Water quality in the Vír Reservoir (the main water source in the Svratka watershed) and its tributaries was investigated in terms of the physico-chemical, biological and microbiological characteristics during 1988-1990. The results of investigation showed that the Vír Reservoir in spite of its great depth (more than 60 m) and favourable morphology 30 years after impoundment has become more eutrophic. Long-term changes were characterized by log normal function which demonstrated the increase of the nitrate concentration from the level 5-10 mg.l−1 at the beginning of this period to the present concentrations, which are more than 20 mg.l−1 in the raw water to the drinking water treatment plant and also in the inflow water (the Svratka River in Dalečín). In a similar way there was an increase in average concentrations of PO43−. The main indicators of the accelerated eutrophication of the Vír reservoir are the occurrence of water blooms of blue-green algae, the summer oxygen depletion and increased concentrations of Fe and Mn in the hypolimnion, while pH and dissolved oxygen increased in the epilimnion. The relationship between concentrations of nitrates and application rates of mineral nitrogen fertilizers in the watershed was highly significant. From this evaluation we drew the conclusion that the main cause of eutrophication is the increased input of nutrients from the watershed.