Changes in the chemical composition of the water in three dam reservoirs are presented against the background of the chemical composition of their affluents, and the effect of these changes on the phytoplankton in terms of the chlorophyll a content, are discussed. Changes in the chemical properties of the water, an increase in trophy, and the existing differences in the fertility of the water between the reservoirs are associated with the ways of utilization of the catchment basins and with their population density. On the basis of the whole collected material a scheme of the eutrophication of the examined reservoirs is presented.