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9.1 Treatise on Fluvial Geomorphology
9.1 Treatise on Fluvial Geomorphology E Wohl, Colorado State University, Fort Collins, CO, USA r 2013 Elsevier Inc. All rights reserved.
9.1.1 Reference
9.1.1
Introduction and Overview
Introduction and Overview
The intent of this treatise in fluvial geomorphology is to review the broad and multidisciplinary body of technical literature that has grown up around river form and process. From the beginning of systematic studies of rivers in the late nineteenth century, individual investigators have approached the physical aspects of rivers – hydrology, hydraulics, sediment transport, and channel geometry – from diverse disciplinary perspectives, including civil engineering, geography, and geology. Ideally, these perspectives complement each other and enrich our understanding of river systems. As in any multidisciplinary endeavor, however, individual investigators or communities of scientists who share a similar background can be challenged to remain current with diverse segments of the literature published in different journals, to attend conferences frequented by different sections of the river-science community, and to appreciate different traditions and approaches to understanding rivers. The intention in selecting individual chapter authors for this volume was to represent these diverse traditions within fluvial geomorphology by inviting authors from different disciplinary backgrounds, countries, genders, and career stages. Each chapter author was asked to review a particular topic and to highlight outstanding challenges in the research and application of this knowledge. The collective aim is to provide a thorough and integrated review of fluvial geomorphology that will provide a basic reference for readers; especially advanced undergraduate students, graduate students, research scientists, and resource managers who already have some familiarity with the field. This volume opens with a chapter by Grant et al. that discusses the conceptual models used to understand rivers and landscapes. Such models are commonly taken for granted, yet they typically create the boundaries that channel our thinking. Any river basin or segment of a river is literally connected to the greater world. Water, sediment, solutes, and organisms move downstream, and organisms can move upstream; these movements connect headwaters and continental interiors to ocean basins. Materials and organisms move from hill slopes into channels and between channels and floodplains, as well as between the surface flow in channels and the shallow hyporheic zone and deeper groundwater. Precipitation and particulate matter are deposited on rivers, aquatic insects emerge from the water, and volatiles diffuse into the atmosphere; some of these constituents can travel literally around Wohl, E., 2013. Treatise on fluvial geomorphology. In: Shroder, J. (Editor in Chief), Wohl, E. (Ed.), Treatise on Geomorphology. Academic Press, San Diego, CA, vol. 9, Fluvial Geomorphology, pp. 1–5.
Treatise on Geomorphology, Volume 9
1 4
the planet before entering a river (Figure 1). The longitudinal, lateral and vertical connectivity of rivers has been emphasized by river ecologists as well as geomorphologists, and is inherent in viewing a river as an ecosystem rather than simply a physical conduit for water and sediment. Any river basin or river segment also has a long and typically complex history that continues to influence contemporary process and form. The longitudinal profile of the river may still be adjusting to base level change that occurred tens of thousands of years ago, creating vertical discontinuities that influence channel geometry, sediment dynamics, hydraulics, and the passage of aquatic organisms. Or, the river may have been altered by timber harvest and log floating or the construction of mill dams that occurred only two centuries ago. Faced with such a complex system, scientists inevitably seek patterns and develop conceptual models, both qualitative and quantitative, to explain those patterns. A particularly useful and appealing model may then become so ingrained in our disciplinary and individual view of rivers that it becomes very difficult to think about river form and process outside of that model. A chapter reviewing such models and their development as river science has grown thus provides a fitting start to the rest of the volume. In Chapter 9.3, Kampf and Mirus examine how surface and subsurface flow give rise to the channelized flow that initiates and integrates river networks. Molnar in Chapter 9.4 reviews how local energy expenditure in individual channel reaches and global energy expenditure across the drainage network influences channel geometry and the structure of river networks, with particular attention to the conceptual model embodied in the optimal channel network. Chapters 9.5–9.12 focus on processes occurring within river channels. Ferguson (Chapter 9.5) examines flow resistance at the scale of the channel reach, with particular attention to relatively steep, coarse-grained channels, and to models widely used to calculate resistance, such as the Manning’s relation. BuffinBe´langer et al. (Chapter 9.6) focus on the different types and scales of coherent and turbulent flow that result from diverse forms of irregularity along the channel margins. Yager and Schott (Chapter 9.7) discuss the factors that influence the initiation of motion for sediment on the channel bed and the development of coarse surface layers in the channel, as well as the numerical models used to quantify and predict sediment motion. Haschenburger (Chapter 9.8) reviews the kinematics of bedload transport, emphasizing channels with mixed grainsize distributions that are particularly challenging to measure and to numerically model, and Kuhnle (Chapter 9.9) reviews the dynamics of sediment moving suspended within the flow. Venditti (Chapter 9.10) discusses how interactions among
http://dx.doi.org/10.1016/B978-0-12-374739-6.00226-8
1
2
Treatise on Fluvial Geomorphology
Atmospherechannel
Upstreamdownstream
Hillslopechannel
Floodplainchannel Hyporheicchannel
Groundwaterchannel
Figure 1 Schematic illustration of the six degrees of connection characterizing any given segment of a river. Materials, energy, and organisms move longitudinally (e.g., water, sediment, nutrients, contaminants, and plant propagules move downstream, and organisms such as insects and fish move up and downstream), laterally at two scales (sediment, wood, nutrients and contaminants move from hillslopes and into rivers; sediment, water, nutrients, contaminants, and organisms move between the floodplain and the river channel), and vertically at three scales (water and solutes move between the channel and the groundwater; water, sediment, solutes, and invertebrates move between the channel and the hyporheic zone; eolian sediment and wet and dry deposition of materials as diverse as nitrates and mercury enter channels from the atmosphere, and aquatic insects emerge into the atmosphere). The inset photos are, clockwise from upper right, landslide sediment entering a river in Nepal; the floodplain forest of the Amazon basin in Brazil during peak flow; larval mayfly on a cobble of the streambed and emerging mayfly (both courtesy of Jeremy Monroe of Freshwaters Illustrated); and suspended sediment from the flooding Paria River entering the Colorado River in Arizona, USA. After Wohl (2010), Figure 5.2.
hydraulics and sediment supply create bedforms in sand-bed channels, which then strongly influence flow resistance. Chapters 9.5–9.10 in particular, represent areas that have long been a central research focus in fluvial geomorphology, although new techniques for measurement and numerical modeling, as well as emphasis on relatively under-studied types of channels (e.g., steep, coarse-grained), continues to promote rapid change in our understanding of these areas. Chapters 9.11 and 9.12 represent areas of relatively recent research foci. Gurnell (Chapter 9.11) reviews research on instream wood. Although forest cover has shrunk dramatically during the past few thousand years as a result of human activities, and people have actively removed wood from channels for centuries, scientists have developed a new appreciation for the geomorphic and ecological effects of wood in channels flowing through forested regions. The body of research on instream wood begun in the late 1970s is now sufficiently mature to give rise to conceptual models. Riggsbee et al. (Chapter 9.12) review the even newer, but rapidly expanding, focus on how aquatic and semiaquatic biota
influence river form and process. Integrating aquatic ecology and fluvial geomorphology, research on this topic demonstrates that, far from being the ‘passive’ agents that respond to habitat and disturbances created by physical processes in river networks, organisms from aquatic plants and insects to fishes, beaver and terrestrial grazing animals can actively shape river form and process. Chapters 9.13–9.15 examine different types of exchanges or fluxes across channel boundaries. Wondzell and Gooseff (Chapter 9.13) discuss fluxes of materials and organisms between the channel and the underlying hyporheic zone. This is another relatively recent area of research in fluvial geomorphology that has developed largely during the past two decades. Merritt (Chapter 9.14) examines the reciprocal relations among riparian vegetation and river form and process. Most of the chapters in this treatise volume could have been placed under several different categories. Chapter 9.14, for example, could as well have been part of the cluster of chapters dealing with river form. The intent in placing it within this cluster of chapters is to emphasize the two-way interactions
Treatise on Fluvial Geomorphology
between riparian plants and rivers. Rivers provide the habitat, nutrients, and hydrologic regime to which riparian plants have adapted, and the plants influence surface and subsurface fluxes to the river from adjacent uplands, bank and overbank resistance and sediment dynamics, and channel form. Korup (Chapter 9.15) reviews large-scale fluxes of sediment into rivers in the form of landslides, which can substantially alter not only sediment supply to the river network, but also channel and valley morphology. Chapters 9.16–9.21 address different aspects of channel pattern. Hooke (Chapter 9.16) reviews meandering, the most common channel pattern on Earth. Ashmore (Chapter 9.17) discusses the morphology and dynamics of braided rivers. For many decades, braided rivers were viewed as something of an aberration that could result from excess sediment supply or destabilized banks; now they are considered the default river type in the absence of riparian vegetation, cohesive sediment, or some other factor that creates sufficiently stable banks to allow meanders to develop. Although numerical models for meandering rivers have existed for several decades, the complex and apparently unpredictable nature of braided rivers limited the use of numerical models to simulate braiding until relatively recently. Eaton (Chapter 9.18) examines hydraulic geometry, one of the earliest quantitative models of local and downstream change in river form. Although the basic numerical relations between discharge and dependent channel form parameters have existed since the 1950s, recent research provides new insight into the influence of grain size, bank strength, and other factors on hydraulic geometry. Nanson (Chapter 9.19) discusses other types of multithread channel that are less well-studied than braided rivers. The individual channels of anabranching and anastomosing rivers are divided by vegetated or otherwise stable alluvial islands. These types of channels are particularly common on large alluvial rivers, where centuries of river engineering have been devoted to simplifying and channelizing river form into a single, uniform channel. Zimmermann (Chapter 9.20) examines just over two decades of research on step-pool channels, a high-gradient channel form that has only recently begun to receive the type of attention given to pool-riffle channels, which Thompson examines in Chapter 9.21. The steep gradients, mixed grainsize distributions, and abundant flow transitions of step-pool channels make them particularly challenging to measure and model, but their common occurrence in high-relief terrain makes them an important component of river networks. As widespread bedforms that occur from sand-bed to boulderbed and bedrock rivers, pool-riffle sequences have a longer history of systematic study than step-pool sequences. Both types of bedforms reflect and influence flow resistance, energy expenditure, and sediment dynamics, as do the sand-bed features discussed in Chapter 9.10. Chapters 9.22 and 9.23 move beyond the channel to address valley-bottom landforms that result largely from fluvial processes. A planned chapter on floodplain processes along small- to medium-sized rivers was not completed in time to include in this volume, but important aspects of floodplain dynamics are covered in Chapter 9.32 on large floodplain rivers. Pazzaglia focuses on terraces as landforms that integrate tectonic, climatic, and geomorphic processes across the watershed, and reflect fundamental
3
nonuniformities in river incision through time. Stock (Chapter 9.23) reviews the depositional landforms that result from a loss of transport capacity where a river leaves a mountain range and creates an alluvial fan. Understanding of former hydrologic and hydraulic conditions is typically implicit in being able to understand contemporary river form and process. Chapters 9.24–9.26 explicitly examine the techniques used to investigate past hydrologic conditions and the body of knowledge that now exists as a result of applying these techniques. Benito and O’Connor (Chapter 9.24) review methods of stratigraphy, geochronology, hydraulics, and statistical frequency analysis used in paleoflood studies. O’Connor and others (Chapter 9.25) then review knowledge of outburst floods from the enormous releases of glacial meltwater at the end of the Pleistocene to smaller, more recent releases of ponded water. Baker (Chapter 9.26) synthesizes global paleohydrologic understanding. For much of the first half century of fluvial geomorphologic research – from roughly 1950 to 2000 – the research community focused on midsized, lowland rivers with predominantly sand to fine-gravel beds. These rivers were more physically accessible for measurements taken while wading or on boats, and many people lived along these rivers in the countries with the most active fluvial geomorphic research communities in North America and western Europe. The conceptual models developed for these rivers continue to form the central core of fluvial geomorphology. During the final decades of the twentieth century, however, investigators increasingly realized that these models were not necessarily adequate for other types of rivers that had received less attention. Chapters 9.27–9.32 discuss some of the river environments that form a distinct subset within rivers. There is inherent overlap between these chapters and preceding chapters; many dryland rivers are braided, for example, many bedrock rivers meander, and large alluvial rivers typically have extensive floodplains. The intent in the chapters in this section of the volume is to emphasize the distinct characteristics that justify designating a particular subset of rivers, such as dryland rivers or tropical rivers. A chapter focusing on rivers of the boreal regions is missing from this section, but a recently published, comprehensive review by Prowse provides an excellent introduction to this type of river. Church’s (Chapter 9.27) discussion of steep, headwater channels complements Ferguson’s treatment of reach-scale resistance and Zimmermann’s coverage of step-pool bedforms in these types of channels. Whipple and others (Chapter 9.28) emphasize the role of bedrock rivers in mediating landscape response to changes in climate and tectonics, and thus landscape evolution, as well as discussing forms and processes characteristic of rivers formed in cohesive substrate. Simon and Rinaldi (Chapter 9.29) examine the other side of the ‘river incision coin’, focusing on landscape evolution as expressed in incised alluvial channels and the implications of these channels for models of landscape equilibrium. Scatena and Gupta (Chapter 9.30) focus on the characteristics of rivers in montane environments of the humid tropics, where high rates of physical and chemical weathering and a high frequency of geomorphically significant events leads to rapid changes in river process and form. Tooth (Chapter 9.31) examines the diversity
4
Treatise on Fluvial Geomorphology
of process and form along rivers in dryland regions. Chapters 9.30 and 9.31 make the point that, although there is no single unique or even characteristic river form in the montane humid tropics or the drylands, investigation of rivers in these environments provides a stronger understanding of the global diversity of rivers. Dunne and Aalto (Chapter 9.32) examine the world’s great alluvial rivers. Although these rivers have long been integral to human existence as a source of resources, their physical characteristics have challenged direct measurement until the relatively recent development of air- and ground-based remote sensing techniques. A larger river does not necessarily mean that processes studied on smaller rivers can simply be scaled up across greater space and time scales and, as with very steep rivers, distinctive conceptual models are being developed for large alluvial rivers. Chapters 9.33–9.35 examine the different approaches to investigating rivers. Wohl (Chapter 9.33) reviews the relatively limited history of in situ, or field, manipulation of river form and process, and the much more extensive history of physical simulations of river form and process in a laboratory setting. Coulthard and Van de Wiel (Chapter 9.34) discuss different types of numerical models applied to small-scale, short-term processes such as particle entrainment or bank erosion, as well as progressively larger-scale and longer-term processes such as drainage network evolution. Oguchi and others (Chapter 9.35) examine the different types of space-based remote sensing that can be used to characterize river form and process. The final group of Chapters 9.36–9.41, deals explicitly with human interactions with rivers. The ubiquitous influence of humans on the Earth’s surface and ecosystems is reflected in the designation of the latest increment of geologic time as the Anthropocene. It is now difficult to find a river that has not been directly influenced by human activities and, as a result of global warming, impossible to find a river that has not been indirectly influenced. This presents a unique challenge for a discipline such as fluvial geomorphology, which inherently has a strong historical component because of the long (relative to human life spans) reaction time of river basins to external changes associated with climate and tectonics. Many investigators prefer to understand how a river segment or river network responds to perturbations in the absence of constraints imposed by humans, such as dams or levees. Much of our society’s effort to rehabilitate or restore rivers to some condition perceived as being more natural or desirable also implicitly relies on knowledge of reference conditions that existed before intensive human alteration of the river. These attitudes are increasingly problematic, however, as constraints imposed by humans become the norm and reference conditions cease to exist. Buffington and Montgomery (Chapter 9.36) review river classification systems, which commonly provide a starting point for river management. James and Lecce (Chapter 9.37) discuss the broad array of human actions that have
altered water, sediment, and other materials entering and moving through river networks, as well as network configuration and channel form, in some areas for thousands of years. Magilligan et al. (Chapter 9.38) focus on the specific influences of dams; Chin et al. (Chapter 9.39) examine urban influences on river form and process; and Overeem et al. (Chapter 9.40) discuss evidence of climate change impacts on rivers. In the final chapter of the volume, Pasternack (Chapter 9.41) reviews river rehabilitation. River rehabilitation, like other forms of river engineering, can provide a direct test of scientific understanding of river form and process, because the interventions associated with a carefully developed rehabilitation project are designed to produce some desired change in form and process. Unlike earlier forms of river engineering, rehabilitation explicitly includes riverine characteristics such as longitudinal, lateral, and vertical connectivity, as well as aquatic and riparian biological communities. Ideally, river rehabilitation thus incorporates the integrated, holistic perspective on rivers that this volume is designed to reflect. The author would like to take this opportunity to thank the numerous individuals who contributed their time and knowledge to this volume as reviewers of individual chapters. As papers and journals proliferate, the fluvial geomorphic community carries an increasingly heavy reviewing load. Every investigator and reviewer acknowledges the vital importance and necessity of thorough, unbiased, constructive peer review, but the energy required to provide such reviews can be daunting. Consequently, the efforts of the following reviewers of this volume is especially appreciated: Brian Barkdoll, James Bathurst, Lee Benda, Walter Bertoldi, Paul Bishop, Gary Brierley, Andrew Brookes, Anthony Brown, Leif Burge, Patrice Carbonneau, Meinhard Cardenas, Paul Carling, Nick Chappell, Jordan Clayton, Nick Clifford, Francesco Comiti, Jose Constantine, David Dust, John England, Noah Finnegan, Mark Fonstad, Antonio Garcia, Nicole Gasparini, Andrew Goudie, Will Graf, Blair Greimann, Greg Hancock, Lee Harrison, Adrian Harvey, Francine Hughes, Erkan Istanbulluoglu, Anne Jefferson, Douglas Jerolmack, Michael Kirkby, Matthew Larsen, Suzanne Leclair, Frederic Liebault, Keith Loague, Luca Mao, Dan Moore, Gerald Nanson, Jonathan Nelson, Fred Ogden, Kyungrock Paik, Greg Pasternack, George Pess, LeRoy Poff, Mark Powell, Ian Reid, Dieter Rickenmann, Andrea Rinaldo, Stewart Rood, John Sabo, Audrey Sawyer, David Sear, Giovanni Seminara, Graeme Smart, Fred Swanson, Michal Tal, Robert Webb, Andrew Wilcox, and treatise series editor Jack Shroder.
Reference Wohl, E., 2010. Mountain Rivers Revisited. American Geophysical Union Water Resources Monograph 19. American Geophysical Union Press, Washington, DC.
Treatise on Fluvial Geomorphology
5
Biographical Sketch Ellen Wohl received her BS in geology from Arizona State University in 1984 and her PhD in geosciences from the University of Arizona in 1988. She joined the faculty at Colorado State University in 1989 and is now a professor of geology there. Her research focuses on form and process in mountain rivers and bedrock canyon rivers and she has conducted field work on every continent except the Antarctica. In addition to numerous journal articles, the books she has written include Mountain Rivers (2000, American Geophysical Union Press), Virtual Rivers (2001, Yale University Press), Disconnected Rivers (2004, Yale University Press), Of Rock and Rivers (2009, University of California Press), and A World of Rivers (2010, University of Chicago Press).
9.1.1 Reference
9.1.1
Introduction and Overview
Introduction and Overview
The intent of this treatise in fluvial geomorphology is to review the broad and multidisciplinary body of technical literature that has grown up around river form and process. From the beginning of systematic studies of rivers in the late nineteenth century, individual investigators have approached the physical aspects of rivers – hydrology, hydraulics, sediment transport, and channel geometry – from diverse disciplinary perspectives, including civil engineering, geography, and geology. Ideally, these perspectives complement each other and enrich our understanding of river systems. As in any multidisciplinary endeavor, however, individual investigators or communities of scientists who share a similar background can be challenged to remain current with diverse segments of the literature published in different journals, to attend conferences frequented by different sections of the river-science community, and to appreciate different traditions and approaches to understanding rivers. The intention in selecting individual chapter authors for this volume was to represent these diverse traditions within fluvial geomorphology by inviting authors from different disciplinary backgrounds, countries, genders, and career stages. Each chapter author was asked to review a particular topic and to highlight outstanding challenges in the research and application of this knowledge. The collective aim is to provide a thorough and integrated review of fluvial geomorphology that will provide a basic reference for readers; especially advanced undergraduate students, graduate students, research scientists, and resource managers who already have some familiarity with the field. This volume opens with a chapter by Grant et al. that discusses the conceptual models used to understand rivers and landscapes. Such models are commonly taken for granted, yet they typically create the boundaries that channel our thinking. Any river basin or segment of a river is literally connected to the greater world. Water, sediment, solutes, and organisms move downstream, and organisms can move upstream; these movements connect headwaters and continental interiors to ocean basins. Materials and organisms move from hill slopes into channels and between channels and floodplains, as well as between the surface flow in channels and the shallow hyporheic zone and deeper groundwater. Precipitation and particulate matter are deposited on rivers, aquatic insects emerge from the water, and volatiles diffuse into the atmosphere; some of these constituents can travel literally around Wohl, E., 2013. Treatise on fluvial geomorphology. In: Shroder, J. (Editor in Chief), Wohl, E. (Ed.), Treatise on Geomorphology. Academic Press, San Diego, CA, vol. 9, Fluvial Geomorphology, pp. 1–5.
Treatise on Geomorphology, Volume 9
1 4
the planet before entering a river (Figure 1). The longitudinal, lateral and vertical connectivity of rivers has been emphasized by river ecologists as well as geomorphologists, and is inherent in viewing a river as an ecosystem rather than simply a physical conduit for water and sediment. Any river basin or river segment also has a long and typically complex history that continues to influence contemporary process and form. The longitudinal profile of the river may still be adjusting to base level change that occurred tens of thousands of years ago, creating vertical discontinuities that influence channel geometry, sediment dynamics, hydraulics, and the passage of aquatic organisms. Or, the river may have been altered by timber harvest and log floating or the construction of mill dams that occurred only two centuries ago. Faced with such a complex system, scientists inevitably seek patterns and develop conceptual models, both qualitative and quantitative, to explain those patterns. A particularly useful and appealing model may then become so ingrained in our disciplinary and individual view of rivers that it becomes very difficult to think about river form and process outside of that model. A chapter reviewing such models and their development as river science has grown thus provides a fitting start to the rest of the volume. In Chapter 9.3, Kampf and Mirus examine how surface and subsurface flow give rise to the channelized flow that initiates and integrates river networks. Molnar in Chapter 9.4 reviews how local energy expenditure in individual channel reaches and global energy expenditure across the drainage network influences channel geometry and the structure of river networks, with particular attention to the conceptual model embodied in the optimal channel network. Chapters 9.5–9.12 focus on processes occurring within river channels. Ferguson (Chapter 9.5) examines flow resistance at the scale of the channel reach, with particular attention to relatively steep, coarse-grained channels, and to models widely used to calculate resistance, such as the Manning’s relation. BuffinBe´langer et al. (Chapter 9.6) focus on the different types and scales of coherent and turbulent flow that result from diverse forms of irregularity along the channel margins. Yager and Schott (Chapter 9.7) discuss the factors that influence the initiation of motion for sediment on the channel bed and the development of coarse surface layers in the channel, as well as the numerical models used to quantify and predict sediment motion. Haschenburger (Chapter 9.8) reviews the kinematics of bedload transport, emphasizing channels with mixed grainsize distributions that are particularly challenging to measure and to numerically model, and Kuhnle (Chapter 9.9) reviews the dynamics of sediment moving suspended within the flow. Venditti (Chapter 9.10) discusses how interactions among
http://dx.doi.org/10.1016/B978-0-12-374739-6.00226-8
1
2
Treatise on Fluvial Geomorphology
Atmospherechannel
Upstreamdownstream
Hillslopechannel
Floodplainchannel Hyporheicchannel
Groundwaterchannel
Figure 1 Schematic illustration of the six degrees of connection characterizing any given segment of a river. Materials, energy, and organisms move longitudinally (e.g., water, sediment, nutrients, contaminants, and plant propagules move downstream, and organisms such as insects and fish move up and downstream), laterally at two scales (sediment, wood, nutrients and contaminants move from hillslopes and into rivers; sediment, water, nutrients, contaminants, and organisms move between the floodplain and the river channel), and vertically at three scales (water and solutes move between the channel and the groundwater; water, sediment, solutes, and invertebrates move between the channel and the hyporheic zone; eolian sediment and wet and dry deposition of materials as diverse as nitrates and mercury enter channels from the atmosphere, and aquatic insects emerge into the atmosphere). The inset photos are, clockwise from upper right, landslide sediment entering a river in Nepal; the floodplain forest of the Amazon basin in Brazil during peak flow; larval mayfly on a cobble of the streambed and emerging mayfly (both courtesy of Jeremy Monroe of Freshwaters Illustrated); and suspended sediment from the flooding Paria River entering the Colorado River in Arizona, USA. After Wohl (2010), Figure 5.2.
hydraulics and sediment supply create bedforms in sand-bed channels, which then strongly influence flow resistance. Chapters 9.5–9.10 in particular, represent areas that have long been a central research focus in fluvial geomorphology, although new techniques for measurement and numerical modeling, as well as emphasis on relatively under-studied types of channels (e.g., steep, coarse-grained), continues to promote rapid change in our understanding of these areas. Chapters 9.11 and 9.12 represent areas of relatively recent research foci. Gurnell (Chapter 9.11) reviews research on instream wood. Although forest cover has shrunk dramatically during the past few thousand years as a result of human activities, and people have actively removed wood from channels for centuries, scientists have developed a new appreciation for the geomorphic and ecological effects of wood in channels flowing through forested regions. The body of research on instream wood begun in the late 1970s is now sufficiently mature to give rise to conceptual models. Riggsbee et al. (Chapter 9.12) review the even newer, but rapidly expanding, focus on how aquatic and semiaquatic biota
influence river form and process. Integrating aquatic ecology and fluvial geomorphology, research on this topic demonstrates that, far from being the ‘passive’ agents that respond to habitat and disturbances created by physical processes in river networks, organisms from aquatic plants and insects to fishes, beaver and terrestrial grazing animals can actively shape river form and process. Chapters 9.13–9.15 examine different types of exchanges or fluxes across channel boundaries. Wondzell and Gooseff (Chapter 9.13) discuss fluxes of materials and organisms between the channel and the underlying hyporheic zone. This is another relatively recent area of research in fluvial geomorphology that has developed largely during the past two decades. Merritt (Chapter 9.14) examines the reciprocal relations among riparian vegetation and river form and process. Most of the chapters in this treatise volume could have been placed under several different categories. Chapter 9.14, for example, could as well have been part of the cluster of chapters dealing with river form. The intent in placing it within this cluster of chapters is to emphasize the two-way interactions
Treatise on Fluvial Geomorphology
between riparian plants and rivers. Rivers provide the habitat, nutrients, and hydrologic regime to which riparian plants have adapted, and the plants influence surface and subsurface fluxes to the river from adjacent uplands, bank and overbank resistance and sediment dynamics, and channel form. Korup (Chapter 9.15) reviews large-scale fluxes of sediment into rivers in the form of landslides, which can substantially alter not only sediment supply to the river network, but also channel and valley morphology. Chapters 9.16–9.21 address different aspects of channel pattern. Hooke (Chapter 9.16) reviews meandering, the most common channel pattern on Earth. Ashmore (Chapter 9.17) discusses the morphology and dynamics of braided rivers. For many decades, braided rivers were viewed as something of an aberration that could result from excess sediment supply or destabilized banks; now they are considered the default river type in the absence of riparian vegetation, cohesive sediment, or some other factor that creates sufficiently stable banks to allow meanders to develop. Although numerical models for meandering rivers have existed for several decades, the complex and apparently unpredictable nature of braided rivers limited the use of numerical models to simulate braiding until relatively recently. Eaton (Chapter 9.18) examines hydraulic geometry, one of the earliest quantitative models of local and downstream change in river form. Although the basic numerical relations between discharge and dependent channel form parameters have existed since the 1950s, recent research provides new insight into the influence of grain size, bank strength, and other factors on hydraulic geometry. Nanson (Chapter 9.19) discusses other types of multithread channel that are less well-studied than braided rivers. The individual channels of anabranching and anastomosing rivers are divided by vegetated or otherwise stable alluvial islands. These types of channels are particularly common on large alluvial rivers, where centuries of river engineering have been devoted to simplifying and channelizing river form into a single, uniform channel. Zimmermann (Chapter 9.20) examines just over two decades of research on step-pool channels, a high-gradient channel form that has only recently begun to receive the type of attention given to pool-riffle channels, which Thompson examines in Chapter 9.21. The steep gradients, mixed grainsize distributions, and abundant flow transitions of step-pool channels make them particularly challenging to measure and model, but their common occurrence in high-relief terrain makes them an important component of river networks. As widespread bedforms that occur from sand-bed to boulderbed and bedrock rivers, pool-riffle sequences have a longer history of systematic study than step-pool sequences. Both types of bedforms reflect and influence flow resistance, energy expenditure, and sediment dynamics, as do the sand-bed features discussed in Chapter 9.10. Chapters 9.22 and 9.23 move beyond the channel to address valley-bottom landforms that result largely from fluvial processes. A planned chapter on floodplain processes along small- to medium-sized rivers was not completed in time to include in this volume, but important aspects of floodplain dynamics are covered in Chapter 9.32 on large floodplain rivers. Pazzaglia focuses on terraces as landforms that integrate tectonic, climatic, and geomorphic processes across the watershed, and reflect fundamental
3
nonuniformities in river incision through time. Stock (Chapter 9.23) reviews the depositional landforms that result from a loss of transport capacity where a river leaves a mountain range and creates an alluvial fan. Understanding of former hydrologic and hydraulic conditions is typically implicit in being able to understand contemporary river form and process. Chapters 9.24–9.26 explicitly examine the techniques used to investigate past hydrologic conditions and the body of knowledge that now exists as a result of applying these techniques. Benito and O’Connor (Chapter 9.24) review methods of stratigraphy, geochronology, hydraulics, and statistical frequency analysis used in paleoflood studies. O’Connor and others (Chapter 9.25) then review knowledge of outburst floods from the enormous releases of glacial meltwater at the end of the Pleistocene to smaller, more recent releases of ponded water. Baker (Chapter 9.26) synthesizes global paleohydrologic understanding. For much of the first half century of fluvial geomorphologic research – from roughly 1950 to 2000 – the research community focused on midsized, lowland rivers with predominantly sand to fine-gravel beds. These rivers were more physically accessible for measurements taken while wading or on boats, and many people lived along these rivers in the countries with the most active fluvial geomorphic research communities in North America and western Europe. The conceptual models developed for these rivers continue to form the central core of fluvial geomorphology. During the final decades of the twentieth century, however, investigators increasingly realized that these models were not necessarily adequate for other types of rivers that had received less attention. Chapters 9.27–9.32 discuss some of the river environments that form a distinct subset within rivers. There is inherent overlap between these chapters and preceding chapters; many dryland rivers are braided, for example, many bedrock rivers meander, and large alluvial rivers typically have extensive floodplains. The intent in the chapters in this section of the volume is to emphasize the distinct characteristics that justify designating a particular subset of rivers, such as dryland rivers or tropical rivers. A chapter focusing on rivers of the boreal regions is missing from this section, but a recently published, comprehensive review by Prowse provides an excellent introduction to this type of river. Church’s (Chapter 9.27) discussion of steep, headwater channels complements Ferguson’s treatment of reach-scale resistance and Zimmermann’s coverage of step-pool bedforms in these types of channels. Whipple and others (Chapter 9.28) emphasize the role of bedrock rivers in mediating landscape response to changes in climate and tectonics, and thus landscape evolution, as well as discussing forms and processes characteristic of rivers formed in cohesive substrate. Simon and Rinaldi (Chapter 9.29) examine the other side of the ‘river incision coin’, focusing on landscape evolution as expressed in incised alluvial channels and the implications of these channels for models of landscape equilibrium. Scatena and Gupta (Chapter 9.30) focus on the characteristics of rivers in montane environments of the humid tropics, where high rates of physical and chemical weathering and a high frequency of geomorphically significant events leads to rapid changes in river process and form. Tooth (Chapter 9.31) examines the diversity
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Treatise on Fluvial Geomorphology
of process and form along rivers in dryland regions. Chapters 9.30 and 9.31 make the point that, although there is no single unique or even characteristic river form in the montane humid tropics or the drylands, investigation of rivers in these environments provides a stronger understanding of the global diversity of rivers. Dunne and Aalto (Chapter 9.32) examine the world’s great alluvial rivers. Although these rivers have long been integral to human existence as a source of resources, their physical characteristics have challenged direct measurement until the relatively recent development of air- and ground-based remote sensing techniques. A larger river does not necessarily mean that processes studied on smaller rivers can simply be scaled up across greater space and time scales and, as with very steep rivers, distinctive conceptual models are being developed for large alluvial rivers. Chapters 9.33–9.35 examine the different approaches to investigating rivers. Wohl (Chapter 9.33) reviews the relatively limited history of in situ, or field, manipulation of river form and process, and the much more extensive history of physical simulations of river form and process in a laboratory setting. Coulthard and Van de Wiel (Chapter 9.34) discuss different types of numerical models applied to small-scale, short-term processes such as particle entrainment or bank erosion, as well as progressively larger-scale and longer-term processes such as drainage network evolution. Oguchi and others (Chapter 9.35) examine the different types of space-based remote sensing that can be used to characterize river form and process. The final group of Chapters 9.36–9.41, deals explicitly with human interactions with rivers. The ubiquitous influence of humans on the Earth’s surface and ecosystems is reflected in the designation of the latest increment of geologic time as the Anthropocene. It is now difficult to find a river that has not been directly influenced by human activities and, as a result of global warming, impossible to find a river that has not been indirectly influenced. This presents a unique challenge for a discipline such as fluvial geomorphology, which inherently has a strong historical component because of the long (relative to human life spans) reaction time of river basins to external changes associated with climate and tectonics. Many investigators prefer to understand how a river segment or river network responds to perturbations in the absence of constraints imposed by humans, such as dams or levees. Much of our society’s effort to rehabilitate or restore rivers to some condition perceived as being more natural or desirable also implicitly relies on knowledge of reference conditions that existed before intensive human alteration of the river. These attitudes are increasingly problematic, however, as constraints imposed by humans become the norm and reference conditions cease to exist. Buffington and Montgomery (Chapter 9.36) review river classification systems, which commonly provide a starting point for river management. James and Lecce (Chapter 9.37) discuss the broad array of human actions that have
altered water, sediment, and other materials entering and moving through river networks, as well as network configuration and channel form, in some areas for thousands of years. Magilligan et al. (Chapter 9.38) focus on the specific influences of dams; Chin et al. (Chapter 9.39) examine urban influences on river form and process; and Overeem et al. (Chapter 9.40) discuss evidence of climate change impacts on rivers. In the final chapter of the volume, Pasternack (Chapter 9.41) reviews river rehabilitation. River rehabilitation, like other forms of river engineering, can provide a direct test of scientific understanding of river form and process, because the interventions associated with a carefully developed rehabilitation project are designed to produce some desired change in form and process. Unlike earlier forms of river engineering, rehabilitation explicitly includes riverine characteristics such as longitudinal, lateral, and vertical connectivity, as well as aquatic and riparian biological communities. Ideally, river rehabilitation thus incorporates the integrated, holistic perspective on rivers that this volume is designed to reflect. The author would like to take this opportunity to thank the numerous individuals who contributed their time and knowledge to this volume as reviewers of individual chapters. As papers and journals proliferate, the fluvial geomorphic community carries an increasingly heavy reviewing load. Every investigator and reviewer acknowledges the vital importance and necessity of thorough, unbiased, constructive peer review, but the energy required to provide such reviews can be daunting. Consequently, the efforts of the following reviewers of this volume is especially appreciated: Brian Barkdoll, James Bathurst, Lee Benda, Walter Bertoldi, Paul Bishop, Gary Brierley, Andrew Brookes, Anthony Brown, Leif Burge, Patrice Carbonneau, Meinhard Cardenas, Paul Carling, Nick Chappell, Jordan Clayton, Nick Clifford, Francesco Comiti, Jose Constantine, David Dust, John England, Noah Finnegan, Mark Fonstad, Antonio Garcia, Nicole Gasparini, Andrew Goudie, Will Graf, Blair Greimann, Greg Hancock, Lee Harrison, Adrian Harvey, Francine Hughes, Erkan Istanbulluoglu, Anne Jefferson, Douglas Jerolmack, Michael Kirkby, Matthew Larsen, Suzanne Leclair, Frederic Liebault, Keith Loague, Luca Mao, Dan Moore, Gerald Nanson, Jonathan Nelson, Fred Ogden, Kyungrock Paik, Greg Pasternack, George Pess, LeRoy Poff, Mark Powell, Ian Reid, Dieter Rickenmann, Andrea Rinaldo, Stewart Rood, John Sabo, Audrey Sawyer, David Sear, Giovanni Seminara, Graeme Smart, Fred Swanson, Michal Tal, Robert Webb, Andrew Wilcox, and treatise series editor Jack Shroder.
Reference Wohl, E., 2010. Mountain Rivers Revisited. American Geophysical Union Water Resources Monograph 19. American Geophysical Union Press, Washington, DC.
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Biographical Sketch Ellen Wohl received her BS in geology from Arizona State University in 1984 and her PhD in geosciences from the University of Arizona in 1988. She joined the faculty at Colorado State University in 1989 and is now a professor of geology there. Her research focuses on form and process in mountain rivers and bedrock canyon rivers and she has conducted field work on every continent except the Antarctica. In addition to numerous journal articles, the books she has written include Mountain Rivers (2000, American Geophysical Union Press), Virtual Rivers (2001, Yale University Press), Disconnected Rivers (2004, Yale University Press), Of Rock and Rivers (2009, University of California Press), and A World of Rivers (2010, University of Chicago Press).