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A Flash Flood is a rapid flooding of geomorphic low-lying areas (washes), rivers and streams, caused by the intense rainfall associated with a thunderstorm, or multiple training thunderstorms. Flash floods can also occur when ice dams block the normal course of a river, or when a man-made structure, such as a dam, collapses, e.g., the Johnstown Flood of 1889. Flash flooding occurs when the ground becomes saturated with water that fell so quickly that it could not be absorbed. 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).