The aim of this study is to identify the water quality of a small reservoir, including its trophic state subjected to anthropogenic influences and its impact on river water quality. The study was carried out on reservoir water sampled from five points, in four quarters of the hydrological year. Following determinations were carried out: turbidity, true colour, electrolytic conductivity, pH, concentrations of ammonia nitrogen, nitrate and nitrite, organic nitrogen and phosphate and total phosphorus. The nature of the trophic state of the ‘Turośń’ reservoir was determined in accordance with the table on the trophic state index according to Carlson. Significant seasonal variations were found in the turbidity and true colour of the reservoir water tested, and to a lesser extent in conductivity and pH values. These parameters were influenced by the location of water sampling. Elevated concentrations of ammonium ions in reservoir water, occurred in the autumn and depended on the sampling point and were higher in outflowing water than in inflowing water. Significant seasonal variations occurred in concentrations of oxidised forms of nitrogen (N-NO3 and N-NO2) and organic nitrogen. The reservoir had a positive effect on the concentrations of both forms of nitrogen, which was reflected in their concentrations in the water leaving the reservoir and, in the case of organic nitrogen, an inverse relationship was noted. Phosphate concentrations were highest in water sampled in summer and total phosphorus in autumn. In samples from the other quarters of the hydrological year, concentrations of both forms of phosphorus were lower. Phosphate concentrations varied little between sampling points, and for total phosphorus there was considerable variation across the reservoir water sampling points. Some phosphate and total phosphorus were retained in the reservoir during the study period. Due to the concentration of nitrogen, the water in the Turośń Reservoir can be predominantly classified as eutrophic. Taking into account total phosphorus, on the basis of the obtained test results, waters in winter and summer are defined as oligotrophic, while in spring and autumn they corresponded to values characterising a mesotrophic state. A number of factors could have contributed to the increasing eutrophication of the water in the studied reservoir.