Waterfall – Sudden descent of a river or stream over a vertical or very steep slope

We all know what a waterfall is, but how do they form?

It often forms where the river meets a band of softer rock after flowing over an area of more resistant material. 

1. Waterfalls typically form in the upper stages of a river. They occur when a band of hard rock overlies a softer rock. Falling water and rock particles erode the soft rock below the waterfall, creating a plunge pool.
2. The soft rock is undercut by erosional processes such as hydraulic action and abrasion, creating a plunge pool where water and debris swirl around, eroding the rock and creating an overhang.
3. The hard rock overhang above the plunge pool collapses as its weight is no longer supported.
4. Erosion continues and the waterfall retreats upstream, leaving behind a gorge.