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

The Board will release slightly higher flows than what Regulation Plan 2012 calls for in October to compensate for extended hydropower outages in recent months.

The Board expects the total St. Marys River flow in October to be 2,560 m³/s (90,400 ft³/s), which is 100 m³/s (3,500 ft ³/s) higher than the outflow of 2,460 m³/s (86,900 ft ³/s) prescribed by Regulation Plan 2012 for the month of October 2025. This additional flow is considered a minor deviation, consistent with the Directive to the International Lake Superior Board of Control on Deviations from the Regulation Plan and is being released to compensate for extended hydropower outages in recent months.

Gate movements and St. Marys Rapids flows

The gate setting of the Compensating Works at the head of the St. Marys Rapids will be lowered this month. The St. Marys Rapids flow will decrease from approximately 570 m³/s (20,100 ft ³/s) (equivalent to approximately four gates fully open) to approximately 310 m³/s (10,900 ft ³/s) (equivalent to approximately two gates fully open). On Tuesday, October 7, Gate #7 and #8 will be lowered to a setting of 20 cm (8 in), and Gate #11 will be lowered to a setting of 63 cm (25 in). Gates #9 and #10 will remain at their current setting of 155 cm (61 in). Gate #1 will remain at its setting which supplies a flow of about 15 m³/s (500 ft³/s) to the channel north of the Fishery Remedial Dike.

Water level changes over the month of September

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

• Lake Superior increased by 1 cm (0.4 in) last month, while the seasonal long-term average pattern is for Lake Superior to decline by 2 cm (0.8 in) in September.
• Lake Michigan-Huron declined by 9 cm (3.5 in) last month, while the seasonal long-term average pattern is for Lake Michigan-Huron to decline by 6 cm (2.4 in) in September.

Water levels as of the beginning of October

• At the beginning of October, the lake-wide average water level of Lake Superior was 5 cm (2.0 in) below the seasonal long-term average (1918-2024) and 7 cm (2.8 in) above the level of a year ago.
• At the beginning of October, the lake-wide average water level of Lake Michigan-Huron was 17 cm (6.7 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 October, Lake Superior may decline by approximately 4 cm (1.6 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 4 cm (1.6 in) and Lake Michigan-Huron may rise by approximately 2 cm (0.8 in) in October.
• If conditions are much drier than average in October, the water level of Lake Superior may fall by 10 cm (3.9 in), and Lake Michigan-Huron may decrease by about 13 cm (5.1 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.