LEVELnews: Great Lakes and St. Lawrence River water levels, May 2025

¹ Water levels are referenced to International Great Lakes (Vertical) Datum 1985 (IGLD85). For more information, please visit International Great Lakes Datum Update – Great Lakes Coordinating Committee.

Typically, all the lakes continue their seasonal rise at this time of year as we head into the summer.

For current information and forecasts, please refer to the sources listed at the end of this newsletter.

The dry conditions last fall caused greater than average seasonal lake level declines on all lakes. Find more details below.

¹ Lake level changes are based on the differences in levels at the beginning of the month, not the monthly average levels.

¹ At the beginning of May, all the Great Lakes except Lake Superior were at least 30 cm above their chart datum level. Lake Superior started May 2 cm below chart datum. Chart datum is a reference elevation for each lake that provides more information on the depth of water for safe boat navigation on the lakes. For more information, please visit Low Water Datum – Great Lakes Coordinating Committee.
² Water levels are referenced to International Great Lakes (Vertical) Datum 1985 (IGLD85). For more information, please visit International Great Lakes Datum Update – Great Lakes Coordinating Committee.

Water levels forecast

Lake Superior ended the month below its average water level and is expected to remain so under most water supply conditions. Lake Superior may rise above average if it receives very wet water supply conditions over the next 6 months.

Lake Michigan-Huron ended the month below its average water level and is expected to remain so under most water supply conditions. Like Lake Superior, Lake Michigan-Huron may rise slightly above average if it receives very wet water supply conditions over the next 6 months.

Lake Erie ended the month above average and is expected to remain close to average if the lake receives average water supply conditions over the next 6 months.

While water levels remain below average in Lake Ontario, forecasts indicate that under average water supply conditions, water levels are expected to return to above-average by August. Wetter-than-average conditions could lead to even higher levels over the next few months.

For more information on the probable range of water levels, consult the LEVELnews note on projections.

For a graphical representation of recent and forecasted water levels on the Great Lakes, refer to the Canadian Hydrographic Service’s monthly water levels bulletin.

¹ As a percentage of the long-term average (LTA).
² Environment and Climate Change Canada – Canadian Precipitation Analysis System. For more information, please visit: Technical documentation: Regional Deterministic Precipitation Analysis.
³ <5% extremely wet; <25% very wet; <45% wet; 45-55% average; >55% dry; >75% very dry; >95% extremely dry.
⁴ Please refer to the LEVELnews – What is net basin supply for a description of net basin supply.

Note: The information contained in this report is provisional and is subject to change. Data are calculated from the best available observations at the time of posting.

Summary of the 2024/25 seasonal decline

By this time of year, all the lakes have typically completed their seasonal declines, which typically begin in late summer or fall (2024). We can now compare these to their averages.

Lake Superior’s seasonal decline of 47 cm was higher than the 1918-2023 average of 34 cm. Its record seasonal decline occurred in 1939, when it decreased by 58 cm. The lake was slightly above average when the 2024/25 seasonal decline started, however the larger than average seasonal decline this year resulted in lake levels remaining below average.

Lake Michigan-Huron experienced a greater than average seasonal decline. Lake levels were above average when starting its seasonal decline, dropping below average in late September 2024 and remaining below average today. The record seasonal decline for Lake Michigan-Huron was 79 cm, which occurred in 1977.

Lake Erie also experienced a greater than average seasonal decline but with lake levels remaining above average, only dropping slightly below average in February 2025. Lake Erie experienced its record seasonal decline in 1920, when lake levels decreased by 99 cm.

Lake Ontario experienced a close to average seasonal decline, dropping slightly below average in October 2024 and remaining below average for the remainder of the season. The previous record high seasonal decline was 119 cm, back in 1998.

It is important to note that lake levels are always fluctuating and very often just pass through average levels on their way up or down, rather than staying at average levels for long periods of time.

Flood information

Great Lakes water levels are difficult to predict weeks in advance due to natural variations in weather. To stay informed about Great Lakes water levels and flooding, visit the Ontario flood forecasting and warning program website.

Additional information can also be found on the International Lake Superior Board of Control website and the International Lake Ontario–St. Lawrence River Board website.

Information on current water levels and marine forecasts

Monthly levels: A monthly water level bulletin, produced by Fisheries and Oceans Canada, is available at: Monthly Water Level Bulletin for the Great Lakes and Montréal Harbour. On this page you will find a link to the full bulletin PDF. This publication is intended to complement the information provided by LEVELnews on a monthly basis.

Daily levels: Current daily lake-wide average levels of all the Great Lakes are available at Great Lakes – Daily Water Levels for This Month in Meters. The daily average water level is an average taken from a number of gauges across each lake and is a good indicator of the overall lake level when it is changing relatively rapidly due to recent high precipitation.

Hourly levels: Hourly lake levels from individual gauge sites can be found on the Government of Canada Great Lakes Water Level Gauging Stations website. These levels are useful for determining real-time water levels at a given site, however, it should be noted that they are subject to local, temporary effects on water levels such as wind and waves.

Marine forecasts: A link to current Government of Canada marine forecasts for wave heights for each of the Great Lakes can be found on the Great Lakes water levels and related data website under the “Wave and wind data” heading. Current marine forecasts for Lakes Superior, Huron, Erie and Ontario are available by clicking on the link of the lake in which you are interested. To view a text bulletin of recent wave height forecasts for all the Great Lakes, click on the “Text bulletin wave height forecasts for the Great Lakes and St. Lawrence River” link.