How do waterfalls form?

Waterfalls form where a river is flowing over hard rock lying on soft rock. The soft rock is eroded faster, so there is a step in the river which deepens over time. It is eroded by hydraulic action and abrasion until there is a plunge pool at the base of the waterfall. Undercutting forms an overhanging of hard rock, which is unsupported underneath. This collapses into the plunge pool. This continues resulting in the waterfall retreating upstream.When answering a question on waterfalls, you may be required to give an example of a waterfall.A diagram would be very helpful for explaining how waterfalls form.