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

The Board expects the total St. Marys River flow in December to be 2,150 m3/s (75,900 ft3/s) as prescribed by Regulation Plan 2012.

The gate setting of the Compensating Works at the head of the St. Marys Rapids will remain at the typical winter gate setting of one-half gate open (Gates #1 and #7 through #10 open 20 cm (8 in) each). The St. Marys Rapids flow will remain at approximately 86 m3/s or 3,000 ft3/s (equivalent to approximately one-half gate open).

Water level changes over the month of November

Water supply conditions were much drier than average in both the Lake Superior basin and the Lake Michigan-Huron basin in November.

  • Lake Superior decreased by 10 cm (3.9 in) last month, while the seasonal long-term average pattern is for Lake Superior to decline by 5 cm (2.0 in) in November.
  • Lake Michigan-Huron declined by 11 cm (4.3 in) last month, while the seasonal long-term average pattern is for Lake Michigan-Huron to decline by 4 cm (1.6 in) in November.

Water levels as of the beginning of December

  • At the beginning of December, the lake-wide average water level of Lake Superior was 13 cm (5.1 in) below the seasonal long-term average (1918-2024) and 3 cm (1.2 in) above the level of a year ago.
  • At the beginning of December, the lake-wide average water level of Lake Michigan-Huron was 30 cm (11.8 in) below the seasonal long-term average (1918-2024) and 20 cm (7.9 in) below the level of a year ago.

Forecast outlook

  • If weather and water supply conditions are near average in December, Lake Superior may decline by approximately 8 cm (3.1 in) and Lake Michigan-Huron may decrease by approximately 5 cm (2.0 in).
  • If conditions are much wetter than average, Lake Superior may decrease by approximately 4 cm (1.6 in) and Lake Michigan-Huron may rise by approximately 2 cm (0.8 in) in December.
  • If conditions are much drier than average in December, the water level of Lake Superior may fall by 13 cm (5.1 in), and Lake Michigan-Huron may decrease by about 10 cm (3.9 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.