Update on Lake Superior Outflows and Expected Conditions - April 2024
The Board expects the total St. Marys River flow in April to be 1,850 m3/s (65,300 cfs), as prescribed by Regulation Plan 2012. Actual hour-to-hour and day-to-day flows may vary depending on hydrologic conditions, as well as variations in flow from the hydropower plants.
The gate setting of the Compensating Works at the head of the St. Marys Rapids will be maintained in April at a setting equivalent to approximately one-half gate fully open (Gates #7 through #10 open 20 cm (7.9 in)) and Gate #1 at its typical setting of 20 cm (7.9 in) open. St. Marys Rapids flow will be approximately 85 m3/s (3,000 cfs) in April.
The MacArthur Lock has been closed for seasonal maintenance and will reopen April 24th. The Poe Lock reopened on March 22nd.
Water level changes over the month of March
Water supply conditions were slightly above average in the Lake Superior basin and slightly below average in the Michigan-Huron basin in March.
Lake Superior remained at its same level as last month, while the seasonal long-term average pattern is for Lake Superior to decline by 1 cm (0.4 in) in March.
Lake Michigan-Huron declined by 1 cm (0.4 in) last month, while the seasonal long-term average pattern is for Lake Michigan-Huron to rise by 5 cm (2.0 in) in March.
Water levels as of the beginning of April
At the beginning of April, the lake-wide average water level of Lake Superior was 2 cm (0.8 in) below the seasonal long-term average (1918-2022) and 20 cm (7.9 in) below the level of a year ago.
At the beginning of April, the lake-wide average water level of Lake Michigan-Huron was 3 cm (1.2 in) above the seasonal long-term average (1918-2022) and 7 cm (2.8 in) below the level of a year ago.
Forecast outlook
Both lakes are expected to rise over the coming month.
If weather and water supply conditions are near average, Lake Superior may rise by approximately 8 cm (3.1 in.) and Lake Michigan-Huron may rise by approximately 10 cm (3.9 in).
If conditions are wetter than average, Lake Superior may rise by as much as 15 cm (5.9 in) and Lake
Michigan-Huron may rise by as much as 19 cm (7.5 in).
If conditions are drier than average, the water level of Lake Superior is expected to rise by approximately 1 cm (0.4 in), and Lake Michigan-Huron is expected to rise by approximately 3 cm (1.2 in).
The International Lake Superior Board of Control is responsible for managing the control works on the St. Marys River and regulating the outflow from Lake Superior into Lake Michigan-Huron. Under any outflow regulation plan, the ability to regulate the flow through the St. Marys River does not mean that full control of the water levels of Lake Superior and Lake Michigan-Huron is possible. This is because the major factors affecting water supply to the Great Lakes (i.e. precipitation, evaporation, and runoff) cannot be controlled, and are difficult to accurately predict. Outflow management cannot eliminate the risk of extreme water levels from occurring during periods of severe weather and water supply conditions. Additional information can be found at the Board’s homepage: https://ijc.org/en/lsbc or on
Facebook at: https://www.facebook.com/InternationalLakeSuperiorBoardOfControl