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

Correct answer: Because slope and velocity decrease

The direct answer is A: after entering the plains, a Himalayan river tends to move from stronger erosion toward greater deposition because its slope and velocity decrease. In the mountains, the river has a steep gradient, so gravity produces fast flow with high carrying power. On reaching flatter plains, the gradient becomes gentler. The river loses speed and energy, so it cannot carry all the sediment brought from the mountains. It drops sand, silt, and other material on the river bed or floodplain. Option A is correct because it states the physical chain: lower slope leads to lower velocity, and lower velocity encourages deposition. Option B is wrong because sea tides are not the main reason for this change, especially far inland. Option C is wrong because the source does not suddenly change when the river enters the plains. Option D is wrong because glacier expansion is not the normal cause of deposition at the mountain-front transition. Remember: steep and fast flow erodes; gentle and slow flow deposits, although a river may still erode its banks in places.