Water levels high in Lake Ontario-St. Lawrence River system

Extreme wet weather in April has resulted in high water levels across the Lake Ontario-St. Lawrence River system. Rainfall has been well above normal across the entire basin, with some areas having received at least twice the average amounts since the start of the month, including the western end of Lake Ontario and large parts of the Ottawa River basin, which flows into the St. Lawrence River upstream of Montreal at Lake St. Louis. The rain, coupled with snowmelt in northern parts of the system, has resulted in increased streamflows, rising water levels and flooding on Lake Ontario, the upper and lower St. Lawrence River, Lake St. Louis and on many inland local tributaries, including the Ottawa River. Lake Ontario rose 39 centimeters (15.4 inches) since the beginning of April and the St. Lawrence River at Lake St. Louis has reached its flood level of 22.33 meters (73.3 feet). 

Flows from the Ottawa River have peaked and are now expected to decline, and flooding in the Montreal area is subsiding, allowing higher outflows from Lake Ontario. Nonetheless, inflows from local streams and Lake Erie remain high, and Lake Ontario levels are expected to continue rising in May—and possibly into June if the wet weather persists. As water levels on Lake Ontario rise, its outflows will increase and high levels downstream at Lake St. Louis will continue. 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 20 years since water levels have been this high (since 1998), higher levels on Lake Ontario and the St. Lawrence River have occurred several times in the past and will occur again in the future.

The current high water levels have not been caused by Plan 2014. Lake and river levels would have been nearly identical this year under the previous regulation plan. Plan 2014 will continue to adjust the Lake Ontario outflows in order to minimize and balance flooding on Lake Ontario and along the St. Lawrence River in the Montreal area.

On 24 April, Lake Ontario was 75.47 meters (247.60 feet), 53 centimeters (20.9 inches) above its long-term average level for this time of year. The level at Lake St. Lawrence was about 10 centimeters (3.9 inches) above average, while the level at Lake St. Louis is about 22.33 meters (73.3 feet). At Montreal Harbor, the level is 89 centimeters (35.0 inches) above average.

The board, in conjunction with its staff, continues to monitor the system. Outflow changes, photos and graphs are posted here and more detailed information is available here.

Maritime Editorial