What causes a waterfall to retreat over time?

A waterfall retreats over time primarily due to hydraulic action. Hydraulic action is a process where the sheer force of flowing water erodes the rock face and the base of the waterfall. As the water tumbles down, it exerts a significant force upon the rocks, which can create cracks over time. This erosive action results in the rock gradually wearing away, causing the waterfall to move back from its original position.

Over time, this continuous erosion at the cliff face can lead to the formation of a plunge pool at the base of the waterfall, deepening the impact of the falling water and accelerating the retreat process. Eventually, this leads to a significant alteration in the landscape, as the waterfall can retreat several meters back as a result of the hydraulic action at work.

While other processes like corrosion may play a role in the overall erosion of rock material, hydraulic action is the primary force responsible for the specific retreat of waterfalls.