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 June 5, 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.
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. Communities should stay in contact with their local government officials and emergency managers and make arrangements to identify an emergency action plan to protect their property should these conditions occur.