depth (m/s) (m3 /s) ID (m/m) (m) width (m) (m) 1

0.075

0.37

1910

9.2

160

11.91 10.80

20,634

2

0.075

0.32

1088

8.5

106

10.24 10.01

10,899

3

0.075

0.45

1194

10.5

83

14.31 11.84

14,139

4

0.075

0.37

654

8

57

11.48 10.60

6936

5

0.15

0.53

616

7.8

54

11.45 10.59

6525

Cross sections 1–4 presented isolated shrubs before the GLOF, while section 5 was in a dense forest area

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Fig. 8.6 a Location of the cross sections used to estimate flow parameters. In light blue the lake surface before the GLOF. In b cross sections showing the reduction of the flow depth and width downstream

8.4 Discussion 8.4.1 Retrospective GLOF Hazard Assessment and GLOF Triggering Factor Average weather conditions were recorded the week before the GLOF, and there was no significant precipitation in this period. Hence, meteorological data alone could have been insufficient for early warning. The failed slope was composed by granitic rocks and evidenced a high density of fractures and reddish areas suggesting weathering processes inside the failed area. Thus, snow and ice melting in an average spring day could have triggered the failure of a steep and already weakened slope. The simple flow routine model used to simulate rock and ice avalanches in the upper catchment indicated that the lake was exposed to mass movement impacts which was corroborated by geomorphic evidence indicative of ice avalanches and rock falls close to the lake. The source area and magnitude of the rock-avalanche, however, could have been more difficult or impossible to predict, as there were not visible signs of slope creeping or geomorphic features indicative of slope instability. Feature tracking techniques or inSAR data could be used to corroborate the slope quiescence before the failure (Dai et al. 2020). The Laguna Espontánea GLOF highlights the need to include in the known GLOF hazard cascade (rock/ice avalanche, seiche wave

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Fig. 8.7 Geomorphic changes associated to the 2018 GLOF. a Shows the lake before and after the GLOF and traces of a seiche wave. b Shows the widening of the active channel and the carving of a new channel in a previous forested slope. c GLOF deposits that dammed the Norte River. Figures in the third panel illustrate the local relief calculated with a moving average window of 13 × 13 pixels in the drone DSM. Note in the upper right panel the convex form of the rock-avalanche deposit

and flood/debris flow), river blockage by GLOF deposits and subsequent hazard upstream (flooding) and downstream in the case of the new dam collapse. Considering the compound hazard effects of multiple processes cascades, holistic flood risk management strategies may be developed (Mazzorana et al. 2019).

8.4.2 Flow Dynamics and Geomorphic Impact The GLOF widened the active channel (from a couple to dozens of meters) and eroded a new path after overrunning a 20 m slope evidencing a large flow momentum. The geomorphic evidence agrees with our flow estimates that indicate a very rapid (≥ 10 m/s) flow with a high peak discharge (> 6000 m3 /s). The flow large magnitude can be explained by the abrupt release of the lake water after the impact of the rock-avalanche. Indeed, the rock-avalanche completely covered the lake surface (Fig. 8.7) allowing to infer an almost instantaneous lake emptying. Large boulders were observed on the flow path. Large boulders could have been embedded in moraine, and alluvial deposits and might have rolled or slipped after erosion;

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however, wood logs covered by large boulders attest for its mobilization during the GLOF event (Fernandez and Coloma 2018). As the flow path shows scoured reaches (Fig. 8.7), the final cross-section areas are likely overestimated and thus critical velocity and peak discharge should be considered with caution. However, even considering a ground level 50% higher when the peak discharge occurred (and assuming that erosion occurred after the peak discharge which is plausible in this very rapid flow), the discharge in all cross sections was in the order of 103 m3 /s which is three orders of magnitude higher than ordinary floods in this small stream. Very high peak discharges as those documented here are common in sudden drainage of glacial lakes as demonstrated by Wilson et al. (2019) in Patagonia and Desloges and Church (1992) in Alaska. Future works can collate our estimates with numerical models to both, confirm our results, and to test the model’s ability to simulate high energy flows (Mergili et al. 2020). DSM comparison shows clear patterns of erosion and aggradation along the GLOF path which agrees well with geomorphic evidence. The steepest sections of the channel and slopes suffered elevation losses, while the rock-avalanche zone and the lower section of the alluvial fan gained elevation. The absolute values of elevation changes, however, are conditioned by vegetation cover. The maximum values of erosion are located on forest areas striped by the GLOF (Fig. 8.7c). According to CONAF/UACH (2012), vegetation in the lower section of the GLOF-affected area was composed by second-growth forest of Nothofagus nitida with heights of 12–20 m. Thus, erosion estimates are probably exaggerated in the lower section of the channel (Fig. 8.8). The use of high-resolution digital terrain models (which exclude vegetation) or the use of filters to remove above-ground objects in DSM (Norhafizi et al. 2022) can help to reduce uncertainties in erosion and accumulation rates estimated in this work. Nevertheless, elevation changes of similar magnitudes (few metres to dozens of metres) have been documented during GLOFs by Wilson et al. (2019) and Zheng et al. (2021).

8.5 Conclusions The analysis of the Laguna Espontánea GLOF landforms and geomorphic features suggests a cascading event initiated by a rock-avalanche which impacted a small lake. It was followed by a seiche wave and violent flood which deposits dammed the Norte River. The GLOF originated an alluvial fan with large boulders (mean diameter 1.3 m) attesting for a flow with high erosive and transport capacity. This is corroborated by a channel eroded by the flow in a forest-covered slope. The geomorphic work was enhanced by the very steep channel and suggests a violent outburst flood after an almost instantaneous lake emptying. Our estimates suggest peak discharges of 103 m3 /s, which are three orders of magnitude larger than ordinary flows in this stream. The flow was very rapid (≥ 10 m/s) and likely reached the Norte River, located 1.2 km downstream, in less than 5 minutes.

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Fig. 8.8 a Elevation changes along the GLOF path. The areas which show highest erosion rates are located were forest was stripped by the flow. The white polygon depicts the lake surface before the GLOF. b Area near the dam that impounded the Laguna Espontánea Lake showing large boulders and works to lower the lake level (image from Cerda (2018))

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Our retrospective GLOF hazard assessment suggests that the lake was highly exposed to mass movements in this narrow and steep valley; however, the source zone, volume, and run-out of the rock-avalanche were difficult to predict as there were no obvious slope instability signals. The weather conditions before the GLOF where on the average historical record and thus could not have been used effectively as a GLOF early warning indicator. The quiescence of the slope before the failure and the average meteorological conditions during the GLOF day show the complexity of GLOF hazard assessments. This is further corroborated by the exceptional river blockage caused by the GLOF. The Laguna Espontánea GLOF shows that this unusual phenomenon (river blockage) should be considered in GLOF hazard assessment scenarios (rock-avalanche, seiche wave, flood and river blockage) when hazardous lakes are located on steep and erodible valleys and GLOFs can brought enough material to dam streams. Acknowledgements This research was funded by the projects ANID/CONICYT FONDECYT Iniciación, Folio 11190389, “Glacial lakes in Chile: evolution and outburst flood hazard assessment” led by Pablo Iribarren and ANID/CONICYT FONDECYT Regular, Folio 1200091, “Unravelling the dynamics and impacts of sediment-laden flows in urban areas in southern Chile as a basis for innovative adaptation (SEDIMPACT)” led by Bruno Mazzorana.

References Alean J (1985) Ice avalanches: some empirical information about their formation and reach. J Glaciol 31:324–333 Byers AC, Rounce DR, Shugar DH et al (2019) A rockfall-induced glacial lake outburst flood, Upper Barun Valley, Nepal. Landslides 16:533–549. https://doi.org/10.1007/s10346-018-1079-9 Carrivick JL (2007) Modelling coupled hydraulics and sediment transport of a high-magnitude flood and associated landscape change. Ann Glaciol 45:143–154 Cenderelli DA, Wohl EE (2003) Flow hydraulics and geomorphic effects of glacial-lake outburst floods in the Mount Everest region, Nepal. Earth Surf Process Landf 28 Cerda C (2018) Cruces en balsa permiten retomar ruta a Bahía Exploradores tras aluvión. Economia y Negocios. 01, Dic 2018. http://www.economiaynegocios.cl/noticias/noticias.asp?id=526287 CONAF/UACH (2012) Monitoreo de Cambios, Corrección Cartográfica y Actualización del Catastro de Bosque Nativo en la XI Región de Aisén. In: Informe Final 1996–2011 Cook KL, Andermann C, Gimbert F, Adhikari BR, Hovius N (2018) Glacial lake outburst floods as drivers of fluvial erosion in the Himalaya. Science 362(6410):53–57. https://doi.org/10.1126/ science.aat4981 Dai C, Higman B, Lynett PJ, Jacquemart M, Howat IM, Liljedahl AK, Dufresne A, Freymueller JT, Geertsema M, Ward Jones M, Haeussler PJ (2020) Detection and assessment of a large and potentially tsunamigenic periglacial landslide in Barry Arm, Alaska. Geophys Res Lett 47(22):e2020GL089800 D’Amboise CJL, Neuhauser M, Teich M, Huber A, Kofler A, Perzl F, Fromm R, Kleemayr K, Fischer JT (2022a) Flow-Py v1.0: a customizable, open-source simulation tool to estimate runout and intensity of gravitational mass flows. Geosci Model Dev 15:2423–2439. https://doi.org/10. 5194/gmd-15-2423-2022

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D’Amboise CJL, Teich M, Hormes A, Steger S, Berger F (2022b) Modeling protective forests for gravitational natural harads and how It relates to risk-based decision support tools. IntechOpen. https://doi.org/10.5772/intechopen.99510 Desloges J, Church M (1992) Geomorphic implications of glacier outburst flooding: Noeick River valley, British Columbia. Can J Earth Sci 29(1992):551–564 Fernandez J, Coloma F (2018) Informa Técnico Remoción en Masa, Ruta x-728, Rio Norte. In: SERNAGEOMIN INF-AYSEN-02.2018, 22 p Heim A (1932) Bergstürze und Menschenleben. In: Beiblatt zur Vierteljahrsschrift der Naturforschenden Gesellschaft in Zürich. Fretz & Wasmuth, Zürich Iribarren Anacona P, Norton KP, Mackintosh A (2014) Moraine-dammed lake failures in Patagonia and assessment of outburst susceptibility in the Baker Basin. Nat Hazards Earth Syst Sci 14:3243– 3259 Iribarren Anacona P, Mackintosh A, Norton KP (2015) Hazardous processes and events from glacier and permafrost areas: lessons from the Chilean and Argentinean Andes. Earth Surf Process Landf 40(1):2–21 Jarrett RD (2016) Summary of peak-discharge computations for the Fall River and Big Thompson River at Estes Park, Colorado, for the September 2013 Flood in the Northern Colorado Front Range. Report to Colorado Water Conservation Board, Denver, p 27 Mazzorana B, Picco L, Rainato R, Iroumé A, Ruiz-Villanueva V, Rojas C, Valdebenito G, Iribarren Anacona P, Melnick D (2019) Cascading processes in a changing environment: disturbances on fluvial ecosystems in Chile and implications for hazard and risk management. Sci Total Environ. https://doi.org/10.1016/j.scitotenv.2018.11.217 Mergili M, Pudasaini SP, Emmer A, Fischer J-T, Cochachin A, Frey H (2020) Reconstruction of the 1941 multi-lake outburst flood at Lake Palcacocha (Cordillera Blanca, Peru). Hydrol Earth Syst Sci 24:93–114. https://doi.org/10.5194/hess-24-93-2020 Mölk M, Rieder B (2017) Rockfall hazard zones in Austria. Experience, problems and solutions in the development of a standardised procedure. Geomech Tunn 10(1):24–33. https://doi.org/10. 1002/geot.201600065 Muñoz R, Huggel C, Frey H, Cochachin A, Haeberli W (2020) Glacial lake depth and volume estimation based on a large bathymetric dataset from the Cordillera Blanca, Peru. Earth Surf Process Landf 45(7):1510–1527 Norhafizi M, Ahmad A, Khanan M, Din A (2022) Surface elevation changes estimation underneath mangrove canopy using SNERL filtering algorithm and DoD technique on UAV-derived DSM data. ISPRS Int J Geo Inf 11(1):32. https://doi.org/10.3390/ijgi11010032 Nuth C, Kääb A (2011) Co-registration and bias corrections of satellite elevation data sets for quantifying glacier thickness change. Cryosphere 5:271–290. https://doi.org/10.5194/tc-5-2712011 Smith MW, Carrivick JL, Quincey DJ (2016) Structure from motion photogrammetry in physical geography. Prog Phys Geogr 40:247–275. https://doi.org/10.1177/0309133315615805 Tomczyk A, Marek E (2020) UAV-based remote sensing of immediate changes in geomorphology following a glacial lake outburst flood at the Zackenberg river, northeast Greenland. J Maps 16(1):86–100. https://doi.org/10.1080/17445647.2020.1749146 Wilson R, Harrison S, Reynolds J, Hubbard A, Glasser N, Wundrich O, Iribarren Anacona P, Mao L, Shannon S (2019) The 2015 Chileno Valley glacial lake outburst flood, Patagonia. Geomorphology 332:51–65 Zheng G, Mergili M, Emmer A, Allen S, Bao A, Guo H, Stoffel M (2021) The 2020 glacial lake outburst flood at Jinwuco, Tibet: causes, impacts, and implications for hazard and risk assessment. Cryosphere 15:3159–3180. https://doi.org/10.5194/tc-2020-379

Chapter 9

Improving the Channel Network Management After a Large Infrequent Disturbance, Taking Advantage of Sediment Connectivity Analysis Lorenzo Martini, Lorenzo Picco, Marco Cavalli, and Andrés Iroumé Abstract Chile is frequently affected by different natural hazards that are constantly reshaping the landscape. Particularly, Large and Infrequent Disturbances (LIDs) such as wildfires and volcanic eruptions are capable of affecting entire river catchments by altering the hydrological cycle, reducing the land cover, and boosting sediment remobilization. Given the multitude of effects caused by such disturbances, the response of the catchments is not easily predictable, and different geomorphic responses are expected. The assessment of sediment connectivity can help to better comprehend the overall effects of wildfires and volcanic eruptions on the sediment transfer dynamics at the catchment scale. Sediment connectivity infers the potential transfer of sediment between compartments of the catchment according to the spatial configurations and the processes of such compartments. After a LID, awareness of the degree of linkage between sediment sources and downstream areas is pivotal to reduce the risk and hazard, improving catchment management. In Chile, analysis of sediment connectivity is extremely valuable even tough the availability of high-resolution topographic data and catchments’ accessibility are not always guaranteed. For this reason, much effort should be employed to adapt approaches, based on high-resolution data, to this context by exploiting freely available global data and satellite images and to find trade-offs between data requirements and reliability of the outcomes. Keywords Large infrequent disturbances · Sediment connectivity · Multi-temporal mapping · Transferable workflow

L. Martini (B) · L. Picco Department of Land, Environment, Agriculture and Forestry, University of Padova, Legnaro, Italy e-mail: [email protected] M. Cavalli National Research Council, Research Institute for Geo-Hydrological Protection, Padova, Italy A. Iroumé Faculty of Forest Sciences and Natural Resources, Universidad Austral de Chile, Valdivia, Chile © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. Oyarzún et al. (eds.), Rivers of Southern Chile and Patagonia, The Latin American Studies Book Series, https://doi.org/10.1007/978-3-031-26647-8_9

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9.1 Introduction Natural perturbations constitute an important component of the landscape since they are responsible for constant alterations of the structure and functioning of river systems. Therefore, understanding how fluvial systems respond to perturbations is fundamental to inform about past, present and future changes in the landscape (Fryirs and Brierley 2013). It is well known that rivers are constantly reshaping and self-regulating their forms and processes after perturbations; thus, they are often recognized for their dynamicity (Knighton 1998). Between subsequent perturbations, iterative adjustments are observed with their magnitude decaying over time and becoming asymptotic toward a new state of equilibrium (Graf 1977; Wu et al. 2012). Indeed, the condition of equilibrium, intended as the absence of change, depends on the time scale considered: a river system might appear stable in a long-time interval, while unstable in the short term due to small fluctuation around the steady state (Wohl 2020). As result, in natural geomorphic systems, an equilibrium barely exists, and the concept is nothing but fleeting. The perturbations acting on the landscape and influencing forest ecosystems are often referred as disturbances, and they can differ in size, spatial distribution, frequency or return interval, as well as intensity and severity (White and Pickett 1985). Particularly large and infrequent disturbances are characterized by their high magnitude and low frequency of occurrence, which makes their effects on geomorphic systems hardly predictable and still poorly understood. Large and Infrequent Disturbances, hereinafter referred simply as LIDs after Turner et al. (1998), were originally documented in for their effects on forest ecosystems but now they are recognized also as major agents of hydro-geomorphic disruption for entire river basins (Foster et al. 1998). Among the most common LIDs, it is possible to recall floods, volcanic eruptions, earthquakes, wildfires, and tropical cyclones. Concerning the Chilean territory, substantial economic losses and environmental damages are caused by wildfires, volcanic eruptions, Glacial Lake Outburst Floods (GLOFs), and earthquakes (Mazzorana et al. 2019). Volcanic eruptions are capable of rapidly altering the flow regime, topography, land cover, sediment, and large wood supply of an entire catchment (Pierson and Major 2014). Specifically, eruptions are responsible for ejecting great volumes of tephra and for triggering cascading events like Pyroclastic Density Currents (PDCs), lahars, landslides, and floods (Kataoka et al. 2009; Manville et al. 2009; Alexander et al. 2010; Capra et al. 2018). In addition, unconsolidated layers of tephra deposited all over the hillslopes are easily re-mobilized by multiple erosional processes, turning into new sediment sources for the stream network according to the degree of connectivity (Martini et al. 2019; Ortíz-Rodríguez et al. 2020). Finally, also the riparian vegetation is affected by volcanic eruptions (Swanson et al. 2013), leading to higher large wood recruitment into the river (Tonon et al. 2017). Another recurring LID in Chile is wildfire. Wildfires are recognized as major agents of land and soil degradation and geomorphological changes (Neary 2005). The high amount of burned material deposited on the soil surface can modify soil properties by increasing and

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then reducing soil infiltration capacity according to the time since the fire (Shakesby 2011). Therefore, the alteration of soil properties can lead to an increase of runoff, soil erosion, and sediment yield, which can be detected even at a long-term scale (Benavides-Solorio and MacDonald 2001). Direct effects on river systems have been documented concerning the increase of in-channel wood recruitment (Benda and Sias 2003), channel aggradation, and mass movements such as landslides and debris flows (Neary et al. 2005). GLOFs are complex and poorly investigated disturbances, occurring in Chile due to glaciers shrinkage and glacial lakes increase (Davies and Glasser 2012; Wilson et al. 2018). They can be generated from the collapse of moraine-dams, ice-dam, or from sudden water release of subglacial reservoirs occasionally accumulated after volcanic eruptions (Björnsson 2003; Westoby et al. 2014). Therefore, GLOFs can convey large amounts of reworked sediment and debris, destabilize steep river flanks, and potentially affect valuable infrastructure and threaten local communities (Carrivick and Tweed 2016; Iribarren Anacona et al. 2018). The occurrence of such a phenomenon is highly variable and depends on the triggering mechanisms (Dussaillant et al. 2010). Increased landslide activity is the one most notable effect of seismic activity in catchments (Keefer 1994). Earthquakes have the potential to trigger large and several landslides even at 100 km from the epicenter (Keefer 1984) and to produce sediment pulses affecting the downstream channel network (Wang et al. 2015). These landslide-driven pulses can create hazards for the settlements and infrastructure on alluvial fans, where aggradation, floods, and channel avulsions are expected. Moreover, when followed by significant rainfall events, the loose material from the earthquake can trigger highly destructive debris flows, which are major concerns in earthquake-hit areas (Korup et al. 2004; Lin et al. 2008). Finally, vegetation can also suffer from the occurrence of earthquake-induced landslides. Cui et al. (2012) found that damaged vegetation was distributed along both river banks and decreased sharply with distance from the river. A quick overview of the LIDs affecting Chile and their hydro-geomorphic impact on river basins has been presented; however, many other effects are under investigation and yet not fully understood. In addition, it is important to underline that such events are sometimes occurring in combination, thus exacerbating the impact and making the causal chain harder to predict (Zscheischler et al. 2018). In order to study the continuous changes of the Earth’s surface, technological innovations have quickly risen. Since the second half of the twentieth century, in fact, new frontiers of Earth’s science have developed primarily due to the advances in remote sensing and surveying methods, the advent of large-scale computation, and revolutions in dating techniques (Church 2010). This new era of knowledge production has facilitated the active quantification, with new tools permitting the acquisition and use of large and rich datasets to study fluvial geomorphology (Piégay et al. 2015). Furthermore, the overlapping use of field, laboratory, and remotely sensed data broke down old boundaries among disciplines, hence promoting new insights and more holistic approaches (Viles 2016). Remote sensing techniques have become more and more available, boosting the amount of information potentially acquired, manipulated, and displayed. Both aboveground platforms (e.g. airborne photography, airborne LiDAR, and space-borne multispectral scanner) and on-ground platforms

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(e.g. terrestrial LiDAR and ground-penetrating radar) have evolved from the necessity of observing the Earth and analysing the topography at various levels with unprecedented resolution (Liu 2008). The observation of the surface and its landforms began with aerial photographs. Interpretation of aerial photographs has become a fundamental practise for geomorphological studies because of their high resolution, high availability in many places, long-term records (since mid-twentieth century), and relatively low cost compared to other forms of remote sensing images (Oguchi et al. 2013). Aerial photographs are largely applied for detecting temporal changes of fluvial morphology and to infer past and ongoing processes such as bank erosion rates (Rhoades et al. 2009), channel migration (Schook et al. 2017), alteration of dominant pattern (Comiti et al. 2011), impact of anthropic presence (Surian and Rinaldi 2003), riparian vegetation cover dynamics (Picco et al. 2017), and more. With increasing spatial scale, satellites can cover large areas in a shorter time span if compared to other remote sensing platforms. Satellite imagery constitutes a special case of digital photography, where satellite sensors can supply their source of energy (active sensors) or they can collect the radiation reflected by the Earth’s surface (passive sensors) in the whole electromagnetic spectrum. One of the most emerging active satellite systems is Synthetic Aperture Radar (SAR), largely used for their relatively high-resolution images obtained independently from weather conditions and daylight and particularly known for their capability of deriving topographic information (Moreira et al. 2013), which will be presented later. On the contrary, passive satellite systems are widely known for their capability of providing multispectral images and for their long history of data collection, as in the case of the Landsat missions. Several commercial and non-commercial satellites have been launched to observe the Earth’s surface, but it is not an aim of this chapter to discuss them, and we refer the readers to existing and exhaustive literature on the topic, e.g. Lillesand et al. (2016). Whether active or passive, remote sensing imagery from satellites is used for a multitude of different applications, and nowadays, it is surely one of the most promising, but not yet fully exploited, options to detect catchment-scale alterations induced by LIDs. With the development of Geographic Information System (GIS), topographic information has become pivotal for research studies dealing with geomorphological and hydrological processes. The most widely used data source to generate digital topography and land surface parameters is the Digital Elevation Model (DEM), which is typically in the form of a set of gridded elevation points in a Cartesian space (Pike et al. 2009; Wilson 2012). DEM is often used in a generic context as a broader term that, in case of Light Detection and Ranging (LiDAR) data (see later), conventionally includes two distinctions, Digital Surface Model (DSM), and Digital Terrain Model (DTM). The former refers to the Earth’s surface with all objects on it; the latter refers to the bare ground without any obstacles like vegetation and buildings (Sofia 2020). Starting from this nomenclature, in this chapter, we will use primarily the broad term DEM without considering the above-ground objects (DSM). The DEM has remained the most popular format of topographic information mainly because of its gridded structure that is controlled by one single parameter, grid size, which makes the application of geomorphic and hydrological algorithms way easier than

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other model structures (e.g. vector data) (Pike et al. 2009). In this way, most of the geomorphic analysis can be applied using landform parameters (e.g. slope, drainage area, and curvature) and indices (e.g. roughness index and topographic position index) derived only from the DEM. In most of the studies approaching the evolution of river catchments after LIDs, the topography stored in a DEM constitutes the starting point, hence the basic information primarily required. Because DEMs are derived from pre-processing of raw data acquired from various methods and platforms, the choice of the data source is fundamental as it influences the quality and frequency of the model and, consequently, the type of analysis that one wants to perform and the results expected. Active remote sensing techniques like the SAR support a wide array of emerging applications ranging from geosciences to climate change research. Radar images can be used to derive topography as in the case of the Shuttle Radar Topography Mission (SRTM), ALOS PALSAR (Rosenqvist et al. 2007), and TanDEM X products (Krieger et al. 2013). These data sources provide global or regional DEM products with spatial resolutions ranging from 12 to 90 m. Also, optimal instruments can be mounted on satellite platforms, hence offering a different data collection method for topography and DEM generation. Among these, we cite ASTER GDEM (Tachikawa et al. 2011) and ALOS Worlds 3D datasets (AW3D) (Tadono et al. 2015), which provide DEMs at 30 m and 5 m to 30 m, respectively. Finally, it is important to underline that all satellite-derived DEMs bring different levels of accuracy (horizontal and vertical) so it is suggested to properly check before choosing one of them. Moreover, newly available high-resolution datasets do not offer free-of-charge solutions, and thus it may influence the final choice. An updated summary of studies dealing with accuracy of different satellite data products can be found in Mudd (2020). Among the remote sensing techniques for terrain data acquisition, airborne and terrestrial LiDAR surely represent one of the most useful and prolific sources in geomorphology (Cavalli and Marchi 2008; Roering et al. 2013; Tarolli 2014). LiDAR technology relies on the emission of laser pulses, emitted and then registered by a scanner that measures the time delay between outgoing pulse and reflected return (Wehr and Lohr 1999). Typically, laser pulses are deployed from airborne and groundbased platforms, termed ALS and TLS, respectively. The advantages of LiDAR over traditional methods are evident in the topographical characterization of bare grounds, hence in the production of DTM, since they are capable of penetrating canopies. Moreover, LiDAR is usually preferred to represent fine-scale elements, as it can obtain high-resolution DEMs due to the great amount obtained, ranging from 1 to 30 pts/m2 (Passalacqua et al. 2015). As part of the most recent advances in digital surveying, structure from motion (hereinafter SfM) represents the most recognized cost-effective technique (Westoby et al. 2012). The acquisition of topographic data improved with the advent of total stations, differential GPS, airborne and terrestrial laser scanning, and lastly, unmanned aerial vehicles (UAVs), which nowadays constitute a cheap alternative to acquire data for SfM technique (Smith et al. 2016). Structure from motion specifically refers to a complete workflow involving 3D computer vision and the most recent developments of photogrammetric algorithms (Snavely et al. 2008). In this sense, the

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workflow is conventionally known as SfM-MVS because of the importance of the Multi-View Stereo (MVS) algorithm in the production of dense point clouds as the final output (Carrivick et al. 2016). Although SfM shares many steps with traditional photogrammetry, it is able to retrieve automatically camera positions and orientation as well as scene geometry without knowing that a priori. This advantage makes the SfM way more flexible and cheaper than other digital survey methods, while providing outstanding point densities (Bangen et al. 2014; Smith et al. 2016). Therefore, SfM is commonly exploited in geomorphology to carry out high-resolution DEMs, which is the result of a gridded 3D point cloud produced in the final stages of the SfM-MVS workflow (Micheletti et al. 2015). However, SfM cannot directly acquire ground information; hence, vegetation filtering methods have to be considered for generating an interpolated DTM. Moreover, SfM employing UAVs is still limited in spatial coverage due to constraints in technology, such as battery life and flying time (Carrivick et al. 2016). The use of remote sensing products has surely facilitated and boosted the analyses of river catchment after LIDs. In particular, in the previous paragraphs, the main DEM sources were presented to shed light on the importance of choosing the most appropriate topographic information. Moreover, as emerged from the short overview on the LIDs affecting river systems in Chile, a wide array of different effects can be identified. Nevertheless, common major effects can be recognized as drivers for secondary impacts. One of these common effects is the growth of sediment available for sudden mobilization, whether from the impacted hillslopes (e.g. exposed soil after wildfires and tephra layers after volcanic eruptions) or directly available in the channel network (e.g. unconsolidated deposits after GLOFs). Therefore, collecting the topographic data is inevitably one of the first steps to characterizing the new sediment sources and to carrying on with further geomorphometric analyses and hydro-morphological considerations addressing the potential sediment transfer to downstream areas. Specifically, the potential sediment transfer from upstream to downstream areas is addressed by the concept of sediment connectivity. The concept of connectivity in the field of geoscience was born to describe the processes of (de)coupling among landscape landforms and through the sediment cascade. Brunsden (1979), first described the role of coupling, between two system components, in the transmission of impulses along the landscape, Brunsden (1993) classified three coupling states: coupled, not coupled, and discoupled. Coupled components show free transmission of energy and material (water and/or sediment); not coupled units show no linkages due to the presence of buffers; and decoupled units refer to a previous coupling state, which is now ceased (Harvey 2001). Notably, buffers are natural landforms, such as alluvial fans and floodplains that impede the transfer of material into the channel network. Therefore, well-connected elements permit the transfer of sediment, whereas in scarcely connected systems this transfer is somehow restricted (Fryirs 2013; Fryirs et al. 2007). In a strongly coupled system, in fact, the effect of environmental changes, enhancing, for instance, the hillslope erosional rates, could be easily transmitted downstream causing high and rapid input

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of material into the channel network. On the other hand, in buffered systems, the same event may lead to local aggradation without any propagation in the lower areas. As consequence, the spatial and temporal response to disturbances is much more uniformly distributed in high-connected systems (Harvey 2001). More recently, the interest in connectivity has soared, and several conceptual frameworks, definitions, methodological approaches, models, and applications to specific case studies have been carried out (Najafi et al. 2021). Initially, it was important to outline broad distinctions. According to the classification by Bracken and Croke (2007), a clear distinction was made between landscape, hydrological, and sediment connectivity, with the latter primarily addressing the sediment transfer in river basins. Concerning the coupling mechanism, (Brierley et al. 2006) distinguished connectivity in longitudinal (along the channel network), vertical (surface–subsurface) or lateral (hillslope-channel). Many definitions relevant to fluvial processes and forms have been proposed (Wohl 2017) so that there is not a universal one. Nevertheless, a widely accepted definition was proposed by Heckmann et al. (2018), who outlined sediment connectivity as the degree to which a geomorphic system facilitates the transfer of sediment through its components. Indeed, the transfer depends upon the relationships between these components, which in drainage basins are hillslopes, channel networks, and valley bottoms (Burt and Allison 2010). Furthermore, a conceptual difference is becoming predominant in this field of research. Structural sediment connectivity regards the physical linkages among sediment sources, channels, and valley bottoms, and how these linkages are spatially arranged in the catchment. It represents a static picture of the catchment’s spatial configuration (Wohl et al. 2018). On the contrary, functional sediment connectivity denotes the dynamism of the sediment transfer and it relates to the processes and temporal component (Wainwright et al. 2011). Therefore, functional connectivity is process-based and investigates the actual transfer of sediment by considering the time component and the frequency-magnitude distributions of sediment detachments (Bracken et al. 2015). An important ongoing challenge is measuring sediment connectivity in a comprehensive way so that structural and functional concepts are addressed. This represents an issue mainly because of the complexity of the concept and the struggle of directly measuring sediment fluxes in multiple temporal and spatial scales, hence functional connectivity (Heckmann et al. 2018). Moreover, the lack of standard protocols for field-based investigations (Turnbull et al. 2018) and the difficulty of summarizing all factors in one metric or one model (Wohl et al. 2018) exacerbate the problem. Anyway, several improvements have been made to measure at least structural connectivity, which is fundamental to represent the spatial configuration of the linkages. Since the development of GIS, the approaches relying on raster-based indices have become successful solutions for the quantitative characterization of structural sediment connectivity. Among raster-based indices, the Index of Connectivity (IC) first developed by Borselli et al. (2008), and then refined by Cavalli et al. (2013) has been widely applied to infer potential sediment transfer from hillslopes to downstream areas, to identify the preferential sediment pathways and to prioritize sediment source

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Fig. 9.1 Detail of an Index of Connectivity (IC) map calculated in a mountain catchment: darker the color, and higher is the connectivity. A schematic representation of the functions composing the IC calculated for a raster unit (reference cell) towards a channel (target)

areas (Fig. 9.1). The IC provides a semi-quantitative approach to investigate structural connectivity by exploiting high-resolution Digital Elevation Models (DEMs) (Cavalli et al. 2013). The original formulation by Borselli et al. (2008), introduced two components representing the degree of connectivity. The logarithmic ratio between the upslope (Dup ) and downslope component (Ddn ) forms the IC, ranging from − ∞ to + ∞. Equation (9.1) reports the original formula: ( IC = log10

) Dup (Ddn )

(9.1)

The upslope component (Eq. 9.2) represents the potential for the mobilization of the sediment given the characteristics of the upslope area: √ Dup = W S A

(9.2)

where W is the average weighting factor, which represents the average value of the impedance to sediment fluxes in the upslope area, S is the average slope (m/m), and A is the area (m2 ) contributing to a specific point under investigation. The downslope component (Eq. 9.3) considers the transfer of the sediment according to the length of the flow path taken to reach the point under investigation (e.g. a sink, channel network): Ddn =

Σ di Wi Si

(9.3)

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where d i is the length (m) of the flow path along the ith , W i is the weighting factor, and Si the slope gradient of the ith cell. Three main adjustments were developed by Cavalli et al. (2013) to adapt the model in mountainous environments: (i) the slope factor was computed according to the steepest downslope direction and limited by a lower and upper limit; (ii) the contributing area was calculated using the D-infinity multiple flow approach; and (iii) the Roughness Index (Cavalli and Marchi 2008) was introduced as weighting factor. As a result of these modifications, the index is better adaptable in steep areas with low presence of vegetation and, moreover, it benefits from the use of high-resolution DTMs. The computation of the IC and the production of IC maps can be done using different tools: (i)

an ArcGIS toolbox (Connectivity Index Toolbox), available at: https://github. com/HydrogeomorphologyTools/Connectivity-Index-ArcGIS-toolbox; (ii) SedInConnect 2.3 free and stand-alone executable (Crema and Cavalli 2018), available at: https://github.com/HydrogeomorphologyTools/SedInConn ect_2.3; (iii) open source R_IC code in R environment (Baggio et al. 2022), available at: https://github.com/TommBagg/R_IC. The tools were originally developed for sharing knowledge of connectivity procedures and to make the connectivity paradigm more accessible in the decision-making process associated with territory management (Crema et al. 2015). The main features of the stand-alone application, leading to positive feedbacks from the scientific community, regards its open source nature and the independence from GIS software. The stand-alone application is typically chosen over the toolbox due to its simplicity and speed of handling. The stand-alone application runs as a single executable Windows file with its Python source code and license (GPL v 2.0), scripting and exploiting libraries. Moreover, TauDEM tools installation is required to run the hydrological functions that operate in SedInConnect 2.3. A new implementation in R language was developed by Baggio et al. (2022), which proposed the R_IC as a novel and flexible way to compute the IC in three different alternatives, hence expanding the potential range of users. Despite the differences of the three scripts composing the R_IC in fact, the main goal is to offer a tool in an environment capable of supporting further statistical and environmental analyses. An important aspect to consider when planning to use IC in any study case is the resolution of the DEM, which may affect deeply the IC maps and the interpretation of the numerical results. Some authors have directly addressed this issue by testing and suggesting the most appropriate DEM grid size for different applications. First of all, when applying the refined version, Cavalli et al. (2013) suggested the use of high-resolution DTMs to optimize the representation of IC in steep alpine catchments. In this type of environment, with low forest cover and steep barren slopes, the applicability and reliability of topography-based approaches and the geomorphological interpretation of the results may benefit from high-resolution data (e.g. ALS). Also, numerically, IC values have a strong relationship with DEM cell size. This

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outcome was initially find by Brardinoni et al. (2015). The authors comparing three different resolutions (2.5, 5, and 10 m) found that IC increases with the increasing pixel size. Similar results were reported by Cantreul et al. (2018), who tested different DEM resolutions (between 0.25 and 10 m) in an agricultural site of 1.24 km2 and recommended a 1 m DEM because it provided the suitable compromise between accuracy and processing time. Indeed, the overall suggestion is to use high-resolution data, for instance from SfM or LiDAR techniques. However, recent applications in large forested catchments, informed about the possible use of low-resolution DEMs retrieved from satellite data. These data were successfully used to compute IC at large scales, for instance by Zanandrea et al. (2019), Singh et al. (2016), and Nicoll and Brierley (2017), who computed IC with 12.5 m, 30 m, and 90 m satellite-derived DEMs, respectively. Low-resolution satellite data are sometimes a viable solution when high-resolution data deficiency, money constraints or site accessibility are forced limits. Nevertheless, it is important to underline that coarse resolution should be treated carefully since it may cause misleading results if implemented to investigate fine-scale landforms or localized processes or if used for connectivity analysis based on surface roughness. Surface roughness in fact, if intended as a proxy of impedance to sediment fluxes, must be computed only using high-resolution data and proper moving window size. On the contrary, too fine resolutions are unsuitable to investigate large-scale patterns of connectivity. Hence, to summarize, even though high-resolution DEMs are often recommended, the choice must consider the extent of the area of interest, the processes and spatial units under investigations, as well as the costs and data constraints (Heckmann et al. 2018). As the IC is becoming a widely used tool to assess sediment connectivity, many applications are focused on the interpretation of IC results providing insights to support river and catchment management conclusions. Since no relevant applications of IC were carried out in Chilean catchments, in the next paragraph we will present particular study cases of IC applications in the mountain basins, mainly in the European Alpine region, in which insights, suggestions or take-home messages could be used to improve the management of mountain catchment. The primary use of IC is surely the assessment of potential hot spots and routes capable to deliver sediment to downstream areas (Cavalli et al. 2013). Integrating the information from IC maps with sediment source inventories is helpful to decide which sources and which sectors of the catchment to manage with higher priority (Messenzehl et al. 2014; Schopper et al. 2019). The prioritization can be made to simply inform managers about the current condition of the catchment or to update after a major disturbance such as LIDs. For example, after the Vaia storm, which affected the Italian Northeastern Alps in late 2018, Pellegrini et al. (2021) mapped the slope instabilities caused by windfalls and assessed their degree of connectivity to the main channel. In another alpine catchment, Rainato et al. (2021) used IC as a supporting tool for estimating which sediment sources actually acted as sediment supplier during the flood induced by Vaia. In the Alps, the human pressure is high, and the presence of infrastructures is an additional element to be considered. To unravel the effects of channel control works like check dams on structural and functional sediment connectivity, Cucchiaro

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et al. (2019) exploited the IC in combination with DEM of Difference (DoD) technique. Different scenarios were considered by Marchi et al. (2019), to conclude that in alpine environments, check dams are constraining lateral sediment connectivity more than under natural conditions, resulting in lower sediment transfer efficiency. Hence, the motivation of these structures, primarily related to risk protection, could be rethought also considering their impact on sediment connectivity. Other common infrastructures in alpine catchments are roads, which potentially act as decoupling agents between slope and channel, re-routing the sediment fluxes towards another target (Tarolli and Sofia 2016). Moreover, according to Kalantari et al. (2017), IC can be integrated into spatial-statistical models for flood probability at road-stream intersections. We presented a few examples of how management interventions may interact with sediment connectivity in mountain catchments and how the use of IC could shed light on these interactions. Of course, many more examples of how catchment management interventions and sediment connectivity interact could be done (e.g. studies dealing with dam construction/removal, levee construction, landuse change due to urbanization or land abandonment, large wood management etc., more information in Poeppl et al. (2017)) but they do not present specific applications of IC and thus they will not be treated. Anyway, it is unquestionable that human interventions have an impact more or less direct on the sediment cascade and connectivity, and that this impact causes consequences and feedback mechanisms that must be addressed (Keesstra et al. 2018). However, river and catchment managers are often neglecting the influence of management actions on sediment connectivity despite the scientific community is pushing for these issues to be considered at the table of discussions (Poeppl et al. 2020). In conclusion, it appears that using IC to support management decisions could be a valuable instrument in the right context and in the right way. For instance, implementing a geomorphometric approach based on the index cannot neglect on-site field activities as originally suggested by Borselli et al. (2008). This is mainly because the capability of describing processes rather than just static linkages is still not achieved. Even though there is an increasing effort towards the fusion of functional and structural connectivity concepts into one single metric (López-Vicente and BenSalem 2019; Zanandrea et al. 2021), the achievement is not fully accomplished, and field validation is, in most of the cases, of utmost importance. In this chapter, we aim at defining and presenting a clear and solid approach to analyse catchment-scale sediment dynamics after LIDs in order to improve management interventions in Chilean river basins. To carry out this approach, we provide a reproducible workflow adaptable to different LIDs, data availability, and contexts.

9.2 Case Studies In the next section, we will present three real study cases carried out in Central and Southern Chile, specifically in the Araucanía Region (IX Region) and in the Los Lagos Region (X Region). For each study case, the chapter provides a description of

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the context and the issues in which the investigation of sediment connectivity and the application of the IC were carried out. Moreover, it will present how those issues were tackled, analysed and what are the main insights and take-home messages obtained. Therefore, the whole section is organized into two sub-sections for each study case: (I) Context of application: an overview on the impact and cascading effects caused by the LID in the catchment, followed by considerations about how the alteration of sediment cascade affected or will affect the local communities. Hence, the section highlights the background and justification that led to the application and adaptation of the geomorphometric approach based on IC. (II) IC implementation: the assessment of sediment connectivity changes made through the IC, starting from the choice of the input data, the methodological approach used, the main outcomes, and potential applications for management practices.

9.2.1 The Rio Blanco Catchment Affected by the Chaitén Eruption 9.2.1.1

Context of Application

In Chile, the presence of active volcanoes is determined by the offshore subduction zone of the Nazca plate and Antarctic plate beneath the South American one, triggering an extensive volcanic arc on the continent. Particularly, along the NazcaSouth America convergence boundary it is possible to find 200 potentially active volcanoes grouped in the Southern Volcanic Zone (SVZ), which extends between latitudes 33° S and 46° S (López-Escobar et al. 1995). Approximately 95 active volcanoes were mapped in the mainland among stratovolcanoes, compound volcanoes or distributed fields as well as monogenetic fields forming single units (Lara et al. 2011). In the Los Lagos Region, the Rio Blanco catchment (73 km2 ) includes the Chaitén volcano, located at 42.84 S latitude and 72.65 W longitude (Fig. 9.2). According to Naranjo and Stern (2004), the volcano did not show intense activity after late Holocene, when the last traceable eruption was dated, and so it was not classified as high-priority and no monitoring activity was promoted prior to May 2008, when an explosive eruption activity begun (volcanic explosivity index, VEI, of 4–5, Carn et al. 2009). From May 2008 to February 2009, three main phases were observed: explosive eruption, dome construction, and dome collapse. Particularly, after the first phase, strong rainfalls forced the mobilization of the pyroclastic material into the Rio Blanco, causing massive aggradation of the riverbed up to 7 m (Pierson et al. 2013). Moreover, all phases were characterized by occurrence of PDCs and lahars that increased the thickness of sediment deposits in the valley floors (Major et al. 2013). A small local community was highly damaged by the cascading events of the Chaitén eruption. The small village of Chaitén, located at the outlet of the catchment, was completely flooded and buried by ~ 3 m of sediment as a result of channel avulsion and aggradation of the Rio Blanco following the first phase. A new

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river delta formed north from the original mouth of the river right after the avulsion. From satellite image interpretation Major et al. (2016), estimated that the new river delta accumulated 1.9 ± 0.2 × 106 m3 of material in less than one month after the explosive phase.

Fig. 9.2 The Rio Blanco catchment (Los Lagos Region) is delineated over a false-color ASTER image (January 2009) showing eruptive plume and extensive forest damages (grey areas) against intact vegetation (blue areas). The location of the old Rio Blanco channel outlet (pre-eruption) and the new channel (post-eruption) outlet are represented in the map. The conventional outlet used for the present study is set in correspondence to the bridge of the Carretera Austral

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Years after the eruption, still enormous amounts of sediment can be supplied to the Rio Blanco by means of the deep pyroclastic deposits in the valley bottom. This represents a potential risk for the downstream village, in which extensive urban sectors are still vulnerable to potential sediment and large wood-laden flows (Basso-Báez et al. 2020). Furthermore, the extensive forest damages (151 km2 around Chaitén according to Korup et al. 2019) have triggered landslide activities along the slopes with potential additional sediment supply for the channel network. In light of these dynamics, the need to monitor sediment connectivity from hillslopes to channel arises, and multi-temporal analyses of land cover changes and IC changes are carried out. The following section regarding the IC implementation draws part of the information from the study published in Martini et al. (2019).

9.2.1.2

IC Implementation

The multi-temporal analysis of sediment connectivity in the Rio Blanco catchment was developed following several steps concerning the choice and implementation of DEMs, weighting factor (W factor) and target which are required for the geomorphometric approach based on IC (Eq. 9.1). Pre- and post-eruption IC maps were derived to detect potential changes in sediment connectivity induced by the LID. The choice of the DEM was made considering the existing constraints of the study area. First, LiDAR surveys were unaffordable due to the size of the catchment (73 km2 ). Second, UAVs surveys were unfeasible because of difficulties in accessing remote areas and again because of the size of the basin. Among satellite data, we selected the SAR-corrected data from ALOS PALSAR source provided by the Alaska Satellite Facility Distributed Active Archive Center (ASF DAAC, https://asf.alaska. edu/) and supported by NASA. In the Rio Blanco area, ALOS PALSAR data offers 12.5 m resolution DEMs for the period 2006–2011. Therefore, two open-source DEMs of 2007 and 2011 were included in this study as they constitute the topography of the pre- and post-eruption conditions. A preliminary DoD was computed to observe potential changes in the topography, caused by the eruption, that could have varied IC before and after the eruption. No significant elevation changes were detected, thus the multi-temporal assessment of IC had to be driven by variation in the weighting factor and/or target. Anyway, for greater accuracy, both DEMs were kept for computing IC in pre- and post-eruption conditions. The weighting factor, representing the impedance to sediment fluxes, can vary according to the area under investigation and to the objective of the study. Original W factors were the Cover and Management factor (C factor) of USLE-RUSLE empirical model and obtained from existing tables of land cover classes (Wischmeier and Smith 1978), or the Roughness Index (RI) (Cavalli and Marchi 2008), calculated from a DTM. Alternative W factors can be freely implemented in the calculation of IC, as long as it is applied equally for the upslope and downslope component and it ranges between 0 and 1. Since the W factor had to represent the spatial and temporal variation in forest cover induced by the eruption, we avoided the use of either the

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C factor or the RI. The former because of the limited choice of values from tabled data, the latter because is recommended only for high-resolution DTMs. For these reasons, an alternative weighting factor based on the Manning’s hydraulic roughness coefficient (n) for the overland flow was implemented following a methodological workflow involving three steps: (I) field surveys; (II) additive procedure; (III) image classification technique. (I)

Field surveys were carried out to identify land cover classes and, for each land cover class, to evaluate specific parameters included in the next step. Field surveys helped the detection of a new class associated with the damages of the forest cover caused by the eruption. Hence, three classes were set for the pre-eruption period: old-growth forest, bare soil and/or ice, sparse vegetation. Four classes for the post-eruption period: old-growth forest, bare soil and/or ice, sparse vegetation, and damaged old-growth forest. (II) Using the additive procedure proposed by Arcement and Schneider (1989), it was possible to calculate a Manning’s n for each land cover class. The procedure requires to evaluate and assign a value, within a pre-fixed range, to the following factors: degree of surface irregularity; effect of obstructions; amount of vegetation. Thanks to the field surveys, it was possible to assign consistent values to each factor. This procedure represents a solid alternative for the construction of a W factor that could be adapted to post-LIDs conditions. (III) The image classification technique was implemented in order to implement the previous steps to the whole spatial and temporal scale of analysis. In other terms, image classification allowed the construction of multi-temporal W factor maps to be used as a proxy of the variation of impedance to the sediment fluxes in the IC. Thanks to the long-time series of data, Landsat missions were found suitable for covering a time period of 19 years (1999–2017). In total, 16 true colour composite images with a resolution of 30 m were downloaded from the U.S. Geological Service data provider EarthExplorer (https://earthexplorer. usgs.gov/). Both unsupervised and supervised image classification algorithms were run in order to have a better control over the results. In the end, the resulting 16 land cover maps were enriched with the Manning’s coefficients previously defined, thus obtaining 16 W factor maps. The last element to consider is the target, which must be implemented as a polygon. The choice of the target is extremely important because IC is highly sensitive to the factor of distance. Therefore, the choice of the target guides the whole analysis towards an objective or another. In this study, we identified the active channel of the Rio Blanco as the target since we focus on the potential connectivity among the affected hillslopes and the river (i.e. lateral sediment connectivity), which in turns transport the sediment towards the town. As for the W factor, also the active channel altered over time, so multiple targets were delineated for each year of the multi-temporal analysis.

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The results showed an evident distinction between high and low IC areas in the Rio Blanco catchment. High IC areas, highlighted in red, indicates potential high sediment connectivity from hillslopes to active channel, whereas blue areas infer low connectivity. In both pre- and post-eruption maps, high IC areas are concentrated primarily on slopes along the upper active channel, where the river is more confined. Moreover, also in the tributary flowing from the caldera to the main Rio Blanco, it is possible to detect hot spot areas. On the contrary, low IC areas are located: (i) along the farthest parts of the catchment, such the headwaters on the East; (ii) in the Eastern sub-catchment, disconnected to the main basin, and (iii) on slopes along the lower active channel, where there is still a floodplain acting as a buffer for the sediment. Another important cold spot is the Chaitén volcano, characterized by a flat rim around the caldera. Considering the differences before and after the eruption, it is clear the role of the active channel in boosting the IC where the valley is more confined. The expansion of the active channel, caused by the input of unconsolidated material, led to a reduction in the distance between slopes and target. The consequence is an overall increase in IC in the middle part of the catchment. Mean and median values grew significantly after 2008, as demonstrated by Fig. 9.3c. The extent of active channel and old-growth forest are reported as well. Main causes of the IC increase is attributable to the expansion of the active channel and to the reduction of forest cover, which lowered the impedance to the potential sediment fluxes. In conclusion, the choice of an alternative W factor, based on forest cover changes, and the dynamic target clearly enhance the assessment of IC changes in the Rio Blanco. The course spatial resolution, given the open-source DEM, permitted the representation of main spatial patterns of IC in a 73 km2 catchment.

9.2.2 The Rio Blanco Este Catchment Affected by the Calbuco Eruption 9.2.2.1

Context of Application

In the Los Lagos Region, the convergence of Nazca and South American plates fuels the volcanic activity of the SVZ. Among the most active volcanoes in the SVZ, the Calbuco stratovolcano is one of the most active, with two sub-Plinian eruptions in 1929 and 1961, and others less powerful in the last century (Castruccio et al. 2016). The volcanic relief of the Calbuco (41.19 S latitude and 72.37 W longitude) rises 2003 m a.s.l. with an area of 180 km2 . Directly from the North-Eastern flanks of the volcano, the Rio Blanco Este flows for approximately 17 km before entering the Rio Hueñu-Hueñu. The whole catchment covers a surface of 39 km2 with mainly

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Fig. 9.3 IC maps for a pre-eruption year (a), and post-eruption year (b) using the derived Manning’s n as weighting factor and the active channel of the Rio Blanco as target. Mean and median IC values of the entire basin are plotted for the entire study period alongside with the multi-temporal variations of active channel (c)

old-growth forests of evergreen native species. While the downstream part is characterized by the flat and wide active channel of the main Rio Blanco Este, the upper catchment is characterized by steeper slopes where three main tributaries are flowing: the Rio Frio on the South-West, the Rio Caliente on the South and the Rio Sin Nombre on the South-East (Fig. 9.4). In 2015, an eruption with a Volcanic Explosivity Index (VEI) of 4 occurred (Romero et al. 2016) and the Rio Blanco Este catchment was severely impacted. During the event, two phases were recognized both involving tephra fall, PDCs and lahars and a total bulk volume of material erupted of 0.38 km3 according to Castruccio et al. (2016) or 0.58 km3 according to Van Eaton et al. (2016). In the

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Fig. 9.4 The Rio Blanco Este catchment (Los Lagos Region) drains directly from the eastern flanks of the Calbuco volcano, which erupted in 2015. The main channel flows for 17 km ca. until the outlet located in correspondence of the bridge of the regional road. After the eruption, a hydropower facility on the left bank was destroyed

Rio Blanco Este catchment pyroclastic flows were conveyed from the flanks into the stream network with several meters of unconsolidated sediment (Bertin et al. 2015). The forest was damaged by tephra fall in most of the catchment and it was wiped away near the volcano. Moreover, hydropower facilities located along the main channel were destroyed due to the consequences of the eruption and years later reconstructed in a similar position (Fig. 9.4). As a result, the catchment has deep deposits of sediment in the upper part of the basin and along the main channel. Thanks to high sediment supply and dynamicity, the Rio Blanco continuously shifts its course, threatening the regional road located downstream the outlet of the watershed. Considering this new setting and dynamic, it is important to understand the role of sediment sources in the upper catchment for the sediment cascade. In summary, managing sediment connectivity in the Rio Blanco Este catchment means focusing on the linkages between upstream sources-bridge (outlet of the basin). Therefore, in this

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scenario, both lateral (hillslope-channel) and longitudinal (channel-outlet) connectivity are implied. The assessment of sediment connectivity provides an overview of the area more prone to deliver sediment downstream.

9.2.2.2

IC Implementation

Similarly to the Rio Blanco affected by Chaitén eruption in Patagonia, also in this case, in addition to the static condition of sediment connectivity, a multi-temporal approach was developed in order to highlight the potential variations driven by land cover changes. The methodological workflow already tested in the Chaitén area was applied to the Rio Blanco Este, carefully considering the choice of DEMs, W factor, and targets. First of all, the choice of DEM source was again forced towards the only viable option, which was satellite DEMs. Amongst the freely accessible sources, the ALOS PALSAR corrected products by the ASF DAAC were selected. However, the ALOS PALSAR time range 2007–2011 did not covered the Calbuco eruption of 2015, therefore a single 12.5 m resolution DEM dated 2011 was selected. Having only one pre-event DEM, the only potential drivers of IC changes are exclusively the W factor, i.e. the land cover changes, and the target. Implementing a W factor involves a series of prior choices. First, the choice of the parameter that most appropriately can represent the impedance to the sediment fluxes in the given scenario. As previously explained, in post-disturbance conditions with low-resolution DEMs, neither the C-factor nor RI were evaluated as the best options. Consequently, we adopted the Manning’s coefficient for the overland flow and the additive procedure to adapt the coefficient to specific land cover classes. How to implement the Manning’s n as W factor for the computation of IC is another important step to consider. Again, three main steps are identified in the methodological workflow: (I) field surveys; (II) additive procedure; and (III) image classification technique. (I)

Even if the pre-eruption land cover can be classified using satellite images and/or pre-existing data from the Chilean national or regional inventories, the identification of new post-eruption land cover classes had to be done thanks to field surveys made throughout the study area. Field surveys focused on the identification of main land cover classes and then, for each class, in the assessment of the parameters involved in the additive procedure at step II. Moreover, in respect to the classification made in the Rio Blanco (Chaitén), grain size distribution was also surveyed to better characterize the bare ground. The resulting pre-eruption land cover classes were: Old-growth forest, sparse vegetation, bare ground of active channel (hereinafter active channel), bare ground of visible pyroclastic deposits (hereinafter bare volcanic ground), and bare ground with no clear evidence of pyroclastic deposits (hereinafter bare ground). For the post-eruption conditions, only the damaged vegetation class was added.

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(II) The additive method proposed by Arcement and Schneider (1989) allowed the construction of ad-hoc Manning’s n for each class by primarily evaluating the degree of surface irregularity, effect of obstructions, and the amount of vegetation. Thanks to the field surveys, it was possible to assign consistent values to each factor. Summing all the values given to each factor, an overall Manning’s n is attributed to each land cover class. (III) The image classification technique was performed over multiple satellite images to derive multi-temporal W factor maps and IC maps. In the Rio Blanco Este, we used five Landsat eight true colour composite images with a resolution of 30 m downloaded from the EarthExplorer website. Each image referred to a single year from 2015 (pre-eruption) to 2020 (post-eruption). Supervised image classification was run for each image, and to each land cover class the corresponding Manning’s n was assigned, according to the procedure in step II, thus obtaining multi-temporal W factor maps. The last step of the workflow involves the target, representing the element towards which the degree of sediment connectivity should be calculated. In the Rio Blanco Este catchment, the geomorphometric approach based on IC was set to investigate both lateral and longitudinal connectivity. Therefore, a polygon was drawn at the outlet of the basin, corresponding to the section passing under the bridge at the downstream regional road. The IC maps depict high connectivity area where the IC values are higher, and colour is red (Fig. 9.5). On the contrary, low connectivity is highlighted in green with lower values of IC. This distinction is evident in the Rio Blanco Este catchment, where high IC spots are mainly located along the flanks of Calbuco volcano, along the Eastern slopes of the upper Rio Blanco Este, and finally in the Eastern subcatchment. Conversely, low IC spots are visible along the Western slopes of the upper Rio Blanco Este and in downstream flatter active channel. Interestingly, these two locations are characterized by large deposits of sediment, but due to low slope, they are poorly connected to the outlet. Therefore, they enhance sediment availability but with low connectivity and lower chances of supplying sediment. Differences are not particularly visible amongst the IC maps computed for the whole study period (2015–2020). The spatial patterns of high and low IC seem stable and only small graduated changes in colour can be observed. However, to point out more objectively the potential changes, IC values have been extrapolated from the maps and plotted during the time. The boxplots show the distributions of IC values for each year and report a first abrupt increase after the eruption, followed by a mild reduction. Because the IC was calculated using the same DEM in all years, such quick variations were not ascribed to morphological changes but rather to the quick real or misleading annual changes in land cover. Even though the time span of the analysis is only six years, it appeared to have captured three major phases: the pre-eruption stability, destabilization time, and

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Fig. 9.5 IC maps for a pre-eruption year (a) and post-eruption year (b) using the derived Manning’s n as weighting factor and the outlet of the Rio Blanco Este as target. IC values of the entire basin are displayed in the boxplots for the entire study period: from 2015 (before eruption) to 2020 (c)

relaxation time. These phases are typically describing the behaviour of a response variable to a disturbance (Pierson and Major 2014). Nevertheless, in the case of LIDs, much more years are needed to fully investigate the sensitivity of IC or any other intrinsic variable. In conclusion, in the Rio Blanco Este catchment, the multi-temporal analysis of IC allowed depicting the main areas connected to the outlet. It emerged that the main deposits located in upper Rio Caliente and Rio Blanco Este show low IC. Hence we assume that the sediment from these deposits can be hardly conveyed downstream through transfer mechanisms contemplated by the IC. On the contrary, the Rio Frio and the Rio Sin Nombre sub-catchments showed high IC due to steeper

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slopes. Moreover, in these two tributaries the presence of large areas of damaged forest, combined with high connectivity, may suggest an exponential increase in large wood recruitment from the slopes to the main channel.

9.2.3 The Rio Toro Catchment Affected by Multiple Wildfires 9.2.3.1

Context of Application

Chile is frequently affected by severe wildfires, which threaten forested catchment covered by native species as well as human settlements located in rural areas. Moreover, in wildfire-affected catchments, both the hydrological cycle and sediment cascade can be highly impacted, with increasing occurrence of hillslope instabilities like landslides and debris flows (Neary et al. 2005). According to CONAF (2019), in the last 50 years, the annual mean area burned in Chile is around 520 km2 , with more than 99% of the wildfires being initiated by human causes. Therefore, natural causes, such as lightnings or volcanic eruptions, are responsible of less than 1% of the ignitions. However, even if lightnings do not cause lots of wildfires, they are often associated with large burned areas (CONAF 2019). In addition, González et al. (2011) reported how the change in climatic condition, with reduction of precipitations and temperature increase, is becoming a critical aspect in central regions (33°–42° S) of Chile. In the Araucaria Region (IX) two wildfire seasons in 2002 and 2015 highly affected the Rio Toro catchment (18 km2 ) located in the Malleco National Reserve. The area was completely covered by native species of Araucaria Araucana and Nothofagus spp., which naturally form mixed forest in the mountain regions of South-Central Chile and Western Argentina (Veblen et al. 1981). The Rio Toro flows for 11 km, from around 1810 m a.s.l. to the outlet located at 760 m a.s.l. In the upper catchment, two tributaries, divided by a central ridge (Fig. 9.6), drain water from a plateau, whereas in the lower catchment the main Rio Toro develops a step-pool/cascade bed morphology (Picco et al. 2021). The first fire occurred in February 2002, burned a surface of 11 km2 alone, thus significantly contributing to the overall 20 km2 burned in the Araucanìa Region in the 2001–2002 summer fire season (González et al. 2005). Additionally, in 2015, when 46 km2 of burned territory was reported in the region, another major fire hit the catchment. Therefore, the Rio Toro catchment represents a peculiar study case, in which sediment dynamics were affected by the re-occurrence of the same LID. Even though significant hillslopes instabilities were not reported right after the second wildfire (Iroumé et al. 2015), the assessment of IC could help to detect areas more prone to deliver sediment in the future. The effects of LIDs along the hillslopes may show a time lag of several years, so that IC, rather than just depicting the static spatial patterns within the catchment, could also function as a metric to foresee trends of sediment connectivity before and after disturbances. The following section regarding the IC implementation in the Rio Toro draws part of the information from the study published in Martini et al. (2020).

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Fig. 9.6 The Rio Toro (Araucanìa Region) is constituted by two major tributaries divided by the Cerro Central in the upper basin and by the main channel in the downstream part (a). The catchment was affected by two wildfires in 2002 and 2015. Aerial photo of the 2002 wildfire (b) was adapted from González et al. (2014) and distributed by CONAF (2019)

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IC Implementation

The assessment of sediment connectivity in the Rio Toro catchment was developed following the already established workflow for the implementation of the geomorphometric approach based on IC. However, differently from the previous Chilean catchments affected by volcanic eruptions, in this study case it became necessary to adapt the workflow to the specific effects of wildfires on the vegetation. Considering the same issues already discussed in the previous catchments, the only feasible source of DEM was the ALOS PALSAR satellite system. A single 12.5 m DEM was used to represent the topographic for the computation of IC before and after the two wildfires. The W factor was calculated using the Manning’s n for the overland flow, which was selected over the original C-factor and RI as discussed in the previous study cases (Sects. 9.2.2.1 and 9.2.2.2). For the implementation of the W factor in the Rio Toro catchment, four steps were carried out: (I) field surveys; (II) additive procedure; (III) computation of spectral vegetation index IFZ; (IV) W factor normalization. (I)

A total of 46 sampling plots were surveyed in the Rio Toro catchment with the aim of evaluating the parameters indicated in the additive procedure at step II. Instead of defining land a specific Manning’s n for different cover classes, in this study the evaluation was made for each sampling plot. With respect to the volcanic-affected catchments, the Rio Toro catchment did not show significant differences in land cover, hence the characterization of land cover classes from image classification could not be performed. (II) The additive procedure by Arcement and Schneider (1989) to derive ad-hoc Manning’s n was applied to each sampling plot by evaluating the degree of surface irregularity, effect of obstructions„ and the amount of vegetation. During field surveys, it was possible to assign consistent values, chosen from a pre-fixed range, to each factor and summing all factors an overall Manning’s n was attributed to each sampling plot. (III) Since no land cover classes were recognized, to extend the Manning’s n to the whole catchment in four scenarios (before and after the two wildfires), we made use of a new approach based on a spectral vegetation index. The Integrated Forest Z-score (IFZ) is a threshold-based index tracking the vegetation changes and recovery after a wildfire (Huang et al. 2010). The IFZ is calculated at pixel-scale using specific spectral bands (Short-wave infrared 1 and Shortwave infrared 2 bands) obtained from multi-spectral remote sensing imagery. Four 30 m Landsat images dated 2002 (pre-first wildfire), 2003 (post-first wildfire), 2015 (pre-second wildfire) and 2016 (post-second wildfire) were selected to represent the four scenarios. The Landsat products were obtained free of charge from the Earth Resources Observation and Science Center (EROS) Science Processing Architecture on Demand Interface (ESPA). The ESPA in fact provides, upon request, satellite data atmospherically corrected at surface

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reflectance. This processing level of satellite data allows the use of multispectral images with a reduced sensor, solar and atmosphere distortion (Young et al. 2017). Four IFZ maps were obtained, in which the higher the value of the pixel, the higher is the likelihood to be forested. Inversely, the higher is the Manning’s n, the lower is the vegetation cover. The relationship between the IFZ and the field-based Manning’s n was used to extend the latter to the whole catchment area. (IV) Differently from volcano-affected areas, in which a clear land cover classification could be made, the range of Manning’s values in the Rio Toro was narrow. Therefore, a normalization equation by Trevisani and Cavalli (2016) was adopted to extend the range of values otherwise constrained by the additive procedure and to enhance the spatial variability of the following W factor and IC maps. Finally, four normalized W factor maps were obtained for two pre- and post-wildfire scenarios. In the Rio Toro catchment, managing the sediment cascade entails the investigation of sediment connectivity from the burned slopes to the channel network. Therefore, the channel network was integrated as the downstream target of the IC and lateral sediment connectivity depicted. Four IC maps were obtained, inferring lateral sediment connectivity before and after the two wildfires that occurred in 2002 and 2015, respectively (Fig. 9.7). Common spatial patterns can be depicted in all four scenarios, with higher IC values located in areas on the west slopes and along the two tributaries. On the contrary, lower IC values are mainly found close to the outlet and in the uppermost part of the catchment, in correspondence to flatter areas. However, relevant differences are visible amongst the scenarios and especially before and after the LIDs. Prior to the first wildfire, the catchment was characterized by widespread low values of IC, demonstrating in fact the presence of a dense vegetation. Few are the hot spots of high IC, indicating a stable geomorphic system and limited sediment cascade. After the first wildfire, large areas increased the IC values due to a sudden reduction of the forest cover, especially the understorey. Interestingly, the time between the two events was sufficient to re-establish part of the understorey so that the IC before the second wildfire is visibly lower than after the first wildfire. This interval shows that the system underwent a relaxation time, which was again interrupted by the occurrence of the 2015 wildfire. The re-occurrence of fire promoted again the increase of IC and the last map shows the largest extension of high IC areas. Therefore, as expected, the vegetation played a fundamental role in altering the potential sediment dynamics of the catchment. Nevertheless, a reliable integration land cover changes in the geomorphometric approach was possible only thanks to the compound use of a spectral vegetation index, capable of tracking the vegetation recovery after wildfires. As result, catchment-scale IC acted as a metric of sensitivity to monitor the sediment connectivity in the Rio Toro after multiple LIDs.

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Fig. 9.7 IC maps of the pre-2002 wildfire (a), post-2002 wildfire (b), pre-2015 wildfire (c), and post-2015 wildfire (d). The IC was calculated using the Manning’s n as W factor and the channel network as target. The outlet is displayed as well to indicated the direction of the fluxes

9.3 Discussion Large infrequent disturbances are responsible for multiple effects on river basins, amongst which the alteration of the sediment cascade is evident. From the creation of new sediment sources to the generation and propagation of sediment fluxes with consequent deposition in downstream areas, LIDs are responsible for severe hydrogeomorphic changes even for centuries (Fryirs and Brierley 2013). Sediment connectivity, which underlies sediment transfer processes in river basins, has a fundamental role in transmitting or impeding sediment fluxes after a disturbance. In addition,

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even the degree of connectivity can be subjected to significant variations driven by LIDs: the system can suddenly switch from highly connected to disconnected and vice versa (Bracken et al. 2015). Therefore, assessing variations in sediment connectivity is fundamental to understand how a catchment can respond to LIDs, foreseeing the potential negative consequences of sediment fluxes. In this context, we presented three peculiar study cases where the different characteristics of the catchments affected by disturbances, and the specific effects caused by these events required the adaptation of the topography-based IC (Cavalli et al. 2013). In the Rio Blanco catchment, the Chaitén eruption that occurred between 2008 and 2009 led to significant changes in the structural properties of the basin at different temporal and spatial scales. For this reason, it was important to conduct a multitemporal analysis of IC as an instrument for documenting potential changes in sediment connectivity over time and to highlight the role of the disturbance in producing those changes. According to the results of the study, it emerged that the eruption increased sediment connectivity, and the driving factors could be traced back to two main effects: land cover changes and widening of the active channel. Particularly, the land cover changes affected sediment connectivity by reducing the impedance to sediment fluxes, which were integrated into the IC by deriving ad-hoc W factors maps. Although land cover and land use changes are already recognized key factors for sediment connectivity changes (López-Vicente et al. 2013; Llena et al. 2019), their effects are often manifested and propagated over a longer time scale compared to the Rio Blanco catchment. In the Chilean study case, in fact, the alteration of land cover is the immediate result of another perturbation rather than the consequence of long-lasting human activities like, for instance, land abandonment, deforestation, or urbanization (Poeppl et al. 2017). The second and fundamental factor responsible for the IC increase is the expansion of the active channel. Hence, fluvial adjustments were included because of their role in promoting or reducing lateral sediment connectivity from the landslides activated along the slopes to the channel network, where, in turn, flood events might transport this extra sediment towards the Chaitén village located at the outlet. A different scenario has been analysed in the Rio Blanco Este, where the IC was computed with reference to the outlet of the basin in order to assess both lateral and longitudinal connectivity before and after the 2015 Calbuco eruption. Similarly to the Rio Blanco catchment, also here ad-hoc W factors were implemented according to the different land cover classes, which included also a further characterization of the bare ground according to the type of sediment deposit. Therefore, also in this study case, the changes in land cover were the key factor to capture multi-temporal changes in connectivity. Even though the multi-temporal analysis was limited to six years, it was already possible to detect three major phases: the pre-eruption stability, destabilization time, and relaxation time. The choice of the target represents a specific management objective, since the outlet of the basin corresponds to the bridge passing over the confluence between Rio Blanco Este and the Rio Hueñu-Hueñu. Besides its already precarious position, the analysis of sediment connectivity showed that the bridge could be even more endangered due to the rapid increase in IC after the eruption. Moreover, considering additional cascading processes triggered by the

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LID (Mazzorana et al. 2019) (e.g. higher sediment availability, alteration of water discharge, large wood transport), the planning of management interventions and/or further constructions along the channel must be carefully considered. In the Rio Toro catchment, the impact of the wildfires was limited to the land cover, which suffered major forest reduction and simplification due to the high severity of the events. Since no channel adjustments were observed (also due to the intrinsic more stable condition of that mountain stream) and no sensible infrastructure was identified to be used as a target, a single polygon representing the static channel network was chosen. The topography and configuration of the channel network is completely different from the ones affected by volcanic eruption. While Rio Blanco and Rio Blanco Este drain large watersheds fed by high annual precipitations and develop dynamic braided channels, the Rio Toro shows a narrow stream network and stable plane-bed/step-pool morphologies. Moreover, the volcanic eruptions conveyed large amount of sediment directly into the system, whereas wildfires did not directly affect the sediment dynamics within channels. Therefore, much attention was paid to improve the modelling of forest cover changes into the W factor, basically by means of field surveys and spectral vegetation index capable of tracking forest recovery after wildfires. The weighting factor is the most adjustable factor in the IC formula, as it represents the impedance to sediment fluxes according to the needs of the user. The application of the W factor is always a critical step in the development of a proper methodology. In the Chilean study cases, the Manning’s n was preferred to compute the W factor over other parameters commonly found in literature such as the Roughness Index (RI) (Cavalli and Marchi 2008) and the C factor of the USLE-RUSLE formula (Wischmeier and Smith 1978). The choice is recapped in: (i) the Manning’s n can be adapted to the features of the study area by using the additive procedure by Arcement and Schneider (1989); (ii) the RI requires high-resolution DTMs and low forest cover to perform accurately, which are not everywhere granted; (iii) the C factor is more suited to agricultural areas and has a limited distribution of values. However, when the conditions are suitable for the other parameters, the choice has to be rethought. Therefore, it is vital to know the characteristics of study area, data availability, and to consider the objective of the research before choosing the W factor. In line with other authors, who investigated how different W factors affect IC after disturbances (Ortíz-Rodríguez et al. 2017; López-Vicente et al. 2020), we advise critical thinking to implement the most appropriate W factor for each situation. The computation and implementation of the W factor constitute a major step of the methodological workflows presented in the studies. Given the array of variables involved in all studies, it became essential to organize the analyses in subsequent phases, blocks or steps. Furthermore, the presence of a workflow helps the reproducibility of the research providing also a track for further improvements. All the studies were analysed using very similar workflows, following a common thread but giving space to refinements in order to better adapt it on the specific case. More specifically, all the case studies were engaged using freely available DEMs, field surveys, and satellite images, but while in the volcanic areas the image classification technique was adopted to classify the land cover affected by the eruptions, in the

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Rio Toro catchment the spatially distributed W factor was derived using the IFZ calculated from multi-spectral satellite data. The reproducibility of the workflow, in fact, depends on another aspect, already mentioned before and related to data availability. Freely available topographic information promotes reproducibility as shown in the Chilean basins, but on the contrary, it limits the level of detail obtained by the results. Low-resolution DEMs are useful to carry out an approach to detect catchment-scale spatial changes of connectivity rather than analyse single source-tochannel processes. For this reason, relating a proper DEM to the final objective of the work is another fundamental aspect. Working with an appropriate spatial resolution helps avoid severe misinterpretations of the outcomes and provides coherent conclusions to the planned objectives. For instance, a 30 m DEM is not suitable to compute IC maps assessing the sediment transfer through gullies or rills; on the contrary, using ultra-high resolution DEMs is inefficient and pointless to detect macro areas of high connectivity in very large catchments. With coarser resolution, the pathways are oversimplified, and with ultra-high resolution, the creation of spurious disconnections is higher (Brardinoni et al. 2015; Cantreul et al. 2018; Martini et al. 2022). In conclusion, according to Heckmann et al. (2018), the optimum resolution depends on multiple factors, including the phenomenon under investigation, the importance of small/large scale features, DEM uncertainty, computational demand, and, overall, the objectives of the study. The rapid growth of studies in the field of connectivity is promoted likewise by the boost of remote sensing techniques. As presented in the introduction, several sources of topography and imagery can be exploited (e.g. satellite, LiDAR, SfM) to carry out geomorphometric approaches addressing sediment connectivity case studies. The propagation of such approaches and application cases leads to benefits for the scientific advancement, but, on the other hand, it may cause the proliferation of analyses overlooking basic rules. For this reason, in the previous paragraphs careful choices to ensure a correct implementation of the IC have been stressed out. To improve the reliability of the approach adopted and to validate the results, field surveys still represent good practice, and good consensus about this conclusion is well documented (Lexartza-Artza and Wainwright 2009; Messenzehl et al. 2014; Hooke et al. 2021). Therefore, field activities were carried out in all three study cases: notably, without ground validation, it would have been impossible to adapt the W factor and to characterize the post-disturbance scenarios. Finally, measuring and quantifying sediment connectivity are still major challenges. Recently, several authors recommended the integration of structural and functional aspects of connectivity to overcome this challenge. Still more efforts need to be done to include also the functions and the external properties of the catchment into the IC-based approaches. However, the three studies presented in this chapter have shown that with proper modifications of the geomorphometric approach is still possible to provide a coherent characterization of the impact of large natural disturbances on sediment connectivity in Chilean basins. Therefore, before focusing fully on the processes and external properties controlling sediment dynamics, more efforts could be done to adjust the structural IC to different conditions.

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9.4 Conclusions River basins must necessarily coexist with multiple disturbances that constantly affect their processes and landforms. In Chile, this coexistence is exacerbated by the multitude of LIDs that cause larger consequences for the river systems and surrounding territory. Particularly in mountain basins, where sediment is originated and then transferred downstream, the occurrence of these events alters of the sediment cascade causing sources for management concerns. In this chapter, we presented a clear and simple approach to analyzing catchment-scale sediment dynamics after Large and Infrequent Disturbances in order to support management interventions in Chilean basins. The application and adaptation of the Index of Connectivity was particularly useful to detect variation in sediment pathways and to identify hot spot areas eligible for interventions (depending on the actual needs of the stakeholders and according to the characteristics of the area). In the three cases discussed, different contexts and issues were considered to justify a multi-temporal analysis of connectivity and to shape the implementation of the IC in the most appropriate way. The adaptation was controlled by a pattern involving specific steps regarding: source of the DEM; implementation of ad-hoc W factors; and target’s choice. In Chile, where the availability of high-resolution data is scarce, freely available satellite-derived DEMs were found sufficiently suitable to compute multi-temporal IC maps and provide reasonable results. Moreover, the integration of the W factor must consider also the specific effects of the LIDs in order to shape a more reliable IC, and finally, an accurate choice of the target must not be neglected since it reflects the objective of the analysis and influences the interpretation of the results. Therefore, to carry out a solid analysis based on the IC, these three steps need to be addressed. Acknowledgements This research was developed within the frame of the project FONDECYT 1200079 funded by the Chilean Government and developed within the project financed with PICC_BIRD2121_02 funds, Dept. TESAF, Università degli Studi di Padova.

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Chapter 10

Mitigating Complex Flood Risks in Southern Chile in a Particular Spatial Planning Context: Towards a Sustainable Strategy Bruno Mazzorana and Francisco Maturana Abstract Effectively mitigating flood risk in fluvial environments in Chile characterized by intense volcanism, cryosphere changes, high relief energy and influenced by multiple disturbances, is particularly challenging, and strategies developed in different contexts such as the European Alps might not be directly transferrable to the Chilean setting. But planning and acting in dire straits might also bring about novel approaches to tackle wicked problems. Here, rather than adopting mitigation concepts that showed, under more favourable conditions, to be only partially effective elsewhere, we discuss a bottom up, participatory approach that aims at holistically achieving flood mitigation facing compound limitations. This approach entails the creation of an inclusive risk culture that could enable anticipated action avoiding settlement expansions that may irreversibly increase exposure and diminish the available management options. In the context of a foresighted spatial planning, we propose sustainable innovative solutions that can achieve substantial risk mitigation effects through a targeted interaction with the hazard processes, rather than aiming at preventing their occurrence. It is argued that a wise fusion of direct and indirect mitigation measures planned in a truly participatory way, constitutes, in the long term, the basis for a sustainable future for societies living in risk prone areas in Southern Chile. Keywords Effective risk mitigation · Inventive problem solving (TRIZ) · Participatory approach · Fluvial processes

10.1 Introduction Urbanization advances unstoppably. According to the UN, over 55% of the population lives in cities and it is expected that by 2050 this percentage will rise 13% B. Mazzorana (B) · F. Maturana Instituto de Ciencias de la Tierra, Facultad de Ciencias, Universidad Austral de Chile, Valdivia, Chile e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. Oyarzún et al. (eds.), Rivers of Southern Chile and Patagonia, The Latin American Studies Book Series, https://doi.org/10.1007/978-3-031-26647-8_10

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(United Nations 2018). The distribution of such urban settlements has not been homogeneous in all regions of the planet, with the particularity that the majority of the population tends to be located in coastal areas or close to water resources (Claval 1981; Neumann et al. 2015). Such areas are frequently exposed to risk of both natural and anthropogenic origin (Gu 2019). In fact, although the general increase of the living standards associated with this unprecedented and ongoing phenomenon is uncontroversial, urban areas have shown to be particularly susceptible to the impacts of extreme flood events (Fuchs et al. 2015) as the most recent disasters in Central Europe and China and the Americas clearly attest. With respect to the American continent, the death toll due to floods is shown in Fig. 10.1 broken down with respect to its four macro-regions (i.e. Caribbean, Central America, North America, and South America). In this regard, South America stands out with particularly high figures. Also worth noting is the peak of fatalities in the year 2011 provoked by the inundation in Teresópolis, Río de Janeiro, Brazil (Data: EM-Dat.). It is widely acknowledged that process cascades resulting into severe flood impacts are often unpredictable as are their further compounding effects generated by interactions with elements of the built environment and by the response of the concerned society. Although part of the complexity of the arising flood risk mitigation problems resides in the variability of the underlying natural processes and their exacerbation due to climate change, the severity of the adverse consequences has deep anthropogenic root causes (Fuchs 2009a).

Fig. 10.1 Number of causalities due to flood hazards in the Americas (broken down in four macroregions) from 2010 to 2020 (graph by Grez et al. unpublished)

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Indiscriminate settlement expansion and other predatory land use practices, on the one hand, and poorly designed critical and vital infrastructure as well as vulnerable road networks and residential areas, on the other hand, decisively contribute to damage causation (Fuchs et al. 2017a, b; Basso-Báez et al. 2020). Without tailored risk mitigation strategies, continuing business as usual modes of land use management and with evermore wealth moving into flood-prone areas, exposure, and vulnerability and, ultimately, risk will increase. This may also trigger a vicious maladaptation cycle in numerous urban areas of the world. Evidence is mounting that resorting to traditional flood mitigation strategies and design practices alone could turn out to be inappropriate in many situations. In fact, past attempts to mitigate flood risks by confining the hazard processes to the active channel networks has failed in many cases (Mazzorana et al. 2018a). This strategy, rather than providing or returning more space to the rivers, has progressively narrowed and simplified their courses (Simoni et al. 2017; Scorpio et al. 2018). In parallel, land occupation has progressed rapidly behind engineered levees and riverbanks cutting off flood plains which might have exerted a buffering effect on flood peaks. Unfortunately, even without floods exceeding the discharge capacity of the engineered river sections, inhabited areas have been severely damaged (Mazzorana et al. 2014) and infrastructure has been disrupted (Eidsvig et al. 2017) due to overlooked process behaviours such as structural failures of susceptible protection system elements and unexpected responses at hydraulic bottlenecks. Adopting analogous design practices in the Chilean context may turn out to be particularly unsuitable when considering the remarkable spectrum of process cascades that can unfold in many river systems that are subjected to different and peculiar disturbance regimes. As Mazzorana et al. (2019) pointed out, multiple process cascades severely affect Chilean river systems and result in a large variety of disturbances perturbing their ecosystems and altering their hydro-morphologic regimes leading to extreme impacts on the affected socio-ecological-technological systems (SETS). The acute, neotectonically pre-determined susceptibility to seismic hazards, the widespread volcanic activity, the currently unstoppable glacier retreat, and the continuous exposure to forest fires clearly disturb entire riverine systems and act together triggering and exacerbatating flood hazards. In the light of this remarkable hazard potential and considering the plausible scenario of rapidly increasing exposure in the next decades, taking proactive measures is imperative. Hence, counteracting flood impacts by preventive adaptation and developing effective response mechanisms in the aftermath of disasters will become indispensable capabilities in the context of rapid global change. For certain, this implies an advanced knowledge of the transformative possibilities associated to the available means (i.e. resources) but also the existence of a democratic debate and consensus on the purpose of ends (Daly and Farley 2010). In relation to the former, it is of utmost importance to envisage “how the world could work”, that is imagining alternative urban system designs (i.e. feasible counterfactuals mandated by the laws of nature and current technology) well adapted to the impacts of potentially unfolding flood hazards and the trade-offs between them. Concerning the latter, legitimate decisions about the ranking of the ends should be

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based on cooperative public engagement assuring broad public participation. The decision space should be rationally delimited in terms of intersections between the following dimensions that are important throughout the public participation and decision process (compare Nardini 2005): (1) societal values; (2) legal and economic constraints; and (3) bio-physical and technical possibilities. With respect to the societal values, making the desiderata of the different segments of society explicit is the first cornerstone of a holistic flood risk management approach which considers the urban settings and the river corridors conjointly. Societal values, in this instance, refer to the values and preferences of concerned citizens, their Willingness to Pay (WTP) for flood risk mitigation and their Willingness to Accept (WTA) potential adverse consequences (Hammitt and Graham 1999; Baranzini and Ferro Luzzi 2001). Stakeholder engagement is a precondition for solving complex problems such as flood risk management and managing urban riverscapes (Thaler and Levin-Keitel 2016). Here, we contend that stakeholder contributions, along with broad interdisciplinary inputs both generated by academia and citizen science initiatives, can improve problem understanding across formal and informal knowledge bases and glue together data and theories originating from different disciplines (Leniak et al. 2013; Stauffacher et al. 2008). With respect to the constraints, the legislative landscape can be complex with multiple and sometimes competing requirements. Enabling policies may conflict with obstacles to action and implementation. Resource constraints also merit proper consideration and often economic restrictions form part of the wicked problems to be solved. In our view, however, they may not always constitute unsurmountable hurdles for the realization of the profound transformations leading to successful adaptations. The amount of resources that will be “freed” in the service of important ends (i.e. adaptation and resilience) will crucially depend on the engine of demand fuelled by the actions taken by an engaged society. Chile is, in this sense, a paradigmatic example. After the decade-long neglect of social justice and environmental problems during a dictatorship and enjoying, afterwards, the short-term benefits of the neo-classic economic bubble, a recent revolt attempted to change the political agenda in favour of the legitimate social demands recognizing also the importance of global and local sustainability issues. Those elements that go hand in hand with a highly exposed occupation of the territory, either due to the historical location of various human settlements or as a result of unbridled urban expansion related to real estate projects resulting from deregulation, generate strong pressures within cities or around them. Additionally, these urban areas are exposed to different hazards such as tsunami (Igualt 2017; Martínez et al. 2020), volcanic eruptions and their effects (Lara et al. 2021), and extreme hydrometeorological events that generate floods, mass movements, or river flooding (Rojas et al. 2019). In this chapter one priority is set on the following question of transformative reach: Which bio-physical system changes can be achieved to prospect viable solutions to the underlying wicked flood risk mitigation problems? Overall, the problem-solving process requires a transdisciplinary approach, integrating a broad field of disciplines encompassing the environmental sciences, the hydro-geo-morphological disciplines, engineering, as well as land use planning and

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design. At the very heart of the flood risk mitigation problems to be solved, the emergence of inventive solution patterns capable of explicitly addressing and tackling the underlying systemic contradictions is essential to achieve a successful adaptation of previously vulnerable urban-river systems. In our view, problem solving attempts should consider the philosophical positions of critical rationalism as a convenient starting point (Miller 2006). Here, we adopt David Deutsch’s optimism concept (Deutsch 2011), which is the proposition that all the evils are due to a lack of knowledge and that knowledge is attainable in form of good explanations by the methods of reason and science. Following Deutsch, a good explanation is an explanation that is hard to vary while still accounting for what it purports to account for. But how is knowledge defined in this context and how it is juxtaposed to the related concept of wealth? David Deutsch refers to explanatory knowledge, namely an understanding that explains how things can be caused to happen and why. Wealth is the repertoire of bio-physical transformations that one is capable of causing. Following this line of thought, problem-solving capability is ultimately restricted only by the stream of knowledge we can access and create and by the wealth at our disposal. Therefore, efficiently navigating the realm of the possible, spotting the set of useful bio-physical transformations for the acute risk mitigation problems is pivotal. In fact, Deutsch (2013) states in his pioneering paper on constructor theory, “for almost every such transformation, the story of how it could happen is the story of how knowledge might be created and applied to cause it”. Further, he clarifies that “part of that story is, in almost all cases, the story of how people (intelligent beings) would create that knowledge and why they would retain the proposal to apply it in that way rejecting or amending rival proposals”. Put differently, we are engaged in transforming, via the created knowledge on urban-river corridors, environments, that can become lethal to us (i.e. due to the impacts of complex flood hazards in our case), into resilient ones. Hence, it is self-evident that being capable of rehabilitating flood exposed urban environments where a large portion of the world’s population is living and aspires to thrive into the future is imperative. In Fig. 10.2, we depict how inventive knowledge intervenes in the transformative design of urban-river systems removing the systemic contradictions determining a poor system performance (i.e. low ideality) in case of extreme floods and favouring their transition towards resilience (i.e. high ideality). Considering the remarkable heterogeneity of exposed urban settings, the complexity and connectedness of vital infrastructure systems, effective solutions patterns are not readily available in most cases and their ideation is, in essence, an inventive task. Creativity plays a fundamental role in it and although it cannot always be brought about readily on demand, the generative process can be structured, and enabling methods can be used to facilitate the creative processes. Therefore, we base our technical considerations on the solid background provided by the Theory of Inventive Problem Solving (TRIZ-TIPS, compare Altshuller 1984; Terninko et al. 1998;

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Fig. 10.2 Key role of inventive knowledge in the transformative design of urban-river systems

Orloff 2006; Zobel 2018a, b). TRIZ is the acronym of the Russian teorya resheniya izobretatelskikh zadatch and constitutes a coherent and rich methodological framework aimed at highlighting, prioritizing, and eventually solving the systemic physical and technical contradictions residing at the core of the inventive task, be it the ideation and development of a new product or the substantial enhancement of an existing one (Zobel and Hartmann 2016; Petrov 2019). In essence, it teaches us how to uncompromisingly define the Ideal Final Result (IFR) of the inventive endeavour and provides powerful tools to solve the aforementioned contradictions as a basis for the elaboration of specific novel solutions (Altshuller 1984; Petrov 2019; Zobel 2019). Hence, in this chapter, we present a structured problem-solving approach which aims at supporting a formed Problem Setting and Solving Team (PSST) in conjecturing valuable solutions and in selecting the best one depending on how well it approximates the IFR. Although we are convinced that the inventiveness of the proposed solutions to the identified risk mitigation problems resides in its capacity to deploy useful functions and effects while eliminating harmful ones, the planning and decision-making contexts are much broader than the embedded inventive tasks. They should create the required space for innovation allocating the means at disposal in the attempt to bring about beneficial system transformations in the service of the flood risk mitigation objectives stated as an urgent societal need. For this reason, in the second part of this chapter we elucidate how such participatory planning and decision-making processes can be launched and “structured” to enhance the chances of enabling the required system transformations. The final section of this chapter is devoted to the description of the spatial planning process in a risk management context as it is common practice in Chile. Herein, we contextualize the potential application of the proposed planning approach.

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10.2 Materials and Methods 10.2.1 Inventive Knowledge The Ideal Final Result (IFR) Figure 10.2 purports the idea that inventive knowledge is, alongside with the application of specific pattern of invention, a key ingredient in the transformations aimed at leveraging the current urban-river system from low to high ideality. Thereby maximum ideality means the absence of any adverse or harmful function or effect, and the deployment of useful ones virtually at no cost. A first attempt to render more amenable the theoretical concept of the IFR for the solution of flood risk mitigation problems can be done by tentatively stating the IFR as follows: The urban-river system by itself deploys the required protection functions which directly arise from its topographical, geometrical, and structural configuration. Synergistic mitigation effects such as optimally deflecting the flow away from vulnerable sectors and/or conveying the flow through flood-proof areas, wherein the valued elements of the built environments are either absent or exhibit total flood resistance, should be attained. In the extreme, the flow field is altered by the urban setting itself as to be innocuous in susceptible areas. Put another way, the urban structure actively controls process magnitudes and intensities thereby generating flood resilience. To reiterate, the IFR should function as the framework for the design process. It should be approximated, but it can never be attained completely. It stimulates the PSST to think the “unthinkable” and to explore novel solution pathways. With respect to an existing urban setting, the PSST may consider redesigning buildings and other constructive elements to deploy the required functional spectrum and the design of new constructions should lead to the emergence of synergistic flood mitigation effects. From a systems perspective, the approximation of the IFR is necessarily associated with (1) cost-efficient and effective risk mitigation measures, (2) reliability and maintainability, and (3) technical quality for optimal function deployment. This IFR description does not mandate that existing flood protection systems should not be intervened. On the contrary, through an in-depth system analysis, functional deficiencies and hydraulic bottlenecks should be spotted and the IFR notion should be used for redesign and adaptation to achieve the desired protection level. The history of numerous technical inventions attests that the creative mind is used to operate, often intuitively, with the IFR concept, defining the desired performance with absolute clarity, identifying the technical and physical contradictions to be solved, and the available resources to be accessed and used. The existing building stock, rather than being viewed as a hindrance only, can be considered a resource since its redesign and adaptation may contribute to flood risk mitigation. Complementing the redesigned building stock with temporary protection measures could

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entail even higher protection levels. Taking this perspective, the urban areas can become the focus of the design efforts following the general trend of function integration observed in several technical innovations. Hence, the exposed system may shift from exhibiting a save to fail performance to a more resilient system also under worsening climate change conditions. Even more strategically, if urban-river corridors were to be redesigned to approximate the IFR, the PSST could further evaluate how engineered river segments (i.e. featuring mostly grey protection infrastructure) could be rehabilitated improving their ecological functionality and in how far floodplains could be restored to limit the humanity’s ecological footprint. Through reliable early warning systems, the number of people at risk could be significantly reduced. Local structural protection could be integrated into the existing building stock. The IFR concept maintains its importance also after the occurrence of a major disaster. The reconstruction phase is a crucial moment when steering towards ideal solutions becomes essential. Too often the damaged urban environments have been reconstructed maintaining the same systemic weaknesses without taking advantage of the unique window of opportunity to redesign the system making it more resilient. Hereafter, we list and describe seven functions/effects that can leverage the urban-river system towards a higher ideality degree in the context of flood mitigation: . F1 Impact disappearance The configuration of the system interacts with the flow in such a way that any interaction of flow and vulnerable structures/areas is avoided. Poignantly stated, the flow field disappears or is decoupled from the susceptible areas. . F2 Self-protection The physical vulnerability of the system attributed to the flood impacts tends to zero. The structural stability as well as serviceability are preserved during and after the flood. Potential harms for people inside the structures and damages to the equipment are avoided. Hence, the structures are impermeable and the building envelopes are not deteriorated. . F3 Safe exit People can safely leave the impacted areas or buildings and reach unexposed areas specifically equipped for the emergency regime. . F4 Useful and controlled failure In case neither F1 nor F2 can be attained, the system has to fail in such a way that the structures can deploy, before and after failure, additional flood mitigation effects as, for example, forming an obstacle that conveniently changes the flow patterns. If this beneficial effect cannot be achieved, the mitigation effect provided by the remaining elements of the built environment is not lowered. . F5 Synergistic mitigation Each element of the exposed system concurs directly or is conducive to a higher overall risk mitigation performance by buffering the impacts.

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. F6 Functional integrality The elements of the built environment are designed to ease the rapid installation of temporal protection measures and to facilitate emergency operations by guarantying sufficient functionality within the system for an optimal response. . F7 Comprehensive monitoring for early warning and action Before, during, and after, the flood event (i.e. processes, impacts, and responses) is monitored to generate crucial knowledge to enhance intervention and early action and to adapt the system accordingly.

10.2.2 Modelling Processes, Impacts, and Responses in the Urban-River System The IFR is a valuable mental construct to direct the focus of the PSST towards the required set of functions that should be brought about. Although a careful elaboration of the IFR may hint at concrete risk mitigation strategies during the earliest problemsolving phases, it is methodologically advisable to carry out a thorough system analysis to understand how flood processes can unfold, which process-structure interactions may take place and which impacts and responses may occur. To this end, it is convenient to setup a chain of models, encompassing hazard process, impact, and response models. It has to be clarified at the outset that the modelling results should never be uncritically accepted. They represent, at best, well-reasoned conjectures with computed scenarios capable of capturing relevant aspects of real events. In this respect, Popper reminds us that in our infinite ignorance we are all equal. In fact, both epistemic and aleatory uncertainties are pervasive in natural hazard risk assessment. This circumstance prevents a conclusive verification of the elaborated scenarios and, rather, leaves open the possibility of their refutation by actual evidence. This may be sobering at first, but the modelling exercise is the best way to structure the available knowledge for the subsequent system design which should also consider, in the light of the aforementioned uncertainties, the importance of the system’s resilience. Hereafter, we briefly outline the qualitative and quantitative models that should be setup to gain the necessary understanding of how floods unfold and interact with elements of the built environment.

10.2.2.1

Hazard Process Models

Qualitative Process Model This model aims at the determination of consistent process scenarios identifying the plausible process chains along the stream network in the hydrologic basin (i.e. contributive dynamics in the river basin and probable process types throughout the

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stream network). Mazzorana et al. (2012) give a thorough account on how to develop consistent flood hazard scenarios in mountain streams.

Quantitative Process Model Through this model process intensities in the response system (i.e. urban-river corridor domain) are determined. The response system includes the manmade and natural environment susceptible to be impacted by flood dynamics. Mazzorana et al. (2014) provide both a detailed description and a case study application of all necessary modelling steps to comprehensively assess the process variables considering their magnitude and their intensity both with respect to their temporal dynamics and spatial distribution through the combined use of mathematical and physical models as well as by documenting and reconstructing past events. Endowed with the quantified process variables (i.e. both with respect to time and space), the analysis of the potential consequences (i.e. impacts, responses and losses) can be carried out. Normally the considered process variables are the relevant flow variables (i.e. flow depths and flow velocities), the normal and shear stresses exerted at the solid boundaries of the flow field and sediment erosion and deposition.

10.2.2.2

Impact Models

Model of the Potentially Impacted Environment (Both Manmade and Natural) The topographical, geotechnical, and structural characteristics of the environment and its convenient parametrization (i.e. digital terrain model, geometrical model of the built environment, technical, and structural characteristics of the exposed building elements and geotechnical characteristics of the soil) are gathered in this model.

Qualitative Impact Model Based on the system characterization resulting from the application of the previously outlined models, a careful identification of the process-structure interfaces follows. These interfaces are the geometrical interfaces where the quantified process variables are “translated” into quantified impact variables (i.e. flow depths and velocities into pressure distributions on the building envelopes). With respect to the characterization of the impact processes, a qualitative impact model has to be assumed. The impact patterns proposed by Suda et al. (2012), for example, provide excellent guidance to determine the qualitative loading conditions for each impacted building (compare Fig. 10.3).

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Fig. 10.3 Example of debris flow loading conditions for an exposed residential building (from Mazzorana et al. 2014). Panel a: inundation pattern (flow depths) at the exposed building location; Panel b: detailed representation of the building envelope (i.e. rolled out in anticlockwise sense) with the exact location of the openings and the process magnitudes (i.e. flow depths and deposition layer thickness)

Quantitative Impact Model The assumed loading configurations in the previous modelling step are now quantitatively substantiated. This entails calculating the impact variables (i.e. based on the quantified process variables), projecting them from the process-structure interfaces onto the building envelope and imposing changes to the geotechnical characteristics (i.e. soil bearing capacity or new geometrical support constraints for the considered structures). These impact variables are essentially acting forces, pressures, and shear stresses exerted by the flow on the building envelopes (compare Suda et al. 2012 for technical details).

10.2.2.3

Response Models

Qualitative Response Model Endowed with the quantified impact variables, with the model of the built environment (i.e. characterized by its geometry, topography and described according to its structural and technical characteristics), the next qualitative step consists in specifying all relevant mechanisms of physical attack and the affected structural elements (i.e. supporting structure, building envelopes, and interiors). In other words, the set of triggers of the physical effects that might lead to damage generation should be identified. Among these physical attack mechanisms, we can distinguish: Friction

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(between the exposed parts of the building envelope and the flow) leading to abrasion of the exposed walls, vorticity occurring in the flow field region surrounding the building envelope, thereby laying off the building’s foundations through scouring, changed structural constraints due to erosion or deposition, drag and lift effects that could alter the static equilibrium of the buildings, flow field extensions into the interior of the building accompanied by solid material intrusion and, the buildup of internal forces and stress resultants deforming the supporting structure.

Quantitative Response Model This modelling phase is devoted to the quantification of the physical effects with damage generation potential. This can be done via a set of verifications following engineering state of the art. First and foremost, one should analyse the potential loss of external stability. Under the physical attack in form of drag, lift and buoyancy a structure can slide, tilt, or float. Furthermore, the potential loss of internal stability should be carefully assessed. The strain and deformation of the structural elements induced by the stress resultants should be quantified and compared with the respective admissible values. It is also known that shear on the exposed building envelope can cause severe damages through abrasion. More generally, the potential loss of global stability and the associated geotechnical failure mechanisms deserve a thorough analysis. In fact, these mechanisms can extend the reach of potential damages beyond the areas exposed to the flow. Undercutting can cause the loss of the global stability of hillslopes where buildings can be located. All these analytic steps can be performed by tailored computational software codes (i.e. finite element models for structural and geotechnical analysis). One challenge, however, consists in accurately determining the material properties of the buildings and the soil. In fact, material conditions of the exposed elements can deteriorate over time and soil parameters can change as well. A recent case study performed in Chaitén (Patagonia, Chile) highlighted the complexity of this task (Basso-Báez et al. 2020).

10.2.2.4

Damage Accounting Task

Based on the quantitative assessments carried out in the previous modelling step, plausible damage patterns should be deduced as a basis for expected loss estimation. These damage patterns should account for (i) loss of structural elements, (ii) destruction of parts of the building envelope, (iii) further deterioration of the building materials entailing a diminished resistance for future impacts, (iv) loss of serviceability and functionality, (v) wetting of the interiors of the building, and (vi) damages to equipment and household items. In Fig. 10.4, the interrelations between the described modelling steps are shown and summarized. It is worth noting that although computational codes support the

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a

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Fig. 10.4 Model chain and interrelations between the previously described qualitative and quantitative models. Adapted from Grez et al. (unpublished)

various quantitative analytic steps, several quantitative steps require careful judgement and rigorous inspection of the potentially affected sites by trained experts. Hence, the procedure can neither be fully automatized nor be carried out in an unsupervised manner.

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10.3 Exploring the Flood Risk of the Transient Urban-River Systems It is widely acknowledged that risk reduction effects can only be partially achieved by hazard process control only. The exposure of the built environment and its vulnerability are key factors to be considered. Vulnerability, as Mazzorana et al. (2018a, b, c) pointed out, is largely determined be the society’s capacity (1) to resist impacts and to respond to the disaster, (2) to recover from past events and cope with new settings, (3) to reconfigure the system and enhance its ability to function even if heavily perturbed by natural hazards, and (4) to reduce to a large extent hazard impacts implementing active and passive mitigation measures (compare also Ballesteros Cánovas et al. 2016). The quality of the design of the exposed system contributes to mitigate all listed determinants of vulnerability. In fact, impact mitigation and disaster response are directly related to the system’s topographical, structural, and functional characteristics. Since post-disaster recovery depends on the legacies of the disaster (i.e. residual functionality of the impacted system), a system design that would have contributed to lessen the adverse effects in the first place would have represented a more convenient starting point for the new adaptation cycle. A required reconfiguration of the impacted system is facilitated by previous system design efforts to approximate the IFR since existing, albeit still insufficient, functional characteristics can be potentiated and further inadequacies can be spotted more easily using the IFR as a focusing lens. It is of utmost importance to monitor the system’s evolution along the “risk cycle”. This may be conveniently done considering four temporal hotspots or planning management milestones (PMMs), where specific actions should be taken to reinforce the flood risk mitigation performance of the implemented innovative urban-river system design: (1) PMM1—adaptation evaluation, (2) PMM2—prevention evaluation, (3) PMM3—resistance and response evaluation, and (4) PMM4—recovery evaluation. Preparedness is essentially the capacity to plan and act effectively and efficiently along the system’s evolution trajectory. In this context, it is worth reiterating the importance of innovation as a central building block of this dynamic notion of risk mitigation. In fact, system design guided by the IFR significantly contributes to bring about the compound beneficial effects of prevention, resistance, and response by putting into practice the inventive knowledge elements.

10.3.1 Essential TRIZ-TIPS Elements for Flood Risk Mitigation 10.3.1.1

Contradiction Terminology

Three fundamental types of contradictions are distinguished (Zobel 2019): (i) the technical-economical contradiction (TEC), (ii) the technical–technological contradiction (TTC), and the technical-physical contradiction (TPC).

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The TEC which underpins an inventive problem can be stated strictly as follows: The expected losses must be reduced, but the value of the system at risk is increasing. Another technical-economical contradiction is closely related to increasing budget restrictions: The costs for the planned protection system must be reduced but they cannot be reduced (to provide a determined protection standard). The PSST is hence forced to ideate both cheaper and more performant protection systems. The TTC considers the specificities of the system to be enhanced. Nevertheless, it is helpful to provide the following general formulation which can be adapted and refined according to the concrete inventive problem: The system should be changed to deploy the required flood mitigation functions and effects, but it is deemed as impossible to change the system because any change would worsen other functions (i.e. connectivity, aesthetic and commercial aspects, etc.). Or perhaps more tangibly: The buildings should be flood-proof, but they should be designed according to longestablished constructive standards and traditional building styles. The TPC is stated in general terms as follows: The system must exhibit mutually exclusive properties, functions, and objects or operate under mutually exclusive conditions. For example, river dynamics need to spatially unfold on urbanized areas and the city needs to expand into the fluvial domain. In the next section, we present the separation principles to tackle the TPCs.

10.3.1.2

Separation Principles

As outlined by Zobel (2019) physical contradictions imply that specific conditions have to coexist with their opposites within the system. This can concern objects, functions, and properties. Objects can equally exhibit positive and negative properties or induce equally positive or negative effects. For example, during extreme flood events, grey protection infrastructure (i.e. the considered object) is essential but during low flows its presence might inhibit hydro-morphological and ecosystem dynamics. Functions can also be associated with both positive and negative effects. The mobility enabling function provided by an optimized road network design for periods without flood events might induce detrimental distributary effects and increase the impacts in the urbanized areas during inundation. Properties as well can exert both positive and negative impacts. The properties of certain construction materials (i.e. lightweight materials) can be appropriate for housing quality in normal circumstances, but these materials may exhibit low resistance to flood impacts and increase susceptibility. Separating as needed the positive (i.e. desired) from the negative (i.e. detrimental) aspects becomes the essential design task and is crucial for the problem-solving process.

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. Spatial separation The conflicting objects, functions, and properties have to be spatially separated in a way that the desired positive effect (activity, dynamics) occurs in a determined spatial domain, whereas the rest of the system remains unaffected. The overall aim is to separate areas characterized by relevant process intensities from areas at risk (i.e. with a relevant accumulation of values at risk). As a corollary, it can be stated that adverse effects should be concentrated on areas with very low vulnerability. . Temporal separation The conflicting objects, functions, and properties have to be temporally separated in a way that the desired positive effect (activity, dynamics) occurs in a determined time window. The functions of the system have to be decoupled in time to avoid conflicting conditions. The overall aim is to separate in time the intensity maxima of liquid discharge and sediment transport on the process side and to displace movable objects at risk from endangered areas during the critical time intervals within the extreme event duration (e.g. by evacuating people at risk). . Separation by change of state of operation The separation of contradictory requirements has to be achieved by changing the state of operation or modifying the operating conditions which would create when synchronously applied detrimental effects. In the bioengineering practice, for example, it is preferred to grow an elastic vegetation cover on river banks rather than favour large and inelastic tree species. During floods, the elastic vegetation is bent and prostrated onto the ground thereby limiting erosion. Modifying system configurations during the critical phases within the event duration (e.g. by avoiding bridge clogging) could also lead to relevant mitigation effects. . Separation within the system and its parts Subsystems can deploy conflicting functions without limiting the overall system performance. It is possible to create subsystems with a lower degree of susceptibility while the residual parts of the system remain unaffected (e.g. local structural protection for individual buildings). 10.3.1.3

Principles to Solve TTCs

A set of inventive principles to solve TTCs are part of the TRIZ-TIPS methodological toolkit (Petrov 2019). These principles can be systematized and ordered according to their generality and degree of abstraction. We closely adhere to the systematization proposed by Zobel (1991) and illustrate the conceptual usefulness of each principle for flood risk mitigation.

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Principle 1: Segmentation (contrary principle: merging) The affected system can be subdivided (i.e. segmented) into different subsystems deploying specific functions to achieve the required protection standards. For example, part of the road network can be modified to conveniently divert the flow away from susceptible areas (ROADAPT consortium 2015). The floodplain can be segmented to optimally buffer flow intensities and store peak flood water volumes. Local flood protection measures can be applied to a group of buildings (i.e. a convenient merge) rather than to single ones (Suda et al. 2012), increasing the cost-effectiveness of the protection investments. Existing boundaries and favourable topography can serve as ideal locations to install temporary protection measures (i.e. modular wall elements, mobile flood protection tube systems) thereby achieving the desired spatial separation (e.g. AquaDam 2017). Existing structural elements like peripheral property walls, but also the building envelopes themselves, can be modified to achieve an effective separation in space. Principle 2: Taking out (remove) Wood and sediment-laden flows in rivers or flows transporting other objects can be very destructive when interacting with the built environment (Ruiz-Villanueva et al. 2019). Mitigating their destructive power can be achieved by entrapping the transported solid volumes (i.e. removing the moving objects from the flow) at convenient locations thereby avoiding collisions with susceptible structures (Mazzorana et al. 2018b). Efficient evacuation of inundated and susceptible areas is another prominent example (Lumbroso and Vinet 2011). Principle 3: Local quality/creating optimal conditions/adapting The envelope of exposed buildings can be adapted to decrease the impacts (Egli 2005; Holub et al. 2012; FEMA 2014; Kreibich et al. 2015). The configuration of the building openings can be changed to avoid material intrusion (Mazzorana et al. 2014). The terrain surrounding a building block can be modified to influence the flow patterns. Principle 4: Combination/multi-functionality Constructions serving main objectives different from flood protection can be designed (adapted) to deploy this effect as well. Road infrastructure (i.e. cycling routes, bypass roads) on artificially elevated terrain can conveniently border entire urban sectors acting as relocated dams. Critical sectors can additionally be protected through temporary protection measures (Attems et al. 2020; Bischiniotis et al. 2020), and open sections can be closed by gates or other temporary closing mechanisms. The combination principle is very powerful, and parks and wetlands are particularly suitable to accomplish an optimal mitigation performance. Principle 5: Prevention This is a very general principle that should inspire all land use decisions and regulations as well as funding policies. Perhaps it is worth reiterating that preventive actions

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to enhance all phases of the risk management cycle are deemed essential (compare Eth Zürich—Swiss Virtual Campus—NAHRIS; Bischof and Lienert 2007; Carter 1991; Alexander 2000; Kienholz et al. 2004; Fuchs 2009b; Bubeck et al. 2016). This principle has already been largely debated in academia, so we do not discuss it further (Di Baldassarre et al. 2013). Principle 6: Inversion (“Viewed the other way around”)/transform “harmful” into “useful” It is widely acknowledged that, without a proper release of sediments either from their sources or from their intermediate deposits and without maintaining sediment connectivity throughout the stream network, the reactivation of hydro-morphological and associated ecological functionalities is physically unfeasible (Palmer et al. 2005; Simoni et al. 2017). The consolidation strategy that inspired torrent control for centuries largely inhibits sediment connectivity (Dell’Agnese et al. 2013). Currently, the inverse strategy is being adopted by attempting to release sediments in a controlled manner through open check dams with an enhanced dosing function (Armanini and Larcher 2001). Principle 7: Dynamization and buffering Overland flow velocities can be influenced by modifying the floodplain roughness and by dissipating energy at obstacles. Certain exposed objects can be modified to maximize their energy dissipation effect “sheltering” other more susceptible ones (Sturm et al. 2018a). Principle 8: Setup models and prototypes Process behaviour and flow structure interaction can be increasingly modelled by accurate computational tools and flood protection design can greatly benefit from it (Begnudelli and Rosatti 2011). The effects of different scenarios and spatial patterns can be studied, and their visualization can support the planning and design process suggesting promising ideas to work on. Once a protection system is planned, its effectiveness can be analysed by means of physical models (i.e. scaled versions of the prototype) and important insights can be gained to refine its design (Mazzorana et al. 2020; Santibañez et al. 2021). Recently scaled alluvial fan models have been employed mirroring the interaction of the flow with the residential buildings (Sturm et al. 2018b). Evaluating the efficacy of the constructive adaptation of the built environment using scaled models before implementation could significantly increase the reliability of such pioneering flood risk mitigation projects. Principle 9: Transition to higher forms and dimensions from geometrical, technological, and physical viewpoints This principle can constitute a phenomenal source of inspiration for the design of flood risk mitigation measures. The conception of “superior designs” can lead to a better approximation of the IFR. Such designs can lead the flow through subterraneous paths rendering previously endangered areas safe (Ito 2020). Thinking

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the other way around, roads can be constructed below dangerous watercourses (McFadden 2021) or rivers can be super-elevated such as in the case of the Pont Canal Du Sart (Province of Hainaut, Wallonia, Belgium). In other situations, part of the road network can be super-elevated guarantying sufficient connectivity and another part of the road network can be redesigned to function as conveyor channels. Combinations of these concepts are possible. Principle 10: Allow for the “impermissible” This principle suggests mitigation concepts based on a “rational sacrifice”, i.e. the controlled flooding of uninhabited areas to better protect the valuable assets at risk. Of course, fair compensation is due to the landowners of the “sacrificed” areas. Sometimes and only in particular cases, “cheap frailty” instead of “costly longevity” can be part of the solution. Imagine a bridge that would fail anyway under the impact of an extreme flood event. In such cases, counting on the failure of a cheap structure could save finances for the reconstruction process. On the contrary, a more resistant structure could get clogged by large wood rendering the unfolding of the flooding process less predictable. In addition to the above-described principles of high generality, ancillary principles of lower generality have been identified which can also be taken as a source of inspiration for the inventive design tasks. For example, the principle of nesting could suggest inserting waterproof modules in a containing frame structure. The principle of asymmetry is another example worth mentioning. Roads can be built asymmetrically to reserve specific lanes for vital connectivity during floods sacrificing the other ones for the conveyance of floodwaters.

10.3.2 The Participatory Decision-Making Process Throughout the urban-river management process, special attention has to be devoted the participatory decision-making process to ensure acceptance and success of the transformative endeavour (Connick and Innes 2003). As stressed by Carr (2015) two basic tenets have to be considered: (1) structured process and (2) consistency of the decision steps throughout the process. We add to this list (3) the constant focus on tackling the systemic contradictions via the inventive knowledge outlined in the previous subsections. Every successful process with public participation (PP) starts with preparation—from setting clear process goals, defining the area and scope of work, and identifying the core team (PSST) and all the participants to be involved in the process (Bisjak et al. 2014a; 2014b). Mazzorana et al. (2018c) report the clusters and associated sub-clusters of individuals who should be actively engaged in the urban-river management process. Once the PP has successfully started, Mazzorana et al. (2018c) suggest the following procedure as a practical way to ensure support, monitoring and rationalization. Based on previous proposal by Nardini (2005), which originally featured six sequential procedural steps, this proposal presents seven steps with iterations (and features a consistency matrix to account for its coherent application).

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The procedural steps are: (1) Diagnosis: This initial step entails a refined scoping based on an integrated urban-river corridor characterization (i.e. from geomorphological, biotic, economic, and social perspectives) and other concurring processes affecting the urban-river corridor. The results of the diagnosis will be used to clearly define and delimit the problems and opportunities and provide a provisional definition of the systemic contradictions to be addressed. Here, it is worth reiterating the paramount importance of understanding how on the one hand the consuetudinary decision-making procedures and management styles and on the other hand the existing stakeholder relations and the legal landscape infringe upon the prospects of devising and implementing transformative flood risk mitigation strategies. With reference to the Chilean setting, we will discuss in depth these issues in the final subsection of this chapter. Methodologically this provisional diagnosis represents a precursor for the inventive problem-solving tasks. Here, we stress that the emerging contradiction statements are necessarily preliminary. Their careful dissection and refined scrutiny and formulation according to the introduced contradiction terminology culminate the problem setting phase and constitute the starting point for inventive problem solving. (2) Vision and objectives: This step entails articulating a common vision and the associated operational target system with clearly identified and measurable objectives (Wiek and Iwaniec 2013; Iwaniec et al. 2014). The formulated vision should describe an urban-river system configuration as a “model to be approximated”. Here, we underline the importance that also, on this more general level, a sort of Ideal Final Vision (IFV) should be elaborated. It should be a grand but still attainable vision, intentionally elevated from mediocracy. By the word “attainable”, here we arrogate that the implied transformations should be judged possible from a bio-physical standpoint given sufficient “wealth” (i.e. following Deutsch’s notion). These concepts merit further discussion. Too often externally imposed budget restrictions may constitute insurmountable obstacles preventing from the outset tangible enhancements and frustrating legitimate societal aspirations for a better world. More than ever, governmental choices related to budget and funding have to be publicly debated and openly questioned in case of perceived mismatches. As outlined in the introduction, too much is at stake to uncritically accept allocation decisions in favour of particular and privileged interest groups. For these interrelated reasons, we explicitly consider knowledge and, by extension, also wealth as variables throughout the transformation process. The resulting model should be revised during successive iterations of the procedure. Thus, it is fundamental that a “neutral space” for discussion and confrontation on contested issues be provided before delving into the problem solving and decision-making steps. (3) Ideation and creation space for the PSST with respect to the domain of flood risk mitigation in the urban-river setting (i.e. here we assume that the Grand Vision

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contemplates this important goal). Among the goals set in the Grand Vision, this chapter focuses in particular on flood risk mitigation within urban-river systems. Here, we propose a procedure in form of a metaheuristic to support the creative processes potentially leading to effectively achieving the abovementioned goal. In this context, the term metaheuristic means a higher-level procedure guiding the problem-solving activities performed by the PSST through system analysis and providing and optimal support for the subsequent system synthesis. Following the routines suggested by the metaheuristic, the PSST is aided in accessing the relevant information about the unfolding of the process chain leading to adverse consequences in the urban-river system, in spotting the systemic contradictions to be addressed, in making the broad resource spectrum visible for problem solving, and in providing specific principles/heuristics to facilitate the solution of the identified contradictions. The first step of the procedure consists in the formulation of the IFR, considering the seven functions/effects, succinctly as follows: preventive transformation and adaptation of the “System X” to achieve an ideal degree of flood resilience by changing the way the flood processes and the system structures interact, thereby avoiding harm to people and minimizing structural damage. Pushing this formulation of the IFR to the limit we could state that the processes interact with the system but the impacts mostly disappear. Hence, one can imagine that the “System X” is fully adapted to flood events, minimizing exposure (i.e. conveniently deploying the effects F1 to F7 ). Attempting to approximate this IFR, the next step of the metaheuristic consists in synoptically considering the transdisciplinary process understanding generated in the diagnostic step, in particular the insights from hydro-morphological analysis to delimit and characterize the river corridor segment which (partially) contains the “System X”. Hydro-morphological analysis frameworks are particularly suitable for this purpose providing clear guidelines on how to delimit the river corridor based on its confinement and how to analyse it considering multiple spatial and temporal scales to gain a profound understanding of its past evolution, its current and potential future adjustments (Habersack and Piegay 2008; Gurnell et al. 2015; Brierley and Fryirs 2016). Active channels, paleo-channels, ongoing morphological changes, evolving fluvial forms, eroded or abandoned terraces, floodplains, and interfering artificial elements can be identified and synoptically visualized (Rinaldi et al. 2016) to aid the PSST in accomplishing the inventive problem-solving tasks. In particular, insights are gained on alternative river corridor configurations which could be potentially compatible with targeted interventions (i.e. which paleo-channels could be re-activated or how could lateral mobility change). Instead of being funnelled towards a narrow search space, the PSST gains a broad perspective on the degrees of freedom available and the potential for rehabilitation and re-naturalization of the river network. The resource spectrum provided by the river system itself in terms of beneficial adjustments is made apparent. Regarding the TRIZ-TIPS body of thought, one could ask: What if the river was capable of adjusting its configuration to

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solve the flood resilience problem for the “System X” by itself? Which minimal interventions would be needed to favour these evolutionary trajectories? Next, careful consideration is devoted to the systematic assessment of hazard impact and the responses of the built environment due to process-structure interactions. In this respect, indications to setup the associated qualitative and quantitative models have been already provided. As a reminder, building on a solid hydro-morphological baseline, the application of process models delivers the relevant flood scenarios. The required result for the inventive problem-solving process is the delimitation of the process propagation domain and its spatial complement, the domain of provisionally non-exposed areas. It is advisable to base this delimitation on relevant flood scenarios including an expert-based definition of a worst-case scenario. It is also crucial to perform sensitivity analyses concerning the uncertain model parametrization and to explore how variations of the boundary conditions affect system response (i.e. inundated areas, spatial patterns of process magnitudes). Does the inundation dynamics present tipping points or unexpected singularities? Of paramount importance are the quantified flow variables and the induced morphological changes. Endowed with the obtained modelling results, the exposed urban area, the exposed infrastructure network, and the exposed domain subjected to other land uses should be spatially demarked. The provisionally not exposed domain can be classified analogously: the not exposed urban domain, the not exposed infrastructure network, and the not exposed areas devoted to other land uses. In particular, the velocity maps are important to represent relevant aspects of the expected distributary behaviour to the flow. In the case of biphasic flows with LW transport, the entrainment, transport, entrapment, and deposition of both the inorganic and organic solid fractions should be assessed. The next step consists in an expert-based assessment of the relevant impacts and their spatial representations and the associated damage generating mechanisms for a rational prioritization of the inventive problem-solving efforts. The question “What is not allowed to happen and which damage mechanisms should be avoided?” can be employed as a useful thinking aid for this purpose. The PSST already recognizes in the early planning/design stages that inventive solutions rely on favouring potentially advantageous flow patterns to effectively mitigate the impacts. Spotting how to conveniently divert the flow by reshaping the terrain or constructing “multifunctional henges and ditches” or super-elevated routes with flood protection/deflection functions can contribute to the approximation of the IFR avoiding the impacts on the susceptible sectors of the built environment. Moreover, useful ideas can be generated on how to effectively buffer the process intensities thereby mitigating the effects of process-structure interaction. The next step of this metaheuristic is devoted to the analysis of all possible physical, spatial, temporal, fields, material, and immaterial resources for an optimal application of the IRF (i.e. in TRIZ-TIPS almost everything can be seen as a potential resource). In this phase, the PSST should go beyond the assessment of available space for hazard mitigation. Traditional methods of dosing transported solid material (i.e. inorganic sediment and LW) or smoothing in space and time the peak flow intensity, could integrate the overall

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flood risk mitigation strategy. One could also consider designing a controlled failure of sacrificial elements of the built environment to create an ad hoc protection effect (i.e. a collapsed object diverts the flow from densely inhabited sectors, a road is used as a conveyor channel saving the surrounding areas). A resource checklist could be very helpful to screen all available resources systematically. Thereafter, the admissible system changes have to be defined. It should be noted that in this metaheuristic the restriction of the search space for feasible risk mitigation solutions is after the definition of the IFR which we consider an important methodological improvement. Next, the inventive solution concepts have to be elaborated in coherency with the IFR, operating with the principles and with the examples of inventive solution patterns presented in the previous section. Each elaborated solution is then evaluated aiming at identifying its quality gaps concerning the IFR. Which useful properties, functions and effects have been enhanced? Which harmful properties, functions, or effects have been reduced, eliminated, or turned into useful ones? Which contradictions (TECs, TTCs, and TPCs) have been solved and which ones persist? What has been worsened? If the answers to these questions are unsatisfactory, the PSST should carefully revise their flood risk mitigation solutions. This may entail restarting from scratch in a sort of Schumpeterian creative destruction (restructuring) approaching the flood risk mitigation problems from different angles and finding new modes of explanation and interpretation remembering Deutsch’s statement that “all evils are due to a lack of knowledge” which has not been generated in the specific context so far. It is crucial, that only very promising flood risk mitigation solutions (i.e. with respect to the approximation of the IFR) be considered thereby enriching the decision space (i.e. next step in the participatory decision-making process). Decision Space: The elaborated designs for flood risk mitigation become part of viable strategies constituting the decision options and management alternatives. In the light of the problem-solving reach of the proposed strategies, the existing constraints (e.g. resources and legislative restrictions) are carefully considered. Evaluation: Methodologically it is decisive not to stifle the elaborated risk mitigation strategies to soon and without meticulously documenting the reasons and motivations for the rejection, which may clearly indicate where exactly the problem-solving inabilities reside and what regrets have to be accepted. Only after these detailed considerations, the solutions are ranked for final selection. Fine-tuning: In this step, the adopted solutions are refined through a detailed design. To reactivate the hydro-morphological and ecological system dynamics, while keeping the risk acceptable and reducing cost flows over the system’s life cycle to sustainable levels, fine-tuning entails an accurate determination of the construction types employed (i.e. green and grey infrastructures) and their dimensions according to the prescribed safety factors (Gulvanessian 2009) and to the required hydrodynamic functionalities (see Mazzorana and Fuchs 2010; Mazzorana et al. 2014). Operationally, an implementation plan can be used to define the necessary tools, time schedule, responsibilities, and resources. Supervised implementation: In this step, solutions are implemented according to the elaborated implementation plan and decisively contribute to create and

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shape the new river corridor. Following a previously elaborated monitoring plan, specific activities are carried out to gather relevant knowledge about ongoing system evolutions and to eventually detect and evaluate unforeseen system changes and intervene, if necessary, with corrective measures which result from a new process iteration. As mentioned, one essential innovative feature of the proposed procedure is the conception of a consistency matrix. The consistency matrix allows tracing all decision-relevant knowledge and information throughout the participatory decisionmaking process. This knowledge should be anchored in an ideal combination of (1) logically structured textual statements (i.e. internally consistent propositions related to the river corridor vision; definition of the goals and objectives); (2) quantified processes and evaluated system states (i.e. hazard intensities; river quality indicator values, hydro-morphological indicator values); and (3) visualized scenarios (i.e. spatialized risk maps; maps showing the beneficial and adverse effects of management alternatives; rendered scenarios of future river corridor configurations). These three forms of knowledge representation complement each other and provide for each key step a consistent pool of textual strings of argumentation (TA) and of quantified knowledge elements, which are processed through a suitable computational architecture (CA) and visualized through an appropriated set of visualization tools (VT, such as GIS, rendering instruments, and sketches). Formative scenario analysis methods support the corroboration of knowledge in the TA domain (Scholz and Tietje 2002; Tietje 2005; Mazzorana et al. 2009). Following the topology proposed by Ducot and Lubben (1980), scenarios can be classified according to (1) their causality; (2) their normative or descriptive nature; and (3) their temporal and spatial dimensions. To make quantified knowledge elements accessible for the decision-making process, we suggest the conceptual scheme of a CA which adopts a compact set of key objectives (i.e. risks, disturbances, risks, costs, natural value, and externalities) to be used as a coherent target system in river corridor management. Visualized information is an important knowledge element throughout all steps of the participatory decision-making process. Useful visualization principles and techniques (or VT) have been proposed in the field of landscape design (Mertens 2009; Wang et al. 2005) and also as a valid support mechanism for participatory decision-making at the river corridor scale. Consistency throughout the participatory decision-making process is of paramount importance. First, it has to be assured that the complementary and overlapping knowledge expressed in the textual, numerical, and visualized form is free of contradictions (i.e. flood hazard scenarios identified by (1) a clear textual description of the process dynamics; (2) by the associated quantifications in terms of local flow depths and velocities; and (3) by convenient visual representations in the form of hazard maps). Second, it is essential to identify relationships between the emerging problems, objectives, strategies and alternatives, and their technical evaluation, as well as the resulting design refinements. Third, the consistency matrix ensures that knowledge generated in a previous procedural step and formalized in a specific form (e.g. visualizations) is accessible to inform subsequent steps and knowledge generation.

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10.4 Design Patterns to Enhance Flood Risk Mitigation In this subsection, we show a series of flood risk mitigation design patterns which may stimulate ad hoc solutions for specific urban-river settings. The first two proposed design patterns are termed enhanced flow sections. These flow sections prevent channel outbursts since additional space for flow conveyance is available in ancillary flow channels arranged below the main channel (see Fig. 10.5) or laterally to it (see Fig. 10.6). Carefully designed lateral spillway overflow guaranties the optimal activation of the ancillary channels in case of extreme events. From an inventive problem-solving perspective, a separation in the 3D space is accomplished saving river corridor areas form being inundated. In Fig. 10.7, a flow channel is created through an unconventional solution below a road. Again, separation in the 3D space is achieved by avoiding from a 2D perspective the spatial overlap between process domain and the road infrastructure. Fig. 10.5 Vertically arranged channel sections (1 and 2). The main channel (above) is connected via a lateral spillway overflow mechanism (3) to the “ancillary channel” (below). The latter is automatically activated when the discharge capacity of the main channel is exceeded (sketch on courtesy by Antonia Valdivia)

Fig. 10.6 Lateral arrangement of the main flow channel (3) and two ancillary lateral channels (4 and 5). These lateral channels are activated via lateral spillway overflow mechanisms once the discharge capacity of the main channel is surpassed. Additional temporary walls (6 and 7) can further enhance the protection of the alluvial plains (1 and 2) (sketch on courtesy by Antonia Valdivia)

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Fig. 10.7 Flow channel is situated below a superstructure sustained by pillars. The road is placed above this superstructure (sketch on courtesy by Antonia Valdivia)

These inventive design patterns for flood mitigation may provide only partial and, hence, insufficient solutions in highly congested urban-river settings. In such situations, it may be advisable to achieve a more comprehensive separation in space by segmenting the river corridor sectors as to optimally buffer flood peaks changing not only the spatiality of the propagation but also its temporal dynamics. In Fig. 10.8, we sketch the idea of the hydraulically convenient river corridor segmentation. Through leeves a set of interconnected storage areas is created which are filled and emptied in a pre-ordered manner to obtain the previously described mitigation effect.

Fig. 10.8 River corridor segmentation into dammed flow areas (1, 2, 3, and 4, respectively) conveniently buffering the flow dynamics protecting the urbanized sectors. The inundation process unfolds in a controlled way (sketch on courtesy by Antonia Valdivia)

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Fig. 10.9 Perimetral structures built for specific purposes (i.e. cycling roads) can be designed as to deploy flood protection functions (i.e. modifying the flow field on the inundated alluvial plains) and to provide connectivity to safe areas (sketch on courtesy by Antonia Valdivia)

Figure 10.9 depicts a similar inventive solution. The technical heuristics segmentation and function integration are used in synergy to design a levee system combined with a cycling road network to achieve the desired double functionality that is flood protection and an infrastructure for sustainable mobility. The reader may note that that this protection system provides also the safe exit function by making safety areas better accessible. In Fig. 10.10, the dynamics of a biphasic flow is altered separating the solid components from the liquid phase. Rope nets intercept floating large wood and transported boulders avoiding their collision with the elements of the built environment. In Fig. 10.11, innovative structural protection elements are shown to mitigate the adverse effects caused by flood impacts. In Panel a, flood wave diversion and reflection elements are installed at the perimeter of the exposed building. In Panel b, the building envelope is equipped with a flood protection rolling shutter. In Panel c, a flood protection coat is risen from below onto the building envelope. Finally, a flood protection balcony banister is lowered to provide the mitigation effect when needed. In Fig. 10.12, we show and integrated flood mitigation concept to protect inhabited areas from flood impacts. In this case, an underground parking facility is designed to function as flood protection measure. Sluice gates can be closed isolating the parking areas and creating a subterraneous floodway conducing the flow harmlessly below the inhabited areas. The reader is referred to Mazzorana et al. (in press) who elaborated further design patterns which may heuristically kick start the elaboration of integrated flood risk mitigation strategies.

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Fig. 10.10 Flow phase separation in biphasic flows achieved by intercepting the transported solid fraction (i.e. large wood and sediments) thereby avoiding the interaction of the transported solid components with the built environment (sketch on courtesy by Antonia Valdivia)

a

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Fig. 10.11 Innovative local protection elements for buildings: a wave diversion and reflection elements installed on the building envelope; b inbuilt flood protection rolling shutter; c risen flood protection envelope, and d lowerable flood protection banister (sketch on courtesy by Antonia Valdivia)

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Fig. 10.12 Integrated flood protection system: an underground parking facility with flood protection capabilities (sketch on courtesy by Antonia Valdivia)

10.5 Unlocking and Overcoming Spatial Planning Deficiencies in Chile as an Essential Condition for Hazard Risk Mitigation Historically, spatial planning has faced great challenges in the domain of risk governance (Camus et al. 2016). Construction regulations, on the contrary, have been progressively developed and enhanced to reach a high international standard, probably due to the large number of events such as earthquakes and subsequent tsunamis that hit the territory (Jorquera et al. 2017). However, in terms of zoning, risk management, evacuation planning, and their nexus with environmental pressures, the debate is rather recent either in terms of considering the impacts of climate change and the implications of the earthquake of 27 February 2010 (Moris et al. 2017). However, there are two fundamental and recent elements that underline the importance that the subject has taken in the country. The first corresponds to the National Policy for Disaster Risk Reduction 2020–2030 (PNRRD) published on 16 March 2021, and the second refers to the first National Policy for Territorial Planning (PNOT) also published the same year (5 July 2021). The first element assumes that Chile is a country permanently exposed to hazards of both natural and anthropic origins and this fact is firmly anchored in the National

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Policy for Disaster Risk Reduction guidelines in matters of management, planning, investments, and intervention instruments (PNRRD 2021). This need emanates from various sectors including national actors contributing to provide a common framework of valid and coherent understanding, in harmony with various national and international related organizations (PNRRD 2021). This policy is framed considering six principles emerged in national and international regulatory frameworks, corresponding to prevention, sustainability, coresponsibility, equity, security and coordination, which synoptically determine five priority axes presented in Table 10.1. As can be seen in this table, the axes provide a comprehensive basis to generate suitable structures and mechanisms to face the challenges involved in risk management and resilience to a particular event. In addition, each one of them is endowed with strategic objectives, where there should be a sectoral articulation, ideally framed in the PNOT (second stated effort) that can contribute to shaping these guidelines both spatially and territorially. The PNOT is intended to provide a long-awaited response to the lack of guidelines that could allow an articulation between desirable land use planning, and a land management (Ferrada Nehme 2011). Its objectives are to advance in an intersectoral approach which should guide the actions of the state, private actors and civil society towards the configuration of a harmonic, integrated, safe, and inclusive territory (in the diverse and wide geography), and also, promote a process of sustainable development that integrates the social, economic, and environmental dimensions, with the territorial identity (PNOT 2021). This policy has the particularity, for the first time in Chile’s history, to assume that there are two general territorial conditions that involve the use and the occupation of land in the territory: the risk of disasters (natural and anthropic origin) and the challenges in adapting to climate change. These conditions are framed in five strategic axes: (i) human settlement systems; (ii) economic-productive systems; (iii) natural systems; (iv) infrastructure and logistics systems; and (v) integrated socio-territorial systems. Particularly in the first axis, reference is made to risk management. In its objective 1.2, it points out the need to promote a resilient occupation and development of the territory that contributes to the reduction of disaster risks and adaptation to climate change. The PNOT reads in this point: Table 10.1 Priority axes of the National Policy for Disaster Risk Reduction Policy

Understanding disaster risk Strengthening the governance of disaster risk management Planning and investing in disaster risk reduction for resilience Providing an efficient and effective response Fostering a sustainable recovery Source: Own elaboration according to National Policy for Disaster Risk Reduction (2021)

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1. Promote a preventive and prospective approach to disaster risk reduction in terms of localization of current and future human settlements, the development of activities and the localization of infrastructures, incorporating and considering the risk of disasters in the territory. 2. Promote communication, operational connectivity, and resilient critical infrastructure. 3. Consider in the different instruments of territorial planning the elements that allow the reduction of risk and increase the resilience of the territories. 4. Incorporate the particularities of natural systems in the mitigation of hazards and employ them as key elements in the adaptation to climate change, in order to reduce the risk of disasters. In terms of land use planning, both policies should be integrated in the planning instruments, considering that these constitute the roadmap for land use planning. For the Chilean case, these instruments have a horizontal and multiscale nature (Cordero 2007). In this scenario, some instruments are presented below, with their scale of application and their current legislative framework that are linked to disaster risk and management (see Table 10.2). The instruments indicated in Table 10.2, have the particularity of being quite decentralized. The PRC, PRI, or PRM emanate from the different SEREMIS of urbanism of the regions (decentralized organisms of the ministries), and they are worked out with and approved by the municipalities in their different instances in coordination with the Regional Government. However, the problem resides in the lack or procastinated updating of the instruments. In the case of the PROTs, being an instrument legally implemented in the last year, they practically do not exist as such, given that, although many Regional Governments made the effort to elaborate them, they have not been considered in their application because the law was not effective yet. Therefore, it is possible to think that, from a risk management point of view, this instrument is practically non-operational. In addition, its scale (i.e. 1:200,000) would not provide greater mitigation possibilities for certain types of risks. Hence, the planning instruments with a greater potential for effective risk mitigation correspond to PRC, PRI, or PRM, although their applicability is limited only to urban space. These instruments (as was pointed out in Table 10.2) have the capacity to regulate land use and therefore provide a real possibility to determine exposed areas and therefore avoid urban expansion therein. In addition, the ordinance of the Law of Urbanism and Constructions establishes the technical parameters of the constructions (i.e. type of material, heights, etc.). Again, the fundamental drawback resides in their slow updating. Unfortunately, either the lack of resources of the municipalities or various technical deficiencies largely prevents their implementation. For example, the city of Valdivia does not have an updated PRC since 1989 and only sectional plans have been elaborated. A second set of instruments acting on a more strategic level could contribute to enhance risk management. Their weakness is that they do not allow for zoning as such, but they should provide directions in terms of policies, objectives, and activities

Law 21074, aimed at strengthening the regionalization of the country, launched the PROTs, which are an instrument that guides the use of the territory of the region to achieve its sustainable development through strategic guidelines and a macro zoning of the territory

Regional Land Use Plans (PROT)

Communal Regulatory Plan According to the law, the PRC/intercommunal PRI or regulatory plan is an instrument constituted by a set metropolitan PRM of rules on adequate conditions of hygiene and safety in buildings and urban spaces, and comfort in the functional relationship between housing, work, equipment, and recreation areas. It will be intercommunal or metropolitan depending on the number of inhabitants in the municipalities

General characteristics

Instrument Scale of application

They have the particularity of Communal, intercommunal being able to zone areas with risk exposure, define the structural and technical elements of building construction, among other land uses. Therefore, it has the power to limit areas that present a risk

Particularly the risk Regional component of this instrument. This is aimed at increasing the resilience of the regions and of their inhabitants to disasters through hazard zoning on a regional scale

Linking disaster risk management and resilience

Table 10.2 Some planning instruments linked to risk and resilience management

(continued)

Regulations/Ley General de Urbanismo y Construcciones, 2021; Ordenanza Ley General de Urbanismo y Consutrcciones, 2021

Indicative, although binding/Law 21.074

Nature/policy framework

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Code1 /Decree 475 with date of December 1994

Nature/policy framework

According to the document Informe N°1 Borde Costero del Departamento de Estudios, División de Planificación, Estudios e Inversión, Ministerio de Desarrollo Social, published in October 2010.

1

Source Own elaboration

Establish the possible uses of Intercommunal the different areas and zones, therefore, it has to consider the exposed zones and theoretically has to be harmonized with the PRC, PRI, or PRM as appropriate

It stems from the National Policy for the Use of the Coastal Edge published in 1994, which establishes the importance of planning this area

Coastal Edge Zoning

Scale of application

Linking disaster risk management and resilience

General characteristics

Instrument

Table 10.2 (continued)

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oriented to such ends in order to achieve the vision of the territory that wants to be built and associate the safeguards needed against possible hazards. Such planning instruments are summarized in Table 10.3 and although their scope in terms of risk assessment and mitigation has not been fully evaluated, it is important to point that their guidelines incorporate such notions so that other instruments such as PROTs or PRCs can spatially implement such policies. This applies particularly considering the link between the PROTs and the Regional Development Strategies. However, for most of the cases presented here, risk is approached from a zonation viewpoint or on strategic level (as shown in Table 10.3), but in terms of its concrete implementation and operability, one must resort to a set of plans that have been developed by the National Emergency Office of the Ministry of the Interior and Public Security (ONEMI). ONEMI has elaborated national and regional plans for the risks of forest fires, volcanic activity, tsunami, mass movements, disposal of hazardous materials, and large-scale mining emergencies (ONEMI 2021). The objective of those documents is to establish the necessary actions to take nationwide in the different phases of an emergency, catastrophe, or disaster resulting from any type of hazard, based on the coordination of the competent technical bodies and considering all the necessary guidelines for this purpose (ONEMI 2021). It should be considered that from the year 2023, ONEMI was replaced by the SENAPRED (National Disaster Prevention and Response Service), Institution that should allow a broader and more robust understanding and management of events. In addition, each region, according to its geographical context, also presents a specific set of plans, which contain the guidelines for responding to a hazard, establishing procedures, competencies, and roles, and it also presents an educational Table 10.3 Some strategic planning instruments where guidelines for hazard management and resilience could be incorporated Instrument

Institution in charge

General definition

Regional Development Strategies (RDS)

Regional government

Medium-term political Strategic non-binding project, which expresses in regulatory terms the major regional objectives and priorities in terms of regional economic and social development

PLADECO Communal Development Plan

Municipality

Instrument that guides Strategic non-binding the development of the in regulatory terms commune. It contemplates the actions oriented to satisfy the needs of the local community and to promote its social, economic, and cultural progress

Nature of the instrument

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objective for the resident population. In this way, these plans present the different types of risk, their theoretical and technical background, their management and form of response. Moreover, they include different cartographies that demark the exposed areas by zoning. The strength of these plans relies on an accurate diagnosis, although some types of risks resulting from increased rainfall or other local risks are not necessarily incorporated.

10.6 Conclusion Chile is a country that has historically been exposed to different threats of natural origin, but its systematic approach to mitigation is rather recent and goes hand in hand with the importance of the different territorial planning instruments developed since the return of democracy. Both the National Policy for Disaster Risk Reduction 2020– 2030 and the first National Territorial Planning Policy clearly underline the concern that the issue has taken in the country. However, the challenge of incorporating such guidelines in a powerful way remains. Territorial planning instruments, particularly those that have a normative character such as the PRC, PRI, or PRM, are useful tools to restrict possible areas of exposure or to generate buffer zones to mitigate certain events. In this sense, it is urgent to update these different plans, but with a sustainable vision, where real estate capital interests do not predominate over social or natural ones. This may become a crucial challenge on the way of generating more resilient territories. In summary, the detailed analysis of the spatial planning instruments applied in Chile highlights the objective difficulty in attempting to mitigate flood risks by preventive land use strategies due to specific issues of reach, scale, responsibilities, and legal cogency. Additionally, these strategies are insufficient in all those spatial contexts where existing urban settings are exposed, and a relocation of the inhabitants is unfeasible. The inventive problem-solving approach embedded in a participatory decision-making context may help to overcome the aforementioned difficulties. Once potential solutions enter the debate in the public participation arena, their dismissal requires a plausible explanation which may spotlight other systemic problems with highest priority. What hinders the applicability of a promising solution pathway? What are the entrenchments and lock-in situations? Which institutional changes are necessary to face the challenges of the twenty-first century enabling a balanced progress in a democratic setting? In fact, profound changes may be deemed as urgent at the institutional level enabling essential reforms. We contend that leveraging by design the exposed urban-river systems towards higher degrees of ideality with respect to flood risk mitigation could be part of a more comprehensive spatial planning strategy aiming at enhancing overall resilience of urban areas. The metaheuristics and flood risk mitigation patterns presented in this chapter, although perfectible, integrate the toolkit at our disposal to face present and future adaptation challenges.

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Acknowledgements: This research was funded by the project ANID/CONICYT FONDECYT Regular, Folio 1200091, “Unravelling the dynamics and impacts of sediment-laden flows in urban areas in southern Chile as a basis for innovative adaptation (SEDIMPACT)” led by Bruno Mazzorana.

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Rinaldi M, Gurnell M, Gonzalez del Tanago M, Bussettini M, Hendricks D (2016) Classification of river morphology and hydrology to support management and restoration. Aquat Sci 78(1):17–33 ROADAPT consortium (2015) ROADAPT—roads for today, adapted for tomorrow. Guidelines. CEDR Transnational Road Research Programme Call 2012: road owners adapting to climate change. Accessed at: https://www.cedr.eu/download/other_public_files/research_progra mme/call_2012/climate_change/roadapt/ROADAPT_integrating_main_guidelines.pdf Rojas O, Latorre T, Pacheco F, Araya M, López J (2019) Inundaciones fluviales en cuencas costeras mediterráneas de Chile: recurrencia, factores físicos y efectos hidrogeomorfológicos de su gestión. In: Martínez C, Hidalgo R, Henríquez C, Arenas F, Rangel-Buitrago N, Contreras-López M (eds) La Zona Costera en Chile: Adaptación y planificación para la Resiliencia. Geolibros, Santiago, pp. 79–103 Ruiz-Villanueva V, Mazzorana B, Bladé E, Bürkli L, Iribarren-Anacona P, Mao L, Nakamura F, Ravazzolo D, Rickenmann D, Sanz-Ramos M, Stoffel M, Wohl E (2019) Characterization of wood-laden flows in rivers. Earth Surf Process Landforms 44:1694–1709. https://doi.org/10. 1002/esp.4603 Santibañez N, Mazzorana B, Iribarren P, Mao L, Rojas I (2021) Dinámica distributiva de flujos bifásicos con carga de madera en un abanico aluvial experimental. Ingeniería Del Agua 25(2):145– 168. https://doi.org/10.4995/ia.2021.14703 Scholz RW, Tietje O (2002) Embedded case study methods: integrating quantitative and qualitative knowledge. Sage, Thousand Oaks, CA Scorpio V, Surian N, Cucato M, Dai Prá E, Zolezzi G, Comiti F (2018) Channel changes of the Adige River (Eastern Italian Alps) over the last 1000 years and identification of the historical fluvial corridor. J Maps 14(2):680–691. https://doi.org/10.1080/17445647.2018.1531074. Available at: https://ssrn.com/abstract=3973783 Simoni S, Vignoli G, Mazzorana B (2017) Enhancing sediment flux control and natural hazard risk mitigation through a structured conceptual planning approach. Geomorphology 291:159–173. https://doi.org/10.1016/j.geomorph.2017.01.026 Stauffacher M, Flüeler T, Krütli P, Scholz RW (2008) Analytic and dynamic approach to collaboration: a transdisciplinary case study on sustainable landscape development in a Swiss Prealpine region. Syst Pract Act Res 21:409–422. https://doi.org/10.1007/s11213-008-9107-7 Sturm M, Gems B, Keller F, Mazzorana B, Fuchs S, Papathoma-Köhle M, Aufleger M (2018a) Understanding impact dynamics on buildings caused by fluviatile sediment transport. Geomorphology 321:45–59. https://doi.org/10.1016/j.geomorph.2018.08.016 Sturm M, Gems B, Keller F, Mazzorana B, Fuchs S, Papathoma-Köhle M, Aufleger M (2018b) Experimental analyses of impact forces on buildings exposed to fluvial hazards. J Hydrol 565:1– 13. https://doi.org/10.1016/j.jhydrol.2018.07.070 Suda J, Holub M, Jaritz W, Ertl-Balga J, Ertl-Balga U, Rudolf-Miklau F, Zimmermann T (2012) Entwurf von Gebäudeschutzmaßnahmen. In: Suda J, Rudolf-Miklau F (eds) Bauen und Naturgefahren. Handbuch für konstruktiven Gebäudeschutz. Springer, Vienna, Austria, pp 249–311 Terninko J, Zusman A, Zlotin B (1998) Systematic innovation: an introduction to TRIZ (theory of inventive problem solving). CRC Press, Boca Raton, FL Thaler T, Levin-Keitel M (2016) Multi-level stakeholder engagement in flood risk management, a question of roles and power: lessons from England. Environ Sci Policy 55:292–301. https://doi. org/10.1016/j.envsci.2015.04.007 Tietje O (2005) Identification of a small reliable and efficient set of consistent scenarios. Eur J Oper Res 162:418–432. https://doi.org/10.1016/j.ejor.2003.08.054 Wang X, Song B, Chen J, Zheng D, Corw TR (2005) Visualizing forest landscapes using different data sources. Landsc Urban Plan 75:111–124. https://doi.org/10.1016/j.landurbplan.2004.12.010 United Nations (2018) Revision of world urbanization prospects. https://population.un.org/wup/. Accessed 12 July 2021 Wiek A, Iwaniec DM (2013) Quality criteria for visions and visioning in sustainability science. Sustain Sci 9(4):497–512. https://doi.org/10.1007/s11625-013-0208-6

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Chapter 11

Cascading Processes and Multiple Hazards and Risks in Chilean Rivers: Lessons Learnt and Remaining Challenges Virginia Ruiz-Villanueva, Bruno Mazzorana, Diego Bahamondes, and Iván Rojas Abstract Characterized by pronounced seismicity, intense volcanism, high relief energy, and cryosphere changes, the Chilean climate, geology, and topography determine the suite of landscape-forming processes and disturbances that under certain circumstances may lead to extreme impacts on society, environment, and infrastructures. Earthquakes, volcanic eruptions, and floods are processes that naturally collide in Chilean river basins, producing process concatenations or cascades and resulting in complex multi-hazards and risks. This calls for a perspective shift, aiming at a multihazard approach that goes beyond the simple overlay of multiple single hazards to an approach that also considers interactions between these hazards and risks. In this chapter, we discuss how individual processes or disturbances may interact acting together to form cascades, using case studies from Chile (e.g. rivers affected by volcanic eruptions), and how the resulting hazards and risks could be assessed by integrating all aspects of hazard interactions together with exposure and vulnerability. Keywords Cascade hazards · Process chain · Debris flood · Wood-laden flows

11.1 Introduction Characterized by pronounced seismicity, intense volcanism, high relief energy, and cryosphere changes, earthquakes, volcanic eruptions, and floods are natural processes V. Ruiz-Villanueva (B) Faculty of Geosciences and the Environment, Institute of Earth Surface Dynamics (IDYST), University of Lausanne, Geopolis, UNIL-Mouline, 1015 Lausanne, Switzerland e-mail: [email protected] B. Mazzorana · D. Bahamondes Facultad de Ciencias, Instituto de Ciencias de la Tierra, Universidad Austral de Chile, Valdivia, Chile I. Rojas Faculty of Forest Sciences, Graduate School, Universidad Austral de Chile, Valdivia, Chile © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. Oyarzún et al. (eds.), Rivers of Southern Chile and Patagonia, The Latin American Studies Book Series, https://doi.org/10.1007/978-3-031-26647-8_11

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that often collide in Chilean river basins (Mazzorana et al. 2018). These processes significantly disrupt the structure of river systems by changing their physical environment and thus can be defined as disturbances (White and Pickett 1985). Such disturbances, under certain circumstances, may lead to extreme impacts on the environment, infrastructures, and society producing complex multi-hazards and risks. Figure 11.1 shows the major Chilean river basins where multiple disturbances (e.g. earthquakes, volcanic eruptions, glacier lake outburst, and wildfires) can occur and therefore the combination of multiple processes might result into complex risks.

Fig. 11.1 a Major river basins classified according to the number of multiple disturbances: b earthquakes of magnitude N7 (1570–2014; http://www.sismologia.cl); c glacial lakes outburst floods (GLOFs) location and glacier extent (Dirección General del Aguas 2018; www.dga.cl); d active volcanoes (http://www.rulamahue.cl); e 2015 wild fires locations (CONAF 2016; http://www.geo portal.cl; f erosivity (i.e. soil susceptibility to be mobilized and eroded; Ministerio del Medio Ambiente de Chile, 2016; http://www.geoportal.cl); and g digital elevation model (45 m pixel size, available from: http://www.diva-gis.org). Modified from Mazzorana et al. (2018)

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Discrete disturbances are described based on three interlinked dimensions: (1) magnitude or intensity, (2) probability of occurrence or frequency, and (3) spatial dimension; hence, river systems can be characterized by their disturbance regime (Formann et al. 2014). Similarly, hazards related to single processes or disturbances are defined according to these three dimensions. However, individual processes or disturbances may interact, acting together and creating cascades. This calls for a perspective shift, aiming at a multi-hazard approach that goes beyond the simple overlay of multiple single hazards to an approach that also encompasses interactions between these hazards and risks. In this chapter, we first review the different and sometimes overlapping terminology used to define the relationships between different processes and disturbances leading to multiple hazards and risks (Sect. 11.2). Secondly, we discuss how the resulted hazards and risks could be addressed by integrating all aspects of hazard interactions together with exposure and vulnerability (Sect. 11.3); finally, we summarize the lessons learnt from the particular case of rivers affected by volcanic eruptions in Chile (Sect. 11.4), highlighting the remaining challenges (Sect. 11.5).

11.2 Defining Multiple Hazard Terminology The interest and reference to the concept of multi-hazard have been widely reviewed by many authors. For example, the general development of the International Decade of Natural Disaster Reduction (IDNDR) and the following permanently installed International Strategy for Disaster Reduction (ISDR) (Zentel and Glade 2013), proposed a multi-hazard approach for disaster management and risk reduction (Gallina et al. 2016). According to this approach, and like for single-risk assessments, the basic components that should be considered in the multi-risk evaluation are the hazards (related to the different disturbances) and the elements at risk including their exposure and vulnerability (UNISDR 2009; IPCC 2012; Komendantova et al. 2014). However, precise definitions of the complex relationships between multiple processes or disturbances resulting in multiple hazards are rare (e.g. Kappes et al. 2012; Gill and Malamud 2016). This section reviews the current terminology used to define these relationships (Table 11.1). The terms “domino”, “chain”, or “cascade” are commonly used to refer to a situation where an original process triggers a sequence of following processes (e.g. Han et al. 2007; Mazzorana et al. 2018; Schaub et al. 2013; Somos-Valenzuela et al. 2016; Worni et al. 2013; Delmonaco et al. 2006a, b; Frey et al. 2016; Joyce et al. 2017; Kumasaki et al. 2016; Mehta et al. 2017; Nguyen et al. 2013; Schneider et al. 2014). “Chain” and “cascade” are in fact similar terms, but “cascade” can be more suitable for one-directional relationships, while “chains” refer to fully coupled processes (two-directional). Disturbances and impacts on river systems can be described as complex cascades along the river continuum (Schauwecker et al. 2019), in which some processes might still be fully coupled. Pescaroli and

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Table 11.1 Non-exhaustive list of terms used to define the complex relationships between multiple processes or disturbances and their impacts Terminology

References

Cascades, cascading effects, cascading failures, or cascade events

Delmonaco et al. (2006b), Carpignano et al. (2009), Zuccaro and Leone (2011), and European Commission (2011)

Chains

Shi (2002) and Erlingsson (2005)

Compound events/hazards

Hewitt and Burton (1971) and Alexander (2000)

Coupled events

Marzocchi et al. (2009)

Domino effects

Luino (2005), Delmonaco et al. (2006b), Perles Roselló and Cantarero Prados (2010), and European Commission (2011)

Multiple hazard

Hewitt and Burton (1971)

Synergic effects

Tarvainen et al. (2006)

Triggering effects

Marzocchi et al. (2009)

Alexander (2018) defined the related disturbances also including their impacts on society, as compound-, interconnected-, interacting-, and cascading-risks. However, “cascading” processes should not be confused with “compound” events (i.e. several processes acting together) a term that is also widely used to refer to the combination of multiple disturbances contributing to multiple risks (Zscheischler et al. 2018; Tilloy et al. 2019). Section 11.3 provides a more detailed definition of these terms. Here, we use the term cascading processes as multiple disturbances (i.e. any event, such as earthquakes, volcanic eruptions, wildfires, or mass movements, disrupting the physical environment; White and Pickett 1985) that may ultimately concur and trigger other disturbances or exacerbate their effects posing human live, land, and property at risk (Mazzorana et al. 2018). One example to illustrate cascading processes is shown in Fig. 11.2.

11.3 Assessing Risks of Multiple Hazards Addressing the risk resulting from the combination of multiple hazards and vulnerability of exposed elements faces challenges that arise from combining all involved components (Kappes et al. 2012). Still, as for single hazards, the multi-hazard risk is usually expressed in damage- or loss-specific units such as economical losses, expected number of lives lost, damage to property and can be estimated in a qualitative (classification), semiquantitative (indices), or quantitative manner (monetary values, probabilities, etc.). A major challenge is to homogenize or standardize the magnitude and frequency of the different hazards to be able to relate them. Thresholds or intensity classes have been proposed to overcome this challenge (see Kappes et al. 2012 for a review), but they usually ponder independent processes and overlay resulting

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Fig. 11.2 Cascading processes (i.e. morphological changes within the river corridor and landslides on the hillslopes) in the Blanco River Basin after the volcanic eruption of the Chaitén volcano in May 2008

levels, without explicitly considering the processes’ interdependencies. That is the case of the traditional univariate risk assessment, which mainly considers one process at a time and ignores the dependence between several disturbances, leading to underestimation or overestimation of the underlying risk (Gill and Malamud 2016). When considering more than one process, according to the selected definition of multiple hazards, two main approaches can be distinguished: (a) the identification where different hazards overlap or (b) the investigation of cascades or chains of one hazard triggering the next (Delmonaco et al. 2006a). In the first case, interrelations are not really considered, while the second case includes hazard interactions and interrelationships (Tilloy et al. 2019). For an effective multiple-risk assessment, the nature of interacting and interconnected relationships between different hazards needs to be defined. Three major hazard interrelation types have been described in the literature: (i)

Independence: There is a spatial or temporal overlapping of the impact of two hazards without any dependence or triggering relationship (Liu et al. 2016). For example, cases where two hazards impact the same area, independently, at different times. However, independent processes are rare as processes acting on the same area might always have an influence on each other. (ii) Cascading: Implies a primary and a secondary hazard being the secondary hazard identical or different from the primary hazard (Gill and Malamud 2014). The relationship could be causal when the primary process triggers the second

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one, or it may also change (i.e. increase) the disposition or probability of occurrence of another process (Gill and Malamud 2016). Both types of cascading relationships occurred in the Blanco River basin, affected by a volcanic eruption, and described in this book chapter. For example, the forest loss after the volcanic eruption changed the hydrological response of the Blanco River basin, which influenced the flood hazard but without directly triggering flooding. (iii) Compound: There is not a primary and a secondary hazard, but the different hazards occur simultaneously or successively (with a relatively short timelagged). The IPCC also includes in this category and the combination of extreme processes that amplify the impacts of the events, or a combination of nonextreme events that lead to an extreme event or impact when combined. Once hazard interrelation has been identified, different modelling approaches can be used to quantify it and characterize the resulting risk. Tilloy et al. (2019) proposed a general framework based on applying stochastic, empirical, or mechanistic approaches. In summary, according to these authors, stochastic models can be applied to quantify the dependence between different hazards, such as for example the statistical dependence of several extreme environmental variables by applying methods like multivariate or extreme value statistics. Thus, stochastic models allow the estimation of joint probabilities of exceedance or return periods. However, limitations exist, for example, when looking at compound events in which not all the variables are characterized as extreme. In that case, statistical methods can lead to either underestimation or overestimation of the joint probability. In that respect, Tootoonchi et al. (2022) proposed a multivariate analysis, particularly copula-based probability distributions to compute the cumulative probability distribution of multiple variables to analyse the risks of compound events. Empirical models, such as dependence measures and regressions, are also particularly useful for cascading hazards assessment, as they include independent (primary hazard(s)) and independent (secondary hazard(s)) variables (Tilloy et al. 2019). Mechanistic models, both conceptual and physically based models, describe or simulate the behaviour of phenomena based upon physical processes and mechanisms driving the considered system. However, to model extreme phenomena may require an extensive amount of data and might be computationally intensive. In addition, the unusual combination of processes makes them difficult to foresee, because they might be rare, and they may not have observed historical analogues, and this issue is likely to be exacerbated because of climate change (Zscheischler et al. 2018). In their review, Tilloy et al. (2019) did not include other data-driven methods, such as those derived from artificial intelligence or machine learning; and they did not consider mixed approaches that combine different model typologies. Artificial neural networks (ANNs) and fuzzy inference systems (FIS) have demonstrated a unique ability in dealing successfully with nonlinearity over a wide range of applications (Valyrakis et al. 2006). Of course, these methods are data dependant, and this could be a limitation in areas with scarce data availability. Combined approaches have been also proposed, for example, Mazzorana et al. (2011) described how a nested approach entailing deterministic simulations as well as

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stochastic evaluation provided a more reliable determination of flood hazard areas and the associated risks. In a similar framework, Mazzorana and Fuchs (2010) presented a Formative Scenario Analysis combined with fuzzy modelling, to derive qualitative and quantitative (expert and local, respectively) knowledge to be integrated into the scenario definition and the evaluation of impacts and risks. Recently, Terzi et al. (2019) reviewed five modelling approaches (i.e. Bayesian networks, agent-based models, system dynamic models, event and fault trees, and hybrid models) that have been used to assess multi-risk derived from climate change and to define adaptation measures in mountain regions. According to these authors, system dynamic and hybrid models demonstrate higher potential for further applications to represent climate change effects on multi-risk processes. Still, those models need to be further explored. Thus, until today, an approach that can simulate and predict cascading hazards, or consider all possible scenarios, does not exist. The challenge is to identify the relevant processes and how they are related to each other, particularly when the different processes may significantly vary in time and space. The concept of process domains may help to select relevant processes in a given region, as was defined as a multi-scale hypothesis to characterize governing geomorphic processes assuming that regional differences in climate, geology, and topography control the occurrence and relative importance of processes (Montgomery 1999). However, the temporal dimension of such processes and their responses are not straightforward to establish and would require further investigation. Piégay et al. (2018) distinguished between pulse and press disturbances and perturbations and described them in terms of direction, magnitude, and persistence to evaluate geomorphic and ecologic systems and relate them to the socio-economic impacts. They proposed a framework based on the concepts of resilience. The concept of resilience has rapidly emerged and is widely used in a variety of settings over a range of spatial and temporal scales (Thoms et al. 2018). When applied to the multi-hazards and risks assessment, resilience can be understood as the capability of the natural (e.g. river basin) as well as the social systems to prepare, respond, and recover from multiple disturbances. The central aspect of resilience is the target system. Therefore, the shift from a single-risk to a multi-risk assessment implies a change from a single process or hazard-centred perspective to a territorial-centred one (Carpignano et al. 2009; Garcia-Aristizabal et al. 2015), in which resilience is key. The widely accepted hazard-centred perspective assumes that the first element of the analysis is the hazard source identification followed by the subsequent definition of the impact area and the assessment of potential risks. Contrastingly, a territorial perspective focuses on the target area for the analysis, followed by the identification of all possible hazards threatening the same area (Komendantova et al. 2016). A sound multi-risk assessment should be resilience and objective-oriented, must provide data characterized by scientific evidence, and take into account uncertainties, allowing the definition and evaluation of different risk management options. We illustrate this approach with the example of the Blanco River.

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11.4 Cascading Processes in a Chilean River Affected by a Volcanic Eruption The Blanco River (70 km2 drainage area and 18 km long) in Los Lagos Region was severely affected by a volcanic eruption in 2008–2009. The Chaitén Volcano suffered a short ash emission phase prior to the initial Plinian explosive phase of about 15 days followed by a longer effusive event of about 20 months. More details on the eruption can be found in Lara (2009), Major and Lara (2013), Major et al. (2013), Pierson et al. (2013), and Swanson et al. (2013). This cascade of volcanic processes strongly affected the riverine environment, modifying the land cover, hydrology, and river morphology. Before the 2008 eruption, the Blanco River catchment was mostly covered (84% area) by native forest composed of Nothofagus dombeyi, Nothofagus nitida, and Nothofagus betuloides, and the river channel was on average 36 m wide and had an almost negligible presence of instream large wood (Ulloa et al. 2015). In February 2009, the volcanic dome partially collapsed, generating an intense and destructive flow that completely changed the riverine geomorphic setting. Lahars were produced and delivered a significant amount of volcanic sediment to the main channel, modifying its morphology and resulting in almost vertical and unstable banks of very fine volcanic material (i.e. ashes and lahars); 4–6 m of ashes were deposited across the entire river basin (Major et al. 2013). The vegetation was strongly affected, with hundreds of trees killed and supplied as large wood to the river, hundreds of dead standing trees still stand along the riverine corridor and a considerable number of them are buried in the volcanic sediments (Ulloa et al. 2015). As a result of the combined effect of ash deposition and vegetation death, dead trees were no longer able to exert their natural and fundamental function of stabilization of hillslopes and riverbanks, and consequently, multiple slope instabilities and mass movements were triggered. These landslides supplied large quantities of both sediment and trees to the river corridor. The river channel was completely filled up with sediment, an aggradation process that was also favoured by the sea level rise at the downstream boundary of the alluvial plane which forced the flow to diminish its transport capacity (Basso-Báez et al. 2020). Consequently, large areas of the Chaitén city were flooded. During this event, the Blanco River avulsed laterally and incised a new channel curving sharply to the right and debouching after a short straight trajectory into the sea, triggering multiple damages and widespread losses throughout the entire urban area (Basso-Báez et al. 2020; Fig. 11.3). One of the hot spots, identified as a critical infrastructure is the “Austral Road” bridge (Fig. 11.3), and the potential risk derived from the morphological changes along the river, the enhanced sediment and wood supply and transport, and related potential blockage and loss of conveyance capacity, eventually resulting in a significantly increased flood risk in the urbanized environment. The cascading processes can be illustrated in an event-based pathway scheme (Schauwecker et al. 2019) using an initial qualitative description, to help understand the complex processes’ interactions (Fig. 11.4).

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Fig. 11.3 Channel avulsion of the Blanco River crossing the city of Chaitén after the volcanic eruption in 2008

The visualization of cascades as the one shown in Fig. 11.4 (increase in runoff due to the loss of vegetation cover and thus a potentially flashier response after rainfall, landslides, enhanced supply and transport of both sediment and wood, river aggradation, blockage of river crossing infrastructures, etc.) is fundamental for both the theoretical and practical understanding of multiple hazards assessment (Gill and Malamud 2016; Schauwecker et al. 2019). Such situations cannot be addressed by applying traditional univariate risk assessment, just considering one process at a time, and ignoring the dependence between the several disturbances. A multi-hazard approach, therefore, should ideally evaluate all identified individual hazards relevant to the area and characterize all possible interactions between them, as explained in the previous section. First, when assessing location-specific hazard potential, clear temporal limits should be established. In the case of the Blanco River basin, the short-term perspective might be more relevant, as the occurrence of a primary hazard (e.g. heavy rainfall) can significantly modify the probability of secondary hazards (e.g. floods, landslides). In the case of the Blanco River, a scenario-based modelling approach was developed and presented by Basso-Báez et al. (2020), in which they analysed the hydrological response of the river basin, determining the expected hydrographs and the associated sediment fluxes for two large flood scenarios (Qpeak of 430 m3 /s

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Fig. 11.4 Process cascade in the Blanco River basin after the volcanic eruption in 2008. Colours determine affected systems or the nature of the triggered hazard (e.g. geophysical, climatic, hydrological, and socio-economical). Shapes show different hazard levels and their impacts and consequences

and 620 m3 /s, respectively). They simulated the river response to these two flood scenarios, obtaining the inundation extent and main hydrodynamic variables (i.e. water depth and flow velocity). They included the worst-case scenario, by assuming extreme sediment and instream transport, and assuming that the main bridge crossing the river (the Austral road bridge) was blocked up to 90% of its capacity. The goal of the study was to evaluate the potential impacts (in terms of forces exerted by the flow and susceptibility to material intrusion) on the buildings in the city of Chaitén. The aim was to contribute to an effective risk dialogue among stakeholders, managers, scientists, and proactive individuals from the concerned civil society and inform decision makers about feasible risk mitigation initiatives. Results clearly showed that the bridge clogging exacerbated the inundation and its impacts in the urban area. A 50% blockage of the “Austral Road” bridge was enough to significantly increase the flooded area, affecting both the northern and southern urban sectors severely affecting the building in that area (Fig. 11.5). The detailed analyses of selected buildings revealed that transported solid material can flow through the openings in the building envelope and partially fill the interior of the buildings and that channelized flow can undermine the foundations of the building and induce structural failure. This first scenario-based modelling approach, however, did not include some key processes, such as hillslope-channel connectivity, sediment and wood supply, wood transport and entrapment explicitly nor simulated erosion and aggradation processes in a movable bed context.

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Fig. 11.5 Modelled flow depth maps of two flood scenarios (Qpeak = 620 m3 /s) without and with bridge blockage. Blue points show analysed buildings

Therefore, a follow-up study examined the inundation effects considering these processes. Preliminary results (Fig. 11.6) showed that the sediment and wood dynamics significantly affect the flood hazard and related risk and thus should be further explored. A better process understanding of the multiple hazards and cascade processes in the Blanco River will benefit, not only the local community of Chaitén but also the general flood and multi-risk community.

Fig. 11.6 Simulated water depth for different flood scenarios, considering only clear water (left), sediment and wood transport (middle), and sediment and wood transport and additional wood supply from upstream (right panel)

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11.5 Lessons Learnt and Remaining Challenges A fist aspect to be carefully considered when dealing with process cascades is the identification of interdependencies and relationships between the processes (see, for example, Fig. 11.4). A clear terminology and definition of the processes’ relationships are key. In this respect, we reviewed and summarized the main typologies and definitions used in the literature (see Sect. 11.2 and Table 11.1). Accurately representing the system boundaries facilitates the assessment of the material and energy influxes responsible for the main triggering mechanisms (i.e. sediment injections through volcanic eruptions). These incoming fluxes may be the ultimate originating causes of the process cascades and often are their main triggering mechanisms. However, they may also contribute to altering intermediate processes (i.e. reduction of soil strength over time through progressive loss of vegetation which increases landslide susceptibility, see Fig. 11.2). In this regard, it is important to consider that these intermediate processes may, in turn, be directly responsible for the initiation of further process cascades involving rapid material displacements or flows within the system. Hence, a clear identification and conceptual representation of both temporal and spatial relations are essential. Moreover, one should always keep in mind that each (river) system is characterized by specific impacts determined not only by the unfolding process cascades but also by the characteristics of the socio-ecological systems and the exposed built environment. Damage generating mechanisms might vary accordingly. In this chapter, we mainly focused on interactions between natural hazards, but we also recognize the importance of anthropogenic processes. It is essential to consider the different timescales of different hazardous processes. The resulted risks may be strongly dependent on the time lag between processes and the different time intervals within the cascade, and the time required for repair, recovery, and reconstruction (again here the concept of resilience is key). Particularly, challenging is assigning a probability of occurrence or recurrence interval to complex process cascades. That would require, first, the definition of the possible processes and scenarios (i.e. the considered event space). In many cases, an exhaustive determination might go beyond the current capabilities, therefore efforts should focus on identifying the most relevant set of scenarios. Relevance, however, depends on the objective and scope of the performed assessment. Also, the level of detail of scenario specification is, at least partially, problem specific. Singularly, in heavily perturbed river systems, scarce data may constitute a major drawback for an exhaustive assessment of all relevant process cascades. Even in the fortuitous case of measured discharge time series, the perturbation and the resulting disturbance regimes substantially change how the system operates, how the different processes interact, and which energy and material fluxes are generated (see, e.g. the processes cascade in Fig. 11.4). In fact, after a remarkable perturbation, many fluvial systems may have undergone systemic shifts where several intrinsic thresholds were exceeded. In such situations, the values of relevant morphometric parameters and variables may outdo the previously applicable ranges proper of dynamic equilibrium.

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A final not minor challenge is related to the quantitative (e.g. physically based) modelling of the qualitatively determined process cascades. This is particularly true for multi-phase flows modelling. Modelling tools for simulating complex multiphasic flow processes are still under development. It should further be acknowledged that laying out how process cascades may work in specific situations always remains somewhat tentative. Proposed schematisations may be revised as new evidence related to unconsidered processes interactions is found. In complex hazard assessment tasks, researchers should always communicate remaining uncertainties, both epistemic and aleatoric, to decision makers and the affected community. In addition, scientists, practitioners, and developers should closely collaborate also on specific case studies. This would enhance the overall quality of cascading process studies and favour continuous learning. Although the cascading process research field is still very young, conceptual knowledge is being developed at a fast pace and complex case studies are being tackled and useful indications for effective risk management are being provided (e.g. Mazzorana et al. 2018, 2019).

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Index

A Annual discharge, 7, 8, 10

B Baker River, 6–8, 10, 11, 15, 23, 33 Biodiversity, 35, 36, 44, 45 Biogeochemistry, 13, 16 Braiding index, 121 Blanco (Chaitén) River, 166 Blanco Este River, 170 Blanco River, 123, 124

C Calbuco volcano, 69, 71, 75, 77, 79, 81–83, 88–90, 93, 94, 99, 101–106, 108, 109 Cascading impacts, 139 Cascading processes, 235, 238, 239, 242, 248 Clear waters, 70, 71–73 Climatology, 1, 3, 8 Compound hazard effects, 149 Conductivity, 67, 69, 71, 72 Confinement index, 121 Cordón Caulle, 68, 69, 70–72, 75, 76

Ch Chaitén volcano, 2, 11, 33, 50, 53, 54, 75, 166, 170, 185, 188–190, 239, 242, 248–250

D Design patterns, 217

E Eco-hydrogeomorphological functioning, 42, 43 Ecosystem disturbance, 14, 15 Explosive volcanic eruption, 13, 15, 25 F Fish communities, 41 Fresh water ecosystems, 13, 15 G Geomorphic evolution, 77 Geomorphological footprint, 111 Glacier Lake Outburst Flood (GLOF), 6, 112, 140 H Hydrological regimes, 1, 6 Hydromorphological evaluation, 80, 81, 83 Hypsometric curves, 87, 90, 106 I Ice-dammed lake, 113 IDRAIM method, 111, 115, 120, 121, 131, 133, 134, 136, 137 Inventive knowledge, 199 L Land use cover, 52 Laguna Espontanea GLOF, 140 Large Infrequent Disturbances (LIDs), 155 Large wood mobility, 58, 63 Large wood volumes, 55

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. Oyarzún et al. (eds.), Rivers of Southern Chile and Patagonia, The Latin American Studies Book Series, https://doi.org/10.1007/978-3-031-26647-8

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252 M Monitoring wood, 51 Monthly mean temperatures, 3, 5, 6 Monthly precipitation, 4, 18 Moraine-dammed lake, 113 Mountain streams, 49, 63, 64 Multi-temporal analysis, 168 N Natural disturbance, 13, 17, 19 Norte River, 139, 146, 150 Northern Patagonian Rivers, 38–41, 47 O Olvidado River, 123, 124 P Pacific Patagonia Rivers, 13 Participatory decision-making, 211 Península de la Montaña River, 123, 124 Principal components, 72, 74 Process cascade, 244, 246, 247, 249 R Radial system, 79, 81–83, 86–92, 94, 99, 103, 106, 107, 109

Index Retrospective hazard assessment, 141

S San Pedro River, 35–37, 39–41, 43, 44 Sediment connectivity, 155 Sinuosity Index, 122 Soler River, 123, 124

T Toro River, 176 TRIZ, 197 Turbid waters, 67, 71, 72

U Ultima Esperanza River, 123, 124

V Valdivia River basin, 35, 36, 38–41, 46

W Water resources, 2 Western Patagonia, 14, 33 Wildfires, 155, 156, 160, 176, 177, 178, 179, 182