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

At the beginning of the summer, water levels on Lakes Superior and Michigan–Huron are typically still rising, while Lakes Erie and Ontario have usually reached their peak, however, when they reach their annual peaks can vary from year to year. Learn more about when each lake reaches its highest level below.

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

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 July, all the Great Lakes were at least 16 cm above their chart datum level. 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

At the end of June, Lake Superior was below its average water level and is expected to remain as such 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 was below its average water level at the end of June and is expected to remain as such under most water supply conditions.

Lake Erie ended the month of June above average and is expected to remain close to average if the lake receives average water supply conditions over the next 6 months. However, the level could also end up either above or below average, depending on future water supplies.

Lake Ontario ended the month of June just above average and is expected to remain as such under most water supply conditions.

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.

The time of year that lake levels peak can vary from year to year

Water levels on each of the Great Lakes tend to peak at different times of the year, with Lake Ontario typically reaching its highest level first and Lake Superior last. These variations are influenced by factors such as colder northern temperatures that delay spring snowmelt, as well as seasonal differences in precipitation and evaporation. However, the peak does not occur in the same month every year for each lake. The table below shows the percentage of years in which each lake reached its peak level in each month. For example, while Lake Michigan–Huron most often peaks in July, this only occurs about half the time. There is also a 19% chance of the peak occurring in June and a 17% chance in August, with probabilities dwindling in other months.

While Lake Michigan–Huron typically rises between June and July, it's important to recognize that the lake may have already reached its peak. The timing depends on year-to-year variations in weather conditions and the flow between upstream and downstream lakes. Similarly, although Lake Erie has usually peaked by this point in the season, a series of significant summer storms over the basin could still result in a later peak.

Keep in mind that when it is said a particular lake may have begun its seasonal decline, this is based on average historical patterns. Actual conditions can vary from year to year depending on natural factors such as weather and runoff.

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.