This thesis assesses phosphorus loads and management practices to control nutrient transport to Lake Uluabat. It analyzes nonpoint sources of pollution, especially agricultural pollution, throughout Uluabat Basin (watershed). AVSWAT (ArcviewTM Interface of Soil and Water Assessment Tool 2000) was used in determination of phosphorus and sediment loads to Lake Uluabat. Contribution of soluble phosphorus (SOLP) loads from agricultural sites was discussed. Seven scenarios were applied through watershed area to see effects of fertilizer and irrigation application rates, landuse changes, point source and watershed inlet loads on phosphorus loads. Calibration of the model was performed annually due to lack of data. First stream flow, next sediment and finally nutrient (SOLP) was calibrated at two gages. At the first gage, simulation results were satisfactory in terms of Nash-Sutcliffe Efficiency (ENS) and percentage deviation between observation and simulation values (Dv). ENS values for stream, sediment and SOLP were &gt / =0.99. Dv values for stream, sediment and SOLP were &lt / &plusmn / 1%. At the second gage, after calibration, following values were obtained for ENS and Dv: Stream flow - ENS=0.75, Dv&lt / 10% / sediment - ENS=0.71, Dv=25% / SOLP - ENS=0.55, Dv&lt / 20%. It was concluded that agricultural sites were among major contributors of phosphorus load to Lake Uluabat. SOLP load to lake was about 4.0 gP/m2/year, greater than the target value of 1.0 gP/m2/year (DHKD, 2002). Scenario results showed that removing agricultural lands around Lake Uluabat and decreasing fertilizer application rates were necessary to reduce SOLP loads to lake. Moreover, phosphorus load from Emet and Orhaneli Watersheds seemed significant.