John Wesley Powell, a prominent Nineteenth Century geologist and explorer of the American West, described a watershed as such:

"...that area of land, a bounded hydrologic system, within which all living things are inextricably linked by their common water course and where, as humans settled, simple logic demanded that they become part of a community."

A watershed is an area of land in which all the water that is under it (groundwater) or drains off of it (surface water) drains into a common waterway such as a stream, lake, estuary, aquifer, or ocean. Powell made an important point: we all live within a watershed.

Watersheds come in all shapes and sizes; they cross county, state, and national boundaries. Large watersheds, such as the 668,220 square kilometer Columbia River watershed, may be made up of many smaller sub-watersheds, and those sub-watersheds made up of even smaller sub-watersheds fed by small tributaries. In this way, any given watershed may be part of a larger watershed, and itself made up of even smaller watersheds.

There are 2,110 watersheds in the continental U.S! Click here to see a map of the large watersheds of the U.S.