Tropical Vibes! Jungle Vibes! Waterfalls in a Rainforest! A waterfall is a steep descent of a river over a rocky ledge into a plunge pool below. Also called cascades, a cascade is a waterfall that descends over a series of rock steps. The process of erosion, the wearing away of earth, plays an important part in the formation of waterfalls. Waterfalls also contribute to erosion. Waterfalls are classified by height, width or the average volume of water in the waterfall.