Explain the formation of a waterfall

Waterfalls are found in the upper course of the river where a harder, more resistant band of rock sits on top of a softer, less resistant band of rock. The softer rock is more easily eroded, so it wears away quicker. This leaves the hard rock overhanging. Hydraulic action and abrasion cause the softer rock to wear away further and create a plunge pool underneath the overhang. Eventually, the harder rock will collapse under gravity as it is not being supported. The process repeats itself, which causes the river to retreat upstream.