Exceptionally wet weather in April has resulted in rapidly rising water levels throughout the Lake Ontario – St. Lawrence River system.
Precipitation was well-above average across the Lake Ontario – St. Lawrence River basin last month, with some areas receiving more than twice their normal April amounts. As a result, according to provisional data, last month will rank as one of the top three wettest April’s on record (since 1900) in terms of total amount of water flowing into Lake Ontario, as the net effects of precipitation and runoff combined with relatively high inflows entering Lake Ontario from Lake Erie.
The wet conditions caused Lake Ontario to rise 44 centimeters (17.3 inches) last month. This represents the third largest increase recorded for the month of April since 1918, just 6 centimeters (2.4 inches) less than the record rise of 50 centimeters (19.7 inches) set in 2014. At the end of April, Lake Ontario’s lake-wide average water level was 75.52 meters (247.77 feet). This is the sixth highest Lake Ontario level recorded at this time of year since 1918, and the highest since 1993.
The water level of Lake Ontario is now at (or above) the International Joint Commission’s upper trigger level that applies at this time of year. Therefore, the Board is now operating under what the IJC defines as criterion H14, which directs the Board to set releases to provide all possible relief to riparians living along the shorelines of the entire Lake Ontario – St. Lawrence River system, balancing water levels upstream and downstream to minimize flood and erosion impacts to the extent possible.
Nonetheless, the Board recognizes that flooding and erosion are occurring both on Lake Ontario and downstream in the St. Lawrence River system. The exceptionally wet conditions in April resulted in record high flows from the Ottawa River, which discharges into the St. Lawrence River just upstream of Montreal at Lake St. Louis. Levels there have also risen dramatically this spring, and have reached levels not seen since 1993.
As a result, the Board is continuing to balance the high levels both upstream and downstream using the guidance provided by Plan 2014, which specifies that when Lake Ontario rises to 75.50 meters (247.7 feet) that its outflows be increased such that levels at Lake St. Louis are increased and maintained at 22.40 meters (73.5 feet), which the Board recognizes is above a flood level at this location. Given it appears very likely that Lake Ontario will remain at or above 75.50 meters (247.7 feet) for some time, Lake St. Louis levels will be maintained at 22.40 meters (73.5 feet) for the foreseeable future.
Coastal jurisdictions should prepare for the possibility of major coastal flooding as storms frequently occur at this time of year. Though it has been nearly 25 years since water levels have been this high (since 1993), higher levels on Lake Ontario and the St. Lawrence River have occurred several times in the past and will occur again in the future.
These high water levels are a result of exceptionally wet conditions and high water supplies, and would have occurred under any regulation plan. Lake and river levels would have been nearly identical this year under the previous regulation plan. Since the Board is now operating under criterion H14, outflows from Lake Ontario will continue to minimize and balance flooding on Lake Ontario and along the St. Lawrence River in the Montreal area.
On May 1, Lake Ontario hit 55 centimeters (21.7 inches) above its long-term average level for this time of year. The level at Lake St. Lawrence, immediately upstream of Moses Saunders Dam in Cornwall, Ontario and Massena, New York, was about 58 centimeter (22.8 inches) below average. Downstream, the St. Lawrence River level at Lake St. Louis is about 74 centimeters (29.1 inches) above average, while at Montreal Harbour the level is 120 centimeters (47.2 inches) above average.
The Board, in conjunction with its staff, continues to monitor the system. Outflow changes, photos and graphs are posted to the Board’s Facebook page at www.facebook.com/InternationalLakeOntarioStLawrenceRiverBoard, and more detailed information is available on its website at http://ijc.org/en_/islrbc .
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