Moses-Saunders Dam

High Outflows from Lake Ontario to Continue Over Winter; Lake St. Lawrence Levels to Fluctuate Widely in Coming Months


While Lake Ontario remains above long-term average, regulated outflows will remain as high as feasible based on river conditions. As a result, residents around Lake St. Lawrence are being warned to expect both extreme high and low water levels this winter, as the Board implements a winter deviation strategy. The strategy aims to take full advantage of all opportunities to safely increase outflows and reduce the impact of future levels on Lake Ontario. 

High water levels are anticipated when ice begins forming at critical areas of the St. Lawrence River.  When ice begins to form, the International Lake Ontario – St. Lawrence River Board must reduce outflows through Moses-Saunders Dam to help create a stable ice cover on the river.  Flow management during ice formation helps to prevent ice jams in the St. Lawrence River that can restrict flows and cause local flooding.

The reduced outflows cause levels of Lake St. Lawrence, immediately upstream of the dam, to rise temporarily.  The largest and most rapid increases occur closest to the dam, in the area just west of Cornwall, ON and Massena, NY.  These effects are reduced moving upstream and tend to be negligible beyond Iroquois, ON and Lisbon, NY.

With Lake Ontario expected to remain well above average for at least the next several weeks, this will also contribute to higher levels of Lake St. Lawrence this winter.  The Board is therefore advising residents of Lake St. Lawrence to prepare for the potential of unusually high levels this winter.

As ice formation is completed, outflows can normally be safely increased under the ice.  This causes Lake St. Lawrence levels to drop, and this drop is also expected to be greater than normal this winter, as the Board continues to investigate potential opportunities to deviate from Plan 2014 and release higher outflows over the next several months in an attempt to lower the risk of high Lake Ontario levels next spring. 

At present, the timing of ice formation remains uncertain, as it depends on both water and air temperatures, which are highly variable from year-to-year.  The Board will continue to communicate with Lake St. Lawrence residents and communities as winter approaches and the timing and risk of fluctuating water levels become clear. 

Since construction of the St. Lawrence Seaway and Moses-Saunders Dam in the 1950s, it has been necessary to reduce Lake Ontario outflow as ice forms on the St. Lawrence River. This reduces the risk of ice jams by slowing down the current to reduce the forces acting on floating ice pans and any tiny ice particles (frazil ice) forming in the water column.  Slower currents keeps ice at or near the surface, and with weather conditions permitting, facilitates them forming into large pans that accumulate, solidify and build a cover upriver.  Once established, outflows can be safely increased to pass under the stable ice cover, allowing higher outflows later in winter and reducing – but not eliminating – the risk of high Lake Ontario levels in spring.

The Board, in conjunction with its staff, continues to monitor conditions on an ongoing basis. Information on hydrologic conditions, water levels and outflows, including graphics and photos, are available on the Board’s website and posted to the Board’s Facebook page at (English), and more detailed information is available on its website at



Rob Caldwell: (613) 938-5864;

Andrew Kornacki: (716) 879-4349, (716) 352-8669;

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 75.92 m (249.1 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 ( 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.