New Report on 2017 Lake Ontario and St. Lawrence River Flooding


report released today by the International Lake Ontario-St. Lawrence River Board (the Board) provides a detailed account of the historic 2017 flood, and the Board’s actions under Plan 2014 to regulate Lake Ontario outflows in the face of high inflows and record-breaking precipitation. 

The report finds that extreme weather and water supply conditions were the primary factors that caused Lake Ontario and St. Lawrence River water levels to rise to record breaking levels last year.  It also finds that Plan 2014 did not cause or exacerbate the devastating floods and associated damages that occurred in 2017.  Under extremely wet weather, the rules of Plan 2014 essentially reflect how the Board operated under the former regulation plan. 

During 2017 Lake Ontario outflows were largely dictated by extreme conditions, including upstream and downstream flooding, highly variable ice formation, and hazardous high flows in the navigation channel.  Throughout the spring, the Board was releasing water from a flooding Lake Ontario into a flooded St. Lawrence River.  The Board would have faced these conditions under the old regulation plan, and outflows would have been very similar.  

The Board recognizes that for many residents and businesses on the shores of Lake Ontario and the St. Lawrence River, the flooding, erosion and other coastal impacts that occurred in 2017 were devastating.  By providing this comprehensive review, the Board hopes to add clarity and transparency to how its outflow regulation under Plan 2014 took the needs of all interests into account and reduced the upstream and downstream impacts to the extent possible. 

While regulating Lake Ontario outflows can help reduce flood damages, it cannot prevent major floods from occurring under extreme water supply conditions.  The Board hopes this report will increase public understanding of that reality.  To reduce future flood damages, shoreline communities must become more resilient, particularly since climate change increases the uncertainty regarding future high water events and their frequency. 

Watch this video to learn more about the Causes of the 2017 Lake Ontario-St. Lawrence River Flood.



Rob Caldwell: (613) 938-5864; 

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 75.88 m (248.95 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.