Carte de Bassin du fleuve Saint-Laurent et Lac Ontario

Lake Ontario outflows increasing again after Ottawa River peaks a second time


Outflows from Lake Ontario have increased once again after they were temporarily reduced after heavy rainfall a week ago caused a second peak in Ottawa River flows, which led to an increase in St. Lawrence River water levels and additional flooding on the St. Lawrence River near Montreal. 

With Ottawa River flows forecasted to decline, Lake Ontario outflow increases will be made to balance high water levels causing flooding near Montreal and downstream in the lower St. Lawrence River, with high water levels causing flooding on Lake Ontario and the upper St. Lawrence River.

Lake Ontario water levels remain below the record highs of 2017, but above the criterion H14 high level that applies this time of year, authorizing the International Lake Ontario – St. Lawrence River Board to deviate from regulation Plan 2014.

High water impacts continue both upstream and downstream, so the Board has decided to continue to adjust outflows according to the F-limit rules of Plan 2014. The F-limit seeks to balance impacts of high levels upstream on Lake Ontario and the upper St. Lawrence, with those downstream on Lake St. Louis and the lower St. Lawrence. 

With the Lake Ontario elevation now surpassing 75.60 meters (248.03 feet), St. Lawrence River levels at Lake St. Louis near Montreal will be maintained around 22.48 meters (73.75 feet). 

A return to relatively drier conditions in the coming days should allow outflows to be increased from Lake Ontario without worsening conditions on the lower St. Lawrence River, but given the level of uncertainty associated with any weather forecast, favorable conditions for further outflow increases could change quickly.

The Board continues to remind people that the main drivers for the high water levels are the extremely high inflows to Lake Ontario from Lake Erie, through the Niagara River, and precipitation, neither of which are controlled.  The Board is constantly monitoring the situation and making use of every opportunity to adjust outflows. The intent is to balance high water levels in the interests of all stakeholders, and to regulate Lake Ontario outflows to provide all possible relief to shoreline property owners and communities both upstream and downstream of the Moses-Saunders Power Dam.

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, (716) 352-8669;

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 ( 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.