To Mira and Mae

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.

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.

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.

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.

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

1. Gupta, A. (ed.) (2007). Large Rivers: Geomorphology and Management, 689. Chichester: Wiley.
2. Hovius, N. (1998). Control of sediment supply by large river. In: Relative Role of Eustasy, Climate, and Tectonism in Continental Rocks, vol. 59 (eds. K.W. Shanley and P.C. McCabe), 3–16. Tulsa: Society for Sedimentary Petrology Special Publication.
3. Inman, D.L. and Nordstrom, C.E. (1971). On the tectonic and morphological classification of coasts. Journal of Geology 79: 1–21.
4. Meade, R.H. (1996). River-sediment inputs to major deltas. In: Sea-Level Rise and Coastal Subsidence: Causes, Consequences and Strategies (eds. J.D. Milliman and B.H. Haq), 63–85. Dordrecht: Kluwer.
5. Meade, R.H. (2007). Transcontinental moving and storage: the Orinoco and Amazon rivers transfer the Andes to the Atlantic. In: Large Rivers: Geomorphology and Management (ed. A. Gupta), 45–63. Chichester: Wiley.
6. 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: Large Rivers: Geomorphology and Management (ed. A. Gupta), 112–144. Chichester: Wiley.
7. Potter, P.E. (1978). Significance and origin of big rivers. Journal of Geology 86: 13–35.