Question

Why does the Brahmaputra in its Tibetan parts have less silt despite a longer course

Answer 1

because here plateau is very high

Answer 2

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In Tibet, the river Brahmaputra known as Tsang Po, carries a small volume of water and less silt as it is a cold and dry area. In India, the river carries a large volume of water and considerable amount of silt because it passes through a region of high rainfall.