International Lake Ontario - St. Lawrence River Board

Board actively managing outflows in response to extreme wet weather, high water levels


Exceptionally wet weather in April has resulted in rapidly rising water levels throughout the Lake Ontario – St. Lawrence River system.  

Precipitation was well-above average across the Lake Ontario – St. Lawrence River basin last month, with some areas receiving more than twice their normal April amounts.  As a result, according to provisional data, last month will rank as one of the top three wettest April’s on record (since 1900) in terms of total amount of water flowing into Lake Ontario, as the net effects of precipitation and runoff combined with relatively high inflows entering Lake Ontario from Lake Erie.   

The wet conditions caused Lake Ontario to rise 44 cm (17.3 inches) last month.  This represents the 3rd largest increase recorded for the month of April since 1918, just 6 cm (2.4 inches) less than the record rise of 50 cm (19.7 inches) set in 2014.  At the end of April, Lake Ontario’s lake-wide average water level was 75.52 m (247.77 ft).  This is the 6th highest Lake Ontario level recorded at this time of year since 1918, and the highest since 1993. 

The water level of Lake Ontario is now at (or above) the International Joint Commission’s upper trigger level that applies at this time of year.   Therefore, the Board is now operating under what the IJC defines as criterion H14, which directs the Board to set releases 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. 

Nonetheless, the Board recognizes that flooding and erosion are occurring both on Lake Ontario and downstream in the St. Lawrence River system.  The exceptionally wet conditions in April resulted in record high flows from the Ottawa River, which discharges into the St. Lawrence River just upstream of Montreal at Lake St. Louis.  Levels there have also risen dramatically this spring, and have reached levels not seen since 1993.  

As a result, the Board is continuing to balance the high levels both upstream and downstream using the guidance provided by Plan 2014, which specifies that when Lake Ontario rises to 75.50 m (247.7 ft.) that its outflows be increased such that levels at Lake St. Louis are increased and maintained at 22.40 m (73.5 ft), which the Board recognizes is above a flood level at this location. Given it appears very likely that Lake Ontario will remain at or above 75.50 m (247.7 ft.) for some time, Lake St. Louis levels will be maintained at 22.40 m (73.5 ft) for the foreseeable future. 

Coastal jurisdictions should prepare for the possibility of major coastal flooding as storms frequently occur at this time of year. Though it has been nearly 25 years since water levels have been this high (since 1993), higher levels on Lake Ontario and the St. Lawrence River have occurred several times in the past and will occur again in the future. These high water levels are a result of exceptionally wet conditions and high water supplies, and would have occurred under any regulation plan. Lake and river levels would have been nearly identical this year under the previous regulation plan. Since the Board is now operating under criterion H14, outflows from Lake Ontario will continue to minimize and balance flooding on Lake Ontario and along the St. Lawrence River in the Montreal area. 

On 1 May 2017, Lake Ontario was 55 cm (21.7 inches) above its long-term average level for this time of year. The level at Lake St. Lawrence, immediately upstream of Moses Saunders Dam in Cornwall, Ontario and Massena, NY, was about 58 cm (22.8 inches) below average.  Downstream, the St. Lawrence River level at Lake St. Louis is about 74 cm (29.1 inches) above average, while at Montreal Harbour the level is 120 cm (47.2 inches) above average. 

The Board, in conjunction with its staff, continues to monitor the system. 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.