Introducing Large Rivers. Avijit Gupta
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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
1 What are the characteristics of large rivers? Do all large rivers have the same characteristics?
2 What kind of geological framework is required for a large river to exist? How do such frameworks originate?
3 How old are large rivers?
4 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.
5 Discuss the sedimentary fill below a large river. Does it rest on a smooth rock surface? Give examples in support of your discussion.
6 Do large rivers stay the same in appearance and behaviour over time?
References
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3 Clift, P.D. and Blusztajn, J.S. (2005). Reorganization of the western Himalayan river system after five million years ago. Nature 438: 1001–1003.
4 Cox, K.G. (1989). The role of mantle plumes in the development of continental drainage patterns. Nature 342: 873–877.
5 Ervin, C.P. and McGinnis, L.D. (1975). Reelfoot rift: reactivated precursor of the Mississippi embayment. Geological Society of America, Bulletin 86: 1287–1295.
6 Fielding, C.R. (2007). Sedimentology and stratigaphy of large river deposits: recognition in the ancient record, and distinction for ‘Incised Valley Fills’. In: Large Rivers: Geomorphology and Management (ed. A. Gupta), 97–113. Wiley.
7 Franzinelli, E. and Igreja, H. (2002). Modern sedimentation in the Lower Negro River, Amazonas State, Brazil. Geomorphology 44: 259–271.
8 Goodbred, S.L. Jr. (2003). Response of the Ganges dispersal system to climate change: a source-to-sink view since the last interstade. Sedimentary Geology 162: 83–104.
9 Gupta, A. (2004). The Mekong River: morphology, evolution and palaeoenvironment. Journal of Geological Society of India 84: 525–533.
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11 Knox, J. (2007). The Mississippi River system. In: Large Rivers: Geomorphology and Management (ed. A. Gupta), 145–182. Wiley.
12 Licht, A. and Giosan, L. (accepted for publication). The Ayeyarwady River. In: Large Rivers: Geomorphology and Management, 2e (ed. A. Gupta). Wiley.
13 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. Wiley.
14 Mertes, L.A.K. 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), 115–144. Wiley.
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18 Said, R. (1994). Origin and evolution of the Nile. In: The Nile: Sharing a Scarce Resource (eds. P.P. Howell and J.A. Allan), 17–26. Cambridge: Cambridge University Press.
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22 Tandon, S.K. and Sinha, R. (2007). Geology of large river systems. In: Large Rivers: Geomorphologyn and Management (ed. A. Gupta), 7–28. Wiley.
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24 Yang, D., Li, X., Ke, X. et al. (2001). A note on the troughs in the Three Gorges Channel of the Changjiang River, China. Geomorphology 41: 137–142.
3 Water and Sediment in Large Rivers
3.1 Introduction
The potential locations of large river basins are determined mainly by plate tectonics. The existence of a large river is possible in such locations provided a high amount of precipitation fell on the basin and is transformed into river discharge. An integrated network of channels is needed to concentrate the basin runoff into a big main channel. A large river basin also includes actively eroding landforms which provide a significant volume of sediment. This chapter introduces the general sources of water and sediment for such rivers.
3.2 Discharge of large Rivers
River discharge (Q) is computed as the volume of water passing a given point on the river in unit time. It is measured in m3 s−1, or for annual or long-term periods for large rivers in km3 year−1. Two rivers can be compared by normalising their discharges into runoff (R) which is discharge per unit area of the basin (R = Q/A). The standard expression of runoff is
where
| R | = | Runoff |
| P | = | Precipitation (includes snowmelt for certain basins) |
| AE | = | Actual evapotranspiration |
| S | = | Surface and subsurface storage of water |
| C | = |
Anthropogenic |