Flash floods happen when slow-moving thunderstorms pour down much more water than usual in a small area. It rains so hard that the water can not soak into the ground fast enough, and the water rushes down the mountainsides or hills into streams and rivers. These streams and rivers cannot carry all the water, so it quickly floods. The most severe flash flood makes the water level rise dangerously in streams, dry places, or canyons. Flash floods can make terrible mud slides and can move very quickly. They can roll big rocks, tear out trees, and destroy buildings and bridges.