Update on Lake Superior Outflows and Expected Conditions – December 2024
Date

The Board expects the total St. Marys River flow in December to be 1,900 m3/s (67,100 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 maintained at the typical winter gate setting (Gates #7 through #10 open 20 cm each) equivalent to approximately one-half gate fully open. Therefore, the average St. Marys Rapids flow in December is expected to be approximately 86 m3/s (3,010 cfs ) Gate #1 will remain at its typical setting which supplies a flow of about 15 m3/s (530 cfs) to the channel north of the Fishery Remedial Dike. 

Water level changes over the month of November

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

  • Lake Superior declined by 2 cm (0.8 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 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 November.

Water levels as of the beginning of December

  • At the beginning of December, the lake-wide average water level of Lake Superior was 16 cm (6.3 in) below the seasonal long-term average (1918-2023) and 11 cm (4.3 in) below the level of a year ago. 
  • At the beginning of December, the lake-wide average water level of Lake Michigan-Huron was 10 cm (3.9 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 December.

  • If weather and water supply conditions are near average, Lake Superior may decline by approximately 8 cm (3.1 in.) and Lake Michigan-Huron may decline by approximately 6 cm (2.4 in). 
  • If conditions are much wetter than average, Lake Superior may decline by as much as 3 cm (1.2 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 drop by approximately 12 cm (4.7 in), and Lake Michigan-Huron may also decrease by as much as 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 or on Facebook at: https://www.facebook.com/InternationalLakeSuperiorBoardOfControl