The trend of decentralized treatment of wastewater from urbanized areas is becoming more and more topical. Sewage is discharged to a sewage treatment plant, and rainwater runoff from paved areas is drained into a watercourse or is absorbed or evaporates to enter the natural cycle of water. This is the case as shown in our research in monitoring the changes in flow rate and changes in water quality in the millrace. After construction of vacuum sewerage systems in the villages of Zbýšov and Hostěrádky- Rešov, the households were gradually connected to the sewerage system, thus reducing the flow of water in the millrace. To monitor changes in water quality, regular measurements were made on 5 profiles and water was sampled, analyzed and evaluated. The results were then compared with the standards set out in the valid legislation of the Czech Republic. Research showed that the water quality of surface water can be improved by reducing the burden of discharged sewage water into the flow. However, reduced flow rates in the flow can lead to higher concentrations of pollutants. This can also be caused in the long term by agricultural activities in the area of interest.