Читать книгу Introducing Large Rivers - Avijit Gupta - Страница 40

The sediment load of the Ganga comes mostly from the tectonic Himalaya Mountains. Chemical weathering is not significant in the mountains and the solution load is low, being diluted even further when the discharge is high in the wet monsoon. The suspended and bed load of the Ganga is very high, the suspended load the second highest in the world, superseded only by the Amazon. The annual suspended load of the Ganga has been estimated by Milliman and Syvitski (1992) as 520 million tonnes. About 90% of the sediment travels during the wet monsoon (Singh 2007). The bed load of a large river is difficult to measure but Wasson has estimated that 600–2500 million tonnes of bed load reaches the delta of the river each year. Most of the sediment arrives in the Ganga from the Himalayas along the large tributaries that originate in the mountains and the foothills (Sinha and Friend 1994). The tributaries that come from the south drain the old cratonic rocks of Peninsular India and contribute a high proportion of coarse sediment.

The change in bed material of the Ganga from Haridwar at the foot of the Himalaya Mountains to Ganga Sagar where one of the major distributary channels, the Hugli, enters the Bay of Bengal is plotted in Figure 3.5. Measured from bar samples, it indicates the general downstream fining characteristic of the Ganga, interrupted by periodic coarsening of the bed by contributions from large tributaries (Singh 1996). The bar sediment of the Ganga is essentially sand, the mineralogy of which is primarily quartz with minor amounts of feldspars, micas, and rock fragments. Material from the weathered source-rocks undergoes further alteration when grains are stored as part of floodplain alluvium between their transportation in high flows (Wasson 2003).