Rainwater and flood zones in Delson
Updated September 28, 2026
Delson is bounded on the north by the St. Lawrence Seaway and crossed from south to north by the Tortue River, which the city calls its most important watercourse. In the neighbourhoods, rainwater from the streets goes to the municipal storm sewer and to retention basins, such as the one in the Cheminots sector. The zoning by-law also distinguishes road ditches, which drain only a street, from shared ditches (fossés mitoyens), which drain two neighbouring lots.
On June 20, 2026, the storm sewer was overloaded and water collected in several sectors. Since then the city has extended its subsidy to equipment such as submersible pumps and window wells, and a draft by-law adds rules on surface-water drainage.
Heavy rain events
What public sources report about heavy rain in Delson, at sector level.
According to the mayor, close to 150 mm of rain fell in a few hours and overloaded the municipal storm sewer, while the sanitary sewer worked with no failures. He reported large accumulations of water in several sectors and, on ICI RDI, spoke of neighbourhoods that had never flooded before. The retention basin in the Cheminots sector filled up. The city's notice asked residents to limit water use and to call 9-1-1 if water backed up into the basement. The city does not say how water got into basements.
Sources: Ville de Delson, le maire fait le point, 2026-06-21 · Ville de Delson, avis de fortes pluies · Radio-Canada, 2026-06-21
After Debby's heavy rain, the city exceptionally opened its dry-materials drop-off site (ecocentre) to residents for the whole following week. According to the CMM, Debby dropped more than 150 mm of rain in 24 hours on Greater Montréal. The sources do not name the sectors affected in Delson or say whether the water came from runoff or from sewer backup.
Sources: Le Reflet, 2024-08-12 · CMM, 2025-12-11
Good to know in Delson
- In September 2026 the city expanded its eco-products subsidy: submersible pumps, gutter extensions and window wells bought since June 19, 2026 now qualify. The rebate cannot exceed 50% of the purchase cost, up to $200 per residence per year. (Ville de Delson, subventions de produits écologiques)
- On September 8, 2026, council tabled draft by-law 901-44, which adds provisions on surface-water drainage and changes the rules on driveway entrances. It is still a draft, not a rule in force. (Ville de Delson, échos du conseil du 8 septembre 2026)
- Construction by-law 903 requires gutters on every building, with a downspout that releases the water onto the lot at least 1.50 m from the building. The water must never run onto neighbouring lots, and gutters may not be connected to the foundation drain. (Ville de Delson, règlement de construction 903, version non officielle (PDF))
- The city notes that a downspout or foundation drain connected to the sewer can help flood the house's own basement, a neighbour's, or lower-lying properties. It describes the backwater valve as an essential device against basement flooding. (Ville de Delson, aqueduc et égouts)
- A municipal by-law generally prohibits fill, excavation, pools, sheds and other structures along the Tortue River. Work allowed near the river requires a prior certificate of authorization. (Ville de Delson, protection de la rivière La Tortue (PDF))
- At the end of 2025 the CMM sent its mapping of low spots (cuvettes) sensitive to urban runoff to every Greater Montréal municipality, Delson included. It is not part of the flood-zone maps, and each municipality decides whether to make it public. (CMM, zones inondables et crues printanières, 2026-02-25)
Rivers and lakes: what the check does not cover
Waterways in Delson: Tortue River and St. Lawrence River (Seaway).
In an update posted by the city, the mayor says the rain of June 20, 2026 caused the Tortue River to overflow; he told ICI RDI he had never seen it so high. Delson's zoning by-law regulates floodplains, including high-risk ice-jam flood zones, without naming the watercourse concerned; almost all construction is prohibited there. The Tortue is not among the water bodies covered by the CMM's new maps, so the old maps remain in force.
Sources: Ville de Delson, le maire fait le point, 2026-06-21 · Radio-Canada, 2026-06-21 · Ville de Delson, règlement de zonage 901, codification de mars 2025 (PDF) · CMM, zones inondables et crues printanières, 2026-02-25
The check is about the rain that falls on the area and runs off over the ground. River and lake flooding is not part of it: in the simulation, a nearby river or lake holds no water. For a waterfront property, the official map comes first.
What we know about the ground in Delson
These values come from the public sources the check uses. They describe the area, not an address.
| 1 m terrain model (NRCan HRDEM) | Available across the whole area |
|---|---|
| Nearest rainfall gauge (ECCC) | McTavish (Montréal), 14 km awayLocal rainfall statistics. |
| Municipal street drains | Not modelled: with no open municipal inventory, the check's depths are an upper bound wherever a working network would carry water away |
| Mapped culverted road crossings | 0Culvert lines mapped in OpenStreetMap (ODbL). |
Data Source: Environment and Climate Change Canada, Short Duration Rainfall Intensity-Duration-Frequency Data, used under the ECCC Data Servers End-use Licence.
Area analysed: adapted from Statistics Canada, Census Subdivision Boundary File (cartographic), 2021 Census. This does not constitute an endorsement by Statistics Canada of this product.
Probed on September 28, 2026.
The official map and what it does not show
Québec's flood-zone maps cover rivers and lakes. They do not show rain water collecting on a lot, in a low spot or at the bottom of a sloping driveway. Check the official map for the address first: it is what says whether a property is in a flood zone.
FloodVerity does not reproduce these maps and does not state any regulatory zone.
Check an address in Delson for $39
We drop a 100-year, one-hour design storm on the ground around the property, route the runoff, and email you the peak depth of rain water at the address, where it collects, a map and a 3D view.
What the check does not do
- This check models rain falling on the area itself and running off over the terrain. Water arriving from a river or lake, sewer backup, ice jams and storm surge are not modelled.
- The pipe network itself, roof and lot drainage, and sewer backup are not modelled.
- Where the structures inventory records a culverted road crossing, it is lowered to a partly blocked opening rather than solved hydraulically.
- It does not replay the storms described on this page: it applies a 100-year, one-hour design storm.
- It is a screening: a simulated storm on public terrain data, not a hydraulic study, not an official flood-zone determination, not an insurance opinion.
Notaries and brokers in Delson
We run the same check over a list of addresses and deliver a spreadsheet ranking them by the modelled rain depth. Priced per address, on quote.
Email [email protected]Other towns and areas
The FloodVerity check is a screening: it applies a design rainstorm to public terrain data and simulates where the rain runs and collects. It is not a hydraulic study or an official flood-zone determination, and it covers neither river and lake flooding nor sewer backup.