Reduction of discharge from Jeziorsko reservoir supplementing flow in Warta River was analysed. Reduction was caused by seepage from river bed along the stretch of 80 km. Twelve hypothetical waves were analyzed according to the size and duration of the discharge. The parameters determining the seepage properties of the soil were determined on the basis of in situ test and field measurements carried out on the area of Majdany polder, in the cross section located in the half the length of analyzed stretch. Cumulative seepage flow and unit discharge through the river bed were determined. For all simulation, losses were compared with discharge volumes from Jeziorsko reservoir. It has been shown that the losses at the analysed river stretch are not large and reach a maximum of 1.3% of the discharge volume. The percentage decreases with increasing discharge volume and its duration. The solutions were obtained using FEM implemented in the Hydrus computer software package, which allows analysis of unsteady seepage flow with time-varying boundary conditions.