What causes waterfalls to retreat upstream?

Waterfalls retreat upstream due to the erosion of the underlying rock at their base. As water flows over the edge, it creates a plunge pool, where the force of the falling water and carried sediments erode the softer rock beneath the harder caprock. Over time, this undercutting weakens the structure, causing sections of the waterfall to collapse and shift upstream. Famous examples include Niagara Falls, which has retreated significantly over thousands of years.