Flood maps · Toronto and region

Toronto flood maps: the TRCA flood plain map, the Ontario flood map, and what neither shows

Three different things come up when someone in Toronto searches for a flood map, and they answer different questions. TRCA’s flood plain map shows where rivers and streams are expected to go in a severe storm. The Province’s “Ontario flood map” shows flood messages in force today. And the City’s basement flooding programme maps the neighbourhoods it has studied for sewer and overland flooding, which neither of the other two shows.

This page sets out how to read TRCA’s map and its coloured lines, what the “regulatory flood plain” is drawn for, why a property can be regulated without being in the flood plain, what TRCA’s property inquiry letter costs a buyer, what no Toronto flood map shows, and what the map means for insurance. The national picture is on flood maps in Canada.

Which map you are looking at

TRCA’s Flood Plain Map Viewer is the address search most people want: the conservation authority’s own mapping of where its rivers and streams would spread in the regulatory storm. Toronto and Region Conservation Authority maps the flood plains for its watersheds under provincial law:

The “Ontario flood map” is something else. The provincial link with that name goes to the flood forecasting and warning program, which shows flood messages issued now — useful in a storm, but it does not tell you whether your lot is in a flood plain. The Province is explicit that it does not draw most of the plain maps itself:

The City of Toronto’s basement flooding study areas are a third map, and the one most relevant to the way Toronto basements actually flood. They are the neighbourhoods where the City has studied sewer and overland flooding and planned work, set out on Toronto’s basement flooding programme and subsidy.

How to read TRCA’s map

Blue is the flood plain. The lines are where people misread it. A red line marks a spill, where flood water leaves the valley and runs across surrounding land, so the edge and the depth are uncertain. A dark red line marks a spill screening area, where a property may be in the flood plain or a spill and the answer may need further study. A pink line marks an undefined area, where the flood plain ran past the edge of the topographic map available when it was drawn. Near any of the three, the honest reading of the map is “ask TRCA”, not “outside”.

The viewer is a generalized picture, and TRCA says so. For a purchase, a building permit or a dispute with an insurer, the document that counts is the approved regulation map, which TRCA supplies on request. The map also changes:

So a screenshot from five years ago, or a seller’s recollection, is not the current map. Land can come out of the flood plain when a study is redone, and land can go in.

What the regulatory flood plain is drawn for

Most of Ontario uses the 100-year flood as its minimum. Much of the Toronto region uses Hurricane Hazel instead, because Hazel was worse: 285 millimetres of rain in 48 hours in October 1954, transposed over each watershed. A property shown outside the blue is outside the Hazel-sized flood as currently modelled — not outside every flood, and not outside the parts that have never been modelled:

The main branches of the rivers are mapped; some headwaters and tributaries are not yet. TRCA’s regulation still applies to a flood hazard there, mapped or not.

Regulated, even outside the flood plain

This is the distinction that catches renovators. The flood plain is one of the hazards inside TRCA’s regulated area, not the whole of it. Valleys, the Lake Ontario shoreline, wetlands and the land around them are regulated too, and development, regrading or work near a watercourse there may need a TRCA permit as well as a City one. TRCA’s Regulated Area Search answers that question by address, separately from the flood plain viewer. Waterproofing and other basement work in Toronto has its own permit rules, set out on basement waterproofing in Toronto.

Buying a house: the property inquiry letter

For a buyer, the letter is the formal record: it records whether the property is likely to be affected by TRCA regulation, what restrictions apply, and any past or outstanding violations, with the regulated-area map where it crosses the lot. TRCA’s FAQ says the same — someone buying who wants to confirm whether a property is regulated should contact its Planning and Permits team. Whether to buy is your decision; TRCA says its staff cannot advise on that.

What no Toronto flood map shows

The City’s two causes — drainage that fails and sewers that back up — are not what TRCA’s viewer is drawn to show. A property outside the blue can still take water in the basement, which is why the devices the City subsidizes exist and why the sewer backup endorsement is sold separately. What a backup involves in Toronto is on sewer backup in Toronto; telling the kinds of basement flood apart is on basement flooding.

Outside TRCA’s area

Beyond TRCA’s boundary the map is usually another conservation authority’s, and the storm it is drawn for may be the 100-year flood rather than Hazel. Ask the conservation authority for your watershed, not the Province, for the plain map. Ottawa’s arrangements are on Ottawa flood maps.

What the map means for insurance

Overland water cover is optional and priced on risk, and a flood plain address is the kind of thing that risk is read from. So a flood plain address can affect whether overland cover is offered and what it costs, and an address outside the blue does not make the cover pointless, because the surface water that floods basements outside river plains is what overland cover is written for. Sewer backup is a different endorsement again. The cover itself is set out on overland water coverage.

Questions people actually ask

Where to read next

• Flood maps across Canada
• Toronto’s basement flooding subsidy
• Sewer backup in Toronto
• Overland water coverage
• Ottawa flood maps