Update on Upcoming Outflows and Expected Conditions – September 2026
Date

The Board expects the total St. Marys River flow in September to be 2,340 m3/s (82,600 ft3/s) as prescribed by Regulation Plan 2012. This flow is near the long-term average for the month of September.

The gate setting of the Compensating Works at the head of the St. Marys Rapids will be adjusted to meet the Plan 2012-prescribed flow in September in consideration of capacity restrictions at the hydropower plants. On Tuesday, September 8, Gate #12 will be lowered to 66 cm (26 in) and Gate #16 will be closed. All other gates will remain at the current setting (Gate #1 open to 20 cm, Gates #7 and #8 open to 122 cm, and Gates #9 through #11 open to 104 cm). 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. The St. Marys Rapids flow will decrease from approximately 505 m3/s (17,800 ft3/s) to approximately 465 m3/s (16,400 ft3/s) after gate adjustments. The setting is equivalent to approximately three gates fully open.

Water level changes over the month of August

  • The August 2026 water level remained constant on Lake Superior while water levels typically rise about 1 cm (0.4 in) in August.
  • During August, the level of Lake Michigan-Huron decreased by 5 cm (2.0 in) which is close to its typical decline of 4 cm (1.6 in) for August.

Water levels as of the beginning of September

As of the beginning of September, water levels for both lakes are within 5 cm (2.0 in) of their long-term averages.

  • The lake-wide average water level of Lake Superior is 5 cm (2.0 in.) below the seasonal long-term average (1918-2025) and 3 cm (1.2 in) above the level of a year ago. 
  • The lake-wide average water level of Lake Michigan-Huron is 2 cm (0.8 in) above the seasonal long-term average (1918-2025) and 16 cm (6.3 in) above the level of a year ago.

Forecast outlook in September

  • If weather and water supply conditions are near average, Lake Superior may decrease by approximately 1 cm (0.4 in) and Lake Michigan-Huron may decrease by 6 cm (2.4 in).
  • If conditions are much wetter than average, Lake Superior may increase by approximately 8 cm (3.9 in) and Lake Michigan-Huron may rise by approximately 1 cm (0.4 in).
  • If conditions are much drier than average, the water level of Lake Superior may decrease 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 occurring during periods of severe weather and water supply conditions. Actual flows will vary hour-to-hour and day-to-day depending on hydrologic conditions and variations in flow at the hydropower facilities. Additional information can be found on the Board’s homepage: https://ijc.org/en/lsbc.