Potholes and Plunge Pools

Potholes and plunge pools are both products of river erosion, but they form in different parts of a stream course and under different conditions. The key idea is that moving water, armed with rock fragments, can carve circular depressions into solid rock — and where the water falls with great force, those depressions become much larger.

On the rocky beds of hill-streams, stream erosion aided by the abrasion of rock fragments creates more or less circular depressions called potholes. The process begins when a small, shallow depression already exists on the riverbed. Pebbles and boulders collect inside it, and the flowing water rotates them continuously. This rotation grinds and scours the bottom and sides of the depression, so the depression grows in both depth and width.

As this continues, a series of such depressions eventually join together. When they merge, the stream valley gets deepened. So potholes are not just isolated curiosities — their growth and coalescence actively contribute to valley deepening in hill-streams.

At the foot of waterfalls, however, the same process operates on a far grander scale. Here, large potholes form that are quite deep and wide. Two forces combine to create them: the sheer impact of the falling water itself, and the rotation of boulders that get trapped at the base of the fall. These large and deep holes at the base of waterfalls are called plunge pools. …