The aim of this study was risk assessment regarding the local scours below sills that stabilize the Warta riverbed below Jeziorsko Reservoir. The studied river sills were constructed due to the reduction of riverbed erosion that took place downstream behind the dam reservoir. The research area embraced two reaches: in the vicinity, sills No. 3 (km 480+902) and No. 4 (km 479+225) were built in order to reduce local and general riverbed erosion resulting from the construction of the reservoir. The current article presents the methodology of the conducted field research and a short description of the applied mathematical model SSIM (a three-dimensional numerical model for simulating sediment movements in water intakes with multiblock options). Based on field geometry measurements of the local scour holes, the distribution of water velocity, and the water levels in the period 2006- 2010, calculations of velocity distributions at the sill have been carried out using SSIIM software. The computer simulations have shown that the calculated water velocity profiles did not differ significantly from the measured ones.