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

This is the time of year when all lakes except Lake Superior would typically continue their seasonal rise under average water supplies. Lake Superior historically has a later seasonal rise due to its northerly location and late freshet in comparison to the remainder of the lakes.

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

A few cold snaps resulted in more ice formation on the Great Lakes throughout this past winter than recent years. A review of the ice conditions throughout the 2024-2025 ice season is provided 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 April, all the Great Lakes except Lake Superior, were at least 20 cm above their chart datum level. Lake Superior started April 11 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 slightly 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 below average under most water supply conditions. Similarly to 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 is expected to remain close to average if the lake receives average water supplies conditions over the next 6 months.

Lake Ontario also ended the month below its average water level. Water levels are expected to remain below average under dry or average water supply conditions. However, wetter than average water supply conditions may result in above average lake levels in 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.

Ice conditions on the Great Lakes

The 2024/2025 ice season saw higher ice cover compared to the record lows of 2023/2024. However, ice cover has remained below average for most of the season, with notable increases in late January and late February. As a response to a cold spell in January, Lake Erie’s ice coverage increased from 30% to over 80% in just 4 days, reaching a peak at more than 90%. As of February 25, the ice cover on the Great Lakes was 32%, which is higher than in recent years. The beginning of March saw the ice coverage of the Great Lakes at 27%, which is slightly lower than the average ice cover of 39% for this period. At the start of April, the Great Lakes were mostly ice free. More information on Great Lakes ice conditions can be found on the Canadian Ice Service website.

Each of the Great Lakes is unique in its typical annual ice coverage. Lake Erie typically has the highest ice coverage due to its shallow depth and Lake Ontario generally has the lowest ice cover as it is very deep. Lake Ontario’s depth allows the lake to retain more heat and resist ice formation throughout the winter months. Lake Superior is the deepest of all the Great Lakes but has a much greater average ice cover due to colder average water temperatures from its northern location.

The accompanying figure shows ice cover over the past 2 years in the Great Lakes region. It shows that ice conditions in the Great Lakes were much higher than the previous season, while remaining below average. The figure also shows how much earlier than average the lakes were ice-free. The figure uses data from the Canadian Ice Service.

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 of the Great Lakes, click on the “Text bulletin wave height forecasts for the Great Lakes and St. Lawrence River” link.