Lake Manassas is a 706-acre public water supply reservoir for the City of Manassas, formed by the impoundment of Broad Run. It is located in western Prince William County at the periphery of the metropolitan Washington, D. c. area. Its watershed drains approximately 46,500 acres of rural land that is expected to undergo rapid development in the next 25 years. The objective of this study was to provide a comprehensive report of the quality of water in Lake Manassas and its tributary streams, analyzing data that had been collected from October, 1984 through April, 1986, as part of a monitoring program funded by the City of Manassas. The capacity of Lake Manassas at full pool was determined to be 4.2 billion gallons (15.8 million cubic meters) and its mean depth 25 feet (7.7 meters). The calculated volume was considerably less than had been previously thought. Trophic state indices indicated that Lake Manassas is eutrophic, and water column chemistry suggested phosphorus to be the limiting nutrient. The Vollenweider model was applied to illucidate the relative impacts of South Run and Broad Run on the lake. While Broad Run contributed the vast majority of flow to the reservoir, higher concentrations of algal nutrients, nitrogen and phosphorus, were transported to the lake by South Run, which receives effluent from the treatment plant at Vint Hill Station, a U.S. Army reservation. Phosphorus loadings from South Run were higher than from Broad Run under baseflow conditions. On an annual basis, stormflow runoff contributed as much as twelve times the tributary baseflow loading of phosphorus. / M.S.