An analysis of the impact of increasing the flood retention of the Jeziorsko reservoir on the low flows in the Warta section below the reservoir has been presented in the paper. Increase in the flood retention approx. 20 million m³ was achieved by changing the normal damming level (NPP) with the elevation of 120.50 m a.s.l. to the elevation of 120.00 m a.s.l. Multi-functional Jeziorsko reservoir, which is not only an important part of flood protection system but also performs the functions of supplementation in periods of low and very low flows in the Warta river. Lowering the NPP reduced the volume dedicated to supply the Warta river below the dam leading undermining the ability of increasing the flows in a low flow periods. The analysis was performed using one-dimensional numerical model of unsteady flow for the section of the Warta from the Jeziorsko reservoir to Oborniki. Hydrographs of discharge from the the Jeziorsko reservoir were determined using a numerical optimization model. The obtained results showed a significant effect of the reduction of NPP on the low flows in the Warta section below the reservoir. Both the number of days with outflow rate equal to reserved flow and number of days with flow lower than medium low flow (SNQ) were increased.