High water levels prompt continued maximum outflow at Moses-Saunders Dam to accommodate commercial vessels

At its weekly conference call, the International Lake Ontario–St. Lawrence River Board decided June 8 to maintain the outflow from Lake Ontario at 10,200 me3/s (360,200 cfs) again for at least another week to address high water levels and associated impacts throughout the system. This high outflow has been released since May 23 to provide relief to residents along the shorelines of Lake Ontario while not exacerbating the impacts to shoreline residents downstream.

Water levels downstream on the St. Lawrence River near Montreal remain at nearly the same high levels seen since early May due to these high outflows and continued rainfall in the Ottawa and other local tributary basins. Despite rains last week, the water level of Lake Ontario has remained relatively stable for the past three weeks. As of June 6, the level had declined three centimeters (1 inch) from the highest daily level reached as recently as May 29. While the levels are forecast to continue to steadily decline, significant additional rainfall may temporarily halt the declines or even cause short-term rises.

Following the second-wettest April and record-wettest May since reliable records began in 1918, it has been generally drier across the Lake Ontario–St. Lawrence River basin. The first week of June has been drier than the long-term average. If these drier conditions continue, the high Lake Ontario outflows are expected to surpass inflows, at which time Lake Ontario’s water level will decline. However, owing to the huge surface area and large volume of water on Lake Ontario, it will take several weeks to significantly reduce levels and longer to return to the average water level for the time of year. The board therefore advises continued caution, especially when onshore winds are predicted, as high water levels will persist for weeks to come. Considerable debris is floating in the waters posing further risks to boaters.

On June 7, Lake Ontario was 80 centimeters (31.5 inches) above its long-term average level for this time of year. The level at Lake St. Lawrence was 5 centimeters (2 inches) above average, while the level at Lake St. Louis was 110 centimeters (43.3 inches) above average. At Montreal Harbor, the level was 137 centimeters (53.9 inches) above average. Downstream, the flooding which has caused evacuations around Lake St. Peter continues.

The board continues to monitor the system and will confer again on June 12. Outflow changes, photos and graphs are posted to the board’s Facebook page at https://www.facebook.com/InternationalLakeOntarioStLawrenceRiverBoard (English) and more detailed information is available on its website at http://ijc.org/en_/islrbc. 

Maritime Editorial