Moses-Saunders Dam

St. Lawrence River Ice Formation Begins, Lake Ontario Outflow Reduced


Cold weather has caused ice formation to start at critical areas on the St. Lawrence River, requiring Lake Ontario outflows to be temporarily reduced in order to reduce the risk of ice jams. 

Cold temperatures yesterday and overnight caused water temperatures to plunge along the St. Lawrence River.  With milder temperatures forecast later this weekend, high outflows from Lake Ontario were maintained and slightly increased yesterday, in an attempt to break up and discharge any ice expected to form at the Beauharnois Canal overnight.  However, despite this, the first ice boom at Beauharnois had filled with ice by this morning.  Ice booms help facilitate ice formation, which prevents major ice flows and ice jams from occurring. 

Outflow reductions from Moses-Saunders Dam will be made today to reduce stress on the ice cover downstream at Beauharnois. Hydro-Quebec is already passing higher flows through the north channel that runs parallel to the Beauharnois Canal in order to limit the reductions required and ensure outflows can remain as high as possible. 

While Lake Ontario remains above its long-term average, regulated outflows will remain as high as feasible based on river conditions, and will be increased again as soon and as rapidly as possible.  Once a solid ice formation is established, higher outflows will be passed under the solid ice cover.  

As a result of the Lake Ontario outflow reductions, residents around Lake St. Lawrence, immediately upstream of Moses-Saunders Dam, are being warned to expect increasing water levels over the next few days. 

As water supplies to Lake Ontario remain very high, Lake Ontario levels are also expected to begin slowly rising.  The Board will be monitoring conditions closely and adjusting flows with the intention of passing the highest possible outflows, while ensuring ice conditions are maintained and do not result in safety concerns.

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 that can damage infrastructure and cause severe flooding, by slowing down the current.  Slower currents keep ice at or near the surface, and, weather permitting, helps them form into large pans that accumulate, solidify and build a stable ice cover.  Once established, outflows can be safely increased to pass under the solid ice cover, allowing higher outflows later in winter.

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.