Sewer overflows in Montréal

Why the sewers overflow

Most of Montréal is served by combined sewers, which carry sewage and rainwater in the same pipe. In heavy rain or during the thaw, the network and the Jean-R.-Marcotte treatment plant can no longer absorb everything: 160 overflow structures then release the untreated surplus into the St. Lawrence, the Rivière des Prairies or Lake Saint-Louis.

Provincial regulation requires the City to measure and declare every day the overflow duration of each structure. MTL Stats counts, for each day, the structures that overflowed, adds up their hours and keeps the highest rainfall reported. The data is published once a year, the following spring: the curve therefore stops on 31 December of the last declared year.

What the curve shows

Rainy days stand out immediately; dry-weather, emergency or planned-works overflows are rare but real. Comparing this page with the shore coliforms a week later shows the direct effect of overflows on swimming.