How does erosion cause a waterfall to move -upstream?

The force of water in a river, and more specifically, in a waterfall, has the ability to carve and erode the banks of the river. This is known as hydraulic action. As water falls over a waterfall, it has a large amount ofgraviatational potential energy. This causes the water to vertically erode the softer rock located beneath the harder rock on the top of the waterfall. As this erosion increases, through abrasion and attrition, as well as hydraulic action, the softer rock is further carved away, causing an undecut to appear. As this undercut becomes more and more apparent, the harder rock above it begins to form a ledge. Eventually, the ledge becomes too heavy to be supported, and will fall into the plunge pool that has been created. This causes teh waterfall to 'move', back up-stream.