Why does the Brahmaputra in its Tibetan part have less silt, despite a longer course?

The Brahmaputra river in its Tibetan part have less slit because it is cold and a dry area. In India it passes through a region of high rainfall, here it carries a large volume of water and a considerable amount of slit. But in Tibetan part of its there is less rainfall and that’s the reason for less slit and water. But during the rainy season, every year the river overflows its banks, that causes widespread devastation due to floods in Assam and Bangladesh. It collects huge volume of the slit on its bed causing the river bed to rise and the river also shifts its channel frequently.