Explain the formation of a waterfall

Waterfalls form in the upper course of the river, as a result of a process called differential erosion. Bands of geology have varying resistances to erosion; as the river channel flows across different bands of geology, the softer and less resistant rock will be eroded faster, thus creating a step in the river. This step enlarges over time, due to the hydraulic action of the river, leading to the formation of a waterfall.
The force caused by the water as it flows over the step leads to the formation of a plunge pool at the base of the waterfall. Turbulent flows and splash-back within the plunge pool undercut the cap of resistant rock above until the undercut is significant enough for the rock above to collapse because of its lack of support. This leaves angular and resistant bed-load in the plunge pool which further deepens the pool due to abrasion. As this process repeats, the waterfall will become higher and will retreat upstream over time.