Image of St. Marys River Control Structures

Update on Lake Superior Outflows and Expected Conditions – May 2019


Wet conditions continued across the upper Great Lakes basin in April.  Water levels of Lake Superior and Lake Michigan-Huron remain well above average, and Lake Superior has the potential to reach or exceed record highs in May should wet conditions continue.  The Board advises all those that may be affected to prepare for coastal impacts similar to those that have occurred during the last few years, as the high levels coupled with potential strong winds and waves are likely to continue to result in an increased risk of shoreline erosion and coastal damages across the upper Great Lakes system.

Lake Superior water levels increased 13 cm last month, while the average water level rise in April is 8 cm. At the beginning of May, Lake Superior is 38 cm above average (1918 – 2018) and 23 cm above its level of a year ago. Lake Michigan-Huron rose 20 cm in April, which is higher than the average water level climb of 11 cm in April. The increases in water levels were driven by above-average precipitation and well-above-average runoff to the lakes from the basin tributaries and watersheds. Lake Michigan-Huron is currently 65 cm above average, and 24 cm above last year’s beginning-of-May level.  Lakes Superior and Michigan-Huron are expected to continue their seasonal rises in May.

The International Lake Superior Board of Control (Board) recently requested approval from the International Joint Commission (IJC) to temporarily deviate from Regulation Plan 2012 from May through November 2019. This deviation strategy is similar to those employed over the past four years, which were also marked by reduced hydropower capacity and high lake levels and outflows. Over the next several months, the Board expects to adjust the gate settings at the Compensating Works in order to offset the effects of maintenance activities at the hydropower plants that are expected to continue through the summer and fall. Over this time period, the total amount of water released through the St. Marys River will be approximately equal to the flow prescribed by Plan 2012 and the deviation strategy will have almost no impact on the water levels of Lake Superior and Lake Michigan-Huron, yet will provide benefits to the St. Marys Rapids directly downstream of the Compensating Works.

The Board expects the total flow in May to be 2,430 m3/s, which is 340 m3/s less than that 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 average St. Marys Rapids flow in May is expected to be approximately 674 m3/s and the gates will be raised to the equivalent of four gates fully open on 6 May. This will be achieved by raising Gates #11 through #14 to a setting of 165 cm open. There will be no change to the setting of Gate #1, which supplies a flow of about 15 m3/s to the channel north of the Fishery Remedial Dike. Gates #2 through #10, #15 and #16 will remain at the current setting of 26 cm open. Anglers and other users of the St. Marys Rapids need to be cautious of the changing flows and water levels that will be experienced in the rapids in May. 

The Board stresses that hydrologic conditions are the primary driver of water level fluctuations. Water levels of the Great Lakes cannot be fully controlled through regulation of outflows, nor can regulation completely eliminate the risk of extreme water levels from occurring during periods of severe weather and water supply conditions.