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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
An accessible introduction to large rivers, including coverage of the geomorphology, hydrology, ecology, and environments of large river systems
This indispensible book takes a structured and global approach to the subject of large rivers, covering geomorphology, hydrology, ecology, and anthropogenic environment. It offers a thorough foundation for readers who are new to the field and presents enlightening discussions about issues of management at the worldwide scale. The book also examines possible future adaptations that may come about due to climate change. The book has benefitted from contributions by Professor W.J. Junk on the ecology of floodplains and Professor Olav Slaymaker on the large arctic rivers.
Introducing Large Rivers is presented in three parts. Part 1 provides an introduction to the world’s large rivers and their basins. It covers source, transfer, and storage of their water and sediment; Pleistocene inheritance; the ecology of channels and floodplains; deltas; and more. Several large rivers are discussed in the next part. These include the Amazon Mississippi, Nile, Ganga-Brahmaputra System, Mekong, and Yangtze. The last part examines changes in large rivers and our management of river systems. It studies anthropogenic alterations such as land use and deforestation in large river basins; structural control systems like dams and reservoirs on channels; and ecological changes. It finishes with chapters on the management of large rivers, covering both technical and political aspects, and the future of the world’s big river systems.
Introducing Large Rivers is ideal as an introductory textbook on large rivers for future earth and environmental scientists and river managers. It will also benefit advanced undergraduate and graduate students studying geography, geology, ecology, and river management.
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Table of Contents
Cover
Preface
Note
1 Introduction
1.1 Large Rivers
1.2 A Book on Large Rivers
References
2 Geological Framework of Large Rivers
2.1 Introduction
2.2 The Geological Framework: Elevated Land and a Large Catchment
2.3 Smaller Tectonic Movements
2.4 The Subsurface Alluvial Fill of Large Rivers
2.5 Geological History of Large Rivers
2.6 Conclusion
References
3 Water and Sediment in Large Rivers
3.1 Introduction
3.2 Discharge of large Rivers
3.3 Global Pattern of Precipitation
3.4 Large River Discharge: Annual Pattern and Long-Term Variability
3.5 Sediment in Large Rivers
3.6 Conclusion
References
4 Morphology of Large Rivers
4.1 Introduction
4.2 Large Rivers from Source to Sink
4.3 The Amazon River
4.4 The Ganga River
4.5 Morphology of Large Rivers: Commonality and Variations
4.6 Conclusion
References
5 Large Rivers and their Floodplains: Structures, Functions, Evolutionary Traits and Management with Special Reference to the Brazilian Rivers
5.1 Introduction
5.2 Origin and Age of Rivers and Floodplains
5.3 Scientific Concepts and their Implications for Rivers and Floodplains
5.4 Water Chemistry and Hydrology of Major Brazilian Rivers and their Floodplains
5.5 Ecological Characterisation of Floodplains and their Macrohabitats
5.6 Ecological Responses of Organisms to Flood-Pulsing Conditions
5.7 Biodiversity
5.8 The Role of Rivers and their Floodplains for Speciation and Species Distribution of Trees
5.9 Biogeochemical Cycles in Floodplains
5.10 Management of Amazonian River Floodplains
5.11 Policies in Brazilian Wetlands
5.12 Discussion and Conclusion
Acknowledgements
References
Notes
6 Large River Deltas
6.1 Introduction
6.2 Large River Deltas: The Distribution
6.3 Formation of Deltas
6.4 Delta Morphology and Sediment
6.5 The Ganga-Brahmaputra Delta: An Example of a Major Deltaic Accumulation
6.6 Conclusion
References
7 Geological History of Large River Systems
7.1 The Age of Large Rivers
7.2 Rivers in the Quaternary
7.3 Changes During the Holocene
7.4 Evolution and Development of the Mississippi River
7.5 The Ganga-Brahmaputra System
7.6 Evolution of the Current Amazon
7.7 Evolutionary Adjustment of Large Rivers
References
8 Anthropogenic Alterations of Large Rivers and Drainage Basins
8.1 Introduction
8.2 Early History of Anthropogenic Alterations
8.3 The Mississippi River: Modifications before Big Dams
8.4 The Arrival of Large Dams
8.5 Evaluating the Impact of Anthropogenic Changes
8.6 Effect of Impoundments on Alluvial Rivers
8.7 Effect of Impoundments on Rivers in Rock
8.8 Large-scale Transfer of River Water
8.9 Conclusion
References
9 Management of Large Rivers
9.1 Introduction
9.2 Biophysical Management
9.3 Social and Political Management
9.4 The Importance of the Channel, Floodplain, and Drainage Basin
9.5 Integrated Water Resources Management
9.6 Techniques for Managing Large River Basins
9.7 Administering the Nile
9.8 Conclusion
References
10 The Mekong: A Case Study on Morphology and Management
10.1 Introduction
10.2 Physical Characteristics of the Mekong Basin
10.3 The Mekong: Source to Sea
10.4 Erosion, Sediment Storage and Sediment Transfer in the Mekong
10.5 Management of the Mekong and its Basin
10.6 Conclusion
References
11 Large Arctic Rivers
Introduction
11.2 Physiography and Quaternary Legacy
11.3 Hydroclimate and Biomes
11.4 Permafrost
11.5 Anthropogenic Effects
11.6 Discharge of Large Arctic Rivers
11.7 Sediment Fluxes
11.8 Nutrients and Contaminants
11.9 Mackenzie, Yukon and Lena Deltas
11.10 Significance of Large Arctic Rivers
Acknowledgment
References
12 Climate Change and Large Rivers
12.1 Introduction
12.2 Global Warming: Basic Concept
12.3 A Summary of Future Changes in Climate
12.4 Impact of Climate Change on Large Rivers
12.5 Climate Change and a Typical Large River of the Future
12.6 Conclusion
References
Index
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Selected characteristics of 24 large rivers.
Chapter 5
Table 5.1 Electrical conductivity and pH values of several major Brazilia...
Table 5.2 Types of flood pulses and affected wetlands.
Table 5.3 Macrohabitat diversity in three large Brazilian floodplains bel...
Table 5.4 Morphological, physiological, and phenological adaptations used...
Table 5.5 Species richness of terrestrial arthropods in Amazonian uplands...
Table 5.6 Large Amazonian river flood plains: effect of flood pulse on th...
Table 5.7 Biomass and net primary production per hectare and year of diff...
Table 5.8 Nitrogen flux in the Camaleão Lake over an area of 650 ha from ...
Table 5.9 Nitrogen input and output fluxes from
várzea
forest at Cama...
Table 5.10 Annual cycle of economic activities of the riparian human popu...
Chapter 6
Table 6.1 Selected major deltas in the world.
Chapter 7
Table 7.1 Source to sink response of the Ganga River since MIS 3.
Chapter 8
Table 8.1 Impact of dams and reservoirs on large rivers.
Chapter 10
Table 10.1 Measurements on the Mekong.
Table 10.2 The Mekong: summary of river units, south of China border.
Chapter 11
Table 11.1 Global rank and countries encompassed by each of the five larg...
Table 11.2 Climate regions based on soil water balance (after Strahler an...
Table 11.3 Climate regions of the Russian large arctic rivers based on qu...
Table 11.4 Global rank of the five largest arctic rivers by discharge.
Table 11.5 Sub-basins of Mackenzie River above Arctic Red River station: areas a...
Table 11.6 Recorded hydrologic changes (1966–1996).
Table 11.7 Sediment in large arctic rivers.
Table 11.8 Specific sediment yield for the mountain tributaries of Macken...
Table 11.9 Estimates of annual suspended load for main tributaries based ...
Table 11.10 Mean monthly fine sediment loads (in 10
6
t) for delta-head ri...
Table 11.11 Estimates of mean annual suspended sediment load at four benc...
Table 11.12 Changing water and sediment pattern in Siberian hydrometric s...
Table 11.13 Baseline infomation for two of the four ACIA regions (Central...
Chapter 12
Table 12.1 Common gases that produce enhanced greenhouse effect.
List of Illustrations
Chapter 1
Figure 1.1 A sketch map showing the location of 24 large rivers in the world...
Figure 1.2 The Mekong. (a) On rock, downstream of Chiang Saen, northern Thai...
Chapter 2
Figure 2.1 Schematic illustration of the relation between structure and morp...
Figure 2.2 Longitudinal profile of the Nile over varying regional structures...
Figure 2.3 Diagrammatic sketches of cross-sections of the Mekong River in ro...
Chapter 3
Figure 3.1 Average discharges of (a) suspended sediment and (b) water in the...
Figure 3.2 Average annual hydrograph of selected large rivers as examples: t...
Figure 3.3 Diagram showing average annual sediment movement between channels...
Figure 3.4 Satellite image showing plumes of sediment entering the Atlantic ...
Figure 3.5 The Ganga River. Changes in the grain size of bar material from H...
Chapter 4
Figure 4.1 At least 10 m high midchannel bar in the Brahmaputra at Sirajganj...
Figure 4.2 Schematic network of a large river. Compare with the satellite im...
Figure 4.3 Schematic sketch of the megafan of the Kosi River.
Figure 4.4 The Amazon: Generalised geology and course.
Figure 4.5 The Amazon from satellite imagery.
Figure 4.6 Structural control at the Amazon-Negro confluence. In Brazil the ...
Figure 4.7 The Ganga from satellite imagery.
Figure 4.8 The Ganga from satellite imagery in alluvium, bars and bends.
Figure 4.9 Diagrammatic sketch of the Irrawaddy leaving an alluvial segment ...
Chapter 5
Figure 5.1 Major Brazilian rivers and wetlands.
Figure 5.2 Mean annual water level fluctuations of major Brazilian rivers be...
Chapter 6
Figure 6.1 Delta landforms. Adapted from Gupta 2011.
Figure 6.2 The Effect of at the tropical cyclone Nargis on the Irrawaddy del...
Figure 6.3 Location of major deltas in the world: 1, Amazon; 2, Ganga-Brahma...
Figure 6.4 Different types of deltas.
Figure 6.5 Photograph of a tidal creek through mangrove forest, Sundarbans, ...
Figure 6.6 Physiographic and tectonic map of the Ganga-Brahmaputra Delta.
Chapter 7
Figure 7.1 Simplified diagram showing drainage reorganisation in South Centr...
Figure 7.2 Global correlation time chart of the Quaternary showing the Marin...
Figure 7.3 Landforms and geomorphological processes in the Blue Nile Basin d...
Figure 7.4 Sketch map of the late Pleistocene glacially diverted drainage sy...
Figure 7.5 The Mississippi Basin.
Figure 7.6 Diagrammatic sketch showing the relative position of the major ri...
Chapter 8
Figure 8.1 Schematic map of the Colorado River Basin showing the major dams ...
Figure 8.2 The concrete arch-gravity Hoover Dam was built in the Black Canyo...
Figure 8.3 Schematic diagram showing the Aswan High Dam in the Nile Basin.
Figure 8.4 Fresh sediment against the rocky bank of the Mekong, Lao PDR. Pho...
Figure 8.5 Sediment change downstream of dams.
Figure 8.6 Degradation and changes in long profile downstream from dams.
Figure 8.7 Effect of the Glen Canyon Dam on the Colorado River: (a) reduced ...
Figure 8.8 The course of the Changjiang and the location of the Three Gorges...
Chapter 9
Figure 9.1 Tonlé Sap and the Mekong.
Chapter 10
Figure 10.1 General geology and river units in the Mekong Basin.
Figure 10.2 Annual hydrographs (1997) of the Mekong at Chang Saen, Thailand ...
Figure 10.3 Mekong flood of 2000 affecting Cambodia and Vietnam. Map prepare...
Figure 10.4 Rock ribs and flood markers in the Mekong channel upstream of Lu...
Figure 10.5 Sand deposition on boulders by the Mekong after a large flood. P...
Chapter 11
Figure 11.1 The five largest arctic river basins drawn to the same scale; th...
Figure 11.2 Ideal fluvial system model (after Schumm 1977). Every fluvial sy...
Figure 11.3 Major tectonic structure of the Lena, Yenisei, and Ob basins (af...
Figure 11.4 Physiographic subdivisions of the Mackenzie basin (after Bostock...
Figure 11.5 Maximum extent of glacial lakes along the margin of the retreati...
Figure 11.6 (a) Climate regions in the five large river basins based on Thor...
Figure 11.7 The boreal forest (taiga) biome in (a) Lena, (b) Mackenzie and (...
Figure 11.8 (a) Zonation of permafrost in the northern hemisphere under the ...
Figure 11.9 The six longest Environment Canada climate records available for...
Figure 11.10 Hydrometric stations on Ob-Irtysh, Yenisei-Angara and Lena rive...
Figure 11.11 Water inputs to the Arctic Ocean by sector and annual water dis...
Figure 11.12 Daily runoff regimes for the Lena and Mackenzie rivers over a t...
Figure 11.13 Runoff regimes of the Mackenzie River at the Arctic Red River s...
Figure 11.14 Great Slave Lake transforms the highly variable discharge of th...
Figure 11.15 Discharge hydrographs of four stations along the Liard River in...
Figure 11.16 Water and sediment discharges in large arctic rivers. Black his...
Figure 11.17 Cordilleran sediment sources (1974–1994) and lake sinks in the ...
Figure 11.18 Correlation of specific sediment yield with discharge area (aft...
Figure 11.19 (a) The locations of eleven major west bank tributaries of the ...
Figure 11.20 Annual fluctuations in fine sediment load inputs to the Mackenz...
Figure 11.21 Concentrations (left columns) and yields (right columns) of nit...
Figure 11.22 Map of the Mackenzie delta anabranching system (after Burn 2010...
Figure 11.23 Lena–Olenek delta: to the west, the Olenek Channel that freezes...
Figure 11.24 Depositional environments of the Yukon–Kushkokwim delta.
Figure 11.25 Schematic diagram of distinctive features of the Yukon–Kushokwi...
Chapter 12
Figure 12.1 Solar and thermal (terrestrial) radiation entering and leaving t...
Figure 12.2 Changing extent of Arctic sea ice in recent years (from satellit...
Figure 12.3 MODIS image of lower Ganga and Brahmaputra valleys and the delta...
Figure 12.4 Possible effects of climate change in the Ganga-Brahmaputra Delt...
Figure 12.5 Probable geomorphic changes in a hypothetical large river due to...
Guide
Cover
Table of Contents
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Introducing Large Rivers
Avijit Gupta
School of Earth, Atmospheric and Life Sciences
University of Wollongong
Australia
With contributions from
Olav Slaymaker
Department of Geography
The University of British Columbia
Vancouver
Canada
Wolfgang J. Junk
National Institute of Science and Technology ofWetlands (INCT-INAU)
Federal University of Mato Grosso (UFMT)
Cuiabá
Brazil
This edition first published 2020
© 2020 John Wiley & Sons Ltd
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Library of Congress Cataloging-in-Publication Data
Name: Gupta, Avijit, author.
Title: Introducing large rivers / by Avijit Gupta.
Description: First edition. | Hoboken, NJ : John Wiley & Sons, Inc., 2020. | Includes bibliographic references and index.
Identifiers: LCCN 2019032032 (print) | LCCN 2019032033 (ebook) | ISBN 9781118451403 (paperback) | ISBN 9781118451427 (adobe pdf) | ISBN 9781118451434 (epub)
Subjects: LCSH: Rivers--Environmental aspects--Research. | Fluvial geomorphology--Environmental aspects--Research.
Classification: LCC GB1205 .G86 2020 (print) | LCC GB1205 (ebook) | DDC 551.48/3--dc23
LC record available at https://lccn.loc.gov/2019032032
LC ebook record available at https://lccn.loc.gov/2019032033
Cover Design: Wiley
Cover Image: © guenterguni/Getty Images
To Mira and Mae
Preface
An edited anthology on geomorphology and management of large rivers was published in 2007.1 The book filled a gap in our knowledge about large rivers as fluvial geomorphology used to be based more on smaller streams of manageable dimensions. We needed to extend our study to big rivers which shape a significant part of the global physiography, carry a high volume of water and sediment to the coastal waters, and support a very large number of people who live on their floodplains and deltas. That was an advanced treatise. This volume is written primarily as a textbook on large rivers, introducing such aspects. A number of line drawings and photographs illustrate the text, and a set of questions at the end of the chapters encourage the reader to explore various issues regarding large rivers.
The book introduces the environmental characteristics of river basins and forms and functions of channels commonly seen among the large rivers of the world. Specific discussions cover their complex geology, water, and sediment. The great lengths of these rivers stretch across a range of different environments. The Mekong, for example, flows on both rock and alluvium with varying form and behaviour. The geological framework of a large river is based primarily on large-scale tectonics commonly derived by plate movements. An uplifted zone, the primary source of sediment in the river, and a nearly subcontinental-scale water catchment area are necessary. A range of morphology exists in large rivers, and the associated floodplains and flood pulses are ecologically important. Large rivers could be geologically long-lived. In future, their forms may change and their functions may alter, following construction of engineering structures and climate change.
The quality of the book has been enhanced by detailed and well-illustrated discussions on two important topics: (i) large rivers and their floodplains: structures, functions, evolutionary traits and management with special reference to the Brazilian rivers by W.J. Junk et al. (Chapter 5), and (ii) large arctic rivers by O. Slaymaker (Chapter 11). I am grateful to all of the authors of these two chapters for their in-depth discussion on these topics. Lastly, the book indicates that the existing rivers possibly are undergoing dynamic adjustments in a world with a changing climate. Rivers change with time, and we usually know a large river only at a particular point in its existence.
Completion of the book has been a demanding task and I am grateful to the editorial and production teams of John Wiley & Sons, Ltd for their remarkable patience, editorial assistance, and continuous encouragement. I would like to thank Athira Menon and Joseph Vimali for guiding me through the intricacies of book production. Lee Li Kheng has produced many of the diagrams from my rough sketches. I have tremendously benefited from the critical readings by Colin Murray-Wallace of Chapter 7 on past rivers and by Colin Woodroffe of Chapter 6 on large river deltas and a discussion on climate change with John Morrison.
Wollongong, Australia, June 2019
Avijit Gupta
Note
1
Gupta, A. (Ed.) (2007).
Large Rivers: Geomorphology and Management
. Wiley: Chichester.
1Introduction
1.1 Large Rivers
We have an intuitive recognition of large rivers although a proper definition is elusive. Even though it is difficult to define a large river, we would probably select the same 15 or 20 rivers as the biggest in the world. Potter identified four characteristic properties of large rivers: they drain big basins; they are very long; they carry a large volume of water; and they transfer a considerable amount of sediment (Potter 1978). It is, however, difficult to attribute quantitative thresholds to these, and not all big rivers exhibit these four characteristics. We associate large rivers with high discharge and sediment transfer, but both water and sediment vary over time and space and their data are difficult to acquire. It is easier to identify large rivers by the size of their drainage basins and their lengths; both are easier to measure.
Based on the areal extent of their drainage basin, Potter (1978) examined 50 of the world's largest rivers, ranked by Inman and Nordstrom (1971), starting with the Amazon. All but one of these rivers are more than 103 km long, and the smallest drainage basin is about 105 km2. These 50 rivers collectively drain about 47% of the land mass, excluding Greenland and Antarctica. The Amazon alone drains about 5% of the continental area. These rivers also have modified the physiography of a large part of the world. Table 1.1 lists the top 24 large rivers (Figure 1.1), ranked according to their average annual water discharge. Their ranks would change if the rivers were listed according to any of the other three properties.
There are other lists. Hovius (1998) tabulated the morphometric, climatic, hydrologic, transport, and denudation data for 97 river basins, all of which measured above 2.5 × 104 km2. Meade (1996) ranked the top 25 rivers twice: first, according to their discharge; and second, according to their suspended sediment load. The two lists do not match well. For example, large rivers such as the Zambezi or Lena carry a large water discharge but a low sediment load. Impoundments too have drastically reduced the once high sediment load of many rivers such as the Mississippi-Missouri. Over approximately the last 100 years, many rivers have been modified by engineering structures such as dams and reservoirs. The Colorado or the Huanghe (Yellow River) at present may not flow to the sea round the year. Such changes have also reduced the amount of sediment that passes from the land to the coastal waters. Large rivers such as the Nile or Indus have been associated with human civilisation for thousands of years and show expected modifications.
Table 1.1 Selected characteristics of 24 large rivers.
River
Average annual water discharge (10
6
m
3
)
Length (km)
Drainage basin area (km
2
)
Current average annual suspended sediment discharge (10
6
t)
1. Amazon
6300
6000
5.9
1000–1300
2. Congo
1250
4370
3.75
43
3. Orinoco
1200
770
1.1
150
4. Ganga-Brahmaputra
970
B-2900 G-2525
1.06 (B-0.63)
900–1200
5. Changjiang
900
6300
1.9
480
6. Yenisey
630
5940
2.62
5
7. Mississippi
530
6000
3.22
210
8. Lena
510
4300
2.49
11
9. Mekong
470
4880
0.79
150–170
10. Paranẚ-Uruguay
470
3965
2.6
100
11. St. Lawrence
450
3100
1.02
3
12. Irrawaddy
430
2010
0.41
260
13. Ob
400
>5570
2.77
16
14. Amur
325
4060
2.05
52
15. MacKenzie
310
4200
2.00
100
16. Zhujiang
300
2197
0.41
80
17. Salween
300
2820
0.27
About 100
18. Columbia
250
2200
0.66
8
19. Indus
240
3000
0.97
50
20. Magdalena
240
1540
0.26
220
21. Zambezi
220
2575
1.32
20
22. Danube
210
2860
0.82
40
23. Yukon
195
3200
0.83
60
24. Niger
190
4100
2.27
40
These figures vary between sources, although perhaps given the dimensions, such variations are proportionally negligible. Discharge and sediment figures are from Meade (1996) and Gupta (2007) and references therein. Drainage areas are rounded off to 106 km to reduce discrepancies between various sources. The Nile is not listed, even though it is 6500 km long. It does not qualify for this table as its water and sediment discharges are relatively low.
The great lengths of these rivers allow them to flow across a range of environments. The Mekong, for example, flows on both rock and alluvium, looking different (Figure 1.2). The end part of the river needs to adjust to all such environmental variations plus the Quaternary changes in sea level.
Fluvial geomorphology generally is based on small and logistically manageable streams. A study of large rivers is necessary, although difficult, for multiple reasons. Large rivers form and modify subcontinental-scale landforms and geomorphological processes. A high number of them convey and discharge a large volume of water and sediment to the coastal seas. An understanding of modern large rivers helps us to explain past sedimentary deposits. Large rivers, such as the Amazon (Mertes and Dunne 2007), and their deposits may reveal basinal and regional tectonics, past and present climate, and sea-level fluctuations. Management of the water resources of a large river is often an essential step toward the supply of water and power to a large number of people. We need to study large rivers for many such reasons.
Figure 1.1 A sketch map showing the location of 24 large rivers in the world: 1, Amazon; 2, Congo; 3, Orinoco; 4, Ganga-Brahmaputra; 5, Changjiang; 6, Yenisei; 7, Mississippi; 8, Lena; 9, Mekong; 10, Parana-Uruguay; 11, St. Lawrence; 12, Irrawaddy; 13, Ob; 14, Amur; 15, Mackenzie; 16, Zhujiang; 17, Salween; 18, Columbia; 19, Indus; 20, Magdalena; 21, Zambezi; 22, Danube; 23, Yukon; 24, Niger.
1.2 A Book on Large Rivers
A number of individual large rivers have been studied and such studies published discretely. A collection of advanced essays on the general characteristics of large rivers, their selected case studies, and their utilisation and management is also available (Gupta 2007). In comparison, this volume is primarily an integrated textbook on large rivers and introduces the reader to the morphology and management of these huge conduits on which both the general physiography of the basins and utilisation of the resources of the rivers depend.
The discussion on large rivers starts with an account of their geological framework (Chapter 2) that determines where they can be located and also what their physical characteristics would be. The geological framework of a large river is based primarily on large-scale tectonics commonly driven by plate movements. An uplifted zone and the adjoining subcontinental-scale water catchment area are necessary requirements for a big river. Smaller tectonic movements may further modify the basin and the channel and explain their detailed morphological characteristics.
Figure 1.2 The Mekong. (a) On rock, downstream of Chiang Saen, northern Thailand. (b) On alluvium near Savanakhet, Lao PDR, photographed from the air. Note the difference in form and behaviour between the two reaches. Large rivers commonly are a combination of a number of similar variations.
Source: A. Gupta.
The regional geology should create a drainage basin large enough to accumulate enough precipitation to support and maintain the big river. Chapter 3 discusses the nature of water and sediment in a large river. The discharge in a large river is determined by various climatic criteria depending on its location: annual rainfall, seasonality in rainfall, and high episodic rain from synoptic disturbances such as tropical cyclones. The supply of water to large rivers could be from almost all parts of the watershed but the sediment supply generally is associated selectively with high mountains. For example, the discharge of the Orinoco is collected from most of the basin, irrespective of geology or relief, but its sediment supply is only from the Andes Mountains and the alluvial Llanos plains formed near the Andean foothills. In certain cases, several large rivers flow through arid landscapes without identifiable addition to their discharge but manage to sustain their flow because of the high discharge arriving from the upper non-arid parts of their drainage basins. Sediment in flood moves in large rivers both in downstream and lateral directions if large floodplains are present. The sediment grains travel a long distance to reach the sea and, in the process, become mature and sorted.
Large rivers have been aptly described as massive conveyance systems that move detrital sediment and dissolved matter over transcontinental distances (Meade 2007). Their morphology is dependent on regional geology, discharge and sediment flux, and may change several times between the headwaters and the sea (Chapter 4). Morphologically a large river usually has a channel flanked by bars, floodplain, and terrace fragments. The channel pattern depends on the gradient of the river and the nature of water and sediment it transports, and the pattern varies among different rivers as they adjust to the local physical environment. Floodplains of large rivers are important not only for their origin and age but also for their ecology which supports a wide variety of species, and their economic utilisation by people. The role of flood pulses in the maintenance of the floodplains and its ecology is crucial. This is discussed in detail by Junk et al., in one of the two invited chapters in this book (Chapter 5). The huge discharge of water and sediment that is deposited by a big river in the sea may create a large delta. Deltas are morphologically fragile and change over time (Chapter 6). Deltas of many large rivers support a large population, and hence are of importance.
Large rivers could be geologically long-lived rivers such as the Mississippi or the Nile. A river that exists for a long time has a history. Tectonic processes commonly influence the origin, geographical location, and modification of major rivers. Understanding of such rivers requires knowledge of their history as rivers have changed episodically through tectonic movements, and especially through climate and sea-level changes in the Quaternary (Chapter 7).
Large rivers are a useful resource to people. A proper utilisation (Chapter 8) and management (Chapter 9) of large rivers is important. The land use of their basins and the use of their water have modified the environment over years of human civilisation. This has led to alteration of large rivers and their basins at various levels, especially over the last hundred years. The form and behaviour of many of the present large rivers have been modified mainly due to construction of large dams and reservoirs. The present state of a large river is conditioned by both the original physical environment of the basin and anthropogenic alterations imposed on the channel.
This requires proper management of the rivers so that basinal economic development and environmental degradation can be balanced in a sustainable way. A management procedure which simultaneously allows both economic development and environmental sustenance needs to be chosen. As a large river usually flows across multiple countries, each with different expectations and varying ability of resource utilisation, there is also a political aspect of large river management.
Chapter 10 deals exclusively with the Mekong River as a case study to illustrate the techniques and problems of managing a multistate river in a complex physical environment. It illustrates the reality of river management which involves dealing with the complexity of the physical characteristics of a big river, meeting the different expectancies of multiple stakeholders of the river basin, and maintaining the quality of the river for future generations, all at the same time.
Chapter 11 is on the special case of major rivers in the arctic. It deals mainly with the Lena, Yenisei and Ob in Siberia and the Mackenzie and Yukon in North America. These rivers flow through a unique environment and are expected to go through large changes in the near future due to global warming. This discussion on arctic rivers by Slaymaker is the second invited contribution in this book.
The last chapter deals with the possible modifications of large rivers in the near future. They may undergo significant changes following climate change and construction of large-scale engineering structures. The general tenets of climate change are known and accepted, but we have limited knowledge regarding its impact on large rivers. We, however, need to consider the future for understanding and management of present large rivers, as such changes would impact the lifestyles of a very large number of people, as the rivers of the future are likely to be different.
References
Gupta, A. (ed.) (2007).
Large Rivers: Geomorphology and Management
, 689. Chichester: Wiley.
Hovius, N. (1998). Control of sediment supply by large river. In:
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Inman, D.L. and Nordstrom, C.E. (1971). On the tectonic and morphological classification of coasts.
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Meade, R.H. (1996). River-sediment inputs to major deltas. In:
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Meade, R.H. (2007). Transcontinental moving and storage: the Orinoco and Amazon rivers transfer the Andes to the Atlantic. In:
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Mertes, L. and Dunne, T. (2007). Effects of tectonism, climate change, and sea-level change on the form and behaviour of the modern Amazon River and its floodplain. In:
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(ed. A. Gupta), 112–144. Chichester: Wiley.
Potter, P.E. (1978). Significance and origin of big rivers.
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2Geological Framework of Large Rivers
2.1 Introduction
A large river is a long river which drains an extensive basin, carries a big discharge, and usually, but not always, transports a huge quantity of sediment (Potter 1978). It possesses a suitable three-dimensional geological framework for achieving these characteristics. A linear depression in rock of considerable length commonly lies below the river. A sedimentary fill of varying depth rests on this depressed rock surface, and along with bedrock constitutes the material below the channel of the river. The fill has been deposited by the main river and its ancestors, and some of its sediment is contributed by tributary streams. On the surface, the long trunk river crosses a range of physical environments and changes form and behaviour several times. For example, the Irrawaddy, Narmada and Danube flow in and out of narrow rocky valleys and wide alluvial basins. The basin of a large river commonly is an accumulation of several sub-basins with different character, exhibiting a polyzonal form and behaviour. The end part of the main river needs to adjust to all such variations in the large basin, plus any change in sea level.
The geological framework of a large river is formed primarily by past large-scale tectonics. Its basin should also be big enough to collect sufficient precipitation to form and support a major river system. Conditions vary spatially within the basin of the large river, and different parts of the basin contribute water and sediment in varying fashion to the mainstream. The main river usually receives water from multiple parts of the basin, but almost all of its sediment is usually derived from higher tectonic parts of the catchment, an area of high relief and disintegrated rocks (Meade 2007; Milliman and Syvitski 1992). Usually, the sediment is derived from such areas by glaciation, slope failures, and eroding headstreams of the river.
In brief, the physical characteristics of a large river depend on its structural framework, its geological history, and its pattern of water and sediment supply. Such characteristics form and maintain the river and its basin. Their nature is modified over time following changes in tectonics and climate, and in current times also by anthropogenic alterations of the river and its basin.
2.2 The Geological Framework: Elevated Land and a Large Catchment
Many of the existing large rivers start at an elevated orogenic zone, drain a subcontinental-scale area, and flow to the ocean through a major delta. The orogeny is usually created by convergence of two tectonic plates. The course of the river may be determined by a large-scale geofracture which it follows, and its mouth may be positioned by a rock basin at the trailing margin of one of the plates. The Amazon is an excellent example. Another example is that of the present Lower Mississippi, still located over a Cretaceous sub-surface rock embayment near its present confluence with the Ohio. It has been suggested that this embayment underneath the sediment of the river is related to a reactivated rift whose history may have started much earlier in the late Precambrian (Ervin and McGinnis 1975; Potter 1978; Knox 2007).
An uplifted zone, often formed by plate collisions, and an adjoining uplifted sub-continental-scale catchment area are the necessary requirements for a major river (Tandon and Sinha 2007). These conditions exist for a time long enough to create and sustain the river system. The present continental land masses are large enough to support the current big rivers, but the size of the land masses has not always been the same. An existing drainage can be modified over time due to changes in tectonic, geomorphic, and hydrologic systems. Rivers larger than those of the present time probably existed during the supercontinents of the Wilson cycle (Pangea and Rhodinia). In contrast, very early rivers on Earth probably did not become large due to limited hydrologic support and restricted basin area on land (Potter 1978). Tectonic movements and the following geomorphic processes determine the required development of the basin topography marked by elevated boundaries and a regional slope. Continental plate tectonics determine the basin framework and trough of the river, and regional disruptions vary its character, locally.
On fewer occasions, a new topography and a confined river is created by rifting, as shown by the Rio Grande, which is a pull-apart system that began in the early Miocene and developed from separate shallow basins to an integrated system. The failed arm at a plate tectonics triple junction also may give rise to rifting which may carry a big river that occupies the long, narrow, deeply filled depression extending into a craton. At the other end of the linear depression, the river builds a delta at the plate boundary marking the edge of the continent. The Niger is an example. Another example is the Blue Nile which rises within the rift system of Ethiopia to flow through a deep gorge for about half of its length.
A new topography and a modified river may also be formed off a mantle plume (Cox 1989) when it rises to create an extensive domal surface with high elevation, as happened in several parts of the Earth during the Late Cretaceous. Certain areas were uplifted by doming and magmatic underplating, giving rise to topographic highs and new drainage systems. The Orange and part of the Zambezi River are examples.
Not all large rivers fall into such clearly generalised classes. Some do not rise from currently active orogenic belts but drain parts of old mountains that have some elevation or are only slightly active tectonically. Tandon and Sinha (2007) described such rivers as located in cratonic settings as these drain major cratons. Examples of this type are the Mississippi, Yukon, Yenisei, Lena, and Ob. Potter (1978) stated that four major morphological patterns may cover the majority of large rivers. These can be described as:
(1) Most sediment derived from mountains marginal to a large craton (the Amazon).
(2) The river flowing marginal and parallel to a fold belt (the Ganga).
(3) A big river flowing along the strike of a mountain range (the Mekong).
(4) A river superimposed across multiple mountain chains (the Danube).
Potter proposed a fifth possibility which may have occurred in the past: a river on a large craton without bordering mountains (Potter 1978).
2.3 Smaller Tectonic Movements
The basin and the channel of a big river are further modified by smaller tectonic disturbances. For example, the Lower Amazon crosses several transverse structures in its course (Figure 2.1). In the downstream direction these are the Iquitos Arch, Jutai Arch, Purus Arch, Mont Alegre Intrusion and Ridge, and Gurupa Arch. When the Amazon crosses such structural highs, certain geomorphic features characterise the river (Mertes and Dunne 2007). The channel runs straight, its floodplain remains relatively narrow, scroll bars are found only near channel margins, and migration of the channel is limited. In contrast, wide floodplains with significant river movement, scroll bars and anabranches distinguish the river flowing on a low gradient between the upwarps. Even when covered by an alluvial fill, these upwarps affect the river. The morphology of a large river therefore varies along its course.
Large-scale fractures also regionally affect the tributary network in the Amazon Basin. Deep-seated basement fracturing appears to have disturbed the overlying sedimentary rocks that affect the drainage net, oriented in northeast and northwest directions (Potter 1978, Figure 8 and references therein; Mertes and Dunne 2007). The alignment of the Lower Negro, one of the major north bank tributaries of the Amazon, has been interpreted as controlled by a NW-SE tectonic lineament (Franzinelli and Igreja 2002). Here sunken crustal blocks and depressions occur along a half-graben, submerged to approximately 20 m with a width of up to 20 km. This controls the pattern of river islands, bars, and the location of sediment storage in the river.
Figure 2.1 Schematic illustration of the relation between structure and morphology, the Lower Amazon Valley. The vertical bars show the approximate location of arches and a tilted block which are structural highs: JA, Jutai Arch; PA, Purus Arch; MI, Monte Allegro Intrusion and Ridge; GA, Gurupa Arch; TFB, approximate location of a tilted fault block.
Source: Mertes and Dunne 2007 and references therein.
Figure 2.2 Longitudinal profile of the Nile over varying regional structures and lithology.
Source: Woodward et al. 2007, from Said 1994.
A number of rivers flow along the structural grains of a folded mountain belt almost to their mouths. The Mekong and Salween are good examples. The Mekong flows for about 80% of its length in a narrow valley flanked by ridges (Figure 1.2a) whereas the Salween flows through a series of gorges. The lower Irrawaddy crosses three gorges in rock separated by wider basins in alluvium, and the morphology and behaviour of the river change between the gorges and basins. The entry and exit to the gorges show adjustment of the river over a short distance. The Irrawaddy finally continues through a flat lowland, discharging into the Gulf of Martaban through a large delta. The entire river is thus a sum of its different parts. The morphology and structural underpinning of large rivers are further discussed in Chapter 4.
Large rivers differ in their form and function, and for many rivers the understanding of such differences is achieved via a history of continental plate tectonics and lesser tectonic movements. The longitudinal profile of large rivers generally reflects a combination of their tectonic settings, basin lithology, and erosional history as their gradient changes (Figure 2.2). The profile of a large river over a long distance thus may be a combination of different sections. Geological structure and lithology tend to control the basic characteristics of the rivers.
2.4 The Subsurface Alluvial Fill of Large Rivers
The rock surface underlying the alluvial valley-fill of a large river generally is not smooth but marked by irregularities such as ribs, furrows, scour holes, etc. Schumm, for example, discussed the variable thickness of the Mississippi Valley alluvium at East St. Louis, Missouri as reported in the literature. The thickness of the alluvium reaches approximately 35–50 m in places (Schumm 1977). The depth of the alluvium and its nature are exhibited in boreholes, but a complete mapping is very difficult. Detailed three-dimensional studies have been carried out on several rivers (Fielding 2007). Construction of dams over large rivers also exposes surface morphology. The section at Hoover Dam on the Colorado River disclosed an inner channel flanked by bedrock terraces. About 200 km of the channel of the Changjiang were mapped between November 1978 and May 1979 near the site chosen later for the construction of the Three Gorges Dam. More than 90 troughs were found at the bottom of the river, cumulatively covering 45% of the total length of the river mapped. All the troughs were more than 40 m below the lowest river level (Yang et al. 2001). The cross-section of the channel of the Upper Mekong is trapezoidal, or has a deep inner channel bounded by rock benches, or a wide scabland-like section with rock ribs and piles rising from the bed (Figure 2.3). Scour pools and rock protrusions occur on the rock benches and the floor of the inner channel (Gupta 2007). The final bed of a big river thus is a composite result of structure and lithology at various scales.
The texture of the river-deposited fill over rock is not uniform. In general, the coarsest sediment is found at the base of the valley-fill and the finest towards the top (Schumm 1977). Sediment also tends to become finer in the downstream direction. However, sedimentation is also likely to indicate: (i) interrupted deposition; (ii) variations along the main channel due to differential contribution by tributaries; and (iii) variations from a general fining-upward sequence probably caused by climate change and tectonics. However, where the available sediment is of similar texture, the fill tends to be texturally rather uniform.
Valleys can be filled longitudinally in three ways. First, basin climate change or uplift of the source area may result in progressive aggradation of relatively coarse material deposited along the valley, downfilling the channel. Secondly, sediment from tributaries joining the trunk stream may vertically fill the valley relatively uniformly over a river reach. Thirdly, a rise in base level may reduce the gradient over the lower parts of the valley, initiating progressive backfilling (Schumm 1977).
Figure 2.3 Diagrammatic sketches of cross-sections of the Mekong River in rock, Lao PDR.
Source: Gupta 2004.
Tandon and Sinha (2007) summarised sedimentation in the Ganga River. Rising from the Himalaya Mountains, the river descends to the plains and flows parallel to the mountains for a considerable part of its course. It then erodes through the igneous Rajmahal Hills; turns south to build part of a major delta; and finally flows into the Bay of Bengal. The upper 20 km of the river downstream below the mountain front carries numerous gravel bars, the riverine alluvium consisting mainly of several metres of gravel. Coarse sand, corresponding to a braided channel belt, replaces gravels downstream (Shukla et al. 2001). This is followed downstream by the meandering and braided middle section of Ganga that deposits mostly fine to very fine sands on medium to coarse sands of older deposits. The fine sands are separated by units of floodplain clay, several metres thick and pedogenically altered, characteristics of a mobile river. Considerable volume of sediment also arrives to the Ganga as fan-deposits of the major Himalayan tributaries such as the Gandak or Kosi. The deposits of these megafans range from the mountain front to the Ganga in zones, from gravelly sand to fine sand and mud. These megafans consist of gravel beds in the upper areas, but most of the fan is made of multistoried sand sheets interbedded with overbank muddy layers (Singh et al. 1993).
Interpretation of both tectonic structures and subsurface fill is required to determine the geological background of a large river and explain its surface configuration. Natural sedimentation in a large river such as the Ganga is controlled primarily by tectonics, climate and sea level. Both tectonics and climate control the nature of the sediment near the mountains, the effect of the climate dominates further downstream along with the sediment supplied by both tributary fans from the mountains and sediment from cratons, and the effect of the changing sea level influences the lower part of the valley.
2.5 Geological History of Large Rivers
The morphology and behaviour of a large river, as discussed, reflect structural control at several levels. The location and origin of the basin are commonly determined by creation of a new surface by plate collision or rifting or doming. The course of the river may follow an older geofracture or be fixed in places where it passes over controls such as a structural embayment. The character of the river changes where it crosses an upwarp transverse to its course or a regional pattern of faults as seen in the central part of the Amazon Basin near Manaus. The dimensions of a major river also change when it passes through steep gorges separated by low-gradient alluvial basins. These variations not only modify the bedrock channel of the rivers, but they also influence valley sedimentation and give rise to variable riverine morphology, bar forms, and presence or absence of floodplains at the surface. Furthermore, in all cases, the new drainage basin needs to be large enough to maintain a major river and be located in a suitable hydrologic environment.
Large rivers vary in their size and age. Mobility of continental plates may result in plate destruction and size reduction or slow passage of the plate to a drier part of the Earth. Such developments modify or may even destroy a major river. In contrast, plates may increase in dimensions or reach more humid areas over time. Potter (1978) suggested that very big rivers may have been formed draining an enormous land mass consisting of welded cratonic blocks which carried at least one system of fold mountains and was well-watered.
The geological history of a major river therefore has a beginning and an end. The Mississippi is recognised to have been in existence since at least the Cretaceous because of a subsurface rock embayment located near its present confluence with the Ohio River mentioned earlier. The embayment is related to later Mississippi River fills and sediment of the Mississippi, dating back to the Jurassic period, has been found in the Gulf of Mexico. The embayment has been dated to late Precambrian (Ervin and McGinnis 1975). A large river therefore has been approximately in a place for nearly 300 million years or about 1/16th of the history of the Earth (Potter 1978). It is an old river, so are several others such as the Nile. The ancestral Nile has been associated with the ancient Pan-African orogenic events of about 550 million years ago or earlier. The river has been modified over time to its present appearance (Woodward et al. 2007 and references therein).
Not all major rivers have a similar long history. The present Amazon came into existence after the rise of the Andes in the Miocene. There was an older drainage system earlier, flowing westward and probably related to the African Plate before the opening of the South Atlantic. The Ganga and Brahmaputra, two major Himalayan rivers, probably came into existence in the Miocene after the formation of the Himalaya Mountains following the collision of the Indian Plate with the Eurasian Plate. There may have been an earlier drainage system, but it has been drastically modified, and the present drainage network has evolved over time. For example, the drainage of the western Himalaya was modified through a set of river captures, changing the direction of flow of the tributaries of the Ganga and Indus (Burbank 1992; Clift and Blusztajn 2005). It is believed that earlier the Sông Hóng (Red River) drainage net included the former upper Changjiang (Yangtze), Mekong, Salween and Tsanpo. The earlier upper drainage of the Sông Hóng then disintegrated to form the headwaters of these separate major rivers (Brookfield 1998). Robinson et al. (2014) have opined that the present Brahmaputra system has changed from the old Yarlung Tsanpo–Brahmaputra–Irrawaddy linkage to the present drainage via river capture and tectonic movements, around 18 Ma ago, and finally was captured by the Bay of Bengal system of the Upper Brahmaputra by the Lohit and newer Himalayan rivers (Licht and Giosan accepted for publication).
A river system may be terminated by other events such as a large-scale marine invasion, a new tectonic deformation, outpouring of lava or continental glaciation (Potter 1978). Tectonic, geomorphic and climatic processes may also modify an earlier drainage. The collision of the Indian and Eurasian Plates resulted in crustal shortening, differential shear and rotation associated with the rise of the Himalaya. An existing river may disappear through river capture, climate change resulting in a drier environment, or deformation due to plate tectonics. The change may affect almost the entire basin or only part of the drainage system. Potter (1978) has described rivers which were an assemblage of parts as having a composite age. Some rivers may persist in spite of major interruptions to experience over a long existence. The major rivers, their deltas, and offshore trenches underwent multiple changes during the Pleistocene. Goodbred Jr (2003) has traced the series of modifications that the Ganga River underwent since Marine Isotope Stage 3 (MIS 3, 58 000 years ago), mainly due to climate change.
2.6 Conclusion
The origin of a large river requires a suitable geological framework that creates (i) a large basin and (ii) a major river that slopes from elevated headwaters to the edge of the continent. Such developments happen primarily via plate tectonics although there could be other explanations. Sufficient precipitation needs to be accumulated in the basin to support and maintain the river. The basic form and function of the river that flows on the surface are spatially modified further by regional and local tectonics.
The size of the basin and the river is determined by plate tectonics and the amount of precipitation received by the area. The size of the river may change because of (i) plate movements which may lead to crustal spread or shortening and (ii) increase or decrease of precipitation. A large river therefore has a beginning and an end, and exists for a length of time. Several rivers such as the Mississippi or the Nile are very old and include parts of an earlier system. Many large rivers of the present are much younger, a number of them coming into existence or being drastically modified after the formation of the young fold mountains such as the Andes or the Himalaya.
Questions
What are the characteristics of large rivers? Do all large rivers have the same characteristics?
What kind of geological framework is required for a large river to exist? How do such frameworks originate?
How old are large rivers?
Mertes and Dunne (
2007
) described the relationship between structure and morphology of the Lower Amazon River. Describe such a relationship for a large river of your choice.
Discuss the sedimentary fill below a large river. Does it rest on a smooth rock surface? Give examples in support of your discussion.
Do large rivers stay the same in appearance and behaviour over time?
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