Lake Allatoona is a large reservoir northeast of Metropolitan Atlanta threatened by excessive algal growth. Rapid population growth has occurred in the southern part of the watershed and poultry combined with beef cattle production is an important activity in the more rural northern part of the watershed. We used the Soil Water Assessment Tool (SWAT) model to estimate the annual P load to Lake Allatoona in 1992 and in 2001 after significant changes occurred in land use. Land cover data from the Multi-Resolution Land Characteristics Consortium from 1992 and 2001 showed that forest land use decreased during this period by about 20% and urban and pasture land uses increased by about 225% and 50%, respectively. Our simulation results showed that the P load to Lake Allatoona increased from 137 Mg to 172 Mg. In the 1990's, forest and point sources were the largest sources of total P, respectively contributing 30.7% and 29.5% of the total P load to Lake Allatoona. Pasture, urban, and row crop agriculture contributed 18.3%, 16.5% and 3.6%, respectively. A decade later, the largest P source was urban land use that contributed 49.7% of the total P loads to Lake Allatoona. Point source P load dropped significantly to 13.3%. Permit limits on poultry processing plants reduced the point source P loads but increasing urban landuse increased non-point sources of P.