This paper evaluates the effects of Torul dam on the stream HarAYit water quality in terms of 13 physico-chemical parameters in the GumuAYhane Province, Eastern Black Sea Basin, Turkey. For this purpose, a study was fortnightly conducted during the four seasons between March 2009 and February 2010. In two monitoring stations selected in the upstream and downstream of the Torul dam, T, pH, DO and EC were determined in situ, and collected water samples were analyzed for TH, COD, NH4 (+)-N, NO2 (-)-N, NO3 (-)-N, TN, TKN, PO4 (3-)-P and MBAS. According to the Turkish Water Pollution Control Regulation (TWPCR), the stream HarAYit was classified, and the obtained results were evaluated for the values proposed by Turkish Standard (TS) 266 and World Health Organization (WHO) guidelines. The results showed that the stream HarAYit has high-quality water in terms of, T, pH, DO, COD, NH4 (+)-N and NO3 (-)-N, but slightly polluted water in terms of NO2 (-)-N, TKN and PO4 (3-)-P, and polluted for MBAS. It was concluded that Torul dam has a positive effect on the stream water quality in terms of decrease in the annual average concentration values. The percent decreases for TH, COD, NH4 (+)-N, NO2 (-)-N, NO3 (-)-N, TN, TKN, PO4 (3-)-P and MBAS were 17.1, 20.3, 56.2, 62.6, 11.7, 11.9, 11.4, 17.8 and 71.4, respectively. The reason for these decreases is probably due to the Torul dam reservoir where the water has a hydraulic residence time and the exposure to chemicals by aquatic organisms or populations that ingest the water. Also, statistical analysis shows that there are significant correlations among the studied parameters.