What Is a Watershed?

Click the image for a larger view.
Watershed Diagram

A watershed is an area of land that drains all of its water to a specific lake or river. As rainwater and melting snow run downhill, they carry sediment and other materials into our streams, lakes, wetlands and groundwater.

The boundary of a watershed is defined by the watershed divide, which is the ridge of highest elevation surrounding a given stream or network of streams. A drop of rainwater falling outside of this boundary will enter a different watershed and will flow to a different body of water.

No matter where you live, work or play you are within a watershed!

Our streams and rivers may flow through many different types of land use in their paths to the ocean. In the illustration to the right from the U.S. Environmental Protection Agency, water flows from agricultural lands to residential areas to industrial zones as it moves downstream. Each land use presents unique impacts and challenges on water quality.

How Big Is a Watershed?

The size of a watershed can be subjective; it depends on the scale that is being considered.

Chesapeake Bay Watershed MapThe image to the right depicts the extent of the Chesapeake Bay watershed, "the big picture" that is linked to our local concerns. This watershed covers 64,000 square miles and crosses into six states: New York, Pennsylvania, Delaware, West Virginia, Maryland, Virginia and the District of Columbia.

Fairfax County, as shown on the map, makes up 400 square miles of the Chesapeake Bay watershed. The 400 square miles is divided into 30 major watersheds, which then flow into the Potomac River watershed, then into the Chesapeake Bay watershed and finally into the Atlantic Ocean. Think of watersheds as being "nested" within each successively larger one, similar to Russian nesting dolls.

For additional information, please contact the Stormwater Planning Division, 703-324-5500, TTY 711.



The Chesapeake Bay watershed and its major river basins.
Courtesy of the Chesapeake Bay Foundation/cbf.org

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