A flood which occurs suddenly and without warning. Flash floods are common after torrential mountain thunderstorms. They affect mountain valleys because there is little soil to soak up the rain, and so it quickly rushes to rivers.

This sign warns people in the Big Thompson valley of the Rocky Mountains what they should do if a flash flood appears.

A flash flood in this valley, about 30 years ago, killed 144 people who were camping by the riverside in summer, when a torrential rainstorm of 250mm of rain fell on the nearby mountains.