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Why do waterfalls retreat?

Waterfalls are an erosional landform which are formed when a band of soft rock is overlaid with a band of hard rock. Over time, the soft rock is eroded through a combination of hydraulic action* and abrasion**. The hard rock is eventually undercut by the erosion until it becomes unsupported and collapses into the water below. The collapsed rocks in turn erodes the foot of the waterfall through abrasion, creating a deep plunge pool.
Over time, as erosion of the soft rock persists and more hard rock collapses into the plunge pool below, the waterfall retreats further up the channel. This creates a steep-sided gorge in the valley.

Key terms*Hydraulic action: The river channel is eroded as the force of the water breaks rock particles away from the beds and the banks of the river
**Abrasion: Eroded rocks picked up by the river scrape against the river channel and wear it away