Waterfalls are most common in the upper course. They are formed when a river crosses a layer of harder rock. This layer is eroded more slowly than the softer rock beneath it. Eventually a step in the profile of the river is formed with a plunge pool formed in the softer rock. As the layer of hard rock is undermined it will collapse and so the waterfall moves upstream.

This forms a narrow valley with steep sides - a gorge.

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