quality:correction, structural:summarization, structural:expansion, information:highlighting
Formation

Waterfalls are commonly formed in the upper course of a river where lakes flow into valleys in steep mountains.

A river sometimes flows over a large step in the rocks that may have been formed by a fault line. Waterfalls can occur along the edge of a glacial trough, where a stream or river flowing into a glacier continues to flow into a valley after the glacier has receded or melted. The large waterfalls in Yosemite Valley are examples of this phenomenon, which is referred to as a hanging valley. Another reason hanging valleys may form is where two rivers join and one is flowing faster than the other.

When warm and cold water meets by a gorge in the ocean, large underwater waterfalls can form as the cold water rushes to the bottom.