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FUNDAMENTALS OF FLUVIAL GEOMORPHOLOGY Fundamentals of Fluvial Geomorphology provides an accessible introduction to the study of river landforms and the processes that shape them. Rivers are significant geomorphological agents which show an amazing diversity of form and behaviour, reflecting the wide range of environments in which they are found. Highly dynamic in nature, river channels adjust and evolve over timescales that range from seconds to tens of thousands of years. This book examines how river systems operate and respond to change and why this understanding is needed for successful river management. The book provides a coherent overview of the main concepts in fluvial geomorphology, together with recent developments in river channel management, clearly illustrating why an understanding of fluvial geomorphology is vital in channel preservation, environmentally sensitive design and the restoration of degraded river channels. Starting with an introduction to the fluvial system, the book moves on to cover: ● ● ● ● ● Flow and sediment regimes: flow generation; flow regimes; sediment sources, transfer and yield. Channel process: flow characteristics; processes of erosion and sediment transport; interactions between flow and the channel boundary; deposition. Channel form and behaviour: controls on channel form; channel adjustments; floodplain development; form and behaviour of alluvial and bedrock-influenced channels. Response to change: how the system responds to change; reconstructing past changes; impacts of climate change, human activity, tectonics and base-level change. River management: the fluvial hydrosystem; environmental degradation; environmentally sensitive engineering techniques; river restoration; the role of the fluvial geomorphologist. Fundamentals of Fluvial Geomorphology is an indispensable text for undergraduate students. It provides straightforward explanations for important concepts and mathematical formulae, backed up with conceptual diagrams and appropriate examples from aroun; d the world to show what they actually mean and why they are important. A colour plate section also shows spectacular examples of fluvial diversity. Ro Charlton lectures in the Geography Department at the National University of Ireland, Maynooth, where she teaches undergraduate and postgraduate courses in fluvial geomorphology and hydrology. Her research interests include meandering river canyons in caves and the impact of climate change on water resources. FUNDAMENTALS OF FLUVIAL GEOMORPHOLOGY Ro Charlton First published 2008 by Routledge 2 Park Square, Milton Park, Abingdon, Oxon, OX14 4RN Simultaneously published in the USA and Canada by Routledge 270 Madison Avenue, New York, NY 10016 This edition published in the Taylor & Francis e-Library, 2007. "To purchase your own copy of this or any of Taylor & Francis or Routledge's collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk." Routledge is an imprint of the Taylor & Francis Group, an informa business ©2008 Rosemary Charlton All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging in Publication Data Charlton, Ro. Fundamentals of fluvial geomorphology/Ro Charlton. p. cm. Includes bibliographical references. 1. Fluvial geomorphology. I. Title. GB561.C475 2007 551.3′55—dc22 2007014030 ISBN 0-203-37108-9 Master e-book ISBN ISBN10: 0-415-33453-5 (hbk) ISBN10: 0-415-33454-3 (pbk) ISBN10: 0-203-37108-9 (ebk) ISBN13: 978-0-415-33453-2 (hbk) ISBN13: 978-0-415-33454-9 (pbk) ISBN13: 978-0-203-37108-4 (ebk) To my parents CONTENTS List of figures List of boxes List of plates List of colour plates Aims and scope of the book Acknowledgements ix xiii xv xvii xviii xxi 1 INTRODUCTION 1 2 THE FLUVIAL SYSTEM 10 3 THE FLOW REGIME 21 4 SEDIMENT SOURCES 37 5 LARGE-SCALE SEDIMENT TRANSFER 52 6 FLOW IN CHANNELS 69 7 PROCESSES OF EROSION, TRANSPORT AND DEPOSITION 93 8 CHANNEL FORM AND BEHAVIOUR 117 9 SYSTEM RESPONSE TO CHANGE 157 10 MANAGING RIVER CHANNELS 177 Notes Glossary of terms References Index 201 202 211 224 LIST OF FIGURES 1.1 Bedrock channels, alluvial channels and a cross-section of a mixed channel reach 2 1.2 The drainage basin 3 1.3 Elements of three-dimensional channel form 5 2.1 The fluvial system subdivided into three zones: the sediment production zone, sediment transfer zone and sediment deposition zone 12 2.2 Simplified representation of the fluvial system 14 2.3 Schematic representation of complex response in a drainage system 16 2.4 Idealised sketch showing the components of the fluvial system. (a) Channel network. (b) River reach. (c) Meander bend. (d) Bedforms. (e) Sedimentary structure. (f ) Sedimentary grain 18 2.5 Types of equilibrium observed over different time scales 19 3.1 Surface and subsurface hydrological pathways 22 3.2 (a) Typical annual hydrograph of a temperate zone river. (b) Characteristics of a storm hydrograph 27 3.3 Flow regimes for selected rivers 29 3.4 Regional growth curves, showing the relation between flood magnitude and frequency 34 4.1 Schematic illustration of different sediment sources 37 4.2 Driving and resisting forces acting on a block of slope material 40 4.3 Examples of mass movement. (a) Translational slide. (b) Rotational slide or slump. (c) Debris flow. (d) Rockfall 41 4.4 Summary of the principal factors controlling water erosion on hillslopes and sediment yield to river channels 43 4.5 Horton's model of overland flow and rill formation 45 5.1 Modes of sediment transport 53 5.2 Sediment yield and specific sediment yield for the Hokitika River, New Zealand, and the Santa Clara River in California, USA 55 5.3 Sediment stores 55 x LIST OF FIGURES 5.4 Global patterns of suspended sediment yield 56 5.5 Climatic variation of sediment yield 57 5.6 Variation of sediment yield with basin area for seven topographic categories of river basin 58 5.7 The components of a sediment budget for a reach of channel 62 6.1 Basic channel parameters 70 6.2 The effect of channel shape on hydraulic radius 71 6.3 (a) Example of a vertical velocity profile, showing changes in time-averaged velocity with depth, (b) Channel cross-sections illustrating the effect of channel shape on the velocity distribution 75 6.4 Illustration of the concept of flow continuity 76 6.5 Flow variations through time and space 77 6.6 Illustration of hydraulic drop and hydraulic jump 81 6.7 Flow separation around obstructions and at a channel bend 83 6.8 (a) Velocity profile and boundary layer for turbulent flow, (b) Fully developed flow profile 85 6.9 (a) Hydraulically smooth surface, (b) Hydraulically rough surface 86 6.10 Hydraulic parameters associated with overbank flow 91 7.1 Bank erosion processes 101 7.2 Driving and resisting forces acting on a grain of sediment resting on the bed of a channel 101 7.3 (a) The Hjulstrøm diagram, showing the mean flow velocity at which particles of a given size are entrained, transported and deposited, (b) A Shields diagram showing the relationship between the critical dimensionless bed shear stress and the boundary Reynolds number 102 7.4 Bed sediment characteristics. (a) Friction angle. (b) Sheltering. (c) Imbrication. (d) Armour layer. 104 7.5 Bedform variation in sand-bed channels 107 7.6 Idealised diagram of a cluster bedform 108 7.7 Characteristics of suspended sediment transport by rivers 112 8.1 The driving variables and boundary conditions that control the form of a channel reach 118 8.2 Interrelationships amongst form, flow and sediments in active gravel-bed river 119 8.3 Illustration of the Lane balance between stream power and sediment supply 120 8.4 (a) Schematic representation of the variation in channel properties through a drainage basin, (b) Long profile of the River Ure, North Yorkshire, UK 127 8.5 Schematic diagram of the time scales of adjustment of various channel form components with given length dimensions in a hypothetical basin of intermediate size 128 8.6 The development of meanders reduces channel slope 129 8.7 Types of channel bar 130 8.8 (a) Cross-sectional view of channel benches, (b) Concave bank bench, (c) Riffle-pool sequence, (d) Step-pool sequence and (e) Bedrock bars 131 8.9 Some of the features associated with (a) medium-energy non-cohesive meandering floodplains and (b) low-energy cohesive floodplains 8.10 The continuum of variants of channel planform 135 137 LIST OF FIGURES 8.11 Sinuosity ratio definition 138 8.12 Aspects of meander geometry. (a) Meander wavelength. (b) Radius of curvature. (c) Typical channel cross-sections 139 8.13 Transformation of a straight channel with a riffle-pool bed into a meandering channel 141 8.14 (a) Flow in a meander bend, (b) Styles of meander migration and cut-off formation 142 8.15 Mechanisms of braid development, based on flume experiments of Leopold and Wolman (1957) and Ashmore (1991), and cartoons from Ferguson (1993) 147 9.1 Pulsed (top) and ramped (middle) disturbances 160 9.2 Palaeochannel patterns in the Prosna valley near Wieruszów, Poland 170 9.3 Sequence of development of channels in urban areas 172 9.4 Aggradation and degradation at a dam 173 9.5 (a) Schematic diagram showing the relation of the Indus valley profile and channel pattern to tectonic elements, (b) Meander belt migration as a result of lateral (cross-valley) tilting 174 10.1 The fluvial hydrosystem 183 10.2 The habitat diversity provided by (a) natural and (b) engineered channels 187 10.3 Environmental management techniques. (a) Distant flood embankments. (b) Multi-stage channel. (c) Bank protection using woven fencing 191 10.4 In-stream structures. (a) Weirs or low dams. (b) Flow deflectors. (c) Sills. (d) Aquatic ledge 195 xi LIST OF BOXES 1.1 Drainage network patterns 4 3.1 The measurement of streamflow 24 3.2 Calculating flood return periods 31 3.3 The frequency and magnitude of channel forming flows 33 5.1 Estimating the suspended sediment load of a river 54 5.2 Using sediment budgets to assess the impacts of land use change in the Coon Creek drainage basin 63 5.3 Case Study: Impacts of gold mining in the Sacramento Valley, California 66 6.1 The du Boys Equation 70 6.2 Energy relationships and the Bernoulli Equation 72 6.3 Flow resistance formulae 78 6.4 Grain Reynolds number 86 6.5 Shear stress distribution within the velocity profile 87 6.6 The logarithmic 'law of the wall' for calculating bed shear stress 89 7.1 Example application of the stream power equations 94 7.2 The Shields parameter 103 7.3 Describing the size distribution of bed sediment 105 7.4 Field techniques for assessing rates of bedload transport 109 7.5 Bedload transport formulae 110 8.1 Hydraulic geometry relationships 125 9.1 Predicting the response to changes in discharge and bedload supply 158 9.2 Terrace formation 164 10.1 Case Study: Experimental flooding in the Grand Canyon, USA 194 10.2 Case Study: The River Skerne restoration project 197 LIST OF PLATES 4.1 Rills developed in a road cutting, South Africa 44 4.2 Ephemeral gully near Ilminster, Somerset, England 46 4.3 Severe gullying in eastern Madagascar 48 6.1 Hydraulic drop and hydraulic jump at a weir on the River Liffey, County Kildare, Irish Republic 82 7.1 95 Potholes near the site of a waterfall, Blyde River Canyon, South Africa 7.2 Sculpted forms in the rock bed of an ephemeral channel, Atlas Mountains, Morocco 96 7.3 Slab failures in cohesive banks, River Ure, North Yorkshire, England 99 7.4 Mixed bank, where selective entrainment of the underlying non-cohesive material has led to collapse of the cohesive layer above, Gunnerside Gill, North Yorkshire, England 100 7.5 Alluvial fan descending the northern slope of Adventdalen, a large glacial trough on the main Spitsbergen island, Norway 115 8.1 A degrading reach along the Connecticut River, United States, where severe bank erosion is threatening a corn field 121 8.2 A confined valley setting, Hundar Gorge, Ladakh, northern India 122 8.3 The Slesse Creek, British Columbia, in 1940, prior to logging operations 123 8.4 The Slesse Creek in 1993, after the removal of riparian forest 123 8.5 Riffle and pool on the upper River Ure, North Yorkshire, England 132 8.6 Steps and pools, Julian Alps, Slovenia 133 8.7 High-energy non-cohesive floodplain, Tien Shan Mountains, Kyrgyzstan 134 8.8 A meandering stream incised into the surface of a glacier 140 8.9 Meander bend, showing how the fastest flow shifts to the outside of the bend, with deposition on the inside of the bend in the form of a point bar, River Ure, North Yorkshire, England 143 8.10 Valley confinement has restricted the lateral extension of these meander bends, resulting in their distorted shape, Maria's River, north central Montana, United States 144 8.11 Confluence between two braid channels during low flow conditions 146 xvi LIST OF PLATES 8.12 Oblique aerial photograph of the Sandover-Bundey River near Ooratippra in the Northern Territory, Australia 150 8.13 Plane bed channel, Mpumalanga Province, South Africa 153 8.14 Aerial photograph of an anabranching bedrock reach of the Sabie River, Mpumalanga Province, South Africa 154 8.15 A mixed anabranching reach of the Sabie River, South Africa 155 10.1 A steep, concrete-lined channel with grade control structures (weirs), Almaty, Kazakhstan 180 10.2 Spur dykes on the lower Mississippi, near Rosedale, Mississippi, United States 181 10.3 The heavily engineered River Tame as it flows beneath Spaghetti Junction, Birmingham, England 188 LIST OF COLOUR PLATES 1 Dry ephemeral channel, Eastern Cape, South Africa 2 Anabranching bedrock channel with rock bars, Mpumalanga Province, South Africa 3 Ephemeral mixed channel with rock bed and alluvial banks, Eastern Cape, South Africa 4 Meandering sand-bed river, the Red River, Texas, United States 5 Incised meanders, Colorado River, Arizona, United States 6 A 15 km stretch of the braided Tsangpo (Brahmaputra) River, near Lhasa, Tibet 7 Braided river during low stage conditions, Tien Shan Mountains, Kyrgyzstan 8 Oblique aerial view of Cooper Creek, Queensland, Australia 9 Columbia River, near Harrowgate, British Columbia, Canada, during the annual summer floods 10 White River, Colorado, United States, an example of a semi-confined valley setting 11 Examples of chute and neck cut-offs. Owl Creek, Alberta, Canada 12 MODIS image of the Ganges and Brahmaputra rivers 13 The undulating walls of Antelope Canyon, Arizona, United States 14 Flash flood in a normally dry channel, Eritrea 15 A dramatic example of gully erosion in an arroyo during flood conditions, eastern Colorado, United States 16 Severe aggradation on the Ok Tedi River (a tributary of the Fly River) in Papua New Guinea, caused by large volumes of sediment from gold mining operations 17 Wandering gravel bed river, North Slope, Alaska, United States 18 Palaeochannels are clearly visible along the valley of the Sweetwater River, Wyoming, United States 19 Unpaired terrace sequence formed in glacial outwash deposits, Tien Shan Mountains, Kyrgyzstan AIMS AND SCOPE OF THE BOOK This book aims to provide an accessible introduction to the subject of fluvial geomorphology for junior undergraduate students of geography and related disciplines. In addition, it will provide supporting material for more senior undergraduates who are looking for basic explanations to aid in the understanding of advanced texts and journal articles. Since fluvial geomorphology is a rapidly expanding, multi-disciplinary area, the book will also be of interest to those who require a broad overview of the subject, such as ecologists, geologists and engineers. Although introductory physical geography and geomorphology textbooks include chapters on fluvial geomorphology, the inevitable constraints imposed by the scope of such texts allows only relatively brief coverage. On the other hand, most of the fluvial geomorphology texts that are currently available are pitched at third-year level and beyond. Students at a more junior level may struggle to understand the underlying concepts, especially those who have a limited background in mathematics and science. This book bridges that gap. It has been written to provide clear, straightforward explanations of concepts and formulae, and little previous knowledge of maths and science is assumed. The material is organised to build on itself throughout the book, with the development of concepts and ideas that have been introduced in earlier chapters. These are illustrated with case studies and examples to develop the student's interest. More difficult concepts and additional formulae have been placed in selfcontained 'boxes', which can be bypassed if desired. Boxes are also included in some chapters to illustrate field techniques and selected case studies. New terms are introduced using bold text and many of these are included in an extensive glossary at the end of the book. In recommending further reading, it is assumed that readers will progress initially to more advanced textbooks for further information, although selected journal articles have also been listed to include illustrative case studies and some 'classics'. Useful web sites are also included. The owners of faculty sites have been contacted to confirm the stability of the site in the foreseeable future and to request that a forwarding link is provided if necessary. Fluvial geomorphology is interpreted in a broad sense in this book, which includes chapters on hydrological and hillslope processes as well as large-scale sediment transfer. This is felt to be appropriate for an introductory text, since an appreciation of the interactions between hillslopes and channel–floodplain systems is necessary to understand how the fluvial system responds to change. A holistic, catchment-wide approach is also widely advocated for successful river channel management. There are ten chapters, starting with an overview of some of the key concepts of fluvial geomorphology in Chapter 1, which introduces the huge variety of fluvial forms that are seen worldwide. Following on from this, AIMS AND SCOPE OF THE BOOK Chapter 2 discusses the fluvial system, examining equilibrium, scale and the complex interrelationships that exist between variables. Chapter 3 focuses on relevant aspects of hydrology, with particular emphasis on the characteristics of different flow regimes, flood frequency–magnitude relationships and channel-forming flows. A general discussion of sediment sources is provided in Chapter 4, which covers weathering, mass wasting and slope erosion processes. Chapter 5 goes on to explain how sediment is transferred through the fluvial system. The importance of sediment storage is highlighted, with an examination of scale effects and the historical legacy of past events, such as glaciations and human activity. Flow processes within the channel are considered in Chapter 6, which provides a basic overview of the properties of fluid flow, flow resistance and boundary layers. Chapter 7 discusses processes of erosion, sediment transport and deposition, highlighting the differences between bedload and suspended load transport. There is also a section on erosion in bedrock channels, which have been neglected by many previous texts. Channel form is examined in Chapter 8, which starts by considering the various controls on morphology, the nature of morphological adjustments and the space and time scales over which they take place. A wide range of alluvial and bedrock-influenced channels are included and relationships between floodplain morphology and channel processes are also examined. Following on from this, Chapter 9 considers the response of fluvial systems to environmental change. Approaches to reconstructing and understanding the nature of past changes are outlined, followed by a discussion of the ways in which river systems have been influenced by climate change, human activity, tectonics and changes in base level. The focus of Chapter 10 is on management, starting with the management problems that are associated with river channels and the reasons why channel engineering is carried out. Some of these management problems have been discussed in preceding chapters and the student is referred back to the relevant sections. Traditional and more recently developed environmental engineering techniques are examined, and the final section focuses on river restoration, providing an overview of the main techniques used and the considerations that need to be taken into account if these are to be successful. xix ACKNOWLEDGEMENTS There are a number of people who I would like to thank for their contribution to this book, not least the many friends and colleagues who have provided support and encouragement during its preparation. Special mention must go to Jim Keenan for his patience and skill in reproducing the artwork. I am very grateful to the three reviewers of the first draft, who provided helpful and constructive criticisms, which I have tried to address in the final version. The comments and suggestions made by the reviewers of the initial proposal were also very helpful. Paddy Charlton and Allan Hale read through the first draft and, as non-specialists, commented on the clarity and accessibility of the text. They also corrected numerous typos and grammatical errors. Thanks are also due to Ian Benson for several interesting discussions on the nature of fluid mechanics and sediment transport and for his comments on the final versions of Chapters 6 and 7. Bernadette Gardiner and the library staff at the National University of Ireland, Maynooth, were always helpful and efficient in dealing with numerous inter-library loans. Some of the material in this book was class-tested on students in the Department of Geography, NUI, Maynooth, who provided helpful feedback. The NUI, Maynooth, Publications Committee awarded a grant towards the cost of photographs and other illustrations. The copyright of the photographs remain held by the individuals who kindly supplied them. The author and publishers would like to thank the following for granting permission to reproduce material in this work: Airphoto – Jim Wark for permission to reproduce Plate 10.2 and Colour Plates 4, 5, 9, 10, 11, 15, 17 and 18. American Society of Civil Engineers, Reston VA, for permission to reproduce Figure 9.5(a), after figure 8.7 in D.J. Harbor, S.A. Schumm and M.D. Harvey, Tectonic control of the Indus River in Sindh, Pakistan, in S.S. Schumm and B.R. Winkley (eds), The Variability of Large Alluvial Rivers (ASCE Press, New York, 1994). Blackwell Publishing, Oxford, for permission to reproduce Figure 8.4(a), after figure 6.3 in M. Church, Channel Morphology and Typology, in P. Calow and G.E. Petts, The Rivers Handbook (Blackwell Scientific, London, 1992), and Figure 8.2, after figure 1 in P.J. Ashworth and R.I. Ferguson, Interrelationships of channel processes, changes and sediments in a proglacial river, Geografiska Annaler, 68 (1986), 361-71, and Figure 8.10, after figure 4.9 in G. Brierley and K. Fryirs, Geomorphology and River Management: Applications of the River Styles Framework (Blackwell Science, London, 2005). Rhett Butler, WildMadagascar.org for permission to reproduce Plate 4.3. xxii ACKNOWLEDGEMENTS Cambridge University Press, Cambridge, for permission to reproduce Figure 6.10, after figure 1 in K. Shiono and D.W. Knight, Turbulent open channel flows with variable depth across the channel, Journal of Fluid Mechanics, 222 (1991), pp. 617–46. Canadian Society of Petroleum Geologists, Calgary, for permission to reproduce Figure 7.6, after figure 2 in A.C. Brayshaw, The Characteristics and origin of Cluster Bedforms in Coarse-grained Alluvial Channels, in C.H. Koster and R.H. Stell, (eds), Sedimentology of Gravels and Conglomerates, Canadian Society of Petroleum Geologists Memoir 10 (1984), pp. 77–85. University of Chicago Press, Chicago, for permission to reproduce Figure 5.6, after figure 6 in J.D. Milliman and P.M. Syvitski, Geomorphic/tectonic control of sediment discharge to the ocean: the importance of small mountainous rivers, Journal of Geology, 100 (1992), pp. 525–44. Elsevier, London, for permission to reproduce Figure 8.4(b), after figure 3 in T.J. Coulthard, J. Lewin and M.G. Macklin, Modelling differential catchment response to environmental change, Geomorphology, 69 (2005), pp. 222–41, copyright 2005 Elsevier. Foerderkreis "Rettet die Elbe", Hamburg (Germany) for permission to reproduce Colour Plate 16. Connell Foley for permission to reproduce Colour Plate 14. Geological Society of America, Boulder CO, for permission to reproduce Figure 8.13, after figure 4 in E.A. Keller, Development of alluvial stream channels: a five-stage model, Geological Society of America Bulletin, 83 (1972), pp. 1531–6. Hodder Education, London, for permission to reproduce Figures 6.3, 8.5, 8.8(c)–(d) and 8.15, after figures 4.1B, 5.3, 5.13 and 5.23 in D. Knighton, Fluvial Forms and Processes: A New Perspective (Arnold, London, 1998), and Figure 6.8(a), after figure 2.2 in A. Robert, River Processes: An Introduction to Fluvial Dynamics (Arnold, London, 2003). Helen Houghton-Carr for permission to reproduce Plate 7.1. Tim McCabe, National Resources Conservation Service for permission to reproduce Plate 8.1. Jeanne Meldon for permission to reproduce Plate 6.1. John Wiley & Sons, Chichester, for permission to reproduce Figure 2.4, after figure 12.1 in S.A. Schumm, Variability of the Fluvial System in Space and Time, in T. Rosswell, R.G. Woodmansee and P.G. Risser (eds), Spatial and Temporal Variability in Biospheric and Geospheric Processes (Wiley, Chichester, 1988), pp. 225–50; Figure 3.3(b) and 3.4, after figures 10.2 and 10.11 in D.A. Knighton and G.D. Nanson, Distinctiveness, Diversity and Uniqueness in Arid Zone River Systems, D.S.G. Thomas (ed.), Arid Zone Geomorphology (Wiley, Chichester, 1997), pp. 185–203; Figure 7.7, after figure 5.10A and C, in I. Reid, J.C. Bathurst, P.A. Carling, D.E. Walling and B.W. Webb, Sediment Erosion, Transport and Deposition, in C.R. Thorne, R.D. Hey and M.D. Newson (eds), Applied Fluvial Geomorphology for River Engineering and Management (Wiley, Chichester, 1997), pp. 95–135; Figure 8.14, after figure 22.2 in A.J. Markham and C.R. Thorne, Geomorphology of Gravel-bed River Bends, in P. Billi, R.D. Hey, C.R. Thorne and P. Taconi (eds), Dynamics of Gravel-bed Rivers (Wiley, Chichester, 1992), pp. 433–50, and Figure 9.4, after figure 7.6, in N.D. Gordon, T.A. McMahon, B.L. Finlayson, C.J. Gippel and R.J. Nathan, Stream Hydrology: An Introduction for Ecologists, second edition (Wiley, Chichester, 2004). ACKNOWLEDGEMENTS Gerald Nanson for permission to reproduce Plate 8.2 and Colour Plate 8. NASA for permission to reproduce Colour Plates 6 and 12. Oxford University Press, Oxford, for permission to reproduce Figure 4.4, after figure 4.1 in R.U. Cooke and J.C. Doornkamp, Geomorphology in Environmental Management: A New Introduction (Clarendon Press, Oxford, 1990). Pearson Education, Harlow, for permission to reproduce Figures 2.3 and 6.1, after figures 9.27 and 8.7 in M. Summerfield, Global Geomorphology: An Introduction to the Study of Landforms (Longman, Harlow, 1991). Province of British Columbia, Victoria, for permission to reproduce Plates 8.3 and 8.4, reprinted from BC Air Photos BC207-55 and BC93026-77. River Restoration Centre, Silsoe, for permission to reproduce in adapted form Figure 10.4(d) and Box 10.2 Figure 1 from figure 3.2.1 and 'The River Skerne Restoration Project' (p. 5) in River Restoration Centre, River Restoration Manual of Techniques (River Restoration Centre, Silsoe, 1999). Mike Simms for permission to reproduce Plates 4.2 and 7.2. South Africa National Parks and Centre for Water in the Environment, University of the Witwatersrand, Johannesburg, for permission to reproduce Plate 8.14. Springer Science, Dordrecht, for permission to reproduce Figure 10.1, after figure 1.1 in G.E. Petts and C. Amoros, Fluvial Hydrosystems (Chapman & Hall, London, 1996). Taylor & Francis, London, for permission to reproduce Figure 4.3, after figure 2.6 in R.J. Hugget, Fundamentals of Geomorphology (Routledge, London, 2003), and Box 6.2 Figure 3, after figure 3.9 in M. Kay, Practical Hydraulics (E. & F.N. Spon, London, 1998). United States Geological Survey, Washington DC, for permission to reproduce Box 7.4 Figure 1, after figure 1 in W.W. Emmett, A Field Calibration of the Sediment-trapping Characteristics of the Helley–Smith Bedload Sampler, US Geological Survey Professional Paper 1139 (1980). Tony Waltham Geophotos for permission to reproduce Plate 7.5 and Colour Plate 13. Every effort has been made to contact copyright holders for their permission to reprint material in this book. The publishers would be grateful to hear from any copyright holder who is not here acknowledged and will undertake to rectify any errors or omissions in future editions of this book. xxiii 1 INTRODUCTION The term fluvial is derived from the Latin fluvius, meaning river. Fluvial geomorphology is the study of the interactions between river channel forms and processes at a range of space and time scales. The influence of past events is also significant in explaining the present form of river channels. Rivers are found in many different environments and show an amazing diversity of form. In this chapter you will learn about: ● ● ● The different scales at which rivers are examined. How the form of a river channel is determined by the balance between the physical force acting on the material forming the channel and the resistance of that material to being moved. The way in which local environmental factors interact with these hydraulic forces to produce a wide variety of channel forms. DIVERSITY OF FORM A quick look through the photographs in this book will give you some idea of the variety that can be seen in rivers and streams worldwide. Rivers drain much of the land area – with the exception of regions that are hyperarid or permanently frozen – and their variety reflects the vast range of different environments in which they are found. Climate, geology, vegetation cover and topography are just some of the factors that influence river systems. Rivers are found in many different climatic zones, ranging from humid to arid, and from equatorial to arctic. Some of the larger rivers even flow across different climatic zones, originating in a humid area before flowing through an arid region. Examples of these 'exotic' rivers include the Nile and Colorado, both of which sustain agriculture and urban centres in desert regions. Perennial rivers flow for all or most of the year, while many of those in dryland environments only transmit water at certain times. The 'trail' shown in Colour Plate 1 is actually an ephemeral channel that was photographed in South Africa during the winter dry season. A small herd of cattle in the distance provide an idea of scale. The material in which the channel is formed is called the channel substrate. An important distinction can be made between bedrock and alluvial substrates (Figure 1.1). Bedrock channels, as their name suggests, are sections of channel that are cut directly into the underlying bedrock, while alluvial channels are formed in alluvium – sediment that has previously been laid down in the valley floor by rivers. Alluvium can include a mixture of unconsolidated particles ranging in size from boulders, gravels and sands to finer deposits of silts and clays. Where the valley floor is wide enough, material laid down in the channel, together with silt deposited by 2 FUNDAMENTALS OF FLUVIAL GEOMORPHOLOGY Bedrock (a) Bedrock Alluvium (b) Alluvium Flood flow Bedrock Low flow channels (c) Figure 1.1 (a) Bedrock channels are cut directly into the underlying bedrock. (b) Alluvial channels are formed in alluvial deposits that have been deposited on the valley floor by fluvial processes. (c) Rivers exist on a continuum from bedrock to alluvial and there are also many mixed alluvial/bedrock channels. This is just one example, showing a crosssection of a mixed channel reach. Bars separate the flow into multiple channels. Each bar has a bedrock core, which is overlain by vegetated alluvial deposits. (c) adapted from van Niekirk et al. (1999). INTRODUCTION floods, form a floodplain adjacent to the river channel (Figure 1.1b). The ephemeral channel shown in Colour Plate 1 is formed in alluvium. Colour Plate 2 shows an example of a bedrock channel, while Colour Plate 3 shows a mixed channel which has a rock bed and alluvial banks. Most rivers flow to the oceans, although some drain to inland seas and lakes, while others dry up completely before reaching the ocean. Each river drains an area of land called its drainage basin – also known as its catchment or watershed – which supplies water and sediment to the channel (see Figure 1.2). This area is bounded by a drainage divide or catchment boundary, something that is clearly visible as a ridge in mountainous areas but which can be rather difficult to discern in areas of more subdued topography. The outlet, where the main channel exits the basin, is at a lower elevation than the rest of the basin area. Drainage basins form a mosaic across the land surface, varying greatly in size from a few hectares to millions of square kilometres. Within each drainage basin is a branching network of channels. The main, or trunk, channel is fed by numerous small tributaries which join to form progressively larger channels. Drainage patterns, as viewed from the air or on maps, vary considerably between basins. The development and evolution of drainage networks is influenced by a number of factors, including geology, climate and long-term drainage basin history. Further information on the ways in which structural controls influence drainage patterns can be found in Box 1.1. In terms of the actual form of different channels, obvious differences can be seen, even along the same river. One of the more noticeable things is the variation in size, from tiny headwater streams that are just a few centimetres wide to large rivers several hundred metres or more in width. The size of a river channel at a given point is largely determined by the discharge supplied from upstream. This is the volume of water that passes through a given channel cross-section in a given period of time. In the upper reaches of a river, the area drained – and hence the discharge – is relatively small. As you move downstream, discharge and channel size generally increase with the upstream drainage area. While many rivers follow a single channel, there are also numerous examples of rivers with multiple channels. Those that flow in a single channel usually tend to a D r ai n ivide ge d Outlet Figure 1.2 The drainage basin is the area of land drained by each river. 3 4 FUNDAMENTALS OF FLUVIAL GEOMORPHOLOGY Box 1.1 D R A I N A G E N E T W O R K PAT T E R N S Water and sediment are transported from the hillslopes to the basin outlet via the drainage network. The shape and density (length of stream channel per unit area) of drainage networks varies considerably, and no two are the same. Figure 1 provides some typical examples of selected drainage patterns. A major control on drainage network morphology is the underlying geology, although topography, soils, tectonic history and climate are also influential. Dendritic drainage networks, which have a random pattern, are found where there are no strong geological controls. Where there is a strong regional dip (slope), parallel drainage patterns develop. Trellis networks are also associated with regional dip, although in this case structural controls are also important; for example, where lines of weakness are provided by well developed joints running at right angles to the dip. Where two sets of structural controls run at right angles to each other, a rectangular pattern develops. Trellis and rectangular networks may also be found where there are alternating bands of hard and soft strata. Radial drainage develops as result of symmetrical erosion around uplifted domes and volcanoes. Although drainage patterns are used in the interpretations of tectonic influences and underlying structural controls, there may not always be a close correspondence. During their long term history, rivers cut down through great thickness of rock, and former drainage patterns are sometimes preserved. Parallel East Wind River Range Sage Creek 0 Dendritic Big Flat, Arkansas 5 Km Rectangular Schroon and Bouquet Rivers, New York Trellis Tuscarora Creek, Pennsylvania 0 Radial Mt Ellsworth, Utah 500 m 0 5 Km Figure 1 Basic drainage patterns. Adapted from Morisawa (1985). INTRODUCTION deviate from a straight line, sometimes following rather an irregular path, in other cases forming more regular meanders (Colour Plates 4 and 5). Braided channels are characterised by numerous bars and islands formed by sediment deposits in the channel (Colour Plates 6 and 7). A rather different type of multiple channel are the anabranching channels shown in Colour Plates 8 and 9. Rather than being separated by bars, the individual channels are cut into the floodplain itself. When considering channel form, individual sections, or reaches, of channel are usually considered. This is because of the downstream changes in channel size and shape that are brought about by factors such as increasing drainage area and variations in channel substrate. As a result, different channel patterns may be found along the same channel. At the reach scale – typically a few tens to hundreds of metres or more – there is a homogeneity of form. Rivers are three-dimensional in shape and, in addition to the channel planform patterns described above, variations are also seen in cross-sectional shape and channel slope (Figure 1.3). For example, braided channels tend to be relatively wide and shallow in comparison to meandering channels, which have a narrower, deeper cross-section. Headwater streams in mountainous areas typically flow in steep channels, with frequent waterfalls, pools and rapids. This is in contrast to rivers that flow across lowland floodplains, where the channel slope is much more gentle. Fluvial forms also exist at the sub-channel scale. These include channel bars, pools excavated by localised scour, and periodic features such as dunes and ripples that form on the bed of sandy channels. You will see later in the book how certain groupings, or assemblages, of these features are associated with different channel types. HOW RIVERS SHAPE THEIR CHANNELS Rivers and streams continuously shape and reform their channels through erosion of the channel boundary (bed and banks) and the reworking and deposition of sediments. For example, erosion and undermining of the banks can lead to channel widening. Scouring of the channel bed deepens the channel, while sediment deposition reduces the depth and can lead to the formation of channel bars. These are just some of the ways in which channel adjustment takes place. A B lope nel s Chan A B C D D C Planform (pattern) Cross-section Figure 1.3 Elements of three-dimensional channel form. The planform is the shape of the river viewed from above, the channel slope is shown for the reach between points A and B, and the channel cross-section between C and D. 5 6 FUNDAMENTALS OF FLUVIAL GEOMORPHOLOGY There is an important balance between the erosive power of the flow and the strength, or resistance, of the bed and bank material to erosion. During major flood events, when the erosive power of the flow is greatly increased, there can be dramatic changes in channel form. Just how dramatic these changes are depends on how much resistance is provided by the bed and banks. Channels formed in unconsolidated alluvium offer much less resistance to erosion than those cut in bedrock. In fact, most flows are able to shape channels formed in sandy alluvium because relatively little energy is required to set the individual sand grains in motion. Because silts and clays are smaller than sand grains, you might expect these particles to be easier to erode. However, they tend to stick together as a result of cohesive (attractive) electrochemical forces between the particles. This means that channel boundaries with a high proportion of silt and clay are actually more resistant to erosion than those formed in sand and fine gravel. The amount and type of vegetation growing along the banks is also significant, since this can provide additional resistance to erosion. In many cases, it is only extreme floods that are capable of significantly modifying bedrock channels. The comparative rarity of these very large floods means that channel adjustments tend to occur sporadically, being interspersed by long periods of little change. Alluvial channels dominated by cobbles and boulders may also be relatively unaffected by most flows, which are not powerful enough to move such coarse material. The energy needed to carry out geomorphological work is provided by the flow of water through the channel. For any length of channel, energy availability is dependent on two things: the flow discharge and the steepness of the channel slope. Increases in either of these will increase the stream power and therefore the potential to carry out geomorphological work. However, before sediment transport and erosion can take place, a surprising amount of energy has to be used simply to move water through the channel. This is because of various types of flow resistance, including friction between the flowing water and the channel boundary. This can be particularly high in rough, boulder bed channels but is also significant for channels formed in finer substrates. Energy is also expended when the channel impinges against the valley walls, when the flow moves around bends and as it cascades over steps and waterfalls. Friction is even generated within the flow itself as a result of eddies and turbulence. It is estimated that 95 per cent of a river's energy is used in overcoming flow resistance, leaving just 5 per cent to carry out geomorphological work. Flow and sediment supply The flow in natural channels constantly fluctuates through a continuous series of normal flows, floods and droughts. Sediment supply also varies through time. Rivers continuously adjust their form in response to these fluctuations, which in turn influences the flow of water and sediment transport through the channel. Because the flow of water in a river provides the energy required to shape the channel, the characteristics of that flow are very important in determining channel form. As previously mentioned, the mean discharge usually increases with the size of the upstream drainage area. However, the mean discharge does not reflect the way in which flow varies through time. These variations are described by the flow regime, which can be thought of as the 'climate' of a river. Characteristics of the flow regime include seasonal variations in flow and the size and frequency of floods. Processes of erosion, transport and deposition within a channel reach are influenced by the supply of sediment at the upstream end as well as sediment that is locally eroded from the bed and banks. It is not only the volume of sediment that is important, but also its size distribution. Processes of sediment transport are very different for coarse and fine sediment, so sediment supply has an important influence on channel form and behaviour. The finer materials – clay particles, silts and sands – are carried in the flow as suspended load. This can be transported over considerable distances. Coarse sediment, because it weighs more, is transported close to the channel bed as bedload. Compared with the suspended load, bedload movement is more localised, involving much shorter travel distances. Deposits of coarse material form the channel bars that characterise many alluvial and bedrock channels, although finergrained sand and silt bars are also common. As you will see later in this book, the form and behaviour of INTRODUCTION bedload-dominated channels is rather different from suspended load dominated channels. Valley setting Channel processes are driven by flow and sediment supply, although the range of channel adjustments that are possible are often restricted by the valley setting. The influence of channel substrate and vegetation on bank erosion and channel migration have already been mentioned. The valley slope is also significant, affecting the steepness of the channel, which, together with discharge, determines stream power. Channels that flow over very gentle gradients can sometimes be extremely restricted in the adjustments they can make because so little energy is available. Another control on channel adjustment is the degree of valley confinement. While some channels are able to migrate freely across a wide floodplain, others are confined to a greater or lesser extent by the valley walls. Various degrees of valley confinement can be seen in Colour Plates 4 to 11. THE FORM OF A CHANNEL With so many environmental variables influencing channel form, an enormous range of different channel forms and behaviour is possible. It should be pointed out that not all rivers fit neatly into one of these categories – there are many examples of transitional rivers that have characteristics associated with more than one channel type. Alluvial channel form Four main types of alluvial channels are generally recognised: straight, meandering, braided and anabranching. Straight channels Although there are many examples of streams and rivers that have been artificially straightened for engineering purposes, naturally straight channels are rare. Even where they do exist, variations are usually seen in flow patterns and bed elevation. Straight channels are relatively static, with rates of channel migration limited by a combination of low energy availability and high bank strength. This is especially true where the channel banks are formed from more resistant material, such as cohesive silts and clays. Meandering channels Meanders form in a variety of bedrock and alluvial substrates. Associated with moderate stream powers, alluvial meanders may develop in gravels, sands, or fine-grained silts and clays. An interesting characteristic of meanders is that they are scaled to the size of the channel, being more widely spaced for larger channels. The degree of meandering varies greatly, from channels that only deviate slightly from a straight line to sequences of highly convoluted meander bends. Variations are also seen in the regularity of meander bends, many of which are rather more irregular than the examples shown in Colour Plates 4 and 5. Meandering channels evolve over time as individual bends migrate across the floodplain. Erosion is usually focused at the outside of meander bends, which gradually eat into the floodplain as the channel migrates laterally. At the same time, deposition on the inside of the bend allows the channel to maintain its width. Cut-offs – short sections of abandoned channel – indicate the path of former meander bends (Colour Plate 11). Braided channels Braided rivers are characterised by wide, relatively shallow, channels in which the flow divides and rejoins around bars and islands (Colour Plates 6 and 7). The appearance of a braided channel varies with changing flow conditions. During high flows, many of the bars become partly or wholly submerged, giving the appearance of a single wide channel. At low flows, extensive areas of bar surface may be exposed (Colour Plate 7). In order for bars to form, an abundant supply of bedload is required. Much of this is supplied from the upstream catchment area, with additional contributions from bank erosion. The bars themselves can be formed from sand, gravel or boulders. Braided rivers, are associated with high rates of energy expenditure, which is involved in the transport 7 8 FUNDAMENTALS OF FLUVIAL GEOMORPHOLOGY of large volumes of sediment. They often have steep channel slopes, although there are several examples of large braided rivers that flow over low gradients, such as the lower reaches of the vast Brahmaputra River in India and Bangladesh (Colour Plate 12). Erodible banks are also required for the channel to become wide enough to allow for the growth and development of channel bars. Braided channels are highly dynamic, with frequent shifts in channel position. Modifications, such as the dissection and reworking of bars and the formation and growth of new bars, occur over relatively short periods of time (days to years). The presence of bars leads to complex patterns of flow within the channel, and there can be sudden shifts in the location of sub-channels. Individual channels can be abandoned or reoccupied in the space of a few days. Anabranching channels Anabranching channels, where the flow is divided into two or more separate channels, are relatively rare in comparison to braided and meandering channels. The separate channels, called anabranches, are typically cut into the floodplain, dividing it up into a number of large islands. Individual anabranches can themselves be straight, meandering or braided. Unlike braided channels, rates of lateral channel migration are typically very low. The islands are stable features and, depending on climatic conditions, are often well vegetated. However, new channels can be cut when floodwaters breach the channel boundary and spill out on to the floodplain. Other channels are abandoned as the flow is diverted elsewhere, or when they become infilled with sediment. Colour Plates 8 and 9 both show examples of a subset of low-energy anabranching channel that is referred to as anastomosing. Although most research on anabranching channels has so far been focused on anastomosing channels, anabranching channels represent the most diverse of the four main channel types. Bedrock channels Bedrock channels also show a wide diversity of form. In comparison with alluvial channels, bedrock and mixed bedrock–alluvial rivers have received relatively little attention until recently. These channels often behave in a different way to alluvial channels, being strongly influenced by the resistant nature of their substrate. Structural controls, such as joints, bedding planes and the underlying geological strata can all have a significant effect on flow processes and river morphology. As with alluvial channels, the flow may follow single or multiple channels. Straight reaches are often associated with structural controls, for example where the channel follows the line of a fault or joint. However, flow characteristics also have an influence in shaping the channel. Colour Plate 13 shows the regularly undulating walls of a slot canyon, which have been shaped by flash floods. Meanders can also form in bedrock-influenced channels, as can be seen from the spectacular incised meanders of the Colorado (Colour Plate 5). Because of the resistant substrate, bedrock meanders tend to be scaled to higher flows than their alluvial counterparts. An example of a multi-channel bedrock river is shown in Colour Plate 2. The individual channels have cut their course to flow around bedrock bars. In some mixed bedrock–alluvial channels, bedrock bars may form a core that becomes covered in alluvial deposits, giving the appearance of an alluvial channel. Chapter summary Rivers are found in many different climate zones. They cut their channels into a range of bedrock and alluvial substrates. Each river drains an area called its drainage basin and is fed by a network of channels, which transports water and sediment from the land area to the basin outlet. The form of a given reach (length) of channel is controlled by the supply of flow and sediment to its upstream end. Also significant are the channel substrate, valley width, valley slope and bankside vegetation. All these controls vary, both between rivers and along the same river. This creates a huge range of fluvial environments and resultant channel forms. The threedimensional shape of a river is described in terms of its planform, slope and cross-sectional shape. Rivers continuously adjust their channels in response to fluctuations in flow and sediment supply. An important balance exists between the erosive force of the flow and the INTRODUCTION resistance of the channel boundary to erosion. Four main types of alluvial channel form can be identified: straight, meandering, braided and anabranching. Bedrock channels also exhibit a wide variety of different forms. FURTHER READING Introductory texts Many introductory geomorphology text books contain good chapters on fluvial geomorphology. These will provide a general overview and may be helpful later on if you would like a more basic explanation of certain concepts. Gilvear, D.J., 2005. Fluvial geomorphology and river management. In: J. Holden (ed.), An Introduction to Physical Geography and the Environment. Pearson Education, Harlow, pp. 327–55. Well written and easy to follow, this clearly explains the key concepts and has an interesting management perspective. Huggett, R.J., 2003. Fundamentals of Geomorphology. Routledge, London. The chapter on fluvial landscapes provides a good, accessible introduction. Summerfield, M.A., 1990. Global Geomorphology: An Introduction to the Study of Landforms. Longman, Harlow, An excellent book, which contains clearly explained chapters on fluvial processes and fluvial landforms. Textbooks for further reference There are a number of textbooks of fluvial geomorphology. Most of them are written for senior undergraduate and postgraduate students. Knighton, D.A., 1998. Fluvial Forms and Processes: A New Perspective. Arnold, London. A well-respected book, written at a more advanced level, which will provide valuable further reading. Leopold, L.B., 1994. A View of the River. Harvard University Press, Cambridge MA. An excellent book, written for a wide audience and liked by my own students. Leopold, L.B., Wolman, M.G. and Miller, J.R., 1964. Fluvial Processes in Geomorphology. Freeman, San Francisco. The 'Bible' of fluvial geomorphology. Although long out of print and available only in libraries, this was a ground-breaking book and is interesting to look through. Morisawa, M., 1985. Rivers. Longman, Harlow. Also out of print, but provides a clear, well illustrated introduction with some interesting examples. Petts, G.E. and Foster, I.D.L., 1985. Rivers and Landscape. Arnold, London. Out of print, but has clear explanations. Well worth referring to if you can get hold of a copy. Richards, K., 1982. Rivers: Form and Process in Alluvial Channels. Methuen, London. Another classic out of print book. Some students may be put off by the mathematical approach but the explanations are clear. Good on sediment transport processes and channel adjustment. Robert, A., 2003. River Processes: An Introduction to Fluvial Dynamics. Arnold, London. Written to complement the book by Knighton, with a focus on channel processes. Thorne, C.R., Hey, R.D. and Newson, M.D. (eds), 1997. Applied Fluvial Geomorphology for River Engineering and Management. John Wiley & Sons, Chichester. An edited volume, with contributions from leading experts in the field, which aims to provide river engineers and managers with an overview of fluvial geomorphology. 9 2 THE FLUVIAL SYSTEM A system is a collection of related objects and the processes that link those objects together. Within fluvial systems, objects such as hillslopes, the channel network and floodplains are linked together by the processes that move water and sediment between them. In common with other systems, the fluvial system is hierarchical, in that there are integrated sub-systems operating within it. This chapter will provide an overview of the fluvial system. Simplified examples are used here because the process–form interactions that are discussed in later chapters have not yet been covered. In this chapter you will learn about: ● ● ● ● ● ● How energy and materials move through the system. Fluvial system variables (e.g. channel slope, discharge, bedload transport rate). The way in which some variables control others and how these relationships depend on the space and time scales considered. How feedbacks between variables can counteract or enhance system response to change. The role of thresholds in system behaviour. How equilibrium can be defined at different time scales. INPUTS, OUTPUTS AND STORES The basic unit of the fluvial system is the drainage basin. Fluvial systems are open systems, which means that energy and materials are exchanged with the surrounding environment. In closed systems, only energy is exchanged with the surrounding environment. Inputs The main inputs to the system are water and sediment derived from the breakdown of the underlying rocks. Additional inputs include biological material and solutes derived from atmospheric inputs, rock weathering and the breakdown of organic material. Most of the energy required to drive the system is provided by the atmospheric processes that lift and condense the water that falls as precipitation over the drainage basin. The pull of gravity then moves this water downslope, creating a flow of energy through the system. This energy is expended in moving water and sediment to river channels and through the channel network. Outputs Water and sediment move through the system to the drainage basin outlet, where material is discharged to the ocean. Not all rivers reach the ocean; some flow into inland lakes and seas, while others, such as the Okavango River in Botswana, dry up before reaching the ocean. This reflects another important output from fluvial systems: the loss of water by evaporation to the atmosphere. THE FLUVIAL SYSTEM Most of the available energy is used in overcoming the considerable frictional forces involved in moving water and sediment from hillslopes into channels and through the channel network. Much of this energy is 'lost' to the atmosphere in the form of heat. Stores A certain amount of material is stored along the way. For example, water is stored for varying lengths of time in lakes and reservoirs, and below the ground in the soil and aquifers. Sediment is stored when it is deposited in channels, lake basins, deltas, alluvial fans and on floodplains. This material may be released from storage at a later stage, perhaps when a channel migrates across its floodplain, eroding into formerly deposited sediments which are then carried downstream. Ferguson (1981) describes the channel as 'a jerky conveyor belt', since sediment is transferred intermittently seawards. TYPES OF SYSTEM Three types of system can be identified in fluvial geomorphology. These are morphological systems, cascading systems and process–response systems. Morphological (form) systems Landforms such as channels, hillslopes and floodplains form a morphological system, also referred to as a form system. The form of each component of a morphological system is related to the form of the other components in the system. For example, if the streams in the headwaters of a drainage basin are closely spaced, the hillslopes dividing them are steeper than they would be if the streams were further apart from each other. Relationships such as this can be quantified statistically. Cascading (process) systems The components of the morphological system are linked by a cascading system, which refers to the flow of water and sediment through the morphological system. Cascading systems are also called process systems or flow systems. These flows follow interconnected pathways from hillslopes to channels and through the channel network. Process–response systems The two systems interact as a process–response system. This describes the adjustments between the processes of the cascading system and the forms of the morphological system. There is a two-way feedback between process and form. In other words, processes shape forms and forms influence the way in which processes operate (rates and intensity). This can be seen where a steep section of channel causes high flow velocities and increased rates of erosion. Over time erosion is focused at this steep section and the channel slope is reduced. Velocity decreases as a result, reducing rates of erosion. In order to examine the components of the fluvial system in more detail, it can be divided into sub-systems, each operating as a system within the integrated whole. One way of doing this is to consider the system in terms of three zones, each of which is a process–response system with its own inputs and outputs (Figure 2.1). Within each zone certain processes dominate. The sediment production zone in the headwater regions is where most of the sediment originates, being supplied to the channel network from the bordering hillslopes by processes of erosion and the mass movement of weathered rock material. This sediment is then moved through the channel network in the sediment transfer zone, where the links between the channel and bordering hillslopes, and hence sediment production, are not so strong. As the river approaches the ocean, its gradient declines and the energy available for sediment transport is greatly reduced in the sediment deposition zone. It is primarily the finest sediment that reaches the ocean, as coarser sediment tends to be deposited further upstream. In fact, only a certain proportion of all the sediment that is produced within a drainage basin actually reaches the basin outlet. F L U V I A L S Y S T E M VA R I A B L E S Variables are quantities whose values change through time. They include such things as drainage density, hillslope angle, soil type, flow discharge, sediment yield, channel pattern and channel depth. 11 12 FUNDAMENTALS OF FLUVIAL GEOMORPHOLOGY Zone 1 production Zone 2 transfer Zone 3 deposition Figure 2.1 The fluvial system can be subdivided into three zones on the basis of the dominant processes operating within each zone. These are the sediment production zone, sediment transfer zone and sediment deposition zone. Adapted from Schumm (1977). Internal and external variables The external basin controls An important distinction exists between internal and external variables. All the examples given above are internal variables, which operate within the fluvial system. Internal variables are influenced by other internal variables, and also by variables that originate from outside the system. These external variables, such as climate, control or regulate the way in which the system operates. Unlike the internal variables, external variables operate independently, in that they are not influenced by what is going on inside the fluvial system. At the basin scale the external variables are climate, base level, tectonics and human activity. If you are considering a sub-system, such as a reach of channel in the transfer zone, the external variables would include the supply of flow and sediment to the channel. This is because these variables originate from outside the channel sub-system, even though they are internal variables at the basin scale. To avoid confusion, the 'ultimate' external variables – climate, base level, tectonics and human activity – will be referred to as external basin controls. The variables defined in this section act as regulators of the whole system. Any change in one of these variables will lead to a complex sequence of changes and adjustments within the fluvial system. ● ● Climate describes the fluctuations in average weather. Although the weather is always changing, longer-term characteristics such as seasonal and inter-annual variations can be defined. Other characteristics include how often storms of a given size can be expected to occur and the frequency and duration of droughts. Where no long-term changes are occurring in the climate, the combination of such attributes defines an envelope of 'normal' behaviour. Climate change occurs when this envelope shifts and a new range of climatic conditions arises. Tectonics refers to the internal forces that deform the Earth's crust. These forces can lead to large scale uplift, localised subsidence, warping, tilting, fracturing and faulting. Where uplift has occurred, THE FLUVIAL SYSTEM ● ● inputs of water have to be lifted to a greater elevation, increasing energy availability; some of the highest rates of sediment production in the world are associated with areas of tectonic uplift. Valley gradients are altered by faulting and localised uplift, which may in turn affect channel pattern. Lateral (sideways) tilting can cause channel migration and affect patterns of valley sedimentation. Base level is the level below which a channel cannot erode. In most cases this is sea level. If there is a fall in sea level relative to the land surface, more energy is available to drive flow and sediment movement. Conversely a relative rise in base level means that less energy is available, resulting in net deposition in the lower reaches of the channel. Over time these effects may be propagated upstream through a complex sequence of internal adjustments and feedbacks. Human activity has had an increasing influence on fluvial systems over the last 5,000 years, especially during recent times. Activities within the drainage basin such as deforestation, agriculture and mining operations all affect the flow of water and production of sediment. These are referred to as indirect or diffuse activities. River channels are also modified directly when channel engineering is carried out. Advances in technology over the last century have meant that dam construction, channel enlargement for navigation and flood control, channel realignment, the building of flood embankments and other engineering works can now be carried out at an unprecedented scale. Today there are very few rivers that have not been affected in some way by the direct and indirect effects of human activity. It can be argued that, under some circumstances, human activity can be considered to be both an internal and an external variable. Many of the direct modifications described above are in response to some local human perception of the system. For example, channels are dredged because they are not deep enough for navigation, or flood defence works carried out because floods occur too frequently. Urban (2002) suggests that direct human intervention can often be classified as an internal variable, although it is more appropriate to consider indirect human activities as external. The influence of the external basin controls on the fluvial system is represented schematically in Figure 2.2. Some internal variables have a greater degree of independence in that they are only affected in a limited way by the fluvial system. These variables are geology, soils and vegetation and topography (which includes relief, altitude and drainage basin size). All are internal variables because they are controlled to some extent by the external basin controls, however their main influence on the operation of the fluvial system is a controlling one. Adjustable (dependent) and controlling (independent) variables From the discussion above it can be seen that some variables control the adjustment of other variables. For example channel pattern is, among other things, affected by the supply of sediment to the channel. In this case, channel pattern is the adjustable or dependent variable while sediment supply is the controlling or independent variable. Things can get a little confusing because controlling variables may in turn be adjusted by other variables. Extending the previous example, sediment supply is itself controlled by hillslope vegetation cover. In this case, sediment supply is the adjustable variable and vegetation cover the controlling variable. All internal variables are adjustable because their operation is ultimately regulated by the external basin controls. They are also influenced to a greater or lesser extent by other internal variables. Because the relationships between variables are so complicated, it can be very difficult to isolate the effect of one variable on another. The hierarchical nature of the fluvial system means that variables operating at larger scales tend to affect the operation of variables at smaller scales. For example, climate affects vegetation cover and hillslope erosion, which in turn determine sediment supply, which influences channel pattern, which affects the small-scale flow dynamics in the channel, which governs the movement of individual grains. This is not a one-way process, however. Over long periods of time, the cumulative effect of small-scale processes, such as the erosion and deposition of individual grains, can lead to larger scale changes. These include changes in channel pattern and, over time 13 FUNDAMENTALS OF FLUVIAL GEOMORPHOLOGY CLIMATE Fluvial system boundary Geology Hillslope system Flow regime Sediment regime Channelfloodplain system HUMAN ACTIVITY Soils & vegetation Topography TECTONICS 14 BASE LEVEL Figure 2.2 Simplified representation of the fluvial system. periods of tens to hundreds of thousands of years, can adjust the slope of the whole river valley. Time itself is an important controlling variable. Every drainage basin has a historical legacy resulting from past changes that have taken place in the basin. This includes the cumulative effect of processes such as erosion, transport and deposition over long periods of time. It also includes the far-reaching effects of changes in the external basin controls, such as the variations in climatic conditions since the Last Glacial Maximum 18,000 years ago, which have greatly affected fluvial systems worldwide. In the temperate zone, many rivers underwent a transition from a braided to a meandering form as climate conditions ameliorated, vegetation became established and sediment loads decreased. However, vast quantities of sediment still remain in formerly glaciated drainage basins, where many fluvial systems are still adjusting to this glacial legacy. Feedbacks A feedback occurs when a change in one variable leads to a change in one or more other variables, which acts to either counteract or reinforce the effects of the original change. Two types of feedback are observed: negative feedback and positive feedback. Both are initiated by a change in one of the system variables, which in turn leads to a sequence of adjustments that eventually counteract the effect of the original change (negative feedback) or enhance it (positive feedback). When there is a change in one of the external controls, negative feedbacks allow the system to recover, damping out the effect of the change. An everyday example of a negative feedback loop is a central heating system controlled by a thermostat, which switches the source of heat on and off as the room cools and warms. An equilibrium is maintained as the temperature fluctuates around an THE FLUVIAL SYSTEM average value. A commonly cited example of negative feedback within the fluvial system occurs when a section of channel is suddenly steepened by tectonic faulting. This leads to a local increase in the flow velocity and rate of bed erosion. Over time this acts to reduce the channel slope, counteracting the effects of the original change. It should be noted that the actual sequence of events is usually rather more complex. This is because change in one part of the system can lead to complex changes, both locally and throughout the rest of the system. The nature of complex response will be discussed later in this chapter. Positive feedbacks have a very different effect. Soil erosion is a natural process and an equilibrium exists if rates of soil removal over a given period are balanced by rates of soil formation over that period. However, an external change, such as the deforestation of steep slopes, can lead to a dramatic increase in soil erosion. The upper soil layers contain the most organic matter, which is important in binding the soil together. It also increases soil permeability, allowing rainfall to soak into the soil rather than running over and eroding the soil surface. If the topsoil is removed, the lower permeability of the underlying soil layers means that more water runs over the surface, increasing erosion and removing still more soil layers. In this way several centimetres of soil can be removed by a single rain storm (Woodward and Foster, 1997). This greatly exceeds the rate of soil formation. Referred to colloquially as 'vicious circles' or 'the snowball effect', positive feedbacks involve a move away from an equilibrium state. They usually involve the crossing of a threshold (see below) as the system moves towards a new equilibrium. A small-scale example of positive feedback is the build up of sediment during the formation of a channel bar. Bar formation is initiated when bedload sediment is deposited at a particular location on the channel bed (the various mechanisms by which this occurs will be discussed in later chapters). This affects local flow dynamics, causing the flow to diverge over and around the initial deposit. As the flow diverges, it becomes less concentrated and therefore less able to transport the coarser sediment. Localised deposition occurs, further disrupting the flow and promoting further deposition and bar growth. Several feedbacks, both positive and negative, exist between channel form, water flow and sediment transport. These are quite complex and, in order to understand them more fully, you will need to be familiar with concepts introduced in Chapters 6 and 7. For this reason, a basic overview is provided here, with a more detailed description in Chapter 8. The form of a channel has an important influence on the way that water and sediment move through it. For example, flow is concentrated where the channel narrows, increasing erosion potential. As you saw above, deposition may occur where the flow diverges around obstacles such as bars. The character of the channel bed is also significant, since the size and arrangement of sediment determines bed roughness and resistance to flow. Where resistance is high, the average velocity of flow in the channel is reduced. This influences hydraulic conditions near the bed of the channel, which are significant for processes of erosion and deposition. Considerable differences are seen across the channel bed, giving rise to spatial variations in erosion and deposition. These processes themselves modify the form of the channel, feeding back to influence flow. Thresholds Thresholds are another important concept in systems theory and you will come across many examples in fluvial geomorphology. For example, a threshold is crossed when a sand grain on the bed of the channel is entrained (set in motion). Movement is resisted by the submerged weight of the grain and friction between it and the neighbouring grains. If the driving force exerted on the grain by the flow is less than these resisting forces, no movement will occur. It is only when the driving force of the flow exceeds the submerged weight of the grain that entrainment will take place. In this example, channel flow is an external variable. When a threshold is crossed there is a sudden change in the system, for example when loose material on a slope becomes unstable and starts to move down the slope as a landslide. The gradual processes by which rock is broken down and loose material builds up on a hillslope take place over time scales of tens to hundreds of years. Why, then, does a landslide occur at a particular point in time? Such a transition can come about when a change in one of the controlling variables leads 15 16 FUNDAMENTALS OF FLUVIAL GEOMORPHOLOGY to instability within the system – as a direct result of an earthquake (tectonics) for example. Thresholds that are crossed as a result of external change are called external thresholds. Instabilities may also develop over time without any external change having occurred. For this reason it is possible for a major landslide to be triggered by a relatively minor rainfall event that falls well within the expected climatic norm, because instability has gradually developed over time and the system is ready 'primed'. This is an example of an internal threshold. Another commonly cited example is the threshold that is crossed when a meander loop is cut off to form an oxbow lake (several of these are shown in Colour Plate 11). Again this is something that can take place without there having been any change in the external variables. Whether or not either kind of threshold is crossed depends on how 'sensitive' the system is, in other words how close to a threshold it is. To illustrate this point, consider a pan filled with water that is heated by 10°C at normal atmospheric pressure. If the water had an initial temperature of 25°C you would not expect to see much change in its appearance. However, if the initial temperature was 90°C, the same increase in temperature would lead to a dramatic change as a threshold was crossed and the water started to boil. In the second example, the sensitivity of the system is much greater because the water temperature is closer to boiling point. Once a threshold has been crossed, the system reaches a new equilibrium. Relating this back to the sand grain example, two scenarios could be considered. In the first, the particle is resting on a flat bed and is fully exposed to the flow. In the second, it is buried beneath a layer of gravel. The threshold for movement will be much lower for the exposed grain than for the buried grain, which cannot move unless the overlying gravel is removed. This leads to a complex sequence of episodic erosion and deposition in the tributary as the system searches for a new equilibrium. A similar sequence of events can be observed from experimental simulations of laboratory drainage networks (Summerfield, 1991) and is illustrated schematically in Figure 2.3. This represents a sequence of events that occurs at the upstream end of a channel (boxes running down the right hand side of the figure) and at the downstream end (boxes running TIME DOWNSTREAM STAGES 1 BASE LEVEL FALLINCISION 2 INCISION EXTENDS UPSTREAM 3 SEDIMENT SUPPLY NO CHANGE UPSTREAM NO CHANGE INCISION INCREASE AGGRADATION INCISION INCREASE 4 5 AGGRADATION EXTENDS UPSTREAM AGGRADATION DECREASE INCISION AGGRADATION DECREASE 6 INCISION EXTENDS UPSTREAM INCISION INCREASE Complex response The response of the fluvial system to change is often complex because of the many interrelationships that exist between the different components of the system. An example is the complex response of a tributary to a lowering in base level elevation at its outlet (Schumm, 1977). Here the main river, into which the tributary flows, has degraded, or lowered its channel elevation by erosion. 7 AGGRADATION Figure 2.3 Schematic representation of complex response in a drainage system as a result of a fall in base level. See text for full explanation. After Summerfield (1991). THE FLUVIAL SYSTEM down the left hand side of the figure). The numbers on the left hand side (1 to 7) indicate sequential time stages. Starting at the top of the page and working down, we can see what is happening upstream and downstream at each time stage by reading across the page. Changes in sediment supply from upstream are also indicated by the arrows running from 'upstream' boxes to 'downstream' boxes. ● ● ● ● ● ● ● Stage 1. A fall in base level results in a local steepening of the channel gradient at the downstream end. This might exist as a sharp break in channel slope or as a steeper section of channel. Erosion is focussed at this steeper part, leading to a localised lowering in channel elevation. As a result of this lowering, the section of channel immediately upstream becomes steeper. The steep zone therefore migrates upstream over time. The initial response of the channel to the fall in base level is therefore incision (downcutting) at the downstream end. At this stage the upstream part of the channel is unaffected, so there is no change in the supply of sediment coming from upstream. Stage 2. Over time, incision is propagated upstream. As a result there is an increase in the amount of sediment coming from further upstream. Stage 3. This increase in sediment supply from upstream leads in turn to a build up of sediment in the channel at the downstream end. This process is called aggradation and results in a reduction in channel gradient. Meanwhile, the zone of incision continues to migrate further upstream. Stage 4. Aggradation is propagated upstream. Since there is now a net deposition of sediment in the upstream part of the channel, there is a decrease in the volume of sediment supplied from upstream. Stage 5. At the downstream end, less sediment means that there is additional energy for erosion and a new cycle of incision begins. Aggradation continues to occur at the upstream end. Stage 6. The wave of incision extends upstream, once again increasing the supply of sediment from upstream. Stage 7. The increased sediment supply leads to aggradation at the downstream end of the channel. This example illustrates the way in which downstream changes can be propagated upstream, while the resultant upstream changes in turn control what happens downstream. In this way there is a complex cycle as periods of episodic erosion are interspersed with periods of deposition. SCALE IN FLUVIAL GEOMORPHOLOGY Scale is an important consideration in fluvial geomorphology, with process–form interactions occurring over a huge range of space and time scales. At one end of this range is the long-term evolution of the landscape. At the other are small-scale processes, such as the setting in motion of an individual grain of sand resting on the bed of a channel. Space scales therefore encompass anything from a few millimetres to hundreds of kilometres. Relevant time scales stretch from a few seconds to hundreds of thousands of years or more. In order to understand how the fluvial system operates we can examine the relationships between processes and form in more detail at finer scales. This can be done by examining individual sub-systems, or sub-systems within sub-systems. When focusing in like this it is important to remember that these sub-systems are all part of an integrated whole and therefore cannot be considered in isolation from the rest of the system. Space scales (spatial scales) In studying the fluvial system, the scale of relevance varies according to the type of investigation. At the largest, drainage basin scale (Figure 2.4a), it is possible to see the form and characteristics of the drainage network and drainage basin topography. These reflect the cumulative effect of processes operating over long time scales, as well as past changes imposed by the external basin controls. At a smaller scale, the form of a reach of meandering channel can be examined in the context of drainage basin history and the influence of controlling variables at the channel scale, such as the supply of water and sediment from upstream (Figure 2.4b). The way in which the form and position of the channel has changed over time scales extending to thousands of years may be 17 18 FUNDAMENTALS OF FLUVIAL GEOMORPHOLOGY preserved as floodplain deposits, which can be used in reconstructing drainage basin history. Moving in to look at an individual meander bend (Figure 2.4c), process–form interactions can be observed at a smaller scale. These include flow hydraulics within the bend and associated sediment dynamics. Investigations of rates of bend migration or bank erosion processes are also carried out at this scale. Depositional channel units such as the point bar (seen on the inside of the meander bend in Figure 2.4c) are of interest to sedimentologists, providing evidence about the flows that formed them. At a finer scale still are individual ripples on the bar surface (Figure 2.4d) formed by (b) (a) (c) (d) (e) (f) Figure 2.4 Idealised sketch showing the components of the fluvial system. (a) Channel network. (b) River reach. (c) Meander bend. (d) Bedforms. (e) Sedimentary structure. (f ) Sediment grain. After Schumm (1988). the most recent high flow and, moving even closer, the internal arrangement of grains (Figure 2.4e). At the finest scale are individual grains of sediment (Figure 2.4f). Time scales (temporal scales) and equilibrium At smaller spatial scales, process–form interactions generally result in more rapid adjustments. At the largest scale, the long term evolution of channel networks occurs over time scales of hundreds of thousands of years or more, while the migration of individual meander bends can be observed over periods of years or decades, and small-scale flow-sediment interactions within minutes. The perspective of the historically oriented geomorphologist concerned with the large-scale, long-term evolution of landforms is therefore very different to that of the process geomorphologist or engineer who is interested in the operation of channel processes at much shorter time scales (Schumm, 1988). Historical studies show that the fluvial system follows an evolutionary sequence of development that is interrupted by major changes induced by the external basin controls. However, over the much shorter time periods involved in the field measurement of processes, there may be little or no significant change in fluvial landforms. This might not matter too much if flow–sediment interactions at very small scales are of interest, although basin history certainly does have an influence at the reach scale, since channel form has been shaped by past changes in flow and sediment supply. The precise definition of equilibrium is also time dependent. Equilibrium refers to a state of balance within a system, or sub-system. Negative feedback mechanisms help to maintain the system in an equilibrium state, buffering the effect of changes in the external variables. However, different types of equilibrium may exist at different time scales. These were defined by Schumm (1977) with reference to changes in the elevation of the bed of a river channel above sea level. If you were to observe a short section of river channel over a period of a few hours you would not see any change in its form (unless there happened to be a flood), although you might see some sediment transport. Over this short time period the channel is said to be in a state of static equilibrium (Figure 2.5a). THE FLUVIAL SYSTEM Static equilibrium Channel bed elevation Steady-state equilibrium Dynamic equilibrium The same river, observed over a longer time scale of a decade, would show some changes. During this time, floods of various sizes pass through the channel, scouring the bed. In the intervening periods, deposition builds up the channel bed again. As a result of these cycles of scour and fill, the elevation of the channel bed fluctuates around a constant average value (Figure 2.5b) and steady state equilibrium exists. Over longer time scales, from thousands to hundreds of thousands of years or more, erosion gradually lowers the landscape. At these time scales, the channel elevation fluctuates around a changing average condition, the underlying trend being a reduction in channel elevation. This is called dynamic equilibrium and is illustrated in Figure 2.5c. As you know, the influence of the external basin controls cannot be ignored. Changes in any of these variables can lead to positive feedbacks within the system and a shift to a new equilibrium state. For example, in tectonically active regions, the section of channel might be elevated by localised uplift. Such episodes of change occur over much shorter time scales than the gradual evolution of the landscape, resulting in abrupt transitions. This type of equilibrium delights in the term dynamic metastable equilibrium (Figure 2.5d). CHAPTER SUMMARY Dynamic metastable equilibrium Time Figure 2.5 Types of equilibrium observed over different time scales. Adapted from Schumm (1977). A system is a group of related objects that are linked by processes. The basic unit of the fluvial system is the drainage basin. Three systems exist within this: morphological, cascading and process–response systems. Process–response systems describe the interactions between the processes (flows of energy and materials) of the cascading system and the forms of the morphological system. Fluvial system variables include hillslope angle, flow discharge and sediment transport rates. These are all quantities that change through time. Some variables (called controlling variables) act to control others (called adjustable variables). The ultimate controls are the external basin controls (climate, tectonics, base level and human activity). These influence the entire fluvial system. Complex interactions occur between internal variables as a result of feedbacks. Negative feedbacks act to mitigate the effects of a disturbance to the system, 19 20 FUNDAMENTALS OF FLUVIAL GEOMORPHOLOGY allowing a state of equilibrium to be maintained. Positive feedbacks lead to instability, exacerbating the effects of disturbances. This involves the crossing of one or more thresholds, causing the system to move towards a new equilibrium. FURTHER READING Introductory texts Huggett, R.J., 2003. Fundamentals of Geomorphology. Routledge, London. The first chapter includes an easy to follow discussion of systems in geomorphology. Summerfield, M.A., 1990. Global Geomorphology: An Introduction to the Study of Landforms. Longman, Harlow. See the first chapter for a good introduction to systems in geomorphology. Schumm, S.A., 1977. The Fluvial System. John Wiley & Sons, New York. Although out of print, this provides clear explanations of the main concepts. Schumm, S.A., 1988. Variability of the fluvial system in space and time. In: T. Rosswall, R.G. Woodmansee and P.G. Risser (eds), Scales and Global Change: Spatial and Temporal Variability in Biosheric and Geospheric Processes. SCOPE: 35. John Wiley & Sons, Chichester, pp. 225–50. Highly recommended, this provides a good overview of the fluvial system. Also available on line at: http://www.icsu-scope.org/downloadpubs/ scope35/chapter12.html. Classics Schumm, S.A. and Lichty, R.W., 1965. Time, space and causality in geomorphology. American Journal of Science, 263: 110–19. Examines how the significance of different variables and their interrelationships varies according to the time scale under consideration. 3 THE FLOW REGIME The flow in river channels exerts hydraulic forces on the boundary (bed and banks). An important balance exists between the erosive force of the flow (driving force) and the resistance of the boundary to erosion (resisting force). This determines the ability of a river to adjust and modify the morphology of its channel. One of the main factors influencing the erosive power of a given flow is its discharge: the volume of flow passing through a given cross-section in a given time. Discharge varies both spatially and temporally in natural river channels, changing in a downstream direction and fluctuating over time in response to inputs of precipitation. Characteristics of the flow regime of a river include seasonal variations in discharge, the size and frequency of floods and frequency and duration of droughts. The characteristics of the flow regime are determined not only by the climate but also by the physical and land use characteristics of the drainage basin. In this chapter you will learn about: ● ● ● ● ● The pathways taken by water as it travels to the channel network. How the flow in a river responds to inputs of precipitation. Seasonal variations in flow that characterise different climatic zones. The size (magnitude) and frequency characteristics of floods. Flows that are significant in shaping the channel. F L O W G E N E R AT I O N Hydrological pathways Inputs of water to a drainage basin are the various forms of precipitation that fall over its area. These include rain, snow, sleet, hail and dew. If you look at a drainage basin on a map, you will see that river channels cover only a very small part of the total area. This means that most of the water reaching the ground surface must find its way from the hillslopes and into the channel network. A number of pathways are involved (Figure 3.1), and a given 'parcel' of water arriving at a stream channel may have taken any number of them. Not all the incoming precipitation actually makes it to the basin outlet, since a certain percentage is evaporated back to the atmosphere. Water that falls on the leaves of vegetation and artificial structures like buildings is intercepted. Some of this water does fall to the ground, as anyone who has sheltered under a tree will know, but much of it is evaporated (water resource managers refer to this output as 'interception loss'). The amount of interception that occurs is dependent on such factors as the extent and type of vegetation (leaf size, structure, density and the arrangement of foliage), wind speed and rainfall intensity (Jones, 1997). Evaporation also takes place from the surface layers of the soil, and from lakes and wetland areas. A related process is transpiration, 22 FUNDAMENTALS OF FLUVIAL GEOMORPHOLOGY Flow convergence in a topographic hollow leading to saturation P P P P 1 4 Rapid through flow occurs where soil pipes are present P Unsaturated zone Variable source areas 2 P 1 P 44 3 44 Saturated zone Hortonian overland flow le r tab Wate 2 1 Bare rock with a low infiltration rate Saturation leading to saturation overland flow 2 Through flow 3 Ground water flow 4 Saturation overland flow Figure 3.1 Surface and subsurface hydrological pathways. whereby plants take up water through their roots and evaporate it through pores, called stomata, on the underside of their leaves. This means that water can be 'lost' to the atmosphere from some depth below the surface. Because evaporation and transpiration are difficult to monitor separately, they are usually considered together as the combined process of evapotranspiration. Water reaching the soil surface may either enter the soil by a process called infiltration or remain at the surface, moving down-slope as overland flow (labelled 1 and 4 in Figure 3.1). Infiltrated water either travels through the soil, parallel to the surface, as throughflow (2), or slowly percolates downwards to the saturated zone, travelling as groundwater flow (3). Rates of water movement are fastest for overland flow (typically between 50 m and 500 m per hour), between 0.005 m and 0.3 m per hour for throughflow and 0.005 m to 1.5 m per day for groundwater flow (Ward and Robinson, 1990). How fast a river rises after rainfall is very much dependent on the relative proportion of water taking faster (surface and near-surface) pathways, and slower (subsurface) pathways. An important control on this is the rate at which water infiltrates into the soil, which is determined by the infiltration capacity. This is defined as the maximum rate at which water enters the soil when it is in a given condition (Horton, 1933). This is an important definition: if the rainfall intensity exceeds the infiltration capacity, the soil cannot absorb all the water and there is excess water, which ponds at the surface. This moves down slope as overland flow, more specifically, Hortonian overland flow (1). As Horton's definition implies, soil properties control the infiltration capacity; these include soil permeability, the presence of vegetation and plant roots and how much water is already in the soil. Following a dry period, the infiltration capacity is highest at the start of a storm, rapidly decreasing as rainfall continues, until a constant infiltration rate is reached (Horton, 1933). THE FLOW REGIME A second type of overland flow, saturation overland flow (4), is generated when the soil is totally saturated and therefore cannot take any more water (Kirkby and Chorley, 1967). Where the water table is relatively shallow, for example in valley bottoms, rainfall can cause it to rise to the ground surface. Where this occurs, a saturated area forms around the channel, increasing in extent as the storm progresses. The saturated area acts as an extension to the channel network, meaning that a significant volume of water is transferred in a short time. These saturated areas are known as variable source areas or dynamic contributing areas and are highly significant in humid environments, where this is the main way in which storm run-off is generated (Hewlett and Hibbert, 1967; Dunne and Black, 1970). Hortonian overland flow is rarely observed in humid environments, unless the surface has a low infiltration rate, for example where there are outcrops of bare rock or artificially paved surfaces. However, in dryland environments, a combination of factors mean that Hortonian overland flow is the dominant mechanism. Rainfall, when it occurs, typically has a high intensity and exceeds the low infiltration capacity of sparsely vegetated soils (Dunne, 1978). In addition, dryland soils often develop a crust at the surface, which further reduces the infiltration capacity. Water that infiltrates the soil may travel at various depths below the surface. While it is fairly obvious that water should move downwards under the force of gravity, it might seem counter-intuitive that it also flows through the soil as throughflow. This happens because a preferential flow path is set up. Soil permeability decreases with depth, meaning that the downward movement of water is slowed. During rainfall, this leads to a backing up effect in the more permeable surface layers. This has been likened to the flow of water down a thatched roof: it is easier for the water to move parallel to the slope of the roof, along the stems of the straw, than to move vertically downwards through it (Ward, 1984; Zaslavsky and Sinai, 1981). Where they exist, soil pipes provide a very rapid throughflow mechanism, and rates of flow can be comparable to those in surface channels. Soil pipes are hydraulically formed conduits that can be up to a metre or more in diameter. They are found in a wide range of environments and are sometimes several hundreds of metres in length (Jones, 1997). As such, they can act as an extension to the channel network, allowing the drainage basin to respond rapidly to precipitation inputs (Jones, 1979). The schematic diagram in Figure 3.1 represents the headwaters of a humid zone river, where the channel intersects the water table surface (it should be noted that aquifers are not always present, for example aquifer development is extremely limited in headwater areas that are underlain by impermeable rocks and characterised by thin soils). In this example, groundwater contributions are made to the channel flow from the underlying aquifer and the channel is called a gaining stream. Even during rainless periods, flow will be maintained, as long as the level of the water table does not fall below that of the channel bed. A rather different situation exists for many dryland channels, where the water table may be several metres, or tens of metres, below the surface. In this case, the direction of flow is reversed, as water is lost through the bed and banks of the channel, percolating downwards to recharge the aquifer. This is termed a losing stream and, although not exclusive to drylands, many examples are found in these environments. It is not unusual for a river to be gaining and losing flow to groundwater along different parts of its course, while seasonal fluctuations in water table levels can mean that losing streams become gaining streams for part of the year. The Euphrates in Iraq provides a good example, with most of the flow being generated in the headwaters in northern Iraq, Turkey and Syria. Further downstream, at the Hit gauging station (150 km west of Baghdad) the river loses flow to groundwater for much of the year (Wilson, 1990). The loss of flow from a channel, due to downward percolation and high evaporation rates, is referred to as transmission loss. The storm hydrograph and drainage basin response Flow discharge (also known as Q) is the volume of water passing through a given channel cross-section in a given time. The units of discharge most commonly used are cubic metres per second (m3 s−1), known as 'cumecs', although for very small flows litres per second may be used. In the United States cubic feet per second, or 'cusecs', are used instead of cumecs. Box 3.1 explains how discharge is monitored. 23 24 FUNDAMENTALS OF FLUVIAL GEOMORPHOLOGY Box 3.1 THE MEASUREMENT OF STREAMFLOW The channel discharge is the volume of water flowing through a given channel cross-section in a given time. A number of different methods have been developed to measure discharge. These can be grouped into instantaneous measurements, where discharge is measured at a particular point in time, and continuous measurements for a record of discharge variations through time. The velocity–area method Discharge is measured in cubic metres per second (m3s−1). It increases with the area of the channel cross section and with the velocity of flow. Discharge can be calculated for a given channel cross-section by measuring its cross-sectional area and the mean fl