Rivers and Streams

What Is A Watershed?

A watershed describes an area of land that contains a common set of streams and rivers that all drain into a single larger body of water, such as a larger river, a lake or an ocean. For example, the Mississippi River watershed is an enormous watershed. All the tributaries to the Mississippi that collect rainwater eventually drain into the Mississippi, which eventually drains into the Gulf of Mexico. Rainwater that falls on more than half of the United States subsequently drains into the Mississippi. 
Map of a WatershedA watershed can cover a small or large land area. In the St. Louis vicinity, for instance, the Meramec River is a small river draining a relatively small amount of land. Small watersheds are usually part of larger watersheds. The Meramec River watershed, which is supplied by even smaller watersheds from dozens of streams, drains into the Mississippi River. All the streams flowing into small rivers, larger rivers, and eventually into the ocean, form an interconnecting network of waterways. 
Not only does water run into the streams and rivers from the surface of a watershed, but water also filters through the soil, and some of this water eventually drains into the same streams and rivers. 
These two processes, surface runoff and infiltration are important for a number of reasons. 

A Stream in Washington State
A Stream in Washington State
For one, they affect water quality. Think about it... The water that runs off the surface of the Earth picks up water pollution and deposits the pollution in streams and rivers as it drains the watershed. Along with many different types of pollution that are carried by surface runoff, soil also becomes a water pollutant as it is eroded from farm lands. Water that filters through the soil can also become contaminated with pollution that is left over from agricultural, industrial, commercial, and other types of human activity. 
The network of streams and rivers that drain our watershed and carry water pollution ultimately empty into larger bodies of water, such as lakes and oceans. As the larger rivers carrying water pollution from the land flow into lakes and oceans, all of the pollution that was in the rivers now is concentrated into these other bodies of water. The oceans of the world become the final resting place for tons of pollution. Through our watersheds, pollution is distributed far away from its original source. And oviously, polluted water affects water quality.

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