Stay Informed and Safe: Fall Can Bring Increased Flooding Threat to Southern Ontario

September 25, 2023, Toronto, ON — Residents in Toronto and the Greater Toronto Area should be aware of the risk of flooding from remnant hurricane systems, particularly during the fall when larger and more widespread rainfall systems can increase the risk from rivers and streams.

rain falls on a window looking out on trees

Although remnant systems typically weaken by the time they reach this area, they may still carry significant moisture that can lead to major flooding.

The hurricane season occurs from June 1 to November 30 in the North Atlantic, with the most powerful hurricanes typically occurring in late summer and early fall.

Less common, but still possible, is rainfall from Eastern Pacific tropical cyclone systems. For example, in August 2023, Southern Ontario experienced rain and severe weather from the remnant moisture of Hurricane Hilary.

Current Conditions in TRCA Watersheds

Existing conditions on rivers and lakes can impact whether a precipitation event leads to flooding. For example, when water levels are already high, the risk of flooding increases.

The summer of 2023 was wetter than average, with Toronto receiving about 270mm total in the months of June, July, and August, which is approximately 120% of normal rainfall amounts for the summer months.

There were no low water level conditions reported in the spring or summer months for TRCA watersheds.

Past Fall Flood Events

On October 15, 1954, the most famous hurricane in Canadian history struck southern Ontario. Hurricane Hazel was projected to dissipate, but instead re-intensified unexpectedly and rapidly, pounding the Toronto region with winds that reached 110 kilometres per hour, as 285 millimetres of rain fell in 48 hours.

black and white archival photograph shows flooding and downed trees in the aftermath of Hurricane Hazel in 1954

Bridges and streets were washed out, and homes and trailers were washed into Lake Ontario. This storm changed the Toronto landscape forever and mobilized the need for managing watersheds on a regional basis.

Learn more about Hurricane Hazel.

How Does TRCA Manage Fall Flooding Risks?

1. Flood Duty Officers

To manage fall flooding risks, Toronto and Region Conservation Authority (TRCA) has a team of Flood Duty Officers who assess and monitor floods daily. They use a range of weather prediction models and other data sources to determine the risk of flooding by rain, snowmelt, thunderstorms, and local Lake Ontario waves and storm surges.

TRCA Flood Duty Officer

TRCA recently developed a Flood Forecasting and Warning System that would automate the Daily Planning Cycle (DPC). With the time saved from these improvements, staff can now allocate more time to decision-making and communication about flood risk. Learn more about the staff.

When flooding is possible or about to occur, TRCA issues flood messages to designated individuals within municipalities, local agencies, school boards, and the media.

the different types of flood messages issued by conservation authorities

To receive flood messages directly from TRCA, follow us on X (formerly Twitter) or sign up here.

2. Flood Monitoring Gauge Network

Flood Duty Officers use TRCA’s flood monitoring gauge network, which continually monitors in-stream water levels at TRCA dams and river stations as well as collecting rainfall amounts.

The measurements are displayed on TRCA’s flood monitoring website, designed as a portal for Flood Duty Officers and our municipal partners, which is publicly accessible.

Visit to explore your local river.

The TRCA real-time gauging map uses shapes colours and symbols to quickly assess the flood risk conditions

3. On-call Dam Operators

TRCA has on-call dam operators for its two largest operable dams: G. Ross Lord Dam and Claireville Dam.

During excessive rainfall or snowmelt, water collects in the “storage” reservoir instead of flowing downstream, which helps prevent floods. The water is then released in controlled amounts.

The dams are checked daily by the operator and are frequently inspected and tested by TRCA’s Flood Infrastructure and Hydrometrics staff.

TRCA dam operator

4. Partnerships with Municipalities

TRCA staff have built strong partnerships with municipalities, as they maintain communications with municipalities during flood events, and attend/provide advice to municipal emergency operations centres (EOCs).

TRCA regularly attends emergency management training sessions and works with partners to communicate flood risk to the public.

Learn More About Flooding Risks