Update on Lake Superior Outflows and Expected Conditions - April 2025

The Board expects the total St. Marys River flow in April to be 1,580 m³/s (55,800 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 (Gate #1 and Gates #7 through #10 open 20 cm each) equivalent to approximately one-half gate open. Therefore, the average St. Marys Rapids flow in April is expected to be approximately 85 m³/s (3,000 cfs). Gate #1 will remain at its typical setting which supplies a flow of about 15 m³/s (500 cfs) to the channel north of the Fishery Remedial Dike. The Poe Lock is open for shipping.

Water level changes over the month of March

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

• Lake Superior rose 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 March.
• Lake Michigan-Huron rose by 12 cm (4.7 in) last month, while the seasonal long-term average pattern is for Lake Michigan-Huron to rise by 5 cm (2.0 in) in March.

Water levels as of the beginning of April

• At the beginning of April, the lake-wide average water level of Lake Superior was 15 cm (5.9 in) below the seasonal long-term average (1918-2023) and 13 cm (5.1 in) below the level of a year ago.
• At the beginning of April, the lake-wide average water level of Lake Michigan-Huron was 13 cm (5.1 in) below the seasonal long-term average (1918-2023) and 16 cm (6.3 in) below the level of a year ago.

Forecast outlook

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

• If weather and water supply conditions are near average, Lake Superior may rise by approximately 9 cm (3.5 in.) and Lake Michigan-Huron may, also, increase by approximately 9 cm (3.5 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 18 cm (7.1 in).
• If conditions are much drier than average, the water level of Lake Superior may rise by only 2 cm (0.8 in), and Lake Michigan-Huron may rise by about 2 cm (0.8 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.