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. A 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. For one, they affect water quality.