Lake Saint-Louis at Risk?

Water Levels and Drinking Water

Christiane Hudon has a PhD in marine biology from Laval University (1982). A specialist in aquatic plant ecology, she has been a research scientist at Environment Canada since 1993.

Dr. Hudon initiated and coordinated the Impacts of Level and Flow Fluctuations in the St. Lawrence River project, which involved about 20 scientists. She was also the Canadian co-chair of the Environmental Technical Working Group (ETWG), established by the Lake Ontario–St. Lawrence River Study Board of the International Joint Commission.

Map: Lake Saint-Louis

Drinking Water

In the Montreal area, 16 municipal water filtration plants produce drinking water for a population of 2.1 million people.

Under low water level conditions, the following situations can develop:

  • a change in the relative size of different water masses, resulting in a change in the quality of raw water for treatment;
  • a reduction in water depth that may disturb the functioning of certain drinking water intakes.
Drinking water intakes of the Montreal region
Map: Location of the drinking water intakes of the Montreal region
1Saint-Zotique15Montreal (borough of Ville-Marie)
2Les Coteaux16Laval (3)
4Salaberry-de-Valleyfield (3)18Saint-Eustache
5Les Cèdres19Sainte-Thérèse
10Montreal (borough of Pierrefonds–Senneville) (2)24Kahnawake
11Montreal (borough of L'Île-Bizard–Sainte-Geneviève–Sainte-Anne-de-Bellevue)25Candiac
12Montreal (borough of Pointe-Claire)26La Prairie
13Montreal (borough of Dorval–L'Île-Dorval) (2)27Longueuil (2)
14Montreal (borough of Lachine) (2)28Varennes

Water Levels and Problems Related to Drinking Water Supply

A survey has revealed that the functioning of drinking water intakes of eight stations out of 16 are vulnerable to reductions in water levels and are likely to experience problems with the odour and taste of water. Five of these stations are vulnerable to variations in inputs from tributaries, which influence the quality of raw water to be treated.

ProblemNumber of stationsPopulation% of population
Water quantity8*200 00010
Influence of tributaries5163 0007.7
Earthy taste and odour8495 41023

Source: École Polytechnique 2003.

* Excluding Montreal (Charles-J.-Des Baillets and Atwater plants – population 1.5 million), who relocated its intake in 2003.

Improvement in Water Quality

Photo: RiverIn 2000, COURD’O, a physico-chemical and bacteriological water quality index, reported a net improvement over the previous 15 years, particularly along the south shore of Montreal Island, where the rating remained between “good” and “very good.” The commissioning of the former Montreal Urban Community’s wastewater treatment plant in 1988 contributed to improving shoreline water quality.
Photo: SwimmersThis has led, in turn to the recovery of some uses of the water, such as swimming. However, when it rains, water quality is likely to deteriorate because of overflowing wastewater collection systems.

Improvement in Water Quality

Photo: Mouth of the Châteauguay river in the Lake Saint-Louis

Mouth of the Châteauguay River in Lake Saint-Louis

The quality of raw water varies according to the origin of the water mass. Water from Lake Ontario requires minimal treatment to make it fit to drink, while water from the tributaries is more expensive to treat, particularly during high flow periods, when levels of particulates and dissolved organic substances are higher.

Aquatic Life

Photo: Aquatic plantNearly 70% of the surface area of the Châteauguay River basin in Quebec is being cultivated. Between 1998 and 2000, the total phosphorus load of the Châteauguay and Ottawa rivers respectively exceeded the threshold for aquatic life in 96% and 23% of the samples.

Water Temperature

High levels of nutrients (nitrogen and phosphorus) favour the proliferation of algae and aquatic plants, particularly under sunny conditions and high water temperatures. The average annual water temperature at the intake to the filtration plants increased by 2°C between 1970 and 2001.

Years of low levels coincide with higher water temperatures.

Average annual water temperature in the St. Lawrence River at the intake to Montreal's water treatment plant (1918–2001)
Illustration: Average annual water temperature in the St. Lawrence River at the intake to Montreal's water treatment plant (1918-2001)

Water Clarity

Photo: Algal bloomsThe water clarity of the St. Lawrence River and the presence of many water masses of different origins makes it possible for more than 350 species of algae to grow, either freely suspended in the water (plankton) or anchored to sediment and submerged plants.

Algal blooms

Illustration: 350 species of algae

Algal Blooms

Photo: Stephanodiscus binderanus

Stephanodiscus binderanus

When the environmental conditions are right, microscopic algae can proliferate to the point where it blocks the filters of the filtration plants, which occurred in Montreal in November 1955.

Algae Abundance

Even though suspended algae less abundant under low water level conditions (1955) than under average water level conditions (1994), the proportion of potentially troublesome algal species, such as chlorophyceae and cyanobacteria, increases.

Abundance of planktonic algae in St. Lawrence River waters
in summer (July–September)

Illustration: Abundance of planktonic algae in the St. Lawrence River waters in summer (July-September)
Source: Hudon et al. 2000.

Degradation of Algae and Aquatic Plants

Photo : Aquatic plant

The degradation of algae and aquatic plants imparts an earthy, musty taste and odour to drinking water due to the production of volatile organic molecules. Eliminating this problem requires additional water treatment and extra costs.

Zebra Mussel Colonization

Zebra Mussels colonize submerged substrates, including drinking water intakes, sometimes even interfering with their functioning. Under low water-level conditions, there is a higher risk of clogging water intakes, given the higher rate at which Zebra Mussels anchor themselves.

Low summer discharge conditions favour Zebra Mussel colonization

Illustration: Zebra Mussel colonization
Source: de Lafontaine 2002.

In Summary

Climate change could have few to many effects on the different aspects of supplying drinking water from the St. Lawrence River.

Anticipated impacts of climate change on drinking water supply

Anticipated impacts of climate change on drinking water supply
ProblemCurrent situationAnticipated situation (low levels)
Water intake location8 stations affected
Work required
Variability of water mass (influence of tributaries)3 stations affected
Could increase
Sewer overflows along the shorelineGeneral improvementStable or declining
Algal bloomsOccasionalStable
Toxic algae (cyanobacteria)RareStable
Earthy, musty taste and odour8 stations affected
Higher incidence
Zebra Mussel colonizationLocal effectsHigher incidence


École Polytechnique de Montréal. 2003. Impacts of Level Fluctuations in the St. Lawrence River on Water Treatment Plant Operations. Report prepared by Annie Carrière and Benoit Barbeau, submitted to the Water Uses Technical Work Group of the International Joint Commission. 208 pages.

de Lafontaine, Y. 2002. Impacts des fluctuations de débits d'eau douce sur la colonisation annuelle par les Moules zébrées dans le fleuve Saint-Laurent. Technical Report submitted to the International Joint Commission.

Hudon, C., S. Lalonde and P. Gagnon. 2000. Ranking the effects of site exposure, plant growth form, water depth, and transparency on aquatic plant biomass. Canadian Journal of Fisheries and Aquatic Sciences 57: 31–42.

Related Links

City of Montreal – Réseau de suivi du milieu aquatique (in French only)

International Joint Commission (IJC)

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