The problem analyzed in our paper is quantification of the impact of sediment deposition and riparian vegetation growth on floodplain inundation frequency. An additional factor is water reservoir operation. A Warta River reach was selected for the study. This natural channel is located above the Jeziorsko Reservoir. The applied methodology is a combination of data collection, hydrodynamic model simulations, and analysis of water stage frequencies. Computations and analyses showed interesting relations between elements of the river system studied. Floodplain inundation frequency was found permanently increasing. Although the main factor responsible for this is sediment deposition, the influence of riparian vegetation is important at higher water stages.