Question

why does Brahmaputra in the Tibetan part have less silt deposit a longer course?

Answer 1

As Tibet is a cold and dry area,the Brahmaputhra river carries less silt and smaller volume of water.When the river enters India through Arunachal Pradesh,it passes through a region of high rainfall.Here the river carries a large volume of water and a considerable amount of silt.
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Answer 2

A large part of Brahmaputra lies in Tibet and it enters India through Arunachal Pradesh. The river is known as Tsang- Po in Tibet. Here the volume of water is less and has less silt but in India it carries large volume of water and thus carries high amount of silt.