International Lake Ontario - St. Lawrence River Board

Expect higher Lake Ontario outflows




18 May 2017


Expect Higher Lake Ontario Outflows


This week the International Lake Ontario – St. Lawrence River Board assessed current and expected conditions to determine the best outflow release strategy which would continue to address high water level and associated impacts throughout the system.  The water level of Lake Ontario is the highest it has been since reliable records began in 1918, breaking the previous record set in June 1952.  Lake Ontario is still above the upper regulatory threshold level for this time of year, which allows the Board to continue to maximize the outflows from Lake Ontario to provide all possible relief to riparians living along the shorelines of the entire Lake Ontario – St. Lawrence River system, balancing water levels upstream and downstream to minimize flood and erosion impacts to the extent possible.  The Ottawa River flow has been declining rapidly since reaching its record-breaking peak on 8 May.  With less flow from the Ottawa River entering  Lake St. Louis (near Montreal), higher Lake Ontario outflows in the St. Lawrence River may maintain the high level in Lake St. Louis, which is above its flood level.  


Further increases to outflows should be possible in the coming days and weeks. As outflows rise, navigation conditions worsen in the St. Lawrence River from the Thousand Islands to Massena, NY /Cornwall, ON.  The Board has notified the St. Lawrence Seaway that these exceptional conditions will likely impact shipping.  The Seaway is examining various strategies to ensure the safety of ships transiting this section of the St. Lawrence River. 

Following one of the wettest months of April on record, the first week of May 2017 was the wettest recorded since 1900 in terms of total inflows to Lake Ontario from Lake Erie and from widespread, heavy rainfall on the lake and across the Ontario drainage basin.  If the drier conditions of the past week continue, outflow is expected to surpass inflow, at which time Lake Ontario’s water level will peak and begin to gradually 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 advises continued caution and preparedness for sustained high water levels in the weeks to come.


On 17 May 2017, Lake Ontario was 75.86 m (248.9 ft), 84 cm (33.1 inches) above its long-term average level for this time of year. The level at Lake St. Lawrence was about 2 cm (0.8 inches) below average, while the level at Lake St. Louis is about 22.51 m (73.8 ft), 97 cm (38.2 inches) above average. At Montreal Harbour, the level is 141 cm (55.5 inches) above average.


The Board continues to monitor the system and will confer again on 22 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 75.73 m (248.5 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.