Dams built to supply electricity, irrigation and fresh water, change the characteristics of the region they are located. The ecological, limnological characteristics and the quality of water in the dam reservoir deteriorate with time. In this study, the physicochemical, toxicological and ecological parameters of Gokcekaya Dam Lake's water, which is situated on Sakarya River were examined. The selected area has different characteristics from the others. This lake is situated between two other dam lakes on Sakarya River. So the main water of the lake comes from Sakarya Dam. During the years 2005-2006, water samples were taken from the lake surface and depths in seasonal periods, and the physical, chemical and biological parameters were examined to determine the modifications in the quality of water. It was seen that while the quality had no certain differences, the level of the nutrients in the water was low. The different characteristics of the coming water enriched the varieties of algae. It was concluded that the structure of the water in Gokcekaya Dam Lake is mezotrophic.