This paper present an assessment of the impact of the Kowalskie reservoir on the hydrological regime alteration of the Główna river. The Główna river is a right tributary of the Warta river. The Kowalskie reservoir was built in 1984. The area of the reservoir in the normal condition is 203 ha and total capacity is 6.58×106 m³. The impact of the Kowalskie reservoir on the hydrological regime was made on the basis of RVA method. The assessment was based on the analysis of flow series from the period of 1971–2012. The calculations were done by means of IHA software version 7.1.0.10 developed by The Nature Conservancy (2009). This study indicate that the Kowalskie reservoir has an impact on two group of flow parameters that characterized frequency and duration of high and low pulses as well as rate and frequency of water condition change. The others flow parameters characterized magnitude and duration of annual extreme water conditions are less affected by the Kowalskie reservoir. This study show that Kowalskie reservoir play important role in flood protection. Additionally the reservoir operation has an positive effect on minimum flows characteristic and base flow index.