The paper presents the way of water quality improvement in storage reservoirs, using pre-dams with plant filters. Pre-dams, built above the main reservoirs, reduce the inflow of pollutants transported by the river and take over most of biochemical processes responsible for eutrophication of the stored water. As a result, water flowing to the main reservoir contains smaller loads of pollutants. The above-mentioned method is more economic, requires relatively short construction time and is less laborious than generally used methods of surface water protection. It is specially recommended to improve the quality of water in small reservoirs, where quick silting and eutrophication occur. Proper functioning of pre-dams depends on proper selection of their parameters, such as: water storage time, water flow velocity, filling depth and type of plant filters. The authors estimated the effectiveness of a pre-dam reservoir situated above the main reservoir in Mściwojów on the Wierzbiak River. The pre-dam reservoir consists of a three-chamber sediment tank and three chambers with plant barriers. The purpose of the chambers is to decrease the quantity of sediments, the biogenie substances and other pollutants carried with water flowing to the main reservoir. Research carried out during the period 2000-2001 showed that pre-dams significantly contributed to the improvement of the water quality in the main reservoir - nitrates were eliminated in ca 65% and phosphates in ca 52%.

The paper presents in form of a case study the results of 10-year long hydro-chemical studies on the Korzeń stream on which the “Skrzyszów” small storage reservoir was built. Studies aimed at evaluating the impact of the reservoir on the surface water quality in a Flysch stream. The basis for the analysis was results of 21 hydro-chemical water quality parameters, from the following groups of indicators: physical and acidity, oxygen and organic pollution, biogenic, salinity, metals. Indicators were determined in one-month intervals in two periods: 2005–2009 (before the reservoir was built) and 2015–2019 (after the reservoir was built). Obtained results were subjected to a statistical analysis. The trend analysis of changes was performed using the Mann–Kendall test or the seasonal Kendall test; significance of differences between indicator values from two periods was evaluated using the nonparametric Mann– Whitney U test. Results of analysis showed significant change trends of water quality parameters, in case of total iron concentration the trend was downward in both periods. Statistically significant differences between the values of definite majority of indicators were found in two analysed periods, indicating both favourable and unfavourable impact of the reservoir on water quality in the stream. Construction of the storage reservoir resulted in a significant change of physical and chemical indicators of water flowing in the stream. Random variation dynamics as well as tendencies and trends of changes over time have changed. In addition to modifying the stream hydro-chemical regime, the reservoir also affected the social and natural conditions.