Lake Ontario-St. Lawrence River Water Level Conditions – August 2025

Ottawa – The International Lake Ontario-St. The Lawrence River Board is providing this notice to summarize current water level conditions. Plan 2014 is a set of rules that govern Lake Ontario outflows through the Moses-Saunders Dam located between Massena, New York and Cornwall, Ontario.

Water Level Conditions – Lake Ontario-St. Lawrence River Basin

• Overall, water levels in Lake Ontario have been near the seasonal long-term average (1918-2022) throughout summer (June, July, and August) 2025 (within +/-5 cm or 2 inches).
• Water levels in the upper St. Lawrence River have generally been above the seasonal long-term average (1960-2022) throughout summer (June, July, and August) 2025 (ranging from 5 cm (2 inches) below to 22 cm (9 inches) above average). Water levels in the upper St. Lawrence River are influenced by both the water level and outflow of Lake Ontario – lower outflow results in higher water levels. The Lake Ontario outflow has declined through the summer, and outflow has not been this low at this time of year since August 2012. As a result of the lower outflow, water levels on the upper St. Lawrence River have remained stable rather than declining as they normally do at this time of year.
• Water levels in the lower St. Lawrence River at Lake St. Louis and Montreal Harbour steadily declined from mid-July through mid-August, as outflows from Lake Ontario and the Ottawa River decreased. The last time water levels were this low in the lower St. Lawrence River in mid-August was 2012.

Drought Conditions

• As of July 31, the Canadian Drought Monitor shows the Lake Ontario and upper St. Lawrence River regions are experiencing a moderate drought, and the lower St. Lawrence River is experiencing abnormally dry conditions.
• For the period of August 6 through 12, the U.S. Drought Monitor shows moderate drought conditions in western Lake Ontario and abnormally dry conditions throughout the central and eastern portions of Lake Ontario through the upper St. Lawrence River.
• Localized conditions may vary as compared to the national drought monitor summary map.

The Board will continue to monitor conditions and ensure outflows are regulated according to the rules of Plan 2014 and are consistent with the Orders of Approval. Specifically, water levels of Lake St. Louis (measured at Pointe Claire) will be monitored in accordance with the M-Limit rules, which maintain minimum water levels on Lake St. Louis in consideration of Lake Ontario, primarily for commercial navigation. Should the M-Limit rules take effect, there would be day-to-day adjustments of Lake Ontario outflows to maintain a specified minimum water level on Lake St. Louis in accordance with Table B2 of the Compendium Report.

The Board provides a weekly forecast of Lake Ontario lake-wide average water levels that shows potential, future water levels based on recent and expected short-term weather conditions and long-term seasonal patterns.

Forecasts for Montreal Harbour predict that water levels could remain below the long-term average through the calendar year 2025.

Overall, water levels in the Great Lakes and St. Lawrence River vary from year to year and throughout the year depending on weather-driven water supply conditions. Everyone should be prepared to live within the full range of levels that have occurred in the past and of those that may occur in the future.

For more information on:

• Lake Ontario Outflow Changes: https://ijc.org/en/loslrb/watershed/outflow-changes
• Water Levels: https://ijc.org/en/loslrb/watershed/water-levels
• Water Supplies: https://ijc.org/en/loslrb/watershed/flows
• Water Level Forecasts:
  • https://ijc.org/en/loslrb/watershed/forecasts
  • https://www.tides.gc.ca/en/monthly-water-level-bulletin-great-lakes-and-montreal-harbour
  • https://e-navigation.canada.ca/topics/water-levels/central/bulletin-en