The authors offer substantiated methodical approaches to complex evaluation of the sanitary reliability of drinking water supply systems. They recommend not only evaluating drinking water quality, but also assessing the sanitary state of water sources (catchment areas), the reliability of water preparation and transportation, the standards of water supply and the reliability of production laboratory control. A range of complex hygienic studies have demonstrated that the problems of Voronezh interurban reservoir as a water source are caused by its multi-purpose use. Under these conditions insufficient hygienic efficiency of the conventional water preparation schemes and low sanitary reliability of water transportation systems favors negative influence of water factor on population mortality. The offered methodical approaches give the systematic idea of factors that determine drinking water quality. Operative administrative decisions concerning hygienic safety of public water use may be made with these methodical approaches taken into consideration.