Update on Lake Superior Outflows and Expected Conditions - June 2025

The Board expects the total St. Marys River flow in June to be 2,020 m³/s (71,300 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 increased from the current setting equivalent to approximately one gate open to a setting equivalent to approximately two gates open, to meet the Plan 2012-prescribed flow in June in consideration of capacity restrictions at the hydropower plants. On Thursday, June 5, Gates #9 through #14 will be raised to 79 cm open and Gate #16 will be opened 15 cm to allow sea lamprey trapping. 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. Gates #7 and #8 will remain open 20 cm. The St. Marys Rapids flow will increase from approximately 138 m³/s (4,900 cfs) to approximately 382 m³/s (13,500 cfs) after the gate adjustments.

Water level changes over the month of May

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

• Lake Superior rose by 7 cm (2.8 in) last month while the seasonal long-term average pattern is for Lake Superior to rise by 10 cm (3.9 in) in May.
• Lake Michigan-Huron rose by 6 cm (2.4 in) last month, and the seasonal long-term average pattern is for Lake Michigan-Huron to rise by 8 cm (3.1 in) in May.

Water levels as of the beginning of June

• At the beginning of June, the lake-wide average water level of Lake Superior was 12 cm (4.7 in) below the seasonal long-term average (1918-2023) and 7 cm (2.8 in) below the level of a year ago.
• At the beginning of June, the lake-wide average water level of Lake Michigan-Huron was 15 cm (5.9 in) below the seasonal long-term average (1918-2023) and 23 cm (9.1 in) below the level of a year ago.

Forecast outlook

Water level changes based on water supplies over the month of June.

• If weather and water supply conditions are near average, Lake Superior may rise by approximately 8 cm (3.1 in) and Lake Michigan-Huron may increase by approximately 5 cm (2.0 in).
• If conditions are much wetter than average, Lake Superior may increase by approximately 16 cm (6.3 in) and Lake Michigan-Huron may rise by approximately 13 cm (5.1 in).
• If conditions are much drier than average, the water level of Lake Superior may rise by only 1 cm (0.4 in), and Lake Michigan-Huron may decrease by about 1 cm (0.4 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.