Image of St. Marys River Control Structures

Update on Lake Superior Outflows and Expected Conditions - December 2020


Lake Superior and Lake Michigan-Huron water levels continue to decline, but are still well above average and the risk of high-water impacts remains.  Lake Superior outflows continue to be set in consideration of high levels upstream and downstream.

Last month, Lake Superior declined 3 cm (1.2 in), while on average it declines 5 cm (2 in) in November. Lake Michigan-Huron declined 6 cm (2.4 in) over the course of the month, which is close to the average water level decline of 5 cm (2 in) in November.  

Both lakes are expected to continue their seasonal declines in December. However, there will continue to be an increased risk of shoreline erosion, lakeshore flooding and coastal damages over the next several weeks and potentially through the winter. 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.

At the beginning of December, Lake Superior is 25 cm (9.8 in) above average (1918 – 2019) and 8 cm (3.2 in) below its level of a year ago. Lake Michigan-Huron is 80 cm (31.5 in) above average and 10 cm (3.9 in) below the level at this time last year.

ILSBC December 2020 Infographic

The gate setting of the control structure will be maintained at the typical winter setting equivalent to one-half gate open in December.

An unanticipated outage due to critical maintenance at one of the hydropower plants is expected to limit the total outflow to 2,140 m3/s (75.6 tcfs) in December. This is 270 m3/s (9.5 tcfs) less than the 2,410 m3/s (85.1 tcfs) prescribed by Lake Superior Regulation Plan 2012.  This is equivalent to approximately 0.9 cm (0.4 in) of water on Lake Superior and approximately 0.6 cm (0.2 in) less water on Lake Michigan-Huron, relative to Plan 2012. Since Plan 2012 considers both upstream and downstream water levels, outflows are expected to automatically adjust to the small effects of this deviation from the regulation plan, and lake level impacts are expected to be slowly reduced to zero in the coming months.

To help put the water level influence of this deviation in perspective, note that provisional forecasts for December show water levels of Lake Superior falling 5 to 14 cm (2 to 6 in) over the course of the month, while Lake Michigan-Huron is projected to remain stable or fall as much as 13 cm (5 in). The wide range for each lake reflects the large influence and also the uncertainty of water supplies and their effects on water levels.

ILSBC December 2020 Deviation Infographic


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: