The North Fork has no named tributaries .
Another unnamed tributary enters from the left just before the North Fork reaches a United States Geological Survey ( USGS ) stream gauge and enters Bull Run Reservoir 1 , which is part of the Bull Run River mainstem .
The Bull Run River   has no named tributaries .
Another unnamed tributary enters from the left just before the North Fork reaches a United States Geological Survey ( USGS ) stream gauge and enters Bull Run Reservoir 1 , which is part of the Bull Run River mainstem .