The runoff was flowing into Manassas Run along Dismal Hollow Road between Gore Road (Rt. 651) and Gap Road (Rt. F-285). There was enough influx of discolored, sediment-laden runoff into Manassas Run to raise the water level at least 4 inches maybe more at the peak. The runoff contaminated at least 3 miles of Manassas Run and then the main-stem of the Shenandoah River.
The orange line on the map indicates where the runoff was entering Manassas Run. Possibly that is a pond just above the railroad tracks that can be seen on the map. Maybe if it is a pond the retaining wall/dam failed or it was being purposely drained.