Update on Lake Superior Outflows and Expected Conditions - May 2024
The Board expects the total St. Marys River flow in May to be 2,000 m3/s (70,600 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 at the head of the St. Marys Rapids will be increased on May 6 from a setting equivalent to approximately one-half gate fully open (Gates #7 through #10 open 20 cm (7.9 in)) to a setting equivalent to approximately three gates fully open, to meet the Plan 2012-prescribed flow in May in consideration of capacity restrictions at the hydropower plants. Gate #1 will remain at its typical setting of 20 cm (7.9 in) open which supplies a flow of about 15 m3/s to the channel north of the Fishery Remedial Dike. The St. Marys Rapids flow will increase from ~85 m3/s to ~575 m3/s after the gate adjustments on Monday, May 6.
The MacArthur Lock reopened on April 25th.
Water level changes over the month of April
Water supply conditions were near average in the Lake Superior basin and above average in the Lake Michigan-Huron
basin in April.
Lake Superior rose by 8 cm (3.1 in) last month which matches the seasonal long-term average pattern in April.
Lake Michigan-Huron rose by 14 cm (5.5 in) last month, while the seasonal long-term average pattern is for Lake Michigan-Huron to rise by 11 cm (4.3 in) in April.
Water levels as of the beginning of May
At the beginning of May, the lake-wide average water level of Lake Superior was 2 cm (0.8 in) below the seasonal long-term average (1918-2023) and 30 cm (11.8 in) below the level of a year ago.
At the beginning of May, the lake-wide average water level of Lake Michigan-Huron was 6 cm (2.4 in) above the seasonal long-term average (1918-2023) and 11 cm (4.3 in) below the level of a year ago.
Forecast outlook
Both lakes are expected to continue their seasonal rise over the coming month.
If weather and water supply conditions are near average, Lake Superior may rise by approximately 11 cm (4.3 in.) and Lake Michigan-Huron may rise by approximately 8 cm (3.1 in).
If conditions are wetter than average, Lake Superior may rise by as much as 19 cm (7.5 in) and Lake Michigan-Huron may rise by as much as 17 cm (6.7 in).
If conditions are drier than average, the water level of Lake Superior is expected to rise by approximately 2 cm (0.8 in), and Lake Michigan-Huron is expected to rise by approximately 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 or on
Facebook at: https://www.facebook.com/InternationalLakeSuperiorBoardOfControl