Why is vertical erosion stronger upstream and lateral erosion stronger in plains of a Himalayan river?

Correct answer: Slope is steeper upstream and gentler in plains

The direct answer is A: the slope is steeper upstream and gentler in the plains. River erosion depends strongly on gradient, which means the steepness of the land. Step by step: in the Himalayan upper course, the river descends a steep slope, so gravity makes the water move rapidly; the fast, energetic flow cuts downward into the bed, producing strong vertical erosion and deep valleys. In the plains, the slope becomes much gentler; downward cutting weakens, while the river swings from side to side and cuts its banks, producing more lateral erosion and wider valleys. Option A is correct because it gives the physical cause of the change. Option B is wrong: there is no sea upstream and no glacier in the plains as a general explanation. Option C is wrong because upper Himalayan rivers do contain water, and lava is not the controlling condition in the plains. Option D is wrong because the slope is not exactly the same along the whole river. Exam cue: steep gradient usually means vertical down-cutting; gentle gradient usually allows sideways bank erosion.