Summary:

The main goal of research presented here is an analysis of nature reserve impact on the performance of the lowland Jeziorsko Reservoir. The Jeziorsko Reservoir is located between sections at km 484+300 and km 503+800 on the Warta River reach. It is multipurpose reservoir working in annual scheme. In 1998 the Nature Reserve "Jeziorsko" was created by Ministry of Environment, Nature Resources and Forestry. The Nature Reserve is located in the upper part of the Jeziorsko Reservoir. Due to that fact one may observe intensive process of sediment accumulation in the upper part of the reservoir. This process is totally free of man's actions, what means it is not disturbed by any hydro-engineering works. In the paper the analysis of vegetation growth and sediment deposition processes is presented. As the result of these processes the water levels in the backwater part of the Jeziorsko Reservoir are significantly are raised, what causes increase of flood risk in the nearby areas.