Image of St. Marys River Control Structures

Update on Lake Superior Outflows and Expected Conditions - August 2020


Basin conditions were generally wetter than average in July. As a result, water levels rose on Lake Superior and remained stable on Lake Michigan-Huron.

Lake Superior rose 7 cm over the course of the month, while on average the water level rises 5 cm in July. At the beginning of August, Lake Superior is 8 cm below the record-high beginning-of-month level set last year in 2019. The level is currently 26 cm above average (1918 – 2019).

Lake Michigan-Huron remained stable over the course of the month, which is typical in July. At the beginning of August, Lake Michigan-Huron is 4 cm above the previous record-high beginning-of-month level set in 1986. The level is currently 88 cm above average. 

ILSBC August 2020 Infographic

Water levels are expected to remain high over at least the next few months and further record highs are possible if wet conditions continue. Furthermore, there will continue to be a significantly increased risk of shoreline erosion, lakeshore flooding and coastal damages as high water levels persist. The International Lake Superior Board of Control (Board) advises all those that may be affected to prepare for potentially severe coastal impacts, especially during periods of strong winds and high waves.

The Board expects the total outflow to be 2,500 m3/s in August, which is as prescribed by Lake Superior Regulation Plan 2012. Actual outflows may vary depending on hydrologic conditions, as well as maintenance activities at the hydropower plants on the St. Marys River. The outflow in August is just 130 m3/s above average and the lowest August outflow since 2012, which is in part a reflection of Plan 2012 adjusting to the relatively wetter conditions and higher levels experienced on Lake Michigan-Huron in comparison to Lake Superior in recent months.

In order to meet the Plan 2012-prescribed flow in August, the gate setting of the control structure will be adjusted to the equivalent of approximately two gates fully open. This will be achieved by raising Gates #11 through #14 to a setting of 132 cm open on Monday, 3 August. This gate configuration will allow safe diving conditions for underwater safety inspections of Gates #3 through #8 that will be conducted next week. The average St. Marys Rapids flow is expected to be 409 m3/s.

Shoreline businesses and property owners are reminded that the Great Lakes – St. Lawrence River Adaptive Management (GLAM) Committee continues to host an online questionnaire to allow for direct reporting on impacts related to recent high water conditions. The 2020 version of the questionnaire is now available: