The upper course of a river is generally more known for which work?

Correct answer: Rapid erosion

The direct answer is B: rapid erosion. A river’s upper course is usually close to its source, where the land has a steep slope. Gravity makes the water flow quickly, and the river has strong vertical cutting power. The reasoning is: the upper course has a steep gradient; steep gradient increases speed; fast water hits the bed and banks strongly; rocks and soil are worn away; therefore erosion is the main characteristic work. Option A is wrong because slow deposition is more common where the river loses energy, especially in flatter areas, not as the main upper-course action. Option B is correct because rapid flow and steep relief encourage strong erosion. Option C is wrong because mixing river water with seawater occurs near the mouth or estuary, not as the defining upper-course work. Option D is wrong because deltas form mainly near a river mouth when sediment is deposited, not in the steep upper course. A river can perform erosion, transportation, and deposition in different places. Memory cue: upper course cuts, middle course carries, and lower course deposits more.