Update on Lake Superior Outflows and Expected Conditions - May 2023
Anglers and other users of the St. Marys Rapids, please be advised that the gate setting of the Compensating Works at the head of the St. Marys Rapids will be increased on Monday May 8, 2023, from the current setting equivalent to approximately five gates fully open to a setting equivalent to approximately eight gates fully open. This will be achieved by raising Gates #9 through #16 (all on the US side of the structure) to a setting of 168 cm (66.1 in) open each. The St. Marys Rapids flow will increase from ~770 m3/s to ~1,094 m3/s after the gate adjustments on May 8.
Low-lying areas of Whitefish Island including recreational trails may flood. Users are encouraged to use extreme caution. The Board advises that even higher flows in the St. Marys Rapids are possible in the coming months, which may increase the level of flooding on Whitefish Island.
The International Lake Superior Board of Control (Board) received approval from the International Joint Commission (IJC) to temporarily deviate from Regulation Plan 2012 from May through November 2023. This deviation strategy is similar to the strategy employed in 2019. Over the next several months, the Board expects to adjust the gate settings at the Compensating Works in order to offset the flow limitations caused by repairs and maintenance at the hydropower plants. The total amount of water released through the St. Marys River will be approximately equal to the flow prescribed by Plan 2012 and the deviation strategy will have almost no cumulative impact on the water levels of Lake Superior and Lake Michigan-Huron yet will provide reduced fluctuations in flows and water levels in the St. Marys Rapids, directly downstream of the Compensating Works.
The Board expects the total St. Marys River flow in May to be 2,890 m3/s (102,100 cfs) which is 50 m3/s (1,800 cfs) less than the flow 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.
Water level changes over the month of April
- Lake Superior rose by 18 cm (7.1 in) last month, while the seasonal long-term average rise is 8 cm (3.1 in) in April.
- Lake Michigan-Huron also rose by 18 cm (7.1 in) last month, while the seasonal long-term average rise is 11 cm (4.3 in) in April.
- Significant snowfall, snowmelt and well above average rainfall during the second half of April directly contributed to the larger than average water level rises.
Water levels as of the beginning of May
- At the beginning of May, the lake-wide average water level of Lake Superior is 28 cm (11.0 in) above the seasonal long-term average (1918-2022) and 28 cm (11.0 in) above the level of a year ago.
- At the beginning of May, the lake-wide average water level of Lake Michigan-Huron is 18 cm (7.1 in) above average and 9 cm (3.5 in) below the level of a year ago.
Water levels of Lake Superior and Lake Michigan-Huron are expected to continue the seasonal rise in May.
Depending on the weather and water supply conditions during the next month,
- Lake Superior may rise by as much as 16 cm (6.3 cm).
- Lake Michigan-Huron may rise by as much as 19 cm (7.5 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