Omerli dam reservoir is very important source of drinking water for Istanbul metropolitan area. The present study covers measurement and evaluation of water quality parameters. Omerli Dam Reservoir has eutrophic character in terms of secchi disc depth (varies between 1.0-3.0 m) and phosphorus parameter (varies between 0.001-3.52 mg/L). It has oligotrophic character in terms of phytoplankton and carbon produced per unit area. Although pH of the reservoir is often alkaline in character, it shows categorically weak acid character after heavy rains. The chlorophyll-a parameter measured in the reservoir varies in vertical and horizontal positions. The average chlorophyll-a in the reservoir is 1.03 mg/L. Based on the water temperature measurements in the reservoir, it was determined that there was stratification in the reservoir during summer months. This stratification (the thermocline) was determined to be destroyed during winter months and the spring. The mean value of the measured dissolved oxygen in the reservoir was 8.49 mg/L. The TKN average in the reservoir was calculated as 1.83 mg/L. With these parameters, Omerli Dam Reservoir water quality could not be considered in a very clean class. Some rehabilitation studies have been carried out to solve the pollution problem in the lake basin. Preventing the pollution and improving water quality have affected the nature of the reservoir basin but it is necessary to continuously monitor the ecological balance of the reservoir continuously, which is significant for drinking water, fisheries and recreation facilities.