A flash flood is typically caused by sudden, excessive rainfall that sends a river, stream or other body of water rapidly out of its banks. Often this occurs in a short amount of time, only several hours or even less.
They can also be caused by ice jams on rivers in conjunction with a winter or spring thaw, or occasionally even a dam break.
The constant influx of water finally causes a treacherous overflow to begin, powerful enough to sweep vehicles away, roll boulders into roadways, uproot trees, level buildings, and drag bridges off their piers. Most frightening is the rapidity with which the water rises.
Torrential rains associated with land falling tropical systems sometimes produce flash floods.
In the western United States, the soil is generally dry, sandy, rocky, and unable to absorb large amounts of water. Heavy rains from showers and thunderstorms can quickly fill dry stream and river beds, sending torrents of water downstream.
Fast-moving water is extremely powerful. The result can be deadly to anyone in the water's path. The force of flash flood waters can be extremely dangerous to motorists who unwittingly or unknowingly drive over water-covered roads - only a foot of running water is needed to sweep away a car. About two-thirds of flood deaths occur in motor vehicles.