The main objective of this study was to determine the impact of current point and nonpoint pollution loads on the water quality of Lake Eirdir. For this purpose, hydrodynamics and water quality status of Lake Eirdir were investigated by using the Delft3D model. The monthly monitoring of pH, dissolved oxygen (DO), chlorophyll-a (Chl-a), and forms of nitrogen and phosphorus was carried out, as was the seasonal monitoring of total organic carbon. In all, seven different sampling points were chosen from the lake and influent streams in order to monitor the lake during the period from December 2010 to November 2011, with the aim of calibrating the model. The performance of the dynamic water quality model was evaluated using the relative root mean square error (RRMSE) values for both the calibration and the validation periods. The results of the model showed that the DO had the lowest, and Chl-a had highest associated RRMSE value. The simulation results showed that Segments 1 and 7 of the system had relatively high concentrations of almost all the pollutants. It was also found that increasing the nonpoint nutrient loads from agricultural sources has a greater effect than increasing the nonpoint nutrient loads from the inhabited areas around the lake.