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Rivers are large natural streams of water that flow over land. Their job is to take rain water back to the sea.

The River Cam from the Green Dragon Bridge

A river basin is an area of land where rain falls and drains into a river. The boundary or edge of a river basin is called a watershed and is usually on high ground. Rainwater runs down either side of this ridge just like on the ridge of a roof, flowing into one river basin or another.

Some rivers have very large basins while others of equal length drain much smaller basins. The world’s two longest rivers, the Amazon and the Nile, are about the same length but the Amazon has a basin more than twice as large.