Hydraulic action

Hydraulic action, most generally, is the ability of moving water (flowing or waves) to dislodge and transport rock particles.

It is a mechanical process, in which the moving water current flows against the banks and bed of a river, thereby removing rock particles.

A primary example of hydraulic action is a wave striking a cliff face which compresses the air in cracks of the rocks.

Attrition is a similar effect caused by eroded particles after they fall to the sea bed where they are subjected to further wave action.

It usually leads to the formation of a plunge pool below the waterfall due in part to corrosion from the stream's load, but more to a scouring action as vortices form in the water as it escapes downstream.