Aim of the study:
In this research, spatial – temporal variations of water quality in Yassıalan Dam Lake were investigated by determining a total of 28 limnologic parameters and by using Water Quality Index (WQI) and Principle Component (PCA).

Material and methods:
Water samples were collected from seven stations selected on the reservoir monthly in 2018 – 2019 and total of 28 water quality parameters were measured in water samples. WQI was used for evaluating the water quality by using the parameters of pH, EC, Cl, NO2, NO3, SO4, Na, Pb, Cu, Cd, Hg, Ni, Zn and PCA was used for evaluating the water quality by using the variables of pH, BOD, EC, salinity, dissolved oxygen, total hardness, temperature, total alkalinity, Hg, Ca, NO3, NO2, NH4, SO4, Mg, Ca, Fe, Ni, Cl, Zn, Cd, S, Na, K, Pb, Cu.

Results and conclusions:
According to data observed, water of the Yassıalan Dam Lake has 1st – 2nd Class water quality in general and according to the results of WQI, although it was determined that the water quality decreased significantly in summer period, the reservoir was found as "A Grade – Excellent" water quality (<50) in all the investigated months and at all the investigated stations. WQI values recorded in the dam lake were found as 15.3 – 26.1 and the detected limnologic parameters did not exceed the specified drinking water standards in all the investigated months and at all the investigated stations. According to the results of PCA, 3 factors explained 84.2% of total variances.