How do waterfalls form?

Waterfalls form when a stream crosses rock layers that wear down at different speeds. The weaker layer is scooped out sooner, so the stronger layer loses support and breaks. That creates a sudden step in the riverbed, and water pours over it instead of sliding along.

The falling water keeps chewing at the bottom, making a deep basin and loosening more stone behind the drop. Over time the falls can retreat upstream, leaving a narrow valley behind. Niagara Falls has shifted for about 12,000 years, which is a lovely scale for a kid to picture.