The paper aims at the assessment of the Goczałkowice Reservoir effect on the changes of surface water quality. For this purpose, tests of 20 physicochemical water quality indices were conducted using reference methods. Water samples were collected every month from April to November 2011 in five measurement-control points: the first was located on the Vistula river inflow to the reservoir, the other three were placed evenly along the reservoir bowl, whereas the fifth was situated on water outflow from the reservoir. The values of each tested indicator were characterized by using descriptive statistics. Moreover, the empirical data were subjected to detailed statistical procedures by means of cluster analysis and t-Student parametric test. The analysis the data obtained from the conducted investigations demonstrated that because of the complexity of the processes, the retention reservoir differently influences the water quality. As a result of the Vistula river water flow through the Goczałkowice Reservoir, concentrations of nitrites and nitrate nitrogen decreased significantly, as has been proved statistically on the significance level α = 0.05. The reservoir also positively affected a decline in the concentrations of phosphates, total phosphorus, total iron and a majority of salinity indices, but worsened oxygen conditions, however, it was not statistically proved.