Update on Upcoming Gate Changes, Outflows, and Expected Conditions - August 2025
Date

Anglers and other users of the St. Marys Rapids, please be advised that the gates of the Compensating Works at the head of the St. Marys Rapids will be adjusted multiple times in August to facilitate underwater inspections and scour surveys and to meet the Plan 2012-prescribed flow in consideration of capacity restrictions at the hydropower plants. Please note that as the gates are adjusted, outflows through the St. Marys River will fluctuate. Low-lying areas of Whitefish Island including recreational trails may flood as the gates are increased in August. Please take extra caution when recreating and navigating the river and adjacent areas.

Flows

The Board expects the total St. Marys River flow in August to be 2,450 m3/s (86,500 cfs), as prescribed by Regulation Plan 2012. Actual hour-to-hour and day-to-day flows will vary depending on hydrologic conditions, variations in flow from the hydropower plants, as well as adjustments of the gates of the Compensating Works.

Gate changes

Gate #1 is the northernmost gate of the Compensating Works structure located closest to the Canadian shoreline. Gate #1 is typically open 20 cm (8 in) to supply a flow of about 15 m3/s to the channel north of the Fishery Remedial Dike. During the temporary closure of Gate #1 next week, natural resources technicians will retrieve all aquatic life that may become trapped in pools as the water recedes. The temporary closure of Gate #1 has been coordinated and approved between the Canadian and United States federal, state, and provincial agencies and by the Batchewana First Nation.

Upcoming gate changes in August will take place as follows:

DateActionSt. Marys Rapids Flow

August 6, 7, 8

 

 

 

 

Gate #1 will be temporarily closed to facilitate underwater inspections. If Gate #1 is not successfully closed on August 6, additional attempts will be made on August 7 and 8, as needed. At the end of each day, Gate #1 will return to its typical position of 20 cm (8 in) open.When Gate #1 is closed, the Rapids flow will temporarily decrease to 290 m3/s (equivalent to 2 gates fully open)
August 8 Gate #9 through #14 will be raised to 94 cm (37 in) and Gate #16 will be closed.417 m3/s (equivalent to 3 gates fully open)
August 11Gates #2 through #8 will be raised to 94 cm (37 in)886 m3/s (equivalent to 6 gates fully open)
August 18To facilitate scour surveys on August 19 and 20, Gates #2 through #5 will be lowered to 20 cm (8 in) open and Gates #6 through #14 will be closed. 88 m3/s (equivalent to one-half gate fully open)
August 21Gates #2 through #14 will be raised to 94 cm (37 in)886 m3/s (equivalent to 6 gates fully open)

St. Marys Rapids

The peak flow that will be experienced in August (886 m3/s) is similar to the flow observed in July 2024 and lower than the July 2023 peak flow of 1,200 m3/s.

Water level changes over the month of July

Water supply conditions were wetter than average in both the Lake Superior and Lake Michigan-Huron basins in July, but particularly in the Lake Superior basin.

  • Lake Superior rose by 13 cm (5.1 in) last month while the seasonal long-term average pattern is for Lake Superior to rise by 4 cm (1.6 in) in July. This is the third largest July rise on record since 1918.
  • Lake Michigan-Huron rose by 2 cm (0.8 in) last month, and the seasonal long-term average pattern is stable for Lake Michigan-Huron in July.

Water levels as of the beginning of August

  • At the beginning of August, the lake-wide average water level of Lake Superior was 5 cm (2.0 in) below the seasonal long-term average (1918-2024) and 1 cm (0.4 in) below the level of a year ago.
  • At the beginning of August, the lake-wide average water level of Lake Michigan-Huron was 10 cm (3.9 in) below the seasonal long-term average (1918-2024) and 23 cm (9.1 in) below the level of a year ago.

Forecast outlook

  • If weather and water supply conditions are near average in August, Lake Superior may rise by approximately 1 cm (0.4 in) and Lake Michigan-Huron may decrease by approximately 3 cm (1.2 in).
  • If conditions are much wetter than average, Lake Superior may increase by approximately 8 cm (3.1 in) and Lake Michigan-Huron may rise by approximately 3 cm (1.2 in) in August.
  • If conditions are much drier than average in August, the water level of Lake Superior may fall by 5 cm (2.0 in), and Lake Michigan-Huron may decrease by about 9 cm (3.5 in).

 

Influence of natural factors compared to the influence of outflow regulation

  • The near-record water level rise in Lake Superior in July was driven by significant rainfall events. From July 15 to July 16, the level of Lake Superior rose 4 cm (approximately 2 inches). That is equivalent to the average water level rise over the entire month of July.
  • A 4 cm (2 inch) increase is the equivalent of approximately 850 billion gallons of water. This is enough water to fill more than a million Olympic size swimming pools!
  • In an improbable hypothetical scenario, it would take an entire month for outflow regulation to have a 4 cm (2 inches) impact on the water level of Lake Superior. There would be a corresponding reduction of approximately 3 cm (1.2 inches) on the water level of Lake Michigan-Huron. This hypothetical action would require advanced approval from the International Joint Commission. The Board is only authorized to conduct “minor deviations” from the flow prescribed by Plan 2012. As directed by the IJC (https://ijc.org/en/lsbc/who/directives/deviations), minor deviations may only have a +/- 0.5 cm impact on the water levels of Lake Superior and Lake Michigan-Huron. Any proposed deviations that would have a larger impact on water levels require an advance approval process in consideration of the impacts on Lake Superior, Lake Michigan-Huron, and the St. Marys River.
  • In short, while a rainfall event can increase the water level of Lake Superior by 4 cm (2 inches) in one day, it would take an entire month to accomplish via outflow regulation.

 

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.