International Lake Ontario - St. Lawrence River Board

Drier conditions, record-high outflows starting to bring relief to Lake Ontario


A stretch of mild, dry weather has accelerated the decline in Lake Ontario water levels in recent weeks, bringing some welcome signs of relief to many of those impacted by wide-spread flooding and coastal damages across the Lake Ontario – St. Lawrence River basin this year. Nonetheless, high levels remain a concern, and the Board continues to maximize outflows with the goal of continuing to reduce Lake Ontario as quickly and safely as possible.

The Board met on 31 August 2017 to assess current conditions and hydrologic forecasts, and review ongoing impacts of high water levels and flows throughout the Lake Ontario – St. Lawrence River system. Lake Ontario fell 35 cm (13.8 inches) over the past month, the greatest decline for the month of August since records began in 1918. Lake Ontario is 70 cm (27.6 in.) below the peak level recorded earlier this spring and 30 cm (11.8 in) below the highest levels previously recorded at this time of year in 1947 as of 6 September. Outflows were 9220 m3/s (325,600 cfs) during the last week until Saturday, 9 September 2017, at midnight, when they will be lowered to 8960 m3/s (316,400 cfs). Despite the decrease, this will be a near record outflow for this time of year, as the Board continues to maximize outflows to further reduce Lake Ontario levels.

Gradual reductions in outflows will continue to be required as Lake Ontario declines in order to balance the impacts to navigation and other interests in the upper St. Lawrence River. The high outflows and falling lake levels together are continuing to cause low levels on Lake St. Lawrence (near Cornwall, Ontario and Massena, NY), the lowest at this time of year since 1998, and levels here are likely to continue declining into the fall. A short term flow reduction scheduled over one weekend in mid-October may be considered to assist with boat haul-outs on Lake St. Lawrence. As Lake Ontario drops, this also results in increased currents and potentially hazardous navigation conditions for all in the international section of the St. Lawrence River. Further downstream, levels of Lake St. Louis (near Montreal) continue near record highs for this time of year but remain below flood levels.

Thus, the Board will continue to maximize outflows in consideration of impacts throughout the system. Water levels are expected to continue to decline rapidly into the fall, and the Board, in conjunction with its staff, will continue to monitor and reassess conditions on an ongoing basis.

Information on hydrologic conditions, water levels and outflows are posted to the Board’s Facebook page and additional information is available on its website at

Gail 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 stlaw-L-subscribe@cciw.cawith the word ’subscribe’ in the title and body of your message.