Occoquan Reservoir is a 2,100-acre reservoir in northeast Virginia, southwest of Washington, D.C., straddling part of the boundary between Fairfax County and Prince William County, west of Alexandria. It is formed by the Occoquan Dam on the Occoquan River .
Managed by the Fairfax County Water Authority, it provides an important water supply to surrounding settlements in northern Virginia, with an output of 17 million US gallons a day. The Occoquan Reservoir is officially under Prince William County, despite being on the border with Fairfax County. The major drainages tributary to the Occoquan Reservoir can be divided into two principal sub-basins: Bull Run and Occoquan Creek.