Proxy Falls, Cascade Mountains, Oregon

A waterfall is a place where water flows over a vertical drop in the course of a stream or river. Waterfalls also occur where meltwater drops over the edge of a tabular iceberg or ice shelf. Waterfalls are commonly formed when a river is young.  At these times the channel is often narrow and deep. When the river courses over resistant bedrock, erosion happens slowly, while downstream the erosion occurs more rapidly.  As the watercourse increases its velocity at the edge of the waterfall, it plucks material from the riverbed.  Source