This paper identifies the parameters governing the microbial quality of water in the distribution system of the City of Montreal. The approach compiled and compared data from different sources (Public Works Department, Water Quality Laboratory, hydraulic modeling) in time and space, and led to the design and exploitation of an integrated visualization tool combining: the structural description of the system (pipes, tanks, valves, pumps); data collected from network management (tank levels, pump operation), water quality data from routine analysis, pipe repairs, etc.,; and, computed operational parameters (flow direction and velocity, water residence times). The visualization tool enabled us to derive information which identifies the causes of coliform events in some locations and lead to the proposal of remedial solutions. Includes 16 references, table, figures.