Update on Lake Superior Outflows and Expected Conditions - September 2023
The gate setting of the Compensating Works at the head of the St. Marys Rapids will be maintained at the current setting in September, which is equivalent to approximately five gates fully open. The St. Marys Rapids flow is approximately 773 m3/s. At this flow, some low-lying areas of Whitefish Island including recreational trails may still be flooded. Users are encouraged to use extreme caution.
The Board expects the total St. Marys River flow in September to be 2,820 m3/s (99,600 cfs) which is the same as 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.
The International Lake Superior Board of Control (Board) received approval from the International Joint Commission (IJC) to temporarily deviate from Regulation Plan 2012 through November 2023. This deviation strategy is like the strategy employed in 2019. As necessary over the next few months, the Board will continue to adjust the gate settings at the Compensating Works to offset the flow limitations caused by repairs and maintenance at the hydropower plants. Since May, the total amount of water released through the St. Marys River has been approximately equal to the flow prescribed by Plan 2012.
Water level changes over the month of August
Water supply conditions were drier than average in Lake Superior basin and wetter than average in the Lake Michigan-Huron basin in August.
- Lake Superior declined by 1 cm (0.4 in.) last month, while the seasonal long-term average pattern is for Lake Superior to rise 1 cm (0.4 in) in August.
- Lake Michigan-Huron declined by 2 cm (0.8 in) last month, while the seasonal long-term average pattern for Lake Michigan- Huron is to decline 4 cm (1.6 in) in August.
Water levels as of the beginning of September
- At the beginning of September, the lake-wide average water level of Lake Superior is 12 cm (4.7 in) above the seasonal long-term average (1918-2022) and 3 cm (1.2 in) above the level of a year ago.
- At the beginning of September, the lake-wide average water level of Lake Michigan-Huron is 13 cm (5.1 in) above the seasonal long-term average and 8 cm (3.1 in) below the level of a year ago.
It is likely that Lakes Superior and Michigan-Huron have reached their seasonal peak water levels.
- If weather and water supply conditions are near average, both Lake Superior and Lake Michigan-Huron are expected to decline in September (by ~3 cm or 1.2 in and 5 cm or 2 in, respectively).
- If conditions are wetter than average, Lake Superior may rise by as much as 6 cm (2.4 cm) and Lake Michigan-Huron may rise by as much as 2 cm (0.8 in).
- If dry conditions continue, the water level of Lake Superior is expected to decline by as much as 10 cm (3.9 in.), and Lake Michigan-Huron is expected to decline by as much as 11 cm (4.3 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/InternationalLakeOntarioStLawrenceRiverBoard/