Egirdir Lake is the second largest fresh water lake with 482 km(2) surface area of Turkey. The lake is an indispensable water source for our country and region because of available water capacity and usage aims such as drinking-irrigation water, tourism, and fishing. However, especially contaminants located in the Egirdir Lake catchment affect the lake water quality negatively in times. Therefore, determination of the water quality of the lake has quite importance for region human health and sustainable usage of the lake. The major factors that control the quality of the Egirdir Lake water are agricultural activities in the basin, water-rock interaction, and domestic and industrial wastes. This study investigates the anthropogenic and geologic impact originated from pollution sources and water-rock interaction in the lake watershed basin to the Egirdir Lake water quality. For this purpose, geological, hydrogeological, and hydrological properties of the lake basin with point and nonpoint pollution sources were investigated. To determine the water quality of the lake, 48 water samples were collected especially from locations representing effects of pollutants in May and October 2009. The analysis results were compared with maximum permissible limit values recommended by World Health Organization and Turkish drinking water standards. The contents of all chemical and physical parameters are higher in dry periods than wet period, and water pollution was observed at discharge points of the streams into the lake. Also, pH, turbidity, potassium, chemical oxygen demand, ammonium, aluminum, iron, and lead concentrations were found to be above drinking water standards.