Image of St. Marys River Control Structures

Update On Lake Superior Outflows And Expected Conditions - September 2019


Water levels across the Great Lakes system have begun their seasonal decline, but they remain near record-highs for this time of year. As a result, there will continue to be a significantly increased risk of shoreline erosion, lakeshore flooding and coastal damages over the next several weeks and potentially through the fall. The Board advises all those that may be affected to prepare for potentially severe coastal impacts, especially during periods of strong winds and high waves.

Relatively dry weather and high outflows allowed Lake Superior water levels to decline by 4 cm last month, while on average the water level rises 1 cm in August.  Nonetheless, the monthly mean level in August of 183.86 m ties the record set in 1952, and at the beginning of September, Lake Superior is just 2 cm below the record-high beginning-of-month level set in 1950. The level is currently 30 cm above average (1918 – 2018) and 18 cm above its level of a year ago.

Lake Michigan-Huron declined 10 cm in August, while on average the water level declines 4 cm in August. Lake Michigan-Huron is currently 8 cm below the record-high beginning-of-month level set in 1986. The level is 73 cm above average, and 30 cm above last year’s beginning-of-September level.  

Lake Superior and Lake Michigan-Huron are expected to continue their seasonal declines in September, but will remain near record-highs for this time of year.

The International Lake Superior Board of Control (Board) expects the total outflow from Lake Superior to be 2,840 m3/s in September, 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 gate setting at the Compensating Works will be maintained at the equivalent of nine gates fully open. Gates #9 and #16 will be adjusted slightly on September 4th.

The average St. Marys Rapids flow in September is expected to be approximately 1,315 m3/s. Anglers and other users of the St. Marys Rapids need to be cautious of the high flows and water levels that will continue to be experienced in the rapids in September. Furthermore, some flooding of low-lying areas of Whitefish Island is expected to continue at these high flows. As a result, some recreational trails and features in these areas will likely be inundated and may sustain damage. Users are encouraged to use extreme caution.