In this study, multivariate statistical analysis was applied to analyze the trend of water quality and water quality in the Fourth reservoir over the last 10 years. Correlation analysis between water quality variables showed that, the correlation coefficient between COD and TOC, which is an indirect indicator of organic matter, showed a high correlation of 0.608, while water temperature showed a positive correlation with pH of 0.515 and a negative correlation with DO of -0.716. Principal component and factor analyses showed that the four major components were extracted from the total water quality and contributed 72.9% of the total variance. When analyzed for cyanobacteria occurrence, three major components were extracted and contributed 67.2% of the total variance. Factor loadings analysis of water quality variables on the factors identified the first factors as COD and TOC. Water quality of the Fourth reservoir was found to be influenced by seasonal effects such as rainfall and organic matter, in addition to the effects of phytoplankton proliferation and nutrient influx.