Difficult Run is a 15.9-mile-long (25.6 km) tributary stream of the Potomac River in Northern Virginia in the United States. The area has had many historical uses dating back to the early 1800s. Today, the area is used recreationally by visitors interested in the watershed's variety of options including hiking, biking, fishing, boating, climbing, and bird watching. The wildlife at Difficult Run is vast as 163 different species can be seen depending on the season. There are 41 different soil types found on the trail and alongside the stream. The stream is part of the greater 57.7- square-mile Drainage basin, or watershed, located in the north-central portion of Fairfax County and drains directly to the Potomac River.