They are called flash floods because they happen very rapidly. Flash floods are caused by sudden rainstorms in hilly or mountainous areas.
The water rushes down the hillsides and into valleys or gullies. Streams overflow, swamping roads and bridges. It all happens so fast that people are taken by surprise.
That’s what happened in Heppner, Oregon, in 1903. After a sudden heavy rain, a wall of water 20 feet high came rushing down a stream and in less than one hour a third of the town was washed away and 325 people had lost their lives.
In 1955, a sudden rainstorm dumped a six-month supply of water on Connecticut, and the Connecticut river rose 20 feet almost overnight. The resulting flood cost many lives and $2 billion of damage.