Update on Lake Superior Outflows and Expected Conditions - September 2024

The Board expects the total St. Marys River flow in September to be 2,210 m³/s (78,000 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 average St. Marys Rapids flow in September is expected to be approximately 540 m³/s, to meet the Plan 2012-prescribed flow in September in consideration of the capacity restrictions at the hydropower plants. Gate #1 will remain at its typical setting which supplies a flow of about 15 m³/s to the channel north of the Fishery Remedial Dike.

Anglers and other users of the St. Marys Rapids need to be cautious of the flows and water levels that will be experienced in the rapids in September. On Monday, September 9, the gate setting of the Compensating Works will be decreased from the current setting equivalent to approximately four gates fully open (Gates #7 through #15 open 97 cm each) to a setting equivalent to approximately three gates fully open (Gates #7 through #15 open 76 cm each).

After the gate adjustments, the St. Marys Rapids flow will decrease from approximately 640 m³/s to approximately 500 m³/s. Concrete core sampling at two Compensating Works piers requires Gates #10 and #14 to be raised to 130 cm early on Monday, September 9. Upon completion of the coring work, on Monday, September 9 or Tuesday, September 10, Gates #10 and #14 will be lowered to their September setting of 76 cm.

Water level changes over the month of August

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

• Lake Superior declined by 2 cm (0.8 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 8 cm (3.1 in) last month, and 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 preliminary lake-wide average water level of Lake Superior was 7 cm (2.8 in) below the seasonal long-term average (1918-2023) and 17 cm (6.7 in) below the level of a year ago.
• At the beginning of September, the preliminary lake-wide average water level of Lake Michigan-Huron was 9 cm (3.5 in) above the seasonal long-term average (1918-2023) and 4 cm (1.6 in) below the level of a year ago.

Forecast outlook

• If weather and water supply conditions are near average, Lake Superior may decline by approximately 1 cm (0.4 in.) and Lake Michigan-Huron may decline by approximately 6 cm (2.4 in).
• If conditions are much wetter than average, Lake Superior may rise by as much as 8 cm (3.1 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 8 cm (3.1 in), and Lake Michigan-Huron may 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

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