Lake Ontario Water Levels Increased Due to Recent Rainfall - International Lake Ontario – St. Lawrence River Board to End Deviations


Although July has experienced above average rainfall in the Lake Ontario-St. Lawrence River system, drought conditions have persisted into July in much of the Great Lakes-St. Lawrence River watershed.  The US drought monitor maps continue to show abnormally dry to moderate drought conditions within the basin, both upstream (around Lake Erie and Lake Ontario) and downstream (along the St. Lawrence River):

Also, Ontario’s South Nation Conservation’s Water Response Team upheld a Level 1 Low Water Condition for most of the watershed:

However, precipitation in the recent two weeks has had a positive impact and the wetter conditions have increased Lake Ontario water levels by approximately 7 cm (2.8 in). Normally, Lake Ontario water levels typically have begun the seasonal decline by this time of year. Water levels are currently at 74.80 m (245.41 ft) which is approximately 22 cm (8.7 inches) below long-term average levels for mid-July.  Due to this increase in water levels, the lake has risen above the low water deviation (Criterion H14) threshold prescribed in the regulation plan known as Plan 2014.  This means that the Board’s authority to deviate from Plan-prescribed flows expires this week.  Therefore, the Board will stop current deviations and return to regulation plan flows on July 17.

The return to regulation plan flows means an increase in Lake Ontario outflows. As a result, water levels will decrease on Lake St. Lawrence approximately 15-20 cm (5.9 -7.9 inches) and rise on Lake St. Louis and at the Port of Montreal approximately 10-15 cm (3.9- 5.9 inches). Recreational boaters, anglers, and other users should be aware of these expected water level fluctuations.  Lake Ontario will increase or decrease depending on natural weather conditions; lake levels will increase if there continues to be above average rainfall and decrease if dry conditions return.  The Board will continue to monitor weather forecasts and water supply conditions and will re-evaluate the regulation strategy regularly

 “The above average rainfall the first two weeks of July was unexpected but truly benefited the region”, said Mr. Steve Durrett, United States Co-Chair of the International Lake Ontario-St. Lawrence River Board.  There is an unpredictable natural supply of water for the Great Lakes and it is important to recognize the full range of high and low water levels that have historically occurred within Lake Ontario.

Information on hydrologic conditions, water levels and outflows, including graphics and photos, are available on the Board’s website and posted to the Board’s Facebook page at (English), and more detailed information is available on its website at



Bryce Carmichael:  (513) 418-8562                   

Sarah Lobrichon:  (613) 794-8592

Or by email :

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.