Here's why:

* Water's Force: The constant flow of water over a cliff face exerts significant force, wearing away the rock through abrasion (the grinding of rock fragments against the rock face) and hydraulic action (the force of water pushing against the rock).

* Undercutting: Waterfalls often form where softer rock layers lie beneath harder ones. The softer rock erodes faster, creating a "cave" or undercut beneath the harder rock. This creates an overhang that eventually collapses, causing the waterfall to retreat upstream.

* Potholes: The force of the water can also create potholes, which are depressions in the rock caused by the swirling action of rocks and debris.

While deposition (the dropping of sediment) does play a role in the formation of some features *associated* with waterfalls, it's not the primary process responsible for the waterfall itself. For example, the debris eroded by the waterfall might be deposited at the base, forming a talus slope.

In summary: A waterfall is primarily a result of erosion, where the force of water wears away the rock, creating a steep drop.