1. A waterfall forms when a river flows from a band of resistant rock to less resistant rock. The band of resistant rock can lie horizontally, vertically, or it can dip downstream. The less resistant rock that lies downstream will be eroded faster than the resistant rock.The defferential erosion causes a change in the gradient of the river.

This will result, over time, in the sudden steepening of the river bed from the resistant rock to the less resistant rock so that the water plunges down almost vertically and rapidly as a waterfall.

2. A waterfall can also be formed where faulting causes land to be displaced, with one block being uplifted relative to the adjacent block. This displacement causes a difference in height, and the water cascades down the fault scarp.

A plunge pool is a pool, lake, or pond that is small in diameter, but deep.

It is also a depression at the foot of a waterfall. The depression is excavated, enlarged and deepened by the hydraulic action of the plunging water as it lands on the bottom river bed with great force. Rock debris, swirled about by the turbulent water at the base of the waterfall, erodes the depression intensively by abrasion to further enlarge and deepen it.