More on waterfalls

A waterfall forms where a river or stream drops suddenly over a steep ledge. Many develop where water wears away softer rock more quickly than the harder rock above it, leaving a drop in the riverbed.

Over time, falling water and the stones it carries can erode a deep pool at the base. As the ledge is worn away, the waterfall may gradually retreat upstream. Others form more suddenly through geological events such as landslides, earthquakes or volcanic activity. In mountainous regions, melting snow can also increase the flow of water in spring and summer.

Information on waterfall formation: National Geographic and USGS.