Lake Ontario peaks as outflows continue to remain high


Spring has come and mostly gone in the Lake Ontario – St. Lawrence River system, and much more moderate weather conditions have left water levels throughout the basin well below the extreme highs that occurred last year.

As of 5 June, Lake Ontario’s level was 4 cm (1.6 in.) lower than its 2018 peak value of 75.30 m (247.05 ft) reached  on 22 May, 20 cm (7.9 in.) above average, but 59 cm (23.2 in.) below the level on this date last year, when Lake Ontario was near its record peak of 2017. Lake Ontario has likely reached its seasonal peak this year, and while further rain events may cause lake levels to temporarily stabilize or rise slightly, water levels are expected to generally continue to fall over the summer months.

Lake Ontario outflows continue to be set according to Plan 2014, which continues to respond to the above-average levels of Lake Ontario and inflows from the upper Great Lakes. The amount of water released from Lake Ontario over the past 12-month period (June 2017 through May 2018), is the second-highest amount recorded during such a period since records began in 1900 (only those in 1986-1987 were slightly higher), and is equivalent to almost 15 metres or 50 feet of water drained out of Lake Ontario in the last 12 months.

Recent outflows had been in accordance with the L-limit rules of Plan 2014 that prescribe the maximum flows that can be released for a given Lake Ontario level while maintaining safe conditions for navigation in the upper section of the St. Lawrence River. Flows continue to be well-above average and are presently the sixth- highest outflow recorded on this date since 1960. On Sunday, 3 June, outflows were reduced dramatically by as much as 16 percent to facilitate the successful refloating of a grounded tanker, the Chem Norma, near Morrisburg, ON. By the time the ship was freed, the flow reductions had raised Lake St. Lawrence levels by up to 30 cm (1 ft.) in the vicinity of the vessel. In so doing, 1 cm (0.4 in.) of water was temporarily stored on Lake Ontario. This small amount of water is already in the process of being removed from Lake Ontario now that the ship is no longer stuck.         

Prior to and following the temporary flow changes over this past weekend, and in response to the falling Lake Ontario levels and high outflows, Lake St. Lawrence’s level has generally been well-below average for this time of year. The current Lake St. Lawrence level is 73.34 m (240.62 ft), which is 31 cm (12.2 in.) below average. Lake St. Lawrence levels are expected to continue to drop slowly in coming weeks.

Ottawa River flows into the lower St. Lawrence River at and near Lake St. Louis have fallen recently after peaking on 10 May. They are now near seasonal averages and expected to generally continue declining into the summer. Lake St. Louis’ levels have generally declined since the final week of May as Ottawa River flows continue to drop. The current level is 21.93 m (71.95 ft), which is 52 cm (20.5 in.) above average.

The Board wishes to remind readers that weather and hydrologic conditions play a more predominant role than water regulation in influencing water levels, and extreme conditions may occur at any time in any given year. During extreme rainfall events, water levels can rise quickly throughout the Lake Ontario – St. Lawrence River system and may be amplified by wind forces. The Board can do little to prevent this occurrence through outflow regulation, so it urges communities to stay in contact with their local government officials and emergency managers and make arrangement to identify an emergency action plan to protect their property should these conditions occur. Likewise, low water level impacts can also occur during periods of dry conditions and severe droughts, so shoreline property and business owners, recreational boaters and local government officials, should always be prepared for the full range of water levels on Lake Ontario and the St. Lawrence River in the future.

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



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 stlaw-L-subscribe@cciw.cawith the word ’subscribe’ in the title and body of your message.