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Rapids are found on rivers. Rapids are where the river bed is rocky, and the river runs fast over and around the rocks. Rapids are found in the mountains near the beginning or "headwaters" of a river. The word "rapids" is always plural, (like "scissors" and "trousers"). The word "rapid" means "very fast".
Because of the splash made by the water going over rocks, rapids are also called "whitewater". Some people enjoying whitewater canoeing or whitewater rafting as a sport.
Rapids at Erdely, Romania