Within the scope of a research programme on the ecological rehabilitation of the large rivers in The Netherlands the impact of hydrology on aquatic communities in floodplain lakes along the Lower Rhine and Meuse has been studied. Apart from the nutrient status of the Lower Rhine and Meuse water the hydrology was found to exercise a major impact on the water chemistry, the phytoplankton development, the vegetation and the zoobenthos communities in the floodplain lakes. The increased input of N and P from the eutrophic main channels to the floodplain lakes via floods has resulted in an increase in phytoplankton biomass and a shift from a macrophyte dominated community towards a community dominated by planktonic algae (Chlorophyta, Cyanobacteria), especially in frequently flooded lakes and with the main channel connected lakes.