After a flood in 1997 that affected the basin of the upper and central Odra River (on selected dam reservoirs, including drinking water reservoirs), the operational water fill ordinates were lowered in order to increase the flood capacity of the reservoirs. One of these reservoirs is the Słup. Its main functions include: compensation of water outflow from the Nysa Szalona and flood protection. This paper presents the analysis of flows of the Nysa Szalona River below and above the Słup Reservoir, analysis of the water table and reservoir capacity, plus quality assessment of water flowing in and out of the reservoir, including stored water. Water quality assessment was based on analyses of the following parameters: NO₃^-, NO₂^-, NH₄⁺, PO₄^3-, BOD₅, COD_Mn, DO, Cl^-, SO₄^2-, TOC, electrolytic conductivity, water temperature, and TSS. The study shows that in the period of 2005-08 the reservoir significantly compensated flows in the Nysa Szalona River. Significant relationship between the volume of water flowing out of the reservoir and volume of water flowing into the reservoir was observed (statistical significance at the level of p<0.05). A statistical analysis of linear correlations among all investigated water quality indicators at particular measurement points was also carried out. Significance of differences among some average water quality indicators with regard to inflowing and outflowing water was found. In order to protect the Słup Reservoir from pollution, construction of a pre-dam in the reservoir backwater is proposed.