Why can deposition suddenly increase when fast upper Himalayan streams reach plains?

Correct answer: Reduced slope and velocity lower the river's carrying capacity

The direct answer is A: reduced slope and velocity lower the river’s carrying capacity. A river can carry sediment only while its speed and energy are sufficient. In the mountains, steep slopes make the current fast, so it transports large stones, sand and silt. On reaching the plains, the slope becomes gentler, friction and spreading reduce velocity, and the river loses transporting power. It then deposits the heaviest particles first, followed by finer material. Option A is correct because it gives the complete process. Option B is wrong because tides mainly affect coastal reaches, not the usual mountain-to-plain transition. Option C is wrong because river water does not completely freeze on entering plains. Option D is wrong because the source does not change into a desert; the river simply enters a lower-gradient region. Exam cue: falling slope and speed mean falling capacity and rising deposition.