Water Levels Continue to Rise in the Lake Ontario-St. Lawrence River Basin

Persistent precipitation this spring has resulted in rising water levels throughout the Great Lakes-St. Lawrence River basin and flood conditions in many tributaries. Since March 1, water levels in Lake Ontario have risen from 74.54 meters (244.55 feet) to 75.30 meters (247.05 feet) by April 30, a total increase of 0.76 meters (2.50 feet) in two months. Meanwhile, the Ottawa River Regulating Committee has warned that outflows from the Ottawa River are high and have exceeded flood levels in certain areas. High outflows from the Ottawa River, which meets the St. Lawrence River near Montreal, can result in flooding along the lower St. Lawrence River.

Extremely wet weather caused water supplies in April to exceed the prescribed plan outflows. Due to the Ottawa River freshet, the annual spring rise in the river primarily caused by rapid snowmelt and heavy rains, and rising water levels in the St. Lawrence River, the International Lake Ontario-St. Lawrence River Board began to apply the F limit on April 16, 2026, as prescribed by Regulation Plan 2014.The F limit sets maximum outflows from Lake Ontario based on the level of Lake St. Louis (measured at Pointe Claire) to limit downstream flooding. As of April 30, the second tier of the F limit is in effect. As Lake Ontario levels increase, the corresponding target level at Lake St. Louis also rises, allowing higher outflows in consideration of high-water impacts both upstream on Lake Ontario and the upper St. Lawrence River, and downstream on the lower St. Lawrence River.

Strong winds, when coupled with even moderately high-water levels, can result in localized flooding of vulnerable areas along the Lake Ontario shoreline as well as downstream along the St. Lawrence River, including Lake St. Louis and Lake St. Pierre.

The International Lake Ontario-St. Lawrence River Board is monitoring the situation.

• Outflow from Lake Erie into Lake Ontario remains slightly above long-term average and below 2024 and 2025 levels.
• Lake Ontario outflow adjustments will continue frequently in accordance with Regulation Plan 2014 which considers both upstream and downstream conditions. The Board is authorized to deviate from the Plan if criterion H14 levels are met. H14 trigger water levels vary throughout the year. The current H14 trigger level is 75.53 m (247.80 feet).
• Lake Ontario typically reaches peak water levels in June, after spring precipitation has subsided, and then begins its seasonal decline.
  • If weather conditions are near average, Lake Ontario is forecasted to reach a seasonal peak between 75.40 m (247.38 feet) and 75.50 m (247.70 feet) in May or June.
  • However, higher water levels are possible if weather conditions are wetter than average. Long-term water levels are difficult to predict, much like weather forecasts.
  • Lake Ontario has reached 75.30 m (247.05 feet) or higher in 33 percent of all years since 1960. While higher than average, this water level is not unusual.

Community members should continue to monitor conditions in their local area. Emergency response is typically provided through local municipalities.

For more information:

• Water levels (updated every weekday): https://ijc.org/en/loslrb/watershed/water-levels
• Flows (updated every weekday): https://ijc.org/en/loslrb/watershed/flows
• Lake Ontario forecast (updated on Thursdays): https://ijc.org/en/loslrb/watershed/forecasts
• Lake Ontario outflow changes (updated in real time as flow changes occur): https://ijc.org/en/loslrb/watershed/outflow-changes#flow_table
• Plan 2014 F limit: https://ijc.org/en/loslrb/watershed/faq/4#4-8-2
• Criterion H14 triggers authorizing deviations: https://ijc.org/en/loslrb/watershed/faq/4#4-8-1
• Ottawa River conditions and forecast (updated daily during freshet): https://ottawariver.ca/

Contacts:

Canada: [email protected]

United States: [email protected]

The International Lake Ontario-St. Lawrence River Board ensures that outflows from Lake Ontario meet the requirements of the International Joint Commission's Orders of Approval. Under any regulation plan, the ability to regulate the outflow from Lake Ontario does not mean that full control of lake levels is possible. This is because the major factors affecting water supply to the Great Lakes, precipitation, evaporation, and runoff cannot be controlled, and are difficult to accurately predict.