Rapids in a river are a waterfall that has reached its old age. They are the remains of what was once a vertical fall of water such as the Niagara Falls. After millions of years the waterfall has worn the cliff away to a sloping bed of rocks.

The water still flows rapidly and tumbles about over the broken surface of the stream. It is dangerous to boats, sometimes making it completely impossible to sail up a river. But rapids are very useful in helping to produce electric power.

Shooting the rapids in canoes or kayaks is a popular sport in America and is also one of the events of the Olympic Games. At the Munich Games in 1972, a long stretch of artificial rapids was built specially for this sporting event.