In order to improve the quality of retentioned waters in dam reservoirs, the pre-dam reservoirs are built, that are localized above the main one. They aim at keeping contaminants transported by a river. The example of such solution can be the largest reservoir in Lublin region - "Nielisz". Reservoir consists of two parts divided with dam. The main reservoir (water surface area 709 ha at NPP, mean depth 2.8 m, and total capacity 18.30 mln m3) is localized in river Wieprz valley. Its catchment area is 1236.2 km2. The pre-dam reservoir of 179 ha area at NPP, mean depth of 0.7 m, and total capacity of 1.18 mln m3, is situated in river Por valley. The damming section is situated on 1.685 km of its course, while the catchment area amounts to 590.3 km2.
Present research aims at presenting the analysis upon the influence of pre-dam reservoir on quality of waters supplying the main reservoir. Water samples were collected with quarter increments at the inflow and outflow from the reservoir. Analysis of results from here conducted survey univocally indicates that analyzed pre-dam reservoir contributes to significant improvement of water flowing into the main one. The largest reduction of pollutant concentrations in pre-dam reservoir on stream Por occurred during the intensive macrophyte growth period. The efficiency of the reservoir work is high. However, it is not able to eliminate all contaminants flowing into it along with water of stream Por.