What phenomenon occurs at the base of rivers, causing holes to be carved into rock?

The phenomenon that occurs at the base of rivers, leading to the formation of holes in rock, is known as potholes. Potholes are created when water, often mixed with sediment, swirls around in depressions at the base of rivers. This swirling action, combined with the abrasive nature of the sediment, can erode the rock beneath, gradually carving out deep, rounded holes.

This process often occurs in areas with strong water currents, particularly in tributaries or sections where the riverbed consists of softer rock that is more vulnerable to erosion. As water continuously flows over and through these areas, it enhances the carving action, leading to the development of several potholes over time.

Other options describe different river features but do not involve the specific carving of holes into rock in the same manner as potholes. Gorges are steep, narrow valleys formed by erosion but represent a much larger scale than individual holes. Waterfalls are dramatic drops where water cascades over a cliff, whereas whirlpools are swirling masses of water but don't specifically cause hole formation in rocks. Thus, potholes are the correct term for the hole-carving phenomenon at the base of rivers.