Update on Lake Superior Outflows and Expected Conditions – May 2025
Date

The Board expects the total St. Marys River flow in May to be 1,880 m3/s (66,400 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 will be increased from the current setting equivalent to approximately one-half gate open to a setting equivalent to approximately one gate open, to meet the Plan 2012-prescribed flow in May in consideration of capacity restrictions at the hydropower plants. On Tuesday, May 6, Gates #11 through #14 will be raised to a setting of 23 cm open. Gate #1 will remain at its typical setting which supplies a flow of about 15 m3/s (500 ft3/s) to the channel north of the Fishery Remedial Dike. Gates #7 through #10 will remain 20 cm open. The St. Marys Rapids flow will increase from approximately 85 m3/s to approximately 137 m3/s after the gate adjustments.

Water level changes over the month of April

Water supply conditions were wetter than average in both Lake Superior and Lake Michigan-Huron basins in April.

  • Lake Superior rose by 13 cm (5.1 in) last month while the seasonal long-term average pattern is for Lake Superior to rise by 8 cm (3.1 in) in April.
  • Lake Michigan-Huron rose by 12 cm (4.7 in) last month, and the seasonal long-term average pattern is for Lake Michigan-Huron to rise by 11 cm (4.3 in) in April.

Water levels as of the beginning of May

  • At the beginning of May, the lake-wide average water level of Lake Superior was 10 cm (3.9 in) below the seasonal long-term average (1918-2023) and 8 cm (3.1 in) below the level of a year ago.
  • At the beginning of May, the lake-wide average water level of Lake Michigan-Huron was 12 cm (4.7 in) below the seasonal long-term average (1918-2023) and 18 cm (7.1 in) below the level of a year ago.

Forecast outlook

Water level changes based on water supplies over the month of May.

  • If weather and water supply conditions are near average, Lake Superior may rise by approximately 11 cm (4.3 in) and Lake Michigan-Huron may increase by approximately 8 cm (3.1 in).
  • If conditions are much wetter than average, Lake Superior may increase by approximately 19 cm (7.5 in) and Lake Michigan-Huron may rise by approximately 17 cm (6.7 in).
  • If conditions are much drier than average, the water level of Lake Superior may rise by only 3 cm (1.2 in), and Lake Michigan-Huron may rise by about 1 cm (0.4 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.