Moses-Saunders Dam

Lake Ontario - St. Lawrence River System: Update and Outlook


The first major spring storm pushed through the Great Lakes – St. Lawrence River basin this past weekend, bringing high winds and precipitation, raising inflows and accelerating the seasonal rise in water levels. 

Strong, southwest winds rapidly increased water levels at the eastern end of Lake Ontario during the storm.  The wind, combined with rising inflows from the Ottawa River and other local tributaries, also raised levels in the lower St. Lawrence River, requiring a temporary reduction in the Lake Ontario outflow. 

However, Lake Ontario outflows have been increasing again since, and are still well above average for this time of year, as the Board continues to get as much water off the lake as possible, while limiting downstream impacts.

Further increases are also expected, as the Ottawa River has reached a peak and is starting to decline.  This is expected to continue over at least the next few days.  Additional information can be found here:

Recent Conditions within the Lake Ontario – St. Lawrence River system:

  • Plan 2014’s Flood Limit (F-limit) continues to be the limiting factor for Lake Ontario outflows, and not the Navigation Limit (L-limit).
  • Lake Ontario’s lake-wide average level is roughly 75.33 m (247.15 ft) currently, which is now slightly below the level at the same time in 2017, and more than 30 cm (1 ft) below the 1973 record-high. 

Future Conditions within the system:

  • Overall Lake Ontario outflows are expected to increase over at least the next few days as Ottawa River flows decline. 
  • Lake Ontario outflow remains high currently, despite recent reductions necessary to address lower St. Lawrence River levels, and outflow will continue to be maximized to the extent possible.
  • The weather is expected to remain cooler and dry for several days.  There are no significant storm systems for at least the next week.

Shoreline impacts were reported across many areas of the Great Lakes during the April 13 storm, due to high winds, compounded by near or above record-high water levels on all but Lake Ontario.  Nonetheless, water levels of Lake Ontario are still well above their seasonal average, and strong winds can still cause significant damage and a surge in water levels.  Communities should continue to invest in long-term coastal resiliency measures to lessen the impact during high and low waters.     

Please note that the Board has created a website page focused on the recent high-water events: (English) [ (French)]. All high-water related materials are now in one easily accessible place. 

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


Rob Caldwell:              (613) 938-5864                          

Andrew Kornacki:       (716) 879-4349             

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 75.92 m (249.1 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 ( 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.