Overview of overflows in the Greater Montreal area

Montreal Metropolitan Community


In order to propose measures to reduce the impact of untreated urban wastewater and stormwater discharges on the water quality of lakes and rivers, the CMM wanted to paint a picture of overflow episodes, also known as overflows, on its territory. CANN Forecast was mandated to:

  1. Identify the structures and rivers with the most significant overflows per season;
  2. Compare the cumulative impact of overflows according to river flow;
  3. Assess the impact of overflows on the quality of the receiving environment using the E. coli indicator.


In order to study overflows, different data sources were used:

  1. Monitoring of Municipal Wastewater Treatment Works (SOMAEU);
  2. Open flow data from the Water Survey of Canada (RHC) and the Center for Water Expertise of Quebec(CEHQ);
  3. the Aquatic Environmental Quality Data Bank (BQMA).

These data allowed CANN Forecast to identify problematic overflow structures. In order to adequately assess the impact of a structure on the watercourse, we considered the dilution capacity of the watercourse using flow data when available. Then the relative impact of the overflow structures by season was calculated using the ratio between the estimated flow at each structure and the median flow of the receiving environment for each season. A significant relationship was identified between the dilution capacity of the river and the impact of overflows. Thus, small streams and rivers are generally more affected by overflows.

In order to confirm the results, the water quality of the rivers was compared for all the structures. In general, the most problematic structures have poorer water quality.

For more information on overflows, read our articles on the subject!