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Difficult Run is a 15.9-mile-long (25.6 km) tributary stream tributary stream of the Potomac River in northern Virginia in the United States. Difficult Run drains directly to the Potomac River.

Difficult Run flows through Fairfax County to Great Falls Park, on the Virginia side of the Potomac River. The portion of the run through the park has been characterized as "a miniature Mather Gorge and Great Falls." In the Great Falls area the run is relatively flat and wide at the top of the trail with calm waters, like the Potomac River above Great Falls. The stream picks up speed as it narrows into a steep gorge with waterfalls and quickly reaches the same level as the Potomac. At the mouth of Difficult Run, one can see a panoramic view of the Potomac River. The U.S. Geological Service’s stream gauge on Difficult Run near Great Falls, Virginia, has a mean daily discharge of 33 cubic feet per second. The mean daily discharge is significantly less at other USGS stream gauges on the river. The stream is named not for the nature of the stream's rapids themselves; rather, long-boatmen in the time of the Patowmack Company found the two sharp bends of the Potomac near Difficult Run's mouth treacherous.

Watershed
The Difficult Run watershed is the largest in Fairfax County at 37179 acre. A map of the watershed shows the following subwatersheds that make up the Difficult Run watershed:


 * Captain Hickory Run
 * Piney Run*
 * Colvin Run*
 * Snakeden Branch*
 * Little Difficult Run*
 * Rocky Run*
 * Angelico Branch
 * Piney Branch*
 * Wolftrap Creek*
 * Rocky Branch*
 * Lower Difficult Run
 * Sharpers Run
 * Middle Difficult Run
 * Upper Difficult Run
 * South Fork Run
 * The Glade
 * Old Courthouse Spring Branch*
 * Dog Run

The Difficult Run watershed includes 145 miles of streams, with the larger tributaries indicated by a * above in the list of subwatersheds.

The watershed includes four large waterbodies: Lake Anne and Lake Fairfax on Colvin Run and Lake Thoreau and Lake Audubon on Snakeden Branch.

Management
The Difficult Run Watershed Management Plan was adopted by the Fairfax County Board of Supervisors on Feb. 26, 2007. Ref The management plan includes action items for Difficult Run’s subwatersheds in addition to watershed-wide activities. The later includes removing dump sites, obstructions (e.g., beaver dam), and fish passage blockages; repairing utility crossings; and restoring riparian buffers.

Watershed Conditions
Although there are many degraded areas of the watershed, it does have some areas of high biological and habitat integrity, especially toward Great Falls, Virginia. Natural resource issues in the watershed include:
 * Pollution from stormwater runoff
 * High quantities of stormwater runoff
 * Uncontrolled stormwater
 * Erosion and streambank instability
 * Stream water quality
 * Need to protect natural resources
 * Stormwater regulatory compliance

Land Use
The Difficult Run watershed has experienced great changes in land use since the area was first developed in the 1600s. Currently, development in the Difficult Run watershed varies greatly, ranging from very developed areas (such as Tysons, Reston, and Vienna) to forested areas (such as Great Falls). Several major highways cross through the watershed including Lee Jackson Memorial Highway (U.S. Rt. 50), the Washington Dulles Toll Road (Rt. 267), and Leesburg Pike (Rt. 7). The greatest land use type in the watershed is residential. Development intensity throughout the watershed ranges from 8.2 to 27.4%. Other land use statistics include: Six percent of the watershed area is wetlands. Twenty percent of the watershed area is forest.

Water Quality
Difficult Run is considered impaired in three locations because it does not meet standards for fish consumption and aquatic life—findings include PBC in fish tissue, E. coli water, and poor benthic-macroinvertebrate assessment.

In the late 1990s the Virginia Department of Forestry conducted a riparian restoration project on Difficult Run to help improve water quality. Along with an outreach project, these efforts are a model for other local stream restoration initiatives in the state.

Fish
A total of 29 fish species have been found within the watershed, including

Parks and trails
Using community planning, a secluded 12 mi hiking and biking trail was built along Difficult Run from Reston to Great Falls Park. The trail is well-known among area mountain biking enthusiasts for its rolling terrain which makes for fast, smooth rides although there are portions requiring technical skills. The portion of the trail that sees the most mountain biking activity starts near the end of Michael Faraday Court, behind SkateQuest-Reston ice skating rink and follows the tributary stream valley to Lake Fairfax. From Lake Fairfax the trail crosses over Hunter Mill Road and follows Difficult Run itself all the way to Great Falls Park. Great Falls Park is a unit of the George Washington Memorial Parkway, and is administered by the National Park Service. In addition, almost the entire length of Difficult Run is paralleled by the Cross County Trail, which is maintained by the Fairfax County Park Authority for continuous coverage along the run.