Difficult Run Watershed

“Difficult Run Watershed,” the largest watershed in Fairfax County, is subdivided into 18 sub-watersheds, including Old Courthouse Spring Branch stream.  As the “sub-watershed” of the Old Courthouse Spring Branch stream already includes the most impervious surfaces countywide, it is important to maintain its surrounding forest and stream valley to protect our water quality.

Difficult Run Watershed map

Difficult Run Watershed map


Difficult Run is a 57.7-square-mile watershed in the north-central portion of Fairfax County and drains directly to the Potomac River and ultimately the Chesapeake Bay.  This watershed is home to Wolf Trap Farm Park and part of Great Falls Park. It is crossed by several major highways, including Leesburg Pike, I-66, Route 50, and the Dulles Access Road, and by the W&OD Trail. It contains four manmade lakes: Lake Anne and Lake Fairfax on Colvin Run, and Lake Thoreau and Lake Audubon on Snakeden Branch.

Importance of the Stream Valley on Water Quality

Difficult Run flows through a wide variety of watershed conditions, from forest to urban areas.  Land use can have a great impact on the stream system.  The type and density of land use in a given area can affect the downstream water quality and stream condition.  More intense land use types, such as the high-density residential, commercial and urban areas of Tysons Urban Center, have more impervious surfaces and contributes higher levels of stormwater runoff and pollutants to the stream system. Less intense types, such as open space and forest, are less impervious, have more natural vegetation, and have less impact on stream quality.

Changes in the land use that result in overall higher intensity uses in the future can result in stream degradation. For example, if the land use shifts from open space to high-intensity commercial use, additional buildings, roadways and parking lots may replace the forest and open fields and impact stream condition through an increase in impervious surfaces.

In particular, Old Courthouse Spring Branch Stream is a headwater system that drains the highly developed urban expanse of Tysons Urban Center.  Under natural conditions, rainwater infiltrates and soaks the soil before it slowly drains away.  However, in the Tysons area, the situation is different as the soil has been largely replaced by artificial impervious surfaces that prevent rainwater from percolating into the soil.  Rainfall and snow melt from Tysons’ rooftops, roadways, and over 160,000 parking spaces is discharged directly into Old Courthouse Spring Branch Stream, via an ad hoc and piece-meal conveyance system of roadside gutters, ditches and storm sewer drains. The resulting concentrated stormwater has increased force which causes the scouring of streamside vegetation and further reducing retention capacity of the stream channel itself.  Along its way, the rainwater picks up oil, grease, heavy metals, trash, sediments, pesticides, feces, urine, fertilizers, and heat from Tyson’s impervious surfaces.  These pollutants and the scouring have decimated the aquatic life in the stream and surrounding wetlands and pose a serious threat to the environment and humans.

In 2007, the Fairfax County Board of Supervisors approved the Difficult Run Watershed Management Plan, which presents a strategy for improving and preserving the water resources and aquatic habitat in the watershed.  Of special interest is the disclosure that Old Courthouse Spring Branch subwatershed has the highest impervious surface levels at 43%, in distant 2nd place is Snakeden Branch at 27%, with the average of all subwatersheds at 18%.  Chemical contaminants such as oils, metals, and sediments, wash off from impervious surfaces.  Also it leads to stream instability and erosion and poor water quality and stream habitat.

Accordingly, saving Tysons Last Forest protects the stream habitat and water quality.

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Stormwater runoff isn’t the only thing that impacts the steam valley.  Sometimes trash, plastic bags, and other litter impacts it too.