Raw water: what flows into Montréal's intakes

What is raw water?

Raw water is the water as the plant pumps it from the river, before any treatment: no settling, filtration, ozone or chlorine. Montréal draws it from Lake Saint-Louis and the St. Lawrence at the Atwater and Charles-J.-Des Baillets plants, and from the Rivière des Prairies and Lake of Two Mountains for the Pierrefonds and Pointe-Claire plants. What it carries depends on what arrives from upstream: the Great Lakes through the river, the Ottawa River through Lake of Two Mountains, and the farm rivers of Montérégie, among them the Châteauguay, which enters Lake Saint-Louis a few kilometres from the intakes.

Environment and Climate Change Canada samples these rivers about once a month and publishes the analyses as an open dataset, updated continuously. MTL Stats keeps six stations and twenty quantities. The number at the top is the latest measurement; the dashed lines are the Québec (blue), Canadian (red) and European (amber) criteria where they exist.

The six stations

Châteauguay mouth

Châteauguay River at the railway bridge, just before it enters Lake Saint-Louis. It is the farm river closest to Montréal's intakes; pesticides have been measured monthly since 2022, with up to 0.996 µg/l of glyphosate and 1.7 µg/l of metolachlor in summer.

Ottawa River at Carillon

Ottawa River one kilometre below the Carillon dam, at the entrance of Lake of Two Mountains: the water feeding the West Island plants and part of Lake Saint-Louis. Followed since 2004 for turbidity, metals and nutrients; pesticides in 2022 and 2023.

St. Lawrence at Cornwall

St. Lawrence at the intake of the Cornwall, Ontario, filtration plant, at the outlet of the Great Lakes: what enters Québec by the river. A single year of measurements, 2022-2023, showing Great Lakes atrazine at 0.04 to 0.06 µg/l.

St. Lawrence at Île Sainte-Thérèse

River east of Île Sainte-Thérèse, between Montréal-Est and Varennes, just downstream of the island of Montréal: the water after it passes the city, atrazine and metolachlor monthly since 2024.

St. Lawrence at Lavaltrie

Intake of the Lavaltrie filtration plant, north shore, ten kilometres downstream of the urban area. The longest series: suspended solids, turbidity, mercury, lead, chloride and nutrients monthly since 2006; pesticides in 2006-2007 and 2022-2023.

Richelieu at Lacolle

Richelieu River at Lacolle, at the outlet of Lake Champlain, just past the US border: what the United States sends into Québec. The Richelieu joins the St. Lawrence at Sorel, downstream of Montréal; it does not supply the city, but it crosses farming Montérégie. A recent station: oxygen, temperature and a few pesticides since 2024.

What each curve measures

Atrazine

A corn herbicide, banned in Europe since 2004 but still registered in Canada. Very soluble and persistent, it is the pesticide most often found in the river, brought mainly by the Great Lakes. Québec and Canadian aquatic-life criterion: 1.8 µg/l; European drinking-water limit: 0.1 µg/l per pesticide.

Desethyl-atrazine

A breakdown product of atrazine, often as abundant as the parent and as toxic. No criterion of its own in Québec or Canada; in Europe the 0.1 µg/l limit applies to metabolites deemed relevant.

Metolachlor

A corn and soybean herbicide applied at planting. Its June and July peaks in the Châteauguay follow the spraying directly. Aquatic-life criterion: 7.8 µg/l in Québec and Canada.

Glyphosate

The world's best-selling herbicide (Roundup). Measured here with a 0.01 µg/l detection limit, a thousand times finer than the municipal laboratory's. Aquatic-life criterion: 800 µg/l; European drinking-water limit: 0.1 µg/l.

AMPA

Aminomethylphosphonic acid, the main breakdown product of glyphosate, but also of phosphonate detergents in wastewater. No quality criterion.

Imidacloprid, thiamethoxam

Neonicotinoid insecticides, used as seed coatings. Québec's 0.0083 µg/l criterion, one of the strictest in the world, protects aquatic insects; the Canadian guideline for imidacloprid is 0.23 µg/l.

Chlorantraniliprole

A diamide insecticide, increasingly used to replace neonicotinoids. Québec criterion: 0.1 µg/l.

2,4-D and dicamba

Lawn and cereal herbicides, banned for cosmetic use in Québec but still common in farming. Québec criteria: 220 and 10 µg/l.

Turbidity

Cloudiness of the water, in nephelometric turbidity units (NTU): the more fine particles, clay and plankton the water carries, the higher the value. It rises with the thaw and floods. It is the first parameter the plant must bring down, below 1 NTU at the outlet.

Suspended solids

Mass of particles retained by a filter, in milligrams per litre. They carry metals and phosphorus; a flood can multiply them a hundredfold.

Temperature

Water temperature at sampling time. It governs aquatic life and how much oxygen the water can hold.

Chloride

Dissolved salt, mostly road de-icing salt, which arrives with the thaw and cannot be treated out. Chronic aquatic-life criterion: 120 mg/l in Québec and Canada; the river sits at 10-30 mg/l.

Total mercury

A toxic metal that builds up in fish. Measured in nanograms per litre, billionths of a gram. Canadian guideline: 26 ng/l; Québec's 770 ng/l criterion targets dissolved mercury and does not guard against methylmercury accumulation.

Dissolved lead

Lead present in the river water, not to be confused with lead at the tap, which comes from plumbing. Criteria depend on water hardness, so no line is drawn; values are hundredths of a microgram.

Phosphorus

A nutrient that feeds algae and drives eutrophication. Québec criterion for rivers: 30 µg/l; Canadian eutrophic threshold: 35 µg/l.

Nitrate

Dissolved nitrogen from fertilisers and wastewater, in milligrams of nitrogen per litre. Aquatic-life criterion: 3 mg/l in Québec and Canada; the drinking-water standard is 10 mg/l.

Dissolved oxygen

Oxygen available to fish. It drops as the water warms or as organic matter decays. Québec warm-water criterion: 4 mg/l; Canadian guideline: 5.5 mg/l.

Reading the curves

Pesticides are drawn on a logarithmic axis, from the laboratory's detection limits up to the criteria, with "not detected" at the floor. They peak in June and July, after spraying and rain, and almost vanish in winter. In the river itself, atrazine is the only constant presence: 0.01 to 0.04 µg/l at Lavaltrie, a quarter of what arrives from the Great Lakes at Cornwall, with a clear decline between 2006 and 2023.

These stations are not Montréal's own intake: Carillon, Cornwall and the Châteauguay are upstream, Île Sainte-Thérèse and Lavaltrie downstream, and the Richelieu at Lacolle enters Québec on its own. Together they bracket what the city receives and what it returns. What the water contains after treatment is on the Tap water page, and the farm rivers themselves on the Pesticides in rivers page.