Often, waterfalls form as streams flow from soft rock to hard rock. This happens both laterally (as a stream flows across the earth) and vertically (as the stream drops in a waterfall). In both cases, the soft rock erodes, leaving a hard ledge over which the stream falls.

Waterfalls are primarily caused by the geological process of differential erosion, occurring where a river or stream flows over layers of rock with varying degrees of hardness. As the water moves, it erodes the softer, underlying rock layers (like shale or sandstone) more quickly than the harder top layer (such as granite or limestone). Over time, this creates a steep drop or "knickpoint." As the softer rock underneath is washed away, the hard "caprock" is left unsupported and eventually collapses into the plunge pool below, causing the waterfall to "retreat" upstream. This process can be accelerated by tectonic activity, such as faulting, which creates a sudden vertical drop in the landscape, or by glacial action, where a massive glacier carves a deep main valley, leaving smaller "hanging valleys" with streams that must drop over the edge. In 2026, climate change is impacting waterfalls globally; increased meltwater from glaciers is temporarily making some falls more powerful, while prolonged droughts are causing others, like the famous Victoria Falls, to experience historically low flows during the dry season.