for my geography corsework i have been asked to study rivers and hear is one question i am strugeling with.
This somewhat explains it, but indirectly. The majority of waterfalls result from layers of rock composed of different degrees of hardness. Hard layers are more resistant to erosion. Soft layers are quickly removed. This is visible all around us in the mountains. Most of our summits are made of resistant limestone or dolomite. The softer shale layers remain protected beneath these resistant caps. If a river flows over a lip composed of a layer of resistant material (like dolomite), that lays atop a softer layer of shale, then the water will remove the soft layer beneath the dolomite at a faster rate. In this way, waterfalls often become undercut as this soft material is worn away. This results in the caves found beneath many waterfalls. Eventually, the undercut becomes so large that the weight of water on the unsupported layer of dolomite will be sufficient to collapse the layer. At this time, amidst a catastrophic collapse, the cave disappears, and the cliff face moves further up the valley. In many cases, the hard-soft layer relationship remains, and the falls simply migrates upstream. This creates a canyon in front of the waterfall. As long as the hardness differential remains, the waterfall will persist. Common examples of this type of waterfall are Athabasca Falls, Sunwapta Falls, and Johnston Canyon.
