When is a river’s erosive power greatest?

Correct answer: When both slope and velocity are high

A river’s erosive power is strongly influenced by its gradient, discharge, speed, and the amount of energy in the flowing water. A steep slope makes water accelerate, and higher velocity gives the river greater ability to loosen, lift, and move soil, sand, and rock. This is why strong erosion is commonly associated with the upper course and steep terrain, although local conditions also matter. Still water, frozen water, or no water cannot actively cut a river channel.

Option A is correct because it identifies both a high slope and high velocity. These conditions increase the river’s kinetic energy and cutting ability. The other choices describe situations in which flowing water is absent or unable to erode in the usual way. The statement does not mean that every steep, fast river has the same erosion rate; it gives the general geographical relationship expected in this school-level question. Therefore answer A is accurate.