International Lake Ontario - St. Lawrence River Board

Lake Ontario water levels stable last week


The International Lake Ontario – St. Lawrence River Board assessed current and expected conditions to determine the best outflow release strategy to address high water level and associated impacts throughout the system.  Months of high precipitation produced the highest recorded waters level on Lake Ontario since reliable records began in 1918. The level was stable for several days as a result of drier weather and increased outflows, and barring significant additional rainfall, the Lake Ontario level may have halted its rapid rise.  The Board has been maximizing the outflows from Lake Ontario, and has been balancing water levels upstream and downstream to minimize flood and erosion impacts to the extent possible. 

The Ottawa River flow has continued to decline since reaching its record-breaking peak on 8 May 2017.  As a result, less water is entering the St. Lawrence River at Lake St. Louis (near Montreal), and more outflow from Lake Ontario has been able to be released into the St. Lawrence River, while still keeping levels downstream from exceeding the flood levels experienced since the beginning of May.  

Lake Ontario outflows have now reached 10,200 m3/s (360,200 cfs).  These extremely high outflows are now impacting navigation conditions in the St. Lawrence Seaway between the Thousand Islands and Massena, NY /Cornwall, ON.  With these higher flows, the Board also had to consider not only maximizing relief on riparians upstream and downstream but to also consider the impacts on the commercial shipping industry. To assist the Board, the Seaway agencies have adopted mitigation measures to ensure the safety of ships transiting this section of the St. Lawrence River. If outflows are increased further, unsafe currents would halt shipping, causing a break in a long time scheduled chain of multi-goods transiting between overseas and the Great Lakes and impacting all those industries and jobs that rely on Seaway shipping.

The Board continues to provide maximum relief to shoreline communities while also attempting to a lesser extent to maintain the Seaways safe operations.  If weather conditions remain as expected, the Board will continue to release an outflow of 10,200 m3/s (360,200 cfs); a flow equivalent to the record maximum weekly flows passed in 1993 and 1998, for several weeks in an effort to reduce the high level of Lake Ontario as quickly and safely as possible.

If the drier conditions of the past weeks continue across the Lake Ontario – St. Lawrence River basin, 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 more average water level conditions.  The Board therefore advises continued caution and preparedness for sustained high water levels in the weeks to come.

On 25 May 2017, Lake Ontario was 75.88 m (248.95 ft), 84 cm (33.1 inches) above its long-term average level for this time of year. The level at Lake St. Lawrence was 6 cm (0.x inches) below average, while the level at Lake St. Louis is about 22.52 m (73.88 ft), 106 cm (41.7 inches) above average. At Montreal Harbour, the level is 140 cm (55.1 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 29 May 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 75.87 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.