Watershed Information

Potomac River Watershed

Map source: Karl Musser
Map source: Karl Musser

Fast Facts

Geological Regions: Appalachian Plateau, Ridge & Valley, Blue Ridge, Piedmont, Coastal Plain

River Miles: Main Stem: 383 (616 km); Main stem plus major tributaries: 12,878.8; 21st largest river in the USA

Discharge: Average flow is 10,800 ft³/s (306 m³/s)

Water Use: 486 million gallons per day or 21 m³/s (ICPRB, 2000) The Potomac River supplies almost 90% of the drinking water to the DC metro area.

Major Tributaries: North Branch, Savage River, South Branch, Cacapon, Shenandoah, Antietam Creek, Monocacy River, Anacostia River

Land Mass: 14,670 square miles. Land Use: 55% forested, 28% agriculture, 5% water and wetlands, 9.7% developed, 2.3% other

Population: 5.8 million (2005 estimated Census) in watershed; 395.4 persons per square mile (8,435 persons per square mile in Washington D.C.)

The Potomac River Watershed covers around 14,670 square miles. The watershed spans 383 miles across several geological regions including the Appalachian Plateau, Ridge & Valleys, Blue Ridge, Piedmont, and Coastal Plain. The main stem plus all major tributaries totals 12,878.8 river miles making the Potomac the 21st largest river in the United States. The river boasts an average daily flow of 10,800 ft³/s. The main tributaries of the Potomac River are North Branch, Savage River, South Branch, Cacapon, Shenandoah, Antietam Creek, Monocacy River, and Anacostia River. The Potomac empties into the Chesapeake Bay downstream. The basin is home to 5.8 million rural and urban residents giving the region an average population density of 395.4 persons per square mile and 8,435 persons per square mile in Washington D.C. About 9.7% of the watershed’s land cover is developed, 55% is forested and 28% is agriculture and only 5% is water and wetlands.

Source: Interstate Commission on the Potomac River Basin (http://www.potomacriver.org/cms/index.php?option=com_content&view=article&id=70&Itemid=57)

Watershed Photo

Image source: NASA (Landsat)
Image source: NASA (Landsat)

More maps:
http://www.potomacriver.org/cms/wp-content/uploads/2013/12/maps_images/Potomac_and_Bay_Watershed.pdf
http://www.potomacriver.org/cms/wp-content/uploads/2013/12/maps_images/LandUse.pdf

Watersheds of Fairfax County

Information and watershed management plans for each of the 30 watersheds of Fairfax County and neighboring jurisdictions, as well as a find-your-watershed search tool, can be found on the County’s website: Click here

Four Mile Run Watershed

Map Source: Northern Virginia Regional Commission
Map Source: Northern Virginia Regional Commission

The Four Mile Run watershed is one of the most heavily urbanized drainage basins in the Northern Virginia region. Although the watershed is fewer than 20 square miles, it is home to approximately 156,000 people (according to the 1990 census) who reside in adjacent portions of four localities: the counties of Arlington and Fairfax and the cities of Alexandria and Falls Church.

Approximately 85 percent of the watershed has been developed and nearly 40 percent of the watershed is covered with impervious surfaces associated with this development (e.g., buildings and pavement). Wherever present, impervious surfaces prevent the natural process of groundwater infiltration from occurring, which in turn greatly increases the volume of surface water runoff that Four Mile Run and its few remaining tributaries must carry downstream.

The urbanization process has replaced most of the watershed’s natural stream channels with an elaborate network of storm sewers. These sewers carry the increased volume of surface runoff downstream much more quickly than would the natural channels that the storm sewers replaced, which causes “flashier” and larger stream flows.

Runoff from the Four Mile Run watershed quickly makes its way into the Potomac River, and eventually drains into the Chesapeake Bay.

Source: Northern Virginia Regional Commission http://www.novaregion.org/index.aspx?NID=536

Cameron Run Watershed

Map Source: Northern Virginia Regional Commission
Map Source: Northern Virginia Regional Commission

The Cameron Run Watershed (including the drainage into Great Hunting Creek) is a 42-square mile watershed located in Northern Virginia. The watershed is highly urbanized and includes portions of Fairfax and Arlington Counties and the Cities of Falls Church and Alexandria.

Cameron Run, a third-order stream, empties to the Potomac River and drains two main tributaries: Holmes Run and Backlick Run. Cameron Run flows eastward from the confluence of Backlick and Holmes Runs from the City of Alexandria into Fairfax County before it opens into Great Hunting Creek at the Potomac River.

The Cameron Run Watershed has changed substantially over the last half century. A developed landscape filled with homes, commercial enterprises, and extensive roadway systems has replaced wetlands and naturally forested areas. More than 40 percent of the watershed consists of impervious surfaces.

The watershed’s floodplains have changed significantly, due the piping of many natural stream channels. A network of storm sewers and culverts now dominates the watershed. In all, the cumulative effects of urbanization and impervious surfaces led to frequent flash flooding throughout the watershed, especially during the 1960s.

Source: Northern Virginia Regional Commission http://www.novaregion.org/index.aspx?NID=419

Smaller Potomac Tributaries

A number of small tributary streams, such as Windy Run and Donaldson Run in Arlington and Alexandria feed directly into the Potomac River, as shown on the map below.

Map source: Arlington County, VA
Map source: Arlington County, VA