Lake Ontario-St. Lawrence River Water Levels Update – Autumn 2023

Ottawa – The International Lake Ontario-St. Lawrence River Board is providing this notice to summarize the water level conditions from September 2023 through the remainder of Autumn.

1. Plan 2014 is a set of rules that govern Lake Ontario outflows through the Moses-Saunders Dam located near Massena, New York and Cornwall, Ontario. Water levels throughout the Great Lakes have been above their long-term averages since Spring 2023. The above-average level on Lake Ontario at the beginning of September exceeded the threshold that triggers the September Rule within Regulation Plan 2014. The September Rule requires increases in flow through the Moses-Saunders Dam when water levels on Lake Ontario measure above 74.8 m (245.3 ft.) at the beginning of September in an effort to lower Lake Ontario levels to 74.8 m or lower by the end of the year.

• Exercising its authority to implement minor deviations from Plan 2014, the International Lake Ontario-St. Lawrence River Board ignored the flow increases specified by the September Rule of Plan 2014 from September 2nd through September 29 to provide higher and more predictable water levels on Lake St. Lawrence and the upper St. Lawrence River. This deviation slowed the seasonal decline upstream of Moses-Saunders Dam by setting outflows below those prescribed by Plan 2014.
• Water levels of Lake St. Lawrence were 5 to 20 cm higher than they would have been had the Board not deviated.
• Water levels on the St. Lawrence River near Montreal were 5 to 15 cm lower than they otherwise would have been.
• By September 30th, the level of Lake Ontario was 2 cm higher than it would have been if the Board had not deviated.

2. As of September 30, water levels on Lake Ontario had declined to 74.8 meters, the threshold in which the September Rule no longer applied. As required by the IJC’s Directive, water accumulated on Lake Ontario as a result of the deviation from the September Rule must be released. This means outflows through the dam will be increased above the prescribed by Plan 2014 Rule Curve until the amount of water that was held back on Lake Ontario is discharged downstream.

3. However, from September 30 through October 13, the Board continued to set flows in accordance with the flows prescribed by Plan 2014. Following Plan 2014 for two more weeks continued to slow the seasonal decline until after the October holiday weekend in Canada (Thanksgiving) and the United States (Indigenous Peoples Day). The weekend of October 7, 8, and 9 experienced favourable wind conditions that pushed waters east throughout the St. Lawrence River. The deviation that slowed the seasonal decline provided a base water level during the holiday weekend of approximately 73.00 m in Lake St. Lawrence. In years when the Board deviated from the Plan to support boat haul-out efforts, the target water level was 73.10 m. The uncontrolled and unpredictable wind event resulted in higher water levels during the holiday weekend on Lake St. Lawrence as well as Lac St. Louis and the Port of Montreal.

• Water levels at Lake St. Lawrence ranged from 73.06 to 73.32 m.
• Water levels downstream of the Moses-Saunders Dam at Lac St. Louis ranged from 21.24 to 21.30 m.
• Water levels at the Port of Montreal ranged from 6.10 to 6.24 m.

4. Now that the majority of recreational boating has ended for the season, and as required by the IJC’s Directive, the deviation from the September Rule will be completely offset from approximately October 14 through November 17. The Board will set flows around 150 m3/s above the applicable Plan 2014 flows, for an estimated five weeks, as conditions permit. The Board’s offsetting deviation strategy may be adjusted depending on weather and water supply conditions.

5. Lake St. Lawrence is the forebay to the Moses-Saunders Dam, a man-made widening in the St. Lawrence River created because of the dam. Without the forebay, the dam could not operate. Being directly upstream of the hydropower dam, the forebay experiences more immediate and pronounced water level changes because of dam operations. When outflows through the dam are increased, water levels in the Lake St. Lawrence forebay decrease. Module 2 – Lake Ontario-Upper St. Lawrence Rivers Levels and Outflows explains the main causes of water level fluctuations in Lake St. Lawrence upstream of the Moses-Saunders Dam.

6. Please consider responding to a short questionnaire so the Board and Great Lakes–St. Lawrence River Adaptive Management (GLAM) Committee can better understand how changing Great Lakes – St. Lawrence River water levels impact people who live and work along the shoreline as well as the ecosystem. The GLAM Committee has developed a short questionnaire to enable impacted shoreline property owners to report on their direct experiences for 2022 and 2023. Responses to the questionnaire will only be reported in a summarized format to ensure the privacy of respondents and your participation is completely voluntary.

2022 Water Level Impacts Questionnaire – https://www.surveymonkey.com/r/GLAM2022
2023 Water Level Impacts Questionnaire – https://www.surveymonkey.com/r/GLAM2023

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 Level Forecast: https://ijc.org/en/loslrb/watershed/forecasts
Review of Plan 2014 https://www.ijc.org/en/glam/glam-expedited-review-plan-2014-phase-2
Water Levels Dynamics: Module 2 – Lake Ontario-Upper St. Lawrence Rivers Levels and Outflows