The Tualatin River Basin is located in the Washington County, Oregon. The river was detected having high chlorophyll a concentration and low dissolved oxygen concentration during summer time, which violated DEQ water quality standards. A mathematical model (HSPF) was used to simulate physical, chemical and biological processes in the basin and to evaluate the effect of the 'best management practice' (BMP) strategy on improving water quality of the basin. Calibration of the model was focused on water temperature, total suspended solids, dissolved oxygen, dissolved ammonia, dissolved nitrate, dissolved phosphate, and chlorophyll a. Comparison against observed data (1990) showed that the model predictions were accurate for most of the constituents. After verification, the model was used to evaluate the strategy of the Best Management Practice (BMP) on the agricultural land at Gales Creek basin, a major sub-basin of the study area. The results showed that the effect of BMP on nutrients loading from land segment is not significant at Gales Creek.