Why does a Himalayan river tend to shift from erosion to deposition after reaching plains?

Correct answer: Slope and velocity decrease

A river’s ability to erode and carry sediment depends greatly on its energy, which is related to slope, discharge and velocity. In the Himalayan mountains, the channel often has a steep gradient, so water flows rapidly and cuts into banks and beds. When the river reaches the plains, the land becomes much flatter. The fall in slope reduces the river’s velocity and transporting power, especially when the channel becomes wider and the flow spreads.

Option C is correct because reduced slope and velocity make the river less able to carry its sediment load. When the river loses energy, sand, silt and other particles settle on the bed or flood plain, producing deposition. The source does not need to change, glaciers do not need to expand, and increasing sea tides are not the main general reason for this inland transition. The key sequence is steep slope and erosion in the mountains, followed by gentler slope, slower flow and deposition in the plains.