International Lake Ontario - St. Lawrence River Board

Maximum outflows being released as levels remain high and winter continues


Several months of maximizing outflows has dropped Lake Ontario at an unprecedented rate and to well below the record peak set at the end of May 2017, yet water levels remain relatively high for this time of year after a cold and wet start to winter. Outflows continue to be maximized in response while making intermittent and temporary reductions to manage challenging ice conditions in several areas of the St. Lawrence River. 

Following last year’s record-wet spring and despite several months of continuing wet weather thereafter, at the start of January, Lake Ontario’s level had fallen 1.12 m (3.7 ft) below its 2017 peak, the largest drop ever recorded from June through the end of December. Extreme cold and snow near the start of 2018 followed by milder temperatures, heavy rain and snowmelt more recently has Lake Ontario’s level rising once again, reaching a level of 74.91 m (245.77 ft) as of 30 January. This is 31 cm (12 inches) above average, although still 28 cm (11.0 in) lower than the record-high for this time of year set in 1952. Lake levels have been higher at this time of the year sixteen times since 1918 (when reliable records began), most recently in 2012.

In response to the high water levels, the Board is still setting outflows from Lake Ontario at the highest rates while managing ice conditions. Recent reductions were required to promote the formation of a solid, stable ice cover in critical sections of the St. Lawrence River, thereafter allowing increased outflow under the ice. Without this stable ice cover, the risk of ice jams is high. These ice jams can clog the river, forcing sudden, significant flow reductions and an increased risk of localized flooding. Outflows are currently 1,730 cubic metres per second above average, 180 cubic metres per second above the previous record-high flow in 1986 and are expected to continue to increase gradually as ice and weather conditions allow. 

While Lake Ontario remains well above average, historically, winter water levels have not provided an accurate indicator of the peak later in spring. Hydrologic conditions have a much greater influence, and while impossible to predict, it is unlikely that last spring's combination of exceptional rainfall, snowmelt and rapidly rising inflows from Lake Erie will all repeat themselves and lead to extremely high water levels again this year. Nonetheless, extreme conditions may occur in any given year, and when they do, the risk of extreme water levels cannot be substantially reduced through regulation of outflows. As a result, shoreline property owners, businesses, and local government officials are advised to always be prepared for the full range of water levels that could occur on Lake Ontario, both highs and lows, now and in the future. 

The Board, in conjunction with its staff, continues to monitor and reassess conditions on an ongoing basis. 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), including this video, which explains why the management of ice conditions on the river is so critical. 



Rob Caldwell: (613) 938-5864;

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 75.88 m (248.95 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.