Update on Upcoming Outflows and Expected Conditions – May 2026

The Board expects the total St. Marys River flow in May to be 2,140 m³/s (75,600 ft³/s) as prescribed by Regulation Plan 2012.

The gate setting of the Compensating Works at the head of the St. Marys Rapids will be increased from the current setting equivalent to approximately one-half gate open to a setting equivalent to approximately four gates open, to meet the Plan 2012-prescirbed flow in May in consideration of capacity restrictions at the hydropower plants. On Thursday, May 7, Gates #7 through #12 will be raised to a setting of 1.22 meters (4 feet) open. 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. The St. Marys Rapids flow will increase from approximately 87 m³/s to approximately 534 m³/s after gate adjustments.

Water level changes over the month of April

There was a record rise in Lake Michigan-Huron water levels during April - this is based on historical records back to 1918 (109 years). Snowmelt runoff, combined with significant rainfall passing directly over the lake, were the main drivers for the record increase in water level.

• From the beginning of April to the beginning of May, the level of Lake Michigan-Huron rose 34 cm (approximately 13.4 inches). The previous record increase occurred in April of 1951, when the Lake Michigan-Huron water level rose 25 cm (close to 10 inches). The seasonal long-term average pattern is for Lake Michigan-Huron to increase by 11 cm (4.3 inches) in April.
• Comparatively, the April 2026 water level rise of Lake Superior was 18 cm (approximately 7.1 inches) which is the 2^nd highest increase on record. The record rise for Lake Superior occurred in April of 2000, when the water level rose 29 cm (11.4 inches). The seasonal long-term average pattern is for Lake Superior to increase by 8 cm (3.1 inches) in April.

Water levels as of the beginning of May

While the water level increases in April 2026 were significant, both lakes Superior and Michigan-Huron started the month below average. As a result, as of the beginning of May, water levels for both lakes are within 4 cm of their long-term averages.

• At the beginning of May, the lake-wide average water level of Lake Superior was 2 cm (0.8 in) above the seasonal long-term average (1918-2024) and 11 cm (4.3 in) above the level of a year ago.
• At the beginning of May, the lake-wide average water level of Lake Michigan-Huron was 4 cm (1.6 in) above the seasonal long-term average (1918-2024) and 17 cm (6.7 in) above the level of a year ago.

Overall, considering the near average water level conditions on both lakes, outflows from Lake Superior into Lake Michigan-Huron will also be near average for the month of May.

Forecast outlook in May

• If weather and water supply conditions are near average, Lake Superior may rise by approximately 10 cm (4.0 in) and Lake Michigan-Huron may increase by approximately 8 cm (3.1 in).
• If conditions are much wetter than average, Lake Superior may increase by approximately 19 cm (7.5 in) and Lake Michigan-Huron may rise by approximately 17 cm (6.7 in).
• If conditions are much drier than average, the water level of Lake Superior may increase by 2 cm (0.8 in), and Lake Michigan-Huron may increase 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 occurring during periods of severe weather and water supply conditions. Actual flows will vary hour-to-hour and day-to-day depending on hydrologic conditions and variations in flow at the hydropower facilities. Additional information can be found on the Board's homepage: https://ijc.org/en/lsbc.