Introducing Large Rivers. Avijit Gupta

Introducing Large Rivers - Avijit Gupta


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changes remain recorded in detail over a long stretch of the lower section of a continental-scale river because the gradient is very low? Do floods and floodplains play an important role regarding the morphology and ecology of large river valleys? Can huge sedimentary alluvial deposits/bars of large rivers such as the Brahmaputra (Figure 4.1) be recognised in later sedimentary rocks (Fielding accepted for publication)? In this chapter we explore these and other queries, and describe the general morphology and behaviour of big rivers.

Photograph depicting 10 m high midchannel bar in the Brahmaputra at Sirajganj, Bangladesh with the top of the bar at several levels.

      Source: Gupta 2007.

Image described by caption and surrounding text.

      This is a short and simplified description. Form, behaviour and sediment of large rivers, however, vary among themselves. Two rivers, the Amazon and Ganga, are described to establish a general picture and several common properties. Several other rivers are then discussed, to highlight an expected assemblage of form and behaviour of large rivers and variations from such expected outcomes. Form and behaviour of a large river is the combined result of such an expected outcome and deviations therefrom.

      Emerging from the mountains to the plain, The Ganga and its Himalayan tributaries also have built alluvial megafans. The best known of the Himalayan megafans is that of the Kosi River which covers 154 × 147 km in area, and slope 0.89–0.025 m km−1 longitudinally from the Himalayan front to the Ganga River. The climate of the region is affected by the Indian monsoon system. The regional average annual rainfall is between 1300 mm and 1800 mm, 80% of which occurs between June and October. Individual large storms are superimposed on the general rainfall pattern. The Kosi fan consists of large volumes of clastic sediment derived from the Himalaya. On the upper part of the fan, the Kosi River

Image described by caption and surrounding text.

      Big rivers of the world have been long utilised as a natural resource, and in the process many of them have been physically modified by anthropogenic activities (Chapter 8). The Amazon, however, remains in near-natural conditions, unmodified by anthropogenic activities. This summary of the geomorphology of the river is based primarily on Dunne et al. (1998), Mertes and Dunne (2007), and Meade (2007). The ecology of its floodplain is discussed in detail by Junk et al. in Chapter 5 of this book.

      4.3.1 The Setting