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Waterfalls form when a sudden elevation drop in the stream channel causes the water to plunge steeply downstream. Many times, hard rocks resistant to erosion will cause the upstream rock to be much higher. Other waterfalls form where hanging valleys of glacial origin enter the main valley or where faulting left sufficient topographic relief. In the long history of a stream or river, a waterfall is a temporary feature which will eventually be eroded. As water impacts at the base of the falls, a zone of undercut removes rock at the base and weakens the overlying rock. Once the undercut creates sufficient instability in the overhang, collapse will occur and the waterfall migrates upstream.
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