Update on International Lake Ontario – St. Lawrence River Board Deviation Strategy - Water Levels Throughout the Basin will Continue Seasonal Decline


The International Lake Ontario-St. Lawrence River Board has been using its authority to deviate from Plan 2014 since September 2nd to provide higher and more predictable water levels on Lake St. Lawrence and the upper St. Lawrence River. Dry weather conditions have caused water levels on Lake Ontario to decline below the 74.80 m threshold. As of September 30, the “September Rule” will no longer apply.

The Board’s deviation strategy helped slow the seasonal decline of water levels on Lake St. Lawrence and the upper St. Lawrence River. Since September 2nd, water levels of Lake St. Lawrence have been 5 to 20 cm higher than they would have been had the Board not deviated. Conversely, water levels downstream of the Moses-Saunders Dam were 5 to 15 cm lower than they otherwise would have been. The level of Lake Ontario is currently 2 cm higher than it would have been if the Board had not deviated in September.

From September 30 through October 13, the Board will continue to set flows in accordance with the applicable Rule Curve flows prescribed by Plan 2014. Outflows are expected to continue to decrease in the coming weeks, as the level of Lake Ontario continues its seasonal decline.

The Board will not conduct an additional flow reduction in early October for boat haul-out in Lake St. Lawrence. As water levels throughout the system continue their seasonal decline, the Board encourages recreational boaters to monitor the water level forecasts to determine the optimal time to remove their vessels and equipment for the season. The Board also encourages boaters to take wind effects into account. Sustained northeasterly winds cause Lake St. Lawrence water levels to decrease.

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 150 m3/s above the applicable Rule Curve flows, for approximately five-weeks, as conditions permit. The Board’s offsetting deviation strategy may be adjusted depending on weather and water supply conditions.

Lake St. Lawrence is the forebay to the Moses-Saunders Dam, a man-made widening in the St. Lawrence River.  The forebay experiences more immediate water level changes as a result of dam operations.  When outflows at the dam increase, 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.

Lake Ontario water level forecast - October 6, 2023 to March 1, 2024

Lake St. Lawrence water levels are expected to decline more rapidly beginning on October 14 when the Board begins offsetting deviations.

For more information on:


United States: ILOSLRB-USSection@usace.army.mil

Canada: ec.cilofsl-iloslrb.ec@canada.ca

Facebook: https://www.facebook.com/InternationalLakeOntarioStLawrenceRiverBoard

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.