A flash flood is a rapid flooding of geomorphic low-lying areas - washes, rivers, dry lakes and basins. It may be caused by heavy rain associated with a storm, hurricane, or tropical storm or meltwater from ice or snow flowing over icesheets or snowfields. Flash floods can also occur after the collapse of an ice dam, debris dam or a human structure, such as a dam, for example, the Johnstown Flood of 1889. Flash floods are distinguished from a regular flood by a timescale less than six hours. The temporary availability of water is often utilised by plants with rapid germination and short growth cycle, and by specially adapted animal life.
Flash flooding occurs when a barrier holding back water fails or when water 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 dozens of 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.
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