Runoff basins: where rainwater pools in Montréal
In an intense storm the sewers fill up and rainwater runs over the ground to the lowest spots, where it pools. The Ville de Montréal mapped these low spots (cuvettes de rétention, or bluespots) for the whole island in 2021, with the Université du Québec à Montréal, from 1-metre elevation data of 2015.
Each basin has a class: the height of water that fills it, from 0–50 mm to more than 300 mm. The model assumes endless rain and full sewers, so it shows where water would gather, not how often it does. It is not a flood zone and does not record past floods.
A low spot next to a house matters because standing water finds its way into basements: window wells, basement doors and stairs, cracks in the foundation, or a driveway that slopes down to a garage. Insurers cover overland water and sewer backup separately, if at all.
Small low spots are everywhere: about half of the island’s lots touch one. Our pages show a basin only when it covers a large part of the lot: a quarter of the lot or more with 100 mm of water or more, and a point to check when more than 300 mm can collect on a third of the lot or more. Before you buy, look at the grading around the foundation, ask the seller about water in the basement and past claims, and have the inspector check the backwater valve and the sump pump.
General information, not legal or professional advice.
More terms
- Notice of deterioration
- The City’s right of first refusal
- Welcome tax (transfer duties)
- Assessment roll
- Building permits in Montréal
- Flood zones
- Contaminated sites
- Lead service lines
- Heritage: protected buildings and the inventory
- Condos: the documents to ask for
- Short-term rentals in Montréal
- The City’s plan for water mains and sewers