Introducing Large Rivers. Avijit Gupta
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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.
2 Geological 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