Why does the velocity of Himalayan rivers decrease after entering plains?

Correct answer: Because the slope becomes gentle

The direct answer is C, because the slope becomes gentle. A river’s velocity is strongly influenced by gradient, which means the steepness of the land. In the Himalayas, the river descends a steep slope, so gravity gives it considerable energy. When it enters the plains, the land is much flatter, the vertical fall per distance decreases, and the river generally flows more slowly. Reasoning chain: mountains have a steep gradient; steep gradient increases the driving force and often the speed; plains have a gentle gradient; gentler gradient reduces the river’s velocity and carrying power. Option C is correct. Option A is wrong because mixing with sea water is not the normal reason for this change, and the river may be far from the sea. Option B is wrong because a change in name cannot change physical flow speed. Option D is wrong because Himalayan rivers usually have many tributaries in the plains; tributaries can even add water. Also, a river’s speed is not determined simply by whether tributaries exist. The accurate exam link is: steep slope usually supports faster flow and strong erosion; gentle slope supports slower flow, meanders, and deposition.