The effect of nutrient loading on trophic conditions and water quality in the Želivka Reservoir, a drinking water source for more than 1 million inhabitants, was evaluated for the period 1974–1994. The input of nutrients to the reservoir from the catchment showed a high inter-annual variation related to hydrology which controlled the intensity of nutrient losses from diffuse sources. A steep gradient of trophic conditions occurred in the reservoir between the eutrophic inflow reach and the mesotrophic lacustrine zone. Water quality in the lacustrine zone was suitable for the waterworks when the reservoir was filled near to its maximum volume, however, it started to be impaired by eutrophication when the water level decreased.. The minimum permissible water volume in the reservoir at which the treatment technology is not yet hampered by serious problems (presence of high concentrations of phytoplankton, organic matter and manganese in raw water) was predicted from empirical and functional models of algal growth.