Lake Ontario, St. Lawrence River remain ‘above long-term average’

LAKE ONTARIO – With the Lake Ontario-St. Lawrence River water network flows and levels are expected to remain above average for some time, they will remain below 2017 and 2019 historic highs.

“A wet weather system has remained over the Lake Ontario-St. Lawrence River basin for several days,” the International Lake Ontario-St. Lawrence River Board’s (ILOSLRB) Lynn Greer released in a statement May 4. “The jet stream is expected to finally move the weather system east by late Friday (May 5).”

The persistent rain and runoff from the system continues to cause water levels in Lake Ontario and the St. Lawrence River to rise.

“Lake levels are largely dependent on inflows received from Mother Nature, namely precipitation, which is very unpredictable more than a few days in advance,” Greer said.

Lake Ontario levels have been rising at a rate of approximately one to 1.5 cm (0.4-0.6 in) per day and have reached 75.32 m (247.11 ft) today (May 4) while levels of the St. Lawrence River at Pointe-Claire (Montreal) have risen above 22.25 m (73 ft). Continued wet weather and runoff from the recent rains may result in flooding of vulnerable areas along the Lake Ontario shoreline as well as downstream along the St. Lawrence River, including Lake St. Louis and Lake St. Pierre.

“At this time, the water levels of Lake Ontario and the St. Lawrence River are expected to remain above long-term average but below the record-highs observed in 2017 and 2019, and below the record high observed in Lake Ontario this same week in 1973 (75.71 m, 248.39 ft),” Greer said.

In addition to increased runoff into Lake Ontario, outflows from the Ottawa River and other tributaries into the St. Lawrence River have increased substantially in the past several days due to the widespread major rainfall. 

“The ILOSLRB is closely monitoring the situation,” Greer said. “The Lake Ontario outflow into the St. Lawrence River continues to be adjusted in accordance with Plan 2014. Flow adjustments are made daily in accordance with the F limit which considers both upstream and downstream conditions.”

Updates will be provided next week and as conditions change. Forecasts of Lake Ontario water levels for the next several months are updated weekly at https://ijc.org/en/loslrb/watershed/forecasts

“Please continue to monitor conditions in your local area, emergency response is typically provided through your local municipality,” Greer said.