Introducing Large Rivers. Avijit Gupta

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СКАЧАТЬ The terms anabranching or wandering have been used instead of anastomosing, but in general all three terms refer to a river divided into multiple channels by one or several semi-stationary bars or islands. These channels and bars or islands, unlike in a braided river, are not particularly mobile. These planform terms have been further subclassified to describe a particular river but such practices may lead to considerable confusion. Confusion also arises as a river can meander or braid over a short distance but because of its low sinuosity, it appears to be anastomosing over a longer stretch. Furthermore, rivers in seasonal climate may rise considerably in the wet season, and then appear to present a different planform. The Brahmaputra has been reported to rise 8 m (Coleman 1969; Baker and Costa 1987), and the Amazon 12 m (Archer 2005). The channel pattern of large rivers therefore may look different between the wet and dry seasons. The wet season river tends to display less sinuosity.

      3 Large-scale avulsions may happen in low-gradient large rivers leading to conspicuous changes in river channels and floodplains.

      4 A large river usually does not display the same channel pattern all along its course because of variations in underlying lithology and structure. Segments of a channel reach may also vary in form and behaviour. In contrast, smaller rivers are often monotonic. Ashworth and Lewin (2012) stated that large rivers do not flow through unified valley systems but appear as chains of interlinked domains with varying fluvial functions and scales. They have provided a generalised list. In sequence, these rivers flow through (i) headwater mountain belts providing sediment and runoff, (ii) intermontane and foreland depositional basins and ramps (megafans), (iii) low-sediment yielding cratons, (iv) transverse tectonic controls that shape riverine forms and behaviour over parts of the river, and (v) coastal environment with the history of regional sea-level change.

      5 Sedimentary forms of different dimensions tend to occur within large rivers. Mid-channel islands match the depth of the entire stream, large bars are associated with the dimensions of an individual anabranch, and large dunes are dependent on channel depth. These morphological features are often exposed during the dry season, when large dunes are found on top of bars or attached transversely to bars. A series of sedimentary features of different sizes are seen. Many of these sedimentary features remain inherited from the previous wet period.

      6 Some of these sedimentary features are scale invariant (Best et al. 2003) but not all. Large rivers thus can be complex phenomena.

      7 A large river in alluvium may possess an active wide channel belt. A significant relationship exists between river channels and floodplains, especially at flood stages.

      8 Different channels in a large river may function differently, depending on river stage. As a result, the same river may appear and behave different at times. A detailed description of such complexity of large rivers has been reviewed in several papers (Ashworth and Lewin 2012; Lewin and Ashworth 2014; Fielding accepted for publication).

      9 Flood pulses and sediment transfer in floods commonly travel both longitudinally and transversely in a large river. Floods commonly move downstream in small rivers. Large rivers tend to have a complex relationship between the main channel and their floodplains as illustrated by the movement of water and sediment in the valley of the Amazon (Mertes and Dunne 2007; and see Chapter 5 of this book).

      10 Ashworth and Lewin (2012) have summarised large rivers to be plural systems because of coupling (Harvey 2002). Partial decoupling has been seen between large rivers and their floodplains and between main and subsidiary channels. Coupling is limited in small rivers.

      11 Basins of large rivers may include polyzonal sub-basins.

4.6 Conclusion

      Large rivers are huge systems that transfer water and sediment from the continents to the oceans. Their nature and behaviour determine the morphology of their drainage basins, which are often subcontinental in size, as the entire drainage network is connected to these massive conduits. Thus large rivers may shape the physiography of the land surface over time.

      Their location, morphology and behaviour depend on plate tectonics, regional and local geology, and large-scale climatic systems. Several of these rivers have existed for a long period, and all of them have been affected by the repeated climate and sea level changes in the Quaternary. The majority of large rivers are related to human habitation because of the availability of water, fertile fine-grained sediment, extensive floodplains, and ease of irrigation. Several floodplains and deltas have been anthropologically modified, even impounded, and altered from their natural forms and functions.

      The following chapters highlight special cases of the morphology of large rivers, their long association with human civilisation, and their probable adjustments to the changing climate of the future.

      Questions

      1 What are the principal properties of a large river? What effect do such properties have on the morphology of a large river?

      2 What is a megafan? Where are megafans found?

      3 Describe the physiography of a megafan.

      4 Describe the movement of flood water and sediment in a large river valley.

      5 Discuss the morphological complexity of a large river.

      6 Large rivers have been described as plural systems by Ashworth and Lewin (2012). Why?

      7 Discuss the polyzonal sub-basins which may form part of a large river basin.

References

      1 Aalto, R., Maurice-Bourgoin, L., Dunne, T. et al. (2003). Episodic sediment accumulation on Amazonian floodplain influenced by El Niño/ Southern Oscillation. Nature 425: 493–497.

      2 Archer, A.W. (2005). Review of Amazonian depositional systems. In: Fluvial Sedimentology VII (eds. M.D. Blum, S.B. Marriott and S. Leclair), 17–39. Blackwell.

      3 Ashworth, P.J. and Lewin, J. (2012). How do big rivers come to be different? Earth Science Reviews 114: 84–107.

      4 Baker, V.R. and Costa, J.E. (1987). Flood power. In: Catastrophic Flooding (eds. L. Mayer and D. Nash), 1–21. London: Allen and Unwin.

      5 Best, J.L., Ashworth, P.J., Bristow, C.S., and Rodin, J. (2003). Three-dimensional sedimentary architecture of a large mid-channel sand braid bar, Jamuna River, Bangladesh. Journal of Sedimentary Research 73: 516–530.

      6 Chakraborty, T., Kar, B., Ghosh, P., and Basu, S. (2011). Kosi megafan: historical records, geomorphology and the recent avulsion of the Kosi River. Quaternary International 227: 143–160.

      7 Coleman, J.M. (1969). Brahmaputra River: channel processes and sedimentation. Sedimentary Geology 3: 129–239.

      8 Das Gupta, S.P. (1984). The Ganga Basin, Part 1. New Delhi: Central Board for the Prevention and Control of Water Pollution.

      9 Dietrich, W.E., Day, G., and Parker, G. (1999). The Fly River, Papua New Guinea: inferences about river dynamics, floodplain sedimentation and fate of sediment. In: Varieties of Fluvial Form (eds. A.J. Miller and A. Gupta), 346–376. Wiley: Chichester.

      10 Douglas, I. and Guyot, J.L. (2005). Erosion and sediment yield in the humid tropics. In: Forests, Water and People in the Humid Tropics (eds. M. Bonell and L.A. Bruijnzeel), 407–421. Cambridge University Press.

      11 Dunne, T., Mertes, L.A.K., Meade, R.H. et al. (1998). Exchanges of sediment between the СКАЧАТЬ