Why is the valley narrow and deep in the upper course of a river?

Correct answer: Because downward cutting is stronger on steep slopes

A river’s upper course usually lies on steep land. Because the slope is strong, water flows rapidly and has considerable energy. Much of this energy is used to erode the river bed vertically, or downward, rather than spreading widely across the valley floor. This process is called vertical erosion. It cuts a narrow channel and produces steep valley sides, often forming a deep V-shaped valley. Weathering and movement of loose material can also help widen the valley slightly, but downward cutting is the main reason for its narrow, deep form.

Option C is correct because it directly identifies stronger downward erosion on steep slopes. Option A is incorrect because deposition is generally more important in lower, gentler parts of a river. Option B is incorrect because upper-course rivers do carry water, even if their channels are smaller. Option D is incorrect because sea waves do not shape most upper river valleys. Thus, the valley shape follows the river’s slope, speed, and erosive power.