What is a watershed?

A watershed is the land area that drains water to a particular stream, river, or lake. It is a land feature that can be identified by tracing a line along the highest elevations between two areas on a map, often a ridge.

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The diagram (right) suggests a typical watershed that starts with small headwater streams in higher elevations of the drainage basin. Water flows down hill into larger streams, eventually joining a river. As more tributary streams join the river, the volume of water increases. This river eventually flows downstream into an even larger river at the confluence.