A waterfall results from river erosion and is found in the upper course of a river. A waterfall forms in areas with bands of hard rock and soft rock. The river erodes the soft rock (less-resistant rock, e.g. shale) faster than the hard rock (resistant rock, e.g. granite), resulting in differential erosion.
The river erodes the rocks with the process of hydraulic action (the force of the water) and abrasion (using its load of sand and pebbles as tools). The river lowers its bed of softer rock resulting in a small notch which is a dip in the soft rock. This increases the speed of the river and the energetic river eventually erodes vertically (downcutting) into the soft rock creating a vertical drop/fall.
A plunge pool forms at the base of the waterfall due to erosion caused by the weight of the water and and it’s load. Water also splashes back against the back wall by the process of solution. Eventually the overhanging piece of hard rock collapses and slowly the waterfall continues to erode backwards in a process called headward erosion. As the waterfall retreats, a gorge replaces the former position of the falls.