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A waterfall is a place where water flows over a vertical drop or a series of drops 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 in the upper course of a river in steep mountains. Because of their landscape position many waterfalls occur over bedrock fed by little contributing area so they are ephemeral and only occur during rainstorms. The further downstream the more perennial a waterfall can be. It is even possible for waterfalls to occur far downstream due to transient discontinuities in a river's longitudinal profile caused by climatic changes and anthropogenic activities. Waterfalls can have a wide range of widths and depths and this diversity is part of what makes them such a charismatic and interesting natural phenomenon. Research into the diversity of natural waterfalls systematics has been carried out in recent years.
When the river courses over resistant bedrock erosion happens slowly and is dominated by impacts of water-borne sediment on the rock, while downstream the erosion occurs more rapidly. As the watercourse increases its velocity at the edge of the waterfall it may pluck material from the riverbed, if the bed is fractured or otherwise more erodable. Hydraulic jets and hydraulic jumps at the toe of a falls can generate large forces to erode the bed especially when forces are amplified by water-borne sediment. Horseshoe-shaped falls focus the erosion to a central point also enhancing riverbed change below a waterfalls. A process known as potholing involves local erosion of a potentially deep hole in bedrock due to turbulent whirlpools spinning stones around on the bed drilling it out. Sand and stones carried by the watercourse therefore increase erosion capacity. This causes the waterfall to carve deeper into the bed and to recede upstream. Often over time the waterfall will recede back to form a canyon or gorge downstream as it recedes upstream, and it will carve deeper into the ridge above it. The rate of retreat for a waterfall can be as high as one and half meters per year.
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