Update on Lake Superior Outflows and Expected Conditions - July 2025
Date

The Board expects the total St. Marys River flow in July to be 2,100 m3/s (74,160 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 adjusted slightly but will remain set to an equivalent of approximately two gates open to meet the Plan 2012-prescribed flow in July in consideration of capacity restrictions at the hydropower plants. On Monday, July 7, Gates #7 through #9 will be closed and Gate #10 will be decreased to 71 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 and Gate #16 will remain opened 15 cm to allow sea lamprey trapping. Gates #11 through #14 will remain open 79 cm. The St. Marys Rapids flow will decrease from approximately 385 m3/s (13,600 cfs) to approximately 298 m3/s (10,500 cfs) after the gate adjustments.

Water level changes over the month of June

Water supply conditions were drier than average in the Lake Superior basin and wetter than average in the Lake Michigan-Huron basin in June.

  • Lake Superior rose by 6 cm (2.4 in) last month while the seasonal long-term average pattern is for Lake Superior to rise by 8 cm (3.1 in) in June.
  • Lake Michigan-Huron rose by 8 cm (3.2 in) last month, and the seasonal long-term average pattern is for Lake Michigan-Huron to rise by 5 cm (2 in) in June.

Water levels as of the beginning of July

  • At the beginning of July, the lake-wide average water level of Lake Superior was 14 cm (5.5 in) below the seasonal long-term average (1918-2023) and 16 cm (6.3 in) below the level of a year ago.
  • At the beginning of July, 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 23 cm (9 in) below the level of a year ago.

Forecast outlook

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

  • If weather and water supply conditions are near average, Lake Superior may rise by approximately 5 cm (2 in) and Lake Michigan-Huron may remain stable.
  • If conditions are much wetter than average, Lake Superior may increase by approximately 12 cm (4.7 in) and Lake Michigan-Huron may rise by approximately 7 cm (2.8 in).
  • If conditions are much drier than average, the water level of Lake Superior may remain stable, and Lake Michigan-Huron may decrease by about 5 cm (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.