Image of St. Marys River Control Structures

Update on Lake Superior Outflow and Expected Conditions - April 2019


Relatively wet conditions continued across the upper Great Lakes basin in March, and water levels of Lake Superior and Lake Michigan-Huron remain well above average. The above-average levels coupled with strong winds and waves continue to result in shoreline erosion and coastal damages across the upper Great Lakes system. Additional shoreline erosion and coastal damages may occur this spring should active weather continue. 

Lake Superior water levels decreased 2 cm last month, which is the average water level decline in March. At the beginning of April, Lake Superior is 33 cm above average (1918 – 2018) and 6 cm above its level of a year ago. Lake Superior is expected to begin its seasonal rise during April. Lake Michigan-Huron rose 4 cm in March, which equals its average water level climb in March. Lake Michigan-Huron is currently 56 cm above average, and 14 cm above last year’s beginning-of-April level.  Lake Michigan-Huron is expected to continue its seasonal rise in April.

In consideration of the continuing high water levels in the upper Great Lakes and to accommodate expected maintenance at the hydropower plants, the International Lake Superior Board of Control (Board) requested and received approval from the International Joint Commission (IJC) to temporarily deviate from Regulation Plan 2012 this winter. All three hydropower plants continued to be directed to flow at their maximum capacity, but the total combined available capacity was less than maximum capacity in December and January due to required maintenance activities. To offset the effects of these activities, over the winter months the Board released more water through the control structure at the head of the St. Marys Rapids by maintaining a gate setting equivalent to two gates fully open instead of the typical winter setting equivalent to one-half gate open.   

The net result of this strategy is that total flows were less than Plan 2012 in December and January, while flows greater than Plan 2012 occurred in February and March. Flows as prescribed by Plan 2012 are expected in April.  The total amount of water released through the St. Marys River this winter was approximately equal to what it would have otherwise been, with minimal net effect on the water levels of Lake Superior and Lake Michigan.

The Board expects the total flow in April to be 2,410 m3/s, which is the flow prescribed by Lake Superior Regulation Plan 2012.  Actual outflows may vary depending on hydrologic and ice conditions, as well as maintenance activities at the hydropower plants on the St. Marys River.  The gate setting of the control structure will be maintained at the current setting (Gates #2 through #16 open 26 cm each). 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.

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. It is not possible to accurately predict such conditions weeks in advance, but given the current levels of the lakes, the Board advises all those that may be impacted to prepare for the above-average water levels, should they continue this spring.