Lake Ontario-St. Lawrence River Water Levels Update: Winter – Spring 2024

Ottawa – The International Lake Ontario-St. Lawrence River Board is providing this notice to summarize winter conditions and water levels as spring is emerging. Plan 2014 is a set of rules that govern Lake Ontario outflows through the Moses-Saunders Dam located between Massena, New York and Cornwall, Ontario. The purpose of the plan is not to keep water levels at their average (which is impossible), but rather the levels will fluctuate with varying weather-driven water supply conditions while trying to moderate extremely high and low water levels.

The seasonal rise of Lake Ontario will depend on future conditions, which are impossible to accurately predict. The Board will continue to monitor conditions and ensure outflows regulated by Plan 2014 are consistent with Orders of Approval. 

Snowpack

• There was a below-average snowpack in the Lake Ontario – St. Lawrence River basin and the Ottawa River basin in winter 2023/2024.
• It is important to note that the correlation between the snowpack in the Lake Ontario basin and its subsequent spring and summer water levels is very low.
• The reason the correlation is weak is that there are many other factors that affect the spring runoff and which are difficult to estimate or predict, including how frozen the ground is when the snow melts, the moisture content of the soil, how fast the snow melts, and most crucial is how much it rains as the snow is melting and afterwards, which can increase runoff dramatically.
• While the snowpack was below average this winter, recent rainfall has increased runoff and raised water levels. The amount of rain received in the coming weeks will have the largest impact on peak water levels this spring.

Water Levels

• Lake Ontario water levels have been near seasonal long-term average values (within +/-10 cm or 4 inches) so far in 2024.
• Lake St. Lawrence levels are slightly below average, similar to those observed last year. Northeast winds during the first week of April caused a temporary decrease in water levels.
• Water levels in the lower St. Lawrence River (Lake St. Louis and Montreal) are also near average and have risen in recent weeks in response to recent rainfall and Ottawa River flows (Ottawa River Regulation Planning Board – Ottawa River Regulation Planning Board )

Spring Outlook

• Long-term weather forecasts are typically informative up to about 10 days and according to the National Oceanic and Atmospheric Administration, a 10-day forecast has an accuracy of about 50 percent.
• A short-term forecast of five days is accurate approximately 90 percent of the time.
• Due to weather forecast variability, a forecast of Lake Ontario lake-wide average water levels is produced weekly and shows potential, and future, water levels based on recent and expected short-term weather conditions and long-term seasonal patterns.
  • The forecast represents the projected range of water levels that may be expected to occur under potentially wet, average, and dry conditions.
  • The red area in the forecast conditions is known as the cone of uncertainty and the further into the future from the current date, the less certainty exists.
  • Actual water levels will depend primarily on weather and water supplies, and during periods of extreme conditions, may fall outside of the projected range, above or below the red area.
  • This forecast employs an ensemble forecasting methodology, whereby a set of over 100 water supply scenarios based on historical records are run through a binational coordinated regulation and routing model to produce a corresponding set of simulated water levels and flows.
• Recent forecasts indicate a greater chance of water levels remaining below the long-term average vs. being above the long-term average through the next six months. The forecast shows a small probability, should there be very dry conditions, of water levels dropping below the low H-14 criterion in mid-August. If water levels decrease below the low H14 thresholds, the Board would have the authority to consider deviating from Plan 2014.
• Water levels in the Great Lakes and St. Lawrence River vary from year to year and throughout the year depending on weather-driven water supply conditions. Everyone should 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.
• Similar mid-April levels have been experienced on Lake Ontario in recent years since 2000, summarized in the table below. In 2001, the Lake Ontario level continued to rise in small increments and peaked at 75.01 m in mid-July. In 2006, the Lake Ontario level peaked in late July at 75.03 m. Peak levels in 2001 and 2006 are similar to peak level that may occur with average water supply conditions this year.
  • The seasonal rise of Lake Ontario will depend on future conditions, which are impossible to accurately predict. The Board will continue to monitor conditions and ensure outflows regulated by Plan 2014 are consistent with Orders of Approval from the International Joint Commission, as directed by the governments that approved the plan.