Lake Ontario has Decreased Below the Low Water Level Threshold that Allows the International Lake Ontario – St. Lawrence River Board to Implement Deviations


Due to Lake Ontario water levels decreasing and falling below the Criterion H14 threshold this week, the International Lake Ontario – St. Lawrence River Board now has the authority to implement deviations from plan-prescribed flows beginning September 24.  In accordance with the International Joint Commission (IJC) Directive and the Supplementary Order of Approval (December 8, 2016),  the regulation plan needs to be followed until water levels reach any of the defined triggers, in this case low Criterion H14 threshold: “In the event that Lake Ontario levels reach or fall below extremely low levels, the works in the International Rapids Section shall be operated to provide all possible relief to municipal water intakes, navigation and power purposes, upstream and downstream.”  The deviation strategy is authorized by the IJC’s Directive

To date, there have been no known or reported impacts to municipal water intakes or hydropower production.  The St. Lawrence Seaway Management Corporation issued a Notice to Shipping on August 17, 2022 regarding speed restrictions at Galop Island and a Notice to Shipping on September 9, 2022 regarding low water levels on Lake St. Louis. The Seaway Corporations and the Board are closely monitoring lower than average water levels both upstream and downstream of the Moses-Saunders Dam. Given these water level conditions, the Board has directed that Lake Ontario outflows the week beginning September 24 be set 100 cubic metres per second (3,532 cubic feet per second) below the flow specified by Plan 2014.

Lake Ontario water levels fell 1 cm (0.4 inches) below the applicable low Criterion H14 threshold for the end of week. It is important to understand that the threshold limits change weekly and are generally decreasing by 3 cm (1.18 inches) per week at this time of year. Lake Ontario water levels have also been dropping about 3-4 cm (1.18- 1.57 inches) per week which is near the average seasonal decline, and water levels remain within the historical fluctuation range.  The historical average (1918-2021) for this time of year is 74.75 m (245.24 feet).  The historical minimum for this week was recorded in 1934 when the water level measured 73.93 m (242.55 feet). 

The water levels of Lake Ontario and the upper St. Lawrence River are mostly dependent on the amount of water entering the system both from the local basin and the inflow from Lake Erie.  The probabilistic forecast that is updated every week by the Board has consistently shown a likelihood of water levels dropping below average and possibly reaching the low Criterion H14 threshold. As a result, the Board has continued to monitor water levels in anticipation of Lake Ontario water levels decreasing to a point at which the low Criterion H14 threshold would be triggered to allow the International Lake Ontario-St. Lawrence River Board to implement deviations.

Although regulation of the outflows and deviations from the regulation plan may result in small increases to the lake levels in certain situations, the levels over the next 6 months will mostly depend on precipitation, evaporation, and non-controlled inflows from the upper Great Lakes.  There is no method currently available to reliably forecast precipitation more than a few weeks into the future and thus decisions that are made by the Board can only be based on the most up to date current information. 

The Board will continue to monitor conditions and the effects of the deviation strategy closely while simultaneously tracking weather forecasts. 

Quick Facts:

  • The International Lake Ontario - St. Lawrence River Board can influence the outflow from Lake Ontario through implementation of regulation plans and deviations, but this influence is far less significant than the natural factors affecting the water levels.  Regulation strategies that affect water levels on Lake Ontario change water levels by centimeters and inches not by meters and feet.


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The International Lake Ontario-St. Lawrence River Board ensures that outflows from Lake Ontario into the St. Lawrence River 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.