The objectives of this study were to: quantify the impact of three covered storage tanks on the quality of distributed water by examining microbial and chemical parameters at the inlet and outlet; evaluate the water quality spatial variability inside the storage reservoirs; and, characterize the chemical and microbial composition of sediments settled on the bottom of the storage tanks. Three covered reservoirs located on the distribution system of a large North American city were chosen for their wide range of operating characteristics: Cl2 residual; residence time; and, hydraulic configuration. These reservoirs are supplied by treated water (direct sand filtration, ozonation and chlorination) taken from the St. Lawrence River. The inlet/outlet sampling were conducted on a weekly basis during July 1998 to September 1998. Two sampling campaigns using boats were also performed during June and September 1998 in order to evaluate the water quality spatial variations inside the reservoirs. In addition, sediments were siphoned from the bottom of the reservoir. Investigated parameters and analytical methods are summarized in this paper. Includes 11 references, tables, figures.