Image of St. Marys River Control Structures

Update on Lake Superior Outflows and Expected Conditions - May 2021


While Lake Superior and Lake Michigan-Huron water levels remain above their respective long-term averages, the risk of shoreline erosion, lakeshore flooding, and coastal damages remains.  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. Lake Superior outflows continue to be set in consideration of high levels upstream and downstream.

Last month, Lake Superior rose 8 cm (3.2 in), which is typical for April when the lake begins its seasonal rise. On the contrary, Lake Michigan-Huron remained relatively stable for the second month in a row. Historically, Lake Michigan-Huron rises an average 11 cm (4.3 in) in April. Lake Superior is expected to continue its seasonal rise and Lake Michigan-Huron is expected to begin its seasonal rise in May.

At the beginning of May, Lake Superior is 20 cm (7.9 in) above average (1918 – 2020), 10 cm (3.9 in) below its level of a year ago, and 22 cm (8.7 in) below its record-high level of 1986. Lake Michigan-Huron is 49 cm (19.3 in) above average and 40 cm (15.7 in) below the record-high level at this time last year.

ILSBC May 2021 Infographic - English

The Board expects the total outflow to be 2,350 m3/s (83 tcfs) in May, which is as prescribed by Lake Superior Regulation Plan 2012. On Tuesday, 4 May, the gate setting of the control structure will be increased from the setting equivalent to one-half gate open to a setting equivalent to approximately three gates fully open by adjusting Gates #9 through #12.  Anglers and other users of the St Marys Rapids need to be cautious of the increased flows and water levels that will be experienced in the rapids in May.

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: