International Lake Ontario - St. Lawrence River Board

High Water Levels in the 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 their 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 cm (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 m (73.3 ft).  

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 2017, Lake Ontario was 75.47 m (247.60 ft), 53 cm (20.9 inches) above its long-term average level for this time of year. The level at Lake St. Lawrence was about 10 cm (3.9 inches) above average, while the level at Lake St. Louis is about 22.33 m (73.3 ft). At Montreal Harbour, the level is 89 cm (35.0 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 (English), and more detailed information is available on its website at


Gail R. Faveri: (905) 336-6007;

Arun Heer: (513) 684-6202;


The International Lake Ontario – St. Lawrence River Board specifies the outflows from Lake Ontario, according to Plan 2014 as required in the 2016 Supplementary Order from the International Joint Commission. This plan was agreed to by the United States and Canada in December 2016 in an effort to improve environmental performance while maintaining most of the benefits provided to other interests by the previous Plan 1958-D, which was in use since 1963. In determining outflows, the Board, in conjunction with its staff, pays close attention to water levels in the Lake Ontario-St. Lawrence River system and on the Great Lakes upstream, and to the effects on stakeholders within the basin . 

Water levels vary from year-to-year and throughout the year depending on weather and water supply conditions.  Such variations benefit coastal wetlands and are critical to a healthy lake environment, but may at times and depending on individual circumstances increase the vulnerability of shoreline structures and reduce opportunities for recreational boating activities.  The Board urges everyone to be prepared to live within the full range of levels that have occurred in the past and of those that may occur in the future.  Based on historical observations and projected future conditions, at a minimum, Lake Ontario water levels are expected to range from a high of 75.73 m (248.5 ft.) to a low of 73.56 m (241.3 ft.) at infrequent intervals.  However, it is also recognized that future climate conditions are uncertain, and more extreme water levels may be reached and these extremes may occur more often.  Levels on the St. Lawrence River tend to vary more widely than on Lake Ontario.  Also, these levels do not include the varying local effects of strong winds and wave action that significantly increase or decrease local water levels on both the lake and river, with temporary changes of over half a meter (two feet) possible in some locations. 

For more information, please see the Board’s website ( and Facebook page ( To receive a weekly email about water levels and flows in the Lake Ontario–St. Lawrence River system, please send a blank e-mail message to, with the word ’subscribe’ in the title and body of your message.