In the period 1990-2006, the decrease in water consumption of Tallinn, the capital of Estonia, determined both the input and output of the water balance of Lake Ü lemiste, the drinking water reservoir of the city as well as its hydraulic retention time (HRT). In 2006, the city's water consumption accounted for only a quarter of that in 1990.The role of regulated inflow in the water balance decreased and that of catchment run-off increased. Lake HRT increased four-fold. Before the 1990s, Lake Ü lemiste resembled a river, now it is more lake-like. Changes in the water regime were correlated with the decline in concentrations of organic matter and chloride suggesting a likely causal relationship between them. Managerial challenges turned on finding new functions for upstream reservoirs and on the opportunity to refill the lake with better quality raw water.