What Is a Watershed?

­A watershed is an area of land that feeds all the water running under it and draining off of it into a body of water. It combines with other watersheds to form a network of rivers and streams that progressively drain into larger water areas.

Topography determines where and how water flows. Ridge tops surrounding a body of water determine the boundary of a watershed. Imagine turning an open umbrella upside down in the rain. Rain that hits anywhere within the umbrella's surface area would go to the bottom at the center of the umbrella. Any rain that didn't hit the umbrella would fall to the ground. The umbrella is like a watershed; it collects everything that falls into it.

Where's My Watershed?
To find out which watershed you live in, take a look at the EPA's Surf Your Watershed. For a map of U.S. watersheds, see the United States Watershed Map. The Water Resources eAtlas provides maps and biodiversity statistics on watersheds around the world.


Waterways within the watershed all feed into that main body of water, which could be a river, lake, or stream. The beginnings of a water source are called headwaters. The spot where headwaters progressively join other water sources is called the confluence, and the endpoint of the waterways that open into the main body of water is called the mouth (source: Environmental Protection Agency).

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watersheds
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A group of sub-watersheds form a watershed.

­Watersheds are just one part of a hierarchical system. Catchments are any natural or manmade structure designed to capture water. A group of catchments make up a sub-watershed, which is simply a smaller watershed. A collection of sub-watersheds forms a watershed, and a group of watersheds forms a basin. Basins are huge land masses. For example, the Amazon Basin in South America, the largest in the world, boasts an area of over 6 million square kilometers in five different countries. It also contains 16 large cities (of over 100,000 residents) and 3,000 different species of fish within its borders [source: Water Resources eAtlas].

To return to the umbrella example, imagine now that there are three groups of umbrellas. One group of large umbrellas (the basin) sits on the ground, while another group of smaller umbrellas (watersheds) floats above them, with a hole in the bottom of each. Yet another group of even smaller umbrellas (catchments) floats above those, also with a hole in the bottom of each. If the rain was caught in the top level of umbrellas, it would drain into the larger umbrellas below, which would drain into the largest umbrellas below them.

Of course, this is a simple model. Water does not simply hit the land and roll off it into a stream. Rainwater (and everything else) is lost through absorption by plants, evaporation and consumption by humans. These factors also depend on the area; the clay-like soil of Georgia will not absorb as much water as the loose soil of Kansas.

So, why do watersheds matter? Why is it imperative that they stay healthy? Read on to find out.