Waterfalls are steep steps in the long journey of a river’s course.
Waterfalls are made when a layer of harder rock overlays on a layer of softer rocks.
As the river passes over the softer rocks it erodes faster, forming a step.
Hydraulic action erodes and makes the plunge pool and notch bigger.
As the notch grows, eventually there is not enough support under the harder rock and it collapses under the plunge pool.
This adds rocks and boulders to the plunge pool and so corrasion starts working with hydraulic action and erode plunge pool and notch.
Erosion continues and harder rock collapses and waterfall retreats and leaves a steep Gorge.