International Lake Ontario - St. Lawrence River Board

Challenges of maintaining record outflow for Lake Ontario


At its conference call on Monday 19 June 2017, the International Lake Ontario – St. Lawrence River Board agreed to continue efforts to further reduce high Lake Ontario levels.

The outflow from Lake Ontario was increased from 10,200 m3/s (360,200 cfs) to 10,400 m3/s (367,300 cfs) on 14 June, resulting in the highest flow that has ever been continuously released from Lake Ontario for a sustained period.  St. Lawrence Seaway authorities have imposed significant limits on navigation and taken additional safety precautions for the duration of the higher flow rates.  Recreational boaters on the St. Lawrence River have also been advised of the high outflows and currents.  Water levels on the lower St. Lawrence River near Montreal have also continued to decline despite the increased outflows.  Based on current observations, additional impacts of the higher flow rates are minimal.

The Board has therefore agreed to continue to release a continuous flow of 10,400 m3/s (367,300 cfs) to provide additional relief to all those affected by record-high water levels on Lake Ontario, without worsening the impacts to other stakeholders within the system.  The Board, St. Lawrence Seaway, operators of Moses-Saunders dam and navigation agencies will continue to closely monitor and evaluate the water levels, water supplies and the outflow during this period of extreme conditions.

The level of Lake Ontario has declined 11 cm (4.3 in.) since the peak level of 75.88 m (248.95 ft) last recorded on 29 May.  Water levels downstream on the St. Lawrence River at Lake St. Louis near Montreal have declined 15 cm (5.9 in.) since 12 June.  With the weather conditions forecast over the coming days, water levels are expected to continue to fall, with the rate of decline depending on rainfall.

On 19 June 2017, Lake Ontario was 75.77 m (248.6 ft), 72 cm (28.3 inches) above its long-term average level for this time of year. The level at Lake St. Lawrence was average, while the level at Lake St. Louis was 22.12 m (72.6 ft), 78 cm (30.7 inches) above average. At Montreal Harbour, the level was 83 cm (32.7 inches) above average.  Downstream, the flooding which has caused evacuations around Lake St. Peter is subsiding.

The Board continues to monitor the system and will confer again on 30 June 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 75.88 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 ( 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.