International Lake Ontario - St. Lawrence River Board

High outflows keep water levels stable


At its weekly conference call, the International Lake Ontario – St. Lawrence River Board decided to maintain the outflow from Lake Ontario at 10,200 m3/s (360,200 cfs) for at least another week to address high water levels and associated impacts throughout the system. 

Water levels downstream on the St. Lawrence River near Montreal remain at nearly the same high levels seen since early May. Despite more heavy rain last week, the water level of Lake Ontario has remained relatively stable and continues to be within a couple of centimetres (1 inch) of its level  two weeks ago.  

The current outflow is equivalent to the historic maximum weekly average outflow from Lake Ontario which occurred in just two weeks previously, once each during the high water years of 1993 and 1998.  The Board has been maximizing the outflows from Lake Ontario in consideration of balancing water levels upstream and downstream to minimize flood and erosion impacts to the extent possible.  The Board is also considering options that would allow it to increase the outflow further, once downstream conditions permit.

Following the wet conditions in April and record rainfall during the first weeks of May, it has been generally drier across the Lake Ontario – St. Lawrence River basin.  If these drier conditions continue, Lake Ontario outflows are expected to soon surpass inflows, at which time Lake Ontario’s water level will begin to decline. However, owing to the huge surface area and large volume of water on Lake Ontario, it will take several weeks to significantly reduce levels, and longer to return to the average water level for the time of year.  The Board therefore advises continued caution, especially when onshore winds are predicted, as high water levels will persist for the weeks to come. 

On 1 June 2017, Lake Ontario was 75.87 m (248.9 ft), 82 cm (32.3 inches) above its long-term average level for this time of year. The level at Lake St. Lawrence was about 28 cm (11.0 inches) above average, while the level at Lake St. Louis was about 22.49 m (73.8 ft), 108 cm (42.5 inches) above average. At Montreal Harbour, the level was 141 cm (55.5 inches) above average.  Downstream, the flooding which has caused evacuations around Lake St. Peter is prolonged.

The Board continues to monitor the system and will confer again on 5 June 2017. Outflow changes, photos, and graphs are posted to the Board’s Facebook page at (English), and more detailed information is available on its website at .



Gail R. Faveri: (905) 336-6007;

Arun Heer: (513) 684-6202;


The International Lake Ontario – St. Lawrence River Board specifies the outflows from Lake Ontario, according to Plan 2014 as required in the 2016 Supplementary Order from the International Joint Commission. This plan was agreed to by the United States and Canada in December 2016 in an effort to improve environmental performance while maintaining most of the benefits provided to other interests by the previous Plan 1958-D, which was in use since 1963. In determining outflows, the Board, in conjunction with its staff, pays close attention to water levels in the Lake Ontario-St. Lawrence River system and on the Great Lakes upstream, and to the effects on stakeholders within the basin .

Water levels vary from year-to-year and throughout the year depending on weather and water supply conditions.  Such variations benefit coastal wetlands and are critical to a healthy lake environment, but may at times and depending on individual circumstances increase the vulnerability of shoreline structures and reduce opportunities for recreational boating activities.  The Board urges everyone to be prepared to live within the full range of levels that have occurred in the past and of those that may occur in the future.  Based on historical observations and projected future conditions, at a minimum, Lake Ontario water levels are expected to range from a high of 75.88 m (248.9 ft.) to a low of 73.56 m (241.3 ft.) at infrequent intervals.  However, it is also recognized that future climate conditions are uncertain, and more extreme water levels may be reached and these extremes may occur more often.  Levels on the St. Lawrence River tend to vary more widely than on Lake Ontario.  Also, these levels do not include the varying local effects of strong winds and wave action that significantly increase or decrease local water levels on both the lake and river, with temporary changes of over half a meter (two feet) possible in some locations.

For more information, please see the Board’s website ( and Facebook page ( To receive a weekly email about water levels and flows in the Lake Ontario–St. Lawrence River system, please send a blank e-mail message to, with the word ’subscribe’ in the title and body of your message.