Waterfall Erosion
The pounding of the water at the base of a waterfall is a powerful force for erosion, especially if the water contains suspended sediment . Even at the lip of the fall, the water gains extra erosive power as it accelerates approaching the brink. For this reason, waterfalls are temporary phenomena, geologically speaking. While the surging water tears away at the base of the falls, removing its rock foundations, the scouring of the lip grinds back the brink of the falls, decreasing the overall height. In some cases, the removal of the underlying rock leads to a collapse of the lip itself.
The time it takes for these processes to erode the river bed to a gentle slope depends on the volume of water flowing over the drop, the amount of sediment available to grind away the bed, and the hardness of the rock over which the river flows. Even waterfalls on smaller rivers can last for millions of years. This is also true of large rivers where the bedrock is resistant to erosion. But in geologic terms, waterfalls are quite temporary, and their presence is a sure sign of the special geologic conditions that produced them. In a sense, they are like the flip side of lakes, which are also temporary, mostly because lakes gradually fill with sediment and eventually turn into marsh and meadow.
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