Watershed
The Difficult Run watershed is the largest in Fairfax County at 37,179 acres (150.46 km2). The following subwatersheds 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
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