Update on Upcoming Gate Changes, Outflows and Expected Conditions – September 2025
Date

The Board expects the total St. Marys River flow in September to be 2,370 m3/s (83,700 ft3/s), 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 lowered this month. On Monday, September 8, Gate #11 will be lowered to a setting of 51 cm (20 in) open and Gates #2 through #4, #6 and #12 through #14 will be closed. 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 open 102 cm (40 in). The St. Marys Rapids flow will decrease from approximately 880 m3/s or 31,100 ft3/s (equivalent to approximately six gates fully open) to approximately 350 m3/s or 12,400 ft3/s (equivalent to approximately two gates fully open) after the gate adjustments.

Water level changes over the month of August

Water supply conditions were drier than average in both the Lake Superior and Lake Michigan-Huron basins in August.

  • Lake Superior declined by 1 cm (0.4 in) last month, while the seasonal long-term average pattern is for Lake Superior to rise by 1 cm (0.4 in) in August.
  • Lake Michigan-Huron declined by 7 cm (2.8 in) last month, while the seasonal long-term average pattern is for Lake Michigan-Huron to decline by 4 cm (1.6 in) in August.

Water levels as of the beginning of September

  • At the beginning of September, the lake-wide average water level of Lake Superior was 8 cm (3.1 in) below the seasonal long-term average (1918-2024) and 1 cm (0.4 in) below the level of a year ago.
  • At the beginning of September, the lake-wide average water level of Lake Michigan-Huron was 14 cm (5.5 in) below the seasonal long-term average (1918-2024) and 23 cm (9.1 in) below the level of a year ago.

Forecast outlook

  • If weather and water supply conditions are near average in September, Lake Superior may decline by approximately 1 cm (0.4 in) and Lake Michigan-Huron may decrease by approximately 6 cm (2.4 in).
  • If conditions are much wetter than average, Lake Superior may increase by approximately 8 cm (3.1 in) and Lake Michigan-Huron may rise by approximately 2 cm (0.8 in) in September.
  • If conditions are much drier than average in September, the water level of Lake Superior may fall by 8 cm (3.1 in), and Lake Michigan-Huron may decrease by about 12 cm (4.7 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.