Water Levels to Decline Throughout the Lake Ontario-St. Lawrence River System with Dry Forecast


Lake Ontario water levels are approaching the seasonal peak. Outflow into the St. Lawrence River is gradually increasing, as Ottawa River flows gradually decrease.  Water levels on Lake Ontario increased the first few days of May due to decreased flows related to rising downstream water levels and a slow-moving wet weather system.  Lake St. Louis water levels have likely peaked and should begin to decline in the next week or two. Further downstream, after reaching their peaks on May 4, the water levels at the Port of Montreal and Lake St. Pierre have been consistently decreasing as the flows from the Ottawa River and other tributaries to the St. Lawrence River decline.  Flooding occurred around Lake St. Pierre last week and some low-lying properties and roads continue to be affected.

Water levels in Lake Ontario and the St. Lawrence River rose quickly and steadily due to the widespread rain throughout the region from April 27 through May 3, combined with outflow reductions in response.  Approximately a month’s worth of rain, 90 mm or 3.5 inches, fell in a matter of days in the heaviest areas.

The gates at Iroquois Dam had been lowered in April by Ontario Power Generation (OPG) to suppress the high-water levels of Lake St. Lawrence (immediately upstream of the Moses-Saunders Dam) while the Lake Ontario outflows were reduced due to the rain event. Now that Lake Ontario outflows are being increased, OPG began raising the gates of Iroquois Dam on May 9.

From May 9 through 11, the Lake Ontario level has remained relatively stable at 75.40 metres (247.4 feet). As of May 11, the level was 39 centimetres (15.4 inches) above average for this time of May, but 41 cm (16.1 in.) below the record high observed on this date in 2017.  The level at Lake St. Louis remains near 22.33 m (73.26 ft.) which is 68 cm (26.8 in.) above average for this time of May, but 17 cm (6.7 in.) below the record high observed on this date in 2017.  Water levels of Lake Ontario and the St. Lawrence River are expected to remain above long-term average but below the record-highs observed in 2017 and 2019.

The International Lake Ontario-St. Lawrence River Board continues to closely monitor the situation, maintain communications with State and Provincial leaders, and update the Board webpages and Facebook page regularly. 

Please continue to monitor conditions in your local area, emergency response is typically provided through your local municipality. 

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.