LEVELnews: Great Lakes and St. Lawrence River water levels, May 2023

Lake Superior experiences extremely wet April

During April, the Great Lakes Basin experienced the following:

• The mean monthly water levels of all the Great Lakes were above average. Of particular note, Lake Superior experienced its ninth highest average water levels for April and its fifth highest beginning of May water level.
• Lake Superior experienced its third highest April water supply conditions (a combination of the precipitation, evaporation, and runoff) on record, while Lake Michigan-Huron also experienced very wet conditions. Lake Erie experienced dry condition, whereas, Lake Ontario experienced wetter than average water supply conditions.
• April precipitation amounts over Lake Superior were one of its highest on record. Lake Michigan Huron experienced higher than average precipitation, while, Lakes Erie and Ontario saw precipitation closer to average.
• Due to the very wet conditions in the basin, Lake Superior experienced its second highest April water level rise on record, while Lake Michigan-Huron’s level rose more than average. Lake Erie experienced a less than average April rise, whereas, Lake Ontario’s level rose approximately 40% above average for the month of April.

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

With water levels remaining above average in all lakes, and the possibility of large storms and winds, low-lying areas are at risk for accelerated coastline erosion and flooding. For current information and forecasts, please refer to the sources listed below.

Later in this issue, we summarize the seasonal Great Lakes level declines over this past season.

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^a 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.

^a Lake level changes are based on the differences in levels at the beginning of the month and not the monthly average levels.

^a At the beginning of May, all of the Great Lakes were at least 40 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.

^b 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 is currently well above its average level and is expected to remain so under most water supply conditions. If there are very wet water supply conditions, lake levels could approach record highs in mid to late summer, while very dry conditions could result in lake levels approaching average.

Lake Michigan-Huron is expected to remain above average under most water supply conditions; it would take very dry conditions to bring the level below average by the end of the summer.

Lake Erie is also expected to stay above average under most water supply scenarios.

Lake Ontario is above average and is expected to remain so under typical water supply conditions within the next few months. Water levels could increase further above average if there are wetter than average water supply conditions or move below average if there are drier than average 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.

^a As a percentage of long-term average (LTA).

^b Environment and Climate Change Canada – Canadian Precipitation Analysis System

^c <5% extremely wet; <25% very wet; <45% wet; 45-55% average; >55% dry; >75% very dry; >95% extremely dry.

^d Please refer to the LEVELnews What is net basin supply for a description of net basin supply.

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

Summary of the 2022/23 seasonal decline

By this time of year, all the lakes have typically completed their seasonal declines since last summer or fall. We can now compare these to their averages.

Lake Superior’s seasonal decline was less than average at 22 cm. Its record seasonal decline occurred in 1939, when it decreased by 58 cm. Lake Superior levels remain well above average with the end of the seasonal decline, resulting in levels much higher than last year but close to those in 2021.

Lake Michigan-Huron declined approximately one third more than its average. This greater than average seasonal decline saw lake levels reach their lowest level since 2014. The record seasonal decline for Lake Michigan-Huron was 79 cm, which occurred in 1977.

Lake Erie saw a slightly higher than average seasonal decline and finished the season with lake levels similar to this time last year. Lake Erie experienced its record decline in 1920, when lake levels decreased by 99 cm.

Lake Ontario experienced a seasonal decline about one third more than average, bringing levels to their lowest since 2013.The previous record high seasonal decline was 119 cm, back in 1998.

Even with greater than average seasonal declines for most lakes, all remain above or well above average. 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

With water levels remaining high on some lakes, there is a high risk of flooding. 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.

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

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 (army.mil). The daily average water level is an average taken from several 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 at the Government of Canada Great Lakes Water Level Gauging Stations Map. 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: Visit the Great Lakes water levels and related data webpage under the “Wave and wind data” heading for marine forecasts and wave heights. Current marine forecasts for Lakes Superior, Huron, Erie and Ontario are available as well as text bulletins of recent wave height forecasts.

For more information:

Frank Seglenieks (Editor) and Nicole O’Brien
Boundary Water Issues
Meteorological Service Canada
Environment and Climate Change Canada
Burlington, ON L7S 1A1
Email: LEVELnews-infoNIVEAU@ec.gc.ca