Frequently Asked Questions

What is a flood plain?

A flood plain is the natural flat area next to a river or stream that is able to convey extra water that spills out of the river channel during a flood event. Normally this area is dry and not covered by water.

High water levels in the river system that result in flooding within the flood plain are referred to as riverine floods. These floods are often the result of higher than normal river flows (or water volume) produced by extreme rainfall and snowmelt events.


Why is the flood plain important?

Knowing which areas will be inundated by flood waters is an important tool for land use planning and emergency management. This knowledge helps guide sensitive land use away from high-risk areas.

In areas where historical development has already occurred, the knowledge helps us identify ways to mitigate flood risk, all with the aim of better protecting people, property, and the resilience of our city for generations to come.


What is the regulatory flood plain?

The regulatory flood plain is the approved standard used in a particular watershed to define the limit of the flood plain for land use planning and regulation purposes. This standard is defined by the Ontario Ministry of Natural Resources and Forestry.

Within Toronto and Region Conservation Authority’s (TRCA) jurisdiction, the regulatory flood plain is based on the more severe of: Hurricane Hazel (also known as the Regional Storm), or the 100-year Storm – whichever is greater (see the following questions for definitions of these terms). For most of our jurisdiction, it is the Regional Storm.

A hydrology model simulates the effect of this amount of rainfall falling over each watershed, taking into account topography, soil type, land use, and other characteristics to determine how much water would end up in our rivers and streams.

A hydraulic model then looks at where this water would go, and what areas would be inundated. The inundated area for the regulatory storm is called the regulatory flood plain.


What does it mean if my property is within the regulatory flood plain?

If your property is within the flood plain, you and your property may be at risk of riverine flooding — although the degree of risk varies from site to site. Learn how TRCA works to reduce the risk of flooding to people and property.

There are also development and redevelopment implications if your property is within the regulatory flood plain. These are detailed in the Living City Policies (LCP). However, for site-specific information, we recommend that you contact TRCA and Planning and Permits.

Similarly, if you are purchasing a property and want to confirm whether it is regulated, you should contact our Planning and Permits team.


Is my property regulated if it is outside the flood plain?

Your property may still be within a Toronto and Region Conservation Authority (TRCA) regulated area even if you are not in the flood plain.

A primary objective of TRCA is to prevent the loss of life and property due to flooding and erosion. Accordingly, TRCA administers Regulation that captures watercourses, river and stream valleys, the Lake Ontario Shoreline, wetlands, and potential areas of interference around wetlands. Any proposed development, interference to wetlands, or alterations to shorelines or watercourses may require a permit from TRCA.

Use TRCA’s search tool to find out if you are in a regulated area.


Why does the flood plain extend onto my property but not my neighbour’s?

Flood plain boundaries are determined by factors like topography, so they can change from one property to another.


How long has the floodplain been identified in my neighbourhood?

It depends on when mapping was completed in your area. Many maps were developed in the 1960s and 1970s under the Flood Damage Reduction Program, but have been refined over the years.

The extent of the flood plain can change as mapping gets continually updated. Learn more about flood plain mapping.


How are the boundaries of the regulatory flood plain determined?

Flood plain mapping studies are technical assessments that use local information from each watershed in order to determine the extent of the flood plain.

Flood plain mapping studies are prepared and approved for TRCA by qualified engineers, using standards and criteria established by the Ontario Ministry of Natural Resources and Forestry.

Flood plain mapping studies use topographical information, surveys of infrastructure (such as the size of bridges and culverts), land use and land cover information, weather, and stream flow data to create detailed hydraulic and hydrologic models of each watershed.

Within TRCA’s jurisdiction it is standard practice to complete comprehensive flood plain mapping updates on a 10-year cycle. This ensures that flood plan maps and associated hydrology and hydraulic modelling incorporate latest land use and land cover information, and incorporate technological advancements in modelling software and techniques so that the resulting flood plain maps remain current and state of the art.


Can the regulatory flood plain boundaries change?

Yes. As each of the variables change (topography, infrastructure, land use, weather data, stream flows, and hydraulic and hydrological modelling techniques), so too will the resulting flood plain boundary.

In order to ensure that flood plain boundaries are current and up to date, flood studies are commissioned every 10 to 15 years for each watershed within Toronto and Region Conservation Authority’s (TRCA) jurisdiction.

When flood plain boundaries change some lands that were formerly within a flood plain could be removed and others added.


Is it possible to be in a flood plain that hasn’t yet been mapped?

Yes. Although the main branches of our rivers have flood plain mapping, which were created by qualified professional engineers and approved by Toronto and Region Conservation Authority (TRCA), some headwater features and tributary areas may not have been studied yet or assessed to the standards and criteria set by the Ontario Ministry of Natural Resources and Forestry.

TRCA would assess these unmapped areas, which include the area adjacent to a watercourse, and determine the need for any required flood plain mapping if any changes were proposed in the vicinity.

Please note that TRCA’s Regulation limit applies to natural hazards including flood hazards, whether or not they have yet been mapped.


What are the spill/spill screening/undefined flood plain areas on the map viewer?

A flood plain spill area exists where flood waters are not physically contained within the valley or stream corridor and exit into surrounding lands. As a consequence, the limit and depth of flooding are difficult to determine. Flood spill areas occur naturally or can occur as a result of downstream barriers to the passage of flood flows such as undersized bridges or culverts. Flood plain spills are symbolised on the flood plain map viewer by a red line segment. Please refer to TRCA’s Living City Policies for further information on flood plain spills.

A flood plain spill screening area is a defined area where properties may be within the flood plain, or within a flood plain spill, and the corresponding sections of theLiving City Policies may apply. Model information, or minimum floodproofing elevation requirements, may be available, or further studies may be required to determine if this property is impacted by a spill. A flood plain screening area is symbolised on the flood plain map viewer by a dark red line segment. Please contact TRCA for further information.

Undefined flood plain areas are identified where the extent of the flood plain is beyond the boundary limits of the topographical map that was available at the time the flood plain map was created. Undefined flood plain areas are symbolised on the flood plain map viewer by a pink line segment.

Areas adjacent to spill and undefined flood plain locations may be within the flood plain, however, at the time the flood plain map was developed, there was insufficient information to accurately define the extent of the flood plain in these areas.

For more information on the areas adjacent to the undefined flood plain or spill areas, please contact TRCA’s Planning and Development staff.


Is flooding possible outside the flood plain?

Yes. The flood plain defines the flood hazard for planning purposes based on the standard set out by the Ontario Ministry of Natural Resources and Forestry. However unlikely, it is still possible to have a larger storm or a different combination of factors (for example, debris blockages) that result in an even bigger flood

Also, the flood plain relates to riverine flooding only (see above). Urban flooding or basement flooding can occur anywhere when there is a lack of an overland flow route or the capacity of the existing drainage system is exceeded.


What’s the basis of TRCA’s authority to map & manage the flood plain?

Regulatory Permitting Role: The Conservation Authorities Act provides the legal basis for Toronto and Region Conservation Authority’s (TRCA) mandate to study watersheds and deliver programs and services that prevent, eliminate, or reduce the risk to life and property from flood hazards and erosion hazards. Section 21.1 (1) of the Act lists programs and services related to the risk of natural hazards as mandatory.

TRCA undertakes flood plain mapping in accordance with its responsibility for implementing Section 28 of the Conservation Authorities ActOntario Regulation 166/06, as amended (Regulation of Development, Interference with Wetlands and Alterations to Shorelines and Watercourses).

Planning Delegated Advisory Role: As outlined in the Conservation Ontario/Ministry of Natural Resources and Forestry/Ministry of Municipal Affairs and Housing Memorandum of Understanding on CA Delegated Responsibilities, conservation authorities (CAs) have been delegated the responsibility of representing the provincial interest on natural hazards encompassed by Section 3.1 of the Provincial Policy Statement, 2020.

This delegation requires CAs to review and provide comments on municipal policy documents and applications circulated to TRCA pursuant to the Planning Act, where they concern natural hazards of flooding and erosion. For further details, see Section 3 of The Living City Policies (LCP).


What is the Regional Storm?

On October 15, 1954, the most famous hurricane in Canadian history struck Southern Ontario. Hurricane Hazel brought winds reaching 110 kilometers per hour to the Toronto region, and dropped 285 millimeters of rain in 48 hours.

When TRCA calculates the regulatory flood plain, TRCA simulates the precipitation produced by Hurricane Hazel over each of the nine watersheds in its jurisdiction.

To learn more about Hurricane Hazel, please visit


What is a 100-year Storm?

The term “100 year storm” is defined as a rainfall event that statistically has a 1% chance of occurring in any given year, at any given place. It does not mean that such a storm will occur only once every 100 years.


What is the difference between Riverine and Urban flooding?

Riverine flooding is what occurs when water levels of rivers, streams, and creeks rise and overflow their banks and spill onto adjacent areas. Conservation authorities are responsible for determining the hazard from riverine flooding.

Urban flooding, such as street flooding and basement flooding, occurs when there is more water than the local drainage system (sewers and streets) can handle, or when there is a lack of a major overland flow route from a low-lying area. Urban storm infrastructure is the responsibility of municipalities.

Toronto and Region Conservation Authority’s (TRCA) regulatory flood plain mapping shows riverine flooding only.


What is a Flood Vulnerable Area?

Flood Vulnerable Areas (FVAs), also known as Flood Vulnerable Clusters (FVCs), are sub-areas within the Regulatory storm flood plain that contain multiple existing structures and/or roads for which a single, comprehensive flood remediation approach may be viable.

FVAs include Special Policy Areas (SPAs), as well as historical flood damage centres. For details about SPAs, please refer to TRCA’s Living City Policies (LCP).

The FVAs within TRCA’s jurisdiction are illustrated on a map located in our Flood Contingency Manual.

Please note that an FVA boundary is a general radius that includes a concentration of structures and roads within the flood plain. Thus, the FVAs conceptually illustrate these particular flood risk areas on at a broader scale.


Stormwater management: does it help with flooding?

Stormwater management helps play a cumulative role in flood protection. While on-site stormwater management cannot sufficiently contain all the floodwaters coming from a river, the act of good stormwater management practices everywhere in our watersheds works to collectively reduce flood impacts.


Who is responsible for issuing flood messages to the public?

In order to provide support to our municipal partners during storm events, and to fulfil responsibilities delegated by the Ontario Ministry of Natural Resources and Forestry, Toronto and Region Conservation Authority (TRCA) operates a Flood Forecasting and Warning Program that runs year-round, with staff on call 24 hours a day, 7 days a week.

This program provides advice and warning to municipalities regarding the forecasted timing and severity of potential flooding events from rivers and streams.


How can I receive flood messages?

Members of the public can receive flood messages directly from Toronto and Region Conservation Authority (TRCA) in two ways:


I got a flood message, but no flooding happened — why?

Flood messages consider weather forecasts for a broad area. It is possible that those conditions leading to flooding do not develop at a specific location.


Where can I find more flooding resources?