waterfall, a sudden unsupported drop in a stream. It is formed when the stream course is interrupted as when a stream passes over a layer of harder rock—often igneous—to an area of softer and therefore more easily eroded rock; the edge of a cliff or plateau; or the edge of a hanging valley formed under glacial conditions (see glacial periods). Normally, as a stream grows older, the waterfall, by undercutting and erosion of the waterfall edge and stream bed above the fall, moves upstream and loses height until it eventually becomes a series of rapids and finally disappears. Waterfalls are often a source of waterpower for cities such as the string of cities in the United States that grew up along the waterfall line where streams from the Appalachians descend suddenly to the coastal plain and early textile and flour mills used power from waterfalls. Angel Falls in Venezuela is the world's highest waterfall.