Flash flooding occurs when precipitation falls too quickly on saturated soil or dry soil that has poor absorption ability. The runoff collects in low-lying areas and rapidly flows downhill. Flash floods most often occur in normally dry areas that have recently received precipitation, but may be seen anywhere downstream from the source of the precipitation, even many miles from the source. In areas on or near volcanoes, flash floods have also occurred after eruptions, when glaciers have been melted by the intense heat.
Flash floods occurs when large rivers that run through flat terrain overflow their banks, they usually do so gradually. The slowly rising water provides warning to communities that a flood is developing. An intense rainstorm, however, may overwhelm smaller rivers and streams, especially those carrying water a short distance or down steep slopes. The sudden increase in the volume of water flowing into these smaller waterways can cause dangerous overflows known as flash floods. In dry regions, heavy downpours can suddenly fill dry streambeds, known as washes, with torrents of water.
Flash floods usually occur when storms drop large amounts of rain within a brief period. They can occur with little or no warning, sometimes reaching their peak in only a few minutes. Waters move very fast during flash floods. The force can roll boulders, uproot trees, destroy buildings, and wash out bridges. Walls of water can reach as high as 10 feet (3 m) to 20 feet (6 m), carrying debris that can kill people and destroy property.