International Lake Ontario - St. Lawrence River Board

Board explains recent high water levels


Wondering why water levels near you seem higher than normal this spring?  Indeed, across most of the Lake Ontario – St. Lawrence River system, water levels have been above their long-term average (calculated from records dating from 1918), largely the result of the mild, wet weather experienced over the past several months.   


The International St. Lawrence River Board of Control (Board), in conjunction with its staff, continually monitors the system to allow it to set the outflows from Lake Ontario at the Moses-Saunders Dam on the St. Lawrence River at Cornwall, ON, and Massena, NY. Hydrological conditions are the main driver of water level fluctuations throughout the year, and over the past several months, generally mild and wet conditions, not only in the Lake Ontario and Ottawa River basins, but also upstream as far as Lake Superior, contributed to the above-average water levels seen lately.


On average, about 85 percent of the water entering Lake Ontario comes from Lake Erie by way of the Niagara River. Outflows through the Niagara River are uncontrolled, and with Lake Erie’s level also above its long-term average, the inflow from that lake into Lake Ontario has also been above normal.


The remaining water Lake Ontario receives comes from its own basin, and is composed of precipitation and runoff, minus lake evaporation. Not only did Lake Ontario receive a lot of precipitation this past winter, the mild temperatures meant a lot of it fell as rain, rather than snow, and a lot of the snow that did fall on the lake’s surrounding drainage basin melted quickly, becoming runoff into the lake itself. Mild temperatures can also result in reduced lake evaporation, as this is typically greatest during periods of cold, dry arctic air passing over the lake, of which there were few occurrences this past winter.


The mild and wet weather extended to the St. Lawrence River as well. Heavy snow that fell in the Ottawa River basin is melting in April and contributing, along with recent rains, to high freshet flows in that river, a tributary to the St. Lawrence River, joining at Lake St. Louis near Montreal, and raising water levels in this location as a result.


The Board regulates the outflow from Lake Ontario through the St. Lawrence River, and this affects water levels throughout the system. The Board typically follows a regulation plan, known as Plan 1958-D, when setting the outflow, and any deviations from this plan must be done in careful consideration of all stakeholders that may be affected. For example, the Board released outflows above Plan 1958-D at the start of March to remove excess water previously stored on Lake Ontario during the critical ice formation period in late January and February. Then, following the start of the Ottawa River freshet in mid-March, the Lake Ontario outflows were adjusted frequently throughout the latter part of the month  in consideration of the remaining stored water, as well as the relatively high water levels both upstream on Lake Ontario and downstream on the lower St. Lawrence River. The Board uses Regulation Plan 1958-D to determine the outflow by incorporating a number of factors, including climate conditions and water levels on Lake Ontario, inflows from Lake Erie through the Niagara River, and water level conditions on the St. Lawrence River upstream and downstream of the dam.


Lake Ontario is currently above its long-term average level for this time of year. The level on 27 April 2016 was 75.12 m (246.5 feet), 16 cm (6.3 inches) above average. This is well within Lake Ontario’s 1.22-metre (four-foot) regulatory range, being 25 cm (9.8 inches) below the lake’s upper limit, and 97 cm (38.2 inches) above the lake’s lower limit. The level at Lake St. Lawrence was about 24 cm (9.4 inches) below average. Downstream, the level at Lake St. Louis is stable at about 4 cm (1.6 inches) above the flood alert level, and is expected to decline; at Montreal Harbour, the level was 28 cm (11.0 inches) above average.


The higher-than-average water levels in the upstream Great Lakes promise higher-than-average inflows to Lake Ontario. Water levels on Lake Ontario are expected to continue to rise over the next several weeks, increasing to an annual peak, which typically occurs around June, before beginning their seasonal decline. Even with very dry conditions in the next six months, Lake Ontario levels are expected to remain above long-term seasonal averages. Lower St. Lawrence River levels are expected to decline somewhat earlier than this, following the end of the Ottawa River freshet, and with average water supplies, the water levels near Montreal will likely be near the long-term 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 Board specifies the outflows from Lake Ontario, generally according to Plan 1958-D as required in the Orders from the International Joint Commission. This plan was agreed to by the United States and Canada when the dam was constructed. Our website contains much useful information including a Frequently Asked Questions section under the News and Information tab that will further explain the details of Plan 1958-D and our Board. In order to know what outflows to specify, the Board, in conjunction with its staff, pays close attention to water levels in the Lake Ontario-St. Lawrence River system and the upstream Great Lakes. The Board is thus prepared to take action when applicable. Outflow changes are posted to the Board’s Facebook site at and its website at under the Maps and Data tab, Lake Ontario outflow changes subheading.

Water levels on both Lake Ontario and the St. Lawrence River vary considerably from year-to-year depending on the weather conditions. The Board urges everyone to be prepared to live within the full range of levels that have occurred.  Although the Board strives to maintain the range of monthly mean levels of Lake Ontario below the upper limit of 75.37 m (247.3 ft) and above the lower limit (from April through November) of 74.15 m (243.3 ft) specified in the Orders of Approval, since regulation began in 1960, actual monthly levels have ranged from a high of 75.73 m (248.5 ft.) to a low of 73.82 m (242.2 ft) due to climate conditions outside the design range. Levels on the river tend to vary more widely. Furthermore, excessive wind set up and wave action may significantly increase or decrease local levels on both the lake and river. Strong winds can change water levels temporarily by over half a metre (two feet) in some locations.

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.