Proper understanding and characterization of water quality behavior in distribution storage reservoirs is critical to ensure meeting regulatory requirements. Field sampling can serve as a diagnostic tool to expedite the processes of detecting problems, assessing their scope, deciding among possible solutions, and evaluating the effectiveness of the selected option. This article describes an extensive sampling study of reservoir water quality conducted in Azusa, California. Primary emphasis was placed on providing a better understanding of the dynamics of hydraulic mixing and free chlorine residual concentration distribution in the reservoir. The results of the study showed a reservoir that approached completely mixed (CSTR) behavior. The two exceptions to this representation were a degree of short-circuiting between the inlet and outlet (which significantly affected the T10 time) and the presence of a stagnant zone in the center core of the reservoir where there was less mixing and resulting older water. These results support the assumption of stratification or partitioning in reservoirs. The authors recommend that field sampling be conducted regularly to facilitate the effective management of distribution system water quality. In particular, interior sampling of reservoirs can provide useful information that could not be inferred otherwise.