The International Lake Superior Board of Control recently increased the total outflow from Lake Superior through the St. Marys River. As is typical for this time of year, the outflow is expected to continue increasing over the summer months before beginning to decline in the fall. To allow for the increase in outflow, the flow through the gated dam at the head of the St. Marys Rapids, known as the Compensating Works, also was increased.
Why is this necessary, and why now?
First, a bit of background. Water released from Lake Superior has been regulated since the completion of the Compensating Works in 1921. The IJC established the basic objectives and limits to the regulation of Lake Superior’s outflow in its 1914 Order of Approval. It acknowledged the needs of various interests on Lake Superior and the St. Marys River, including navigation, hydropower, and riparian owners.
Since 1978, the Commission has issued several additions to the original Order. It now specifies that the level of Lake Michigan-Huron must also be considered when determining the Lake Superior outflow.
What all this means is that outflows from Lake Superior are set in consideration of various interests upstream and downstream.
But the ability to regulate the outflow from Lake Superior doesn’t mean that full control of lake levels is possible. This is because the major factors affecting water supply to the Great Lakes --- precipitation, evaporation, and runoff --- cannot be controlled. We know that water levels tend to rise in the spring and summer due to snowmelt and increased rain, but it’s hard to predict by how much.
The Lake Superior regulation plan takes all of this into account. It looks at where water levels are now, what water supplies and levels might be like in the coming months, and then tries to provide a balance in consideration of all interests and all sources of uncertainty. It’s not an easy task.
An aerial view showing the St. Marys River control structures. Credit: U.S. Army Corps of Engineers.
Flashback to present
After more than a decade of low water levels, recent wet conditions have resulted in Lake Superior rebounding to above-average levels not seen since 1998. Lake Michigan-Huron interests, meanwhile, also have experienced a sustained period of low levels, with the lake reaching record lows in January 2013. But it’s been wetter since then, and as a result, like Lake Superior, levels of Lake Michigan-Huron also have risen, though they still remain below average.
As water levels have gone up, the outflow from Lake Superior has increased. And, with Superior levels now relatively higher than Michigan-Huron’s, the regulation plan has been “nudging” flows up a bit to try to bring lake levels into balance. This balancing principle benefits both lakes, as flows can also be “nudged” down when the pendulum swings the other way, and Michigan-Huron is relatively higher than Lake Superior.
So what about the Compensating Works?
The release of water from Lake Superior each month is made through the various structures located on the St. Marys River. Less than 1 percent of the total flow is needed for domestic water supply and navigation through the locks. A minimum flow is then needed to supply and maintain fish habitat in the main rapids and through the Fishery Remedial Works channel below the Compensating Works. The amount is usually less than 10 percent of the total Lake Superior outflow.
The remainder and vast majority of water (more than 90 percent on average) is used for hydropower production, split equally between Canada and the U.S. However, at times the flow allocated to hydropower exceeds the capacity of the plants, in which case the excess must be released through the St. Marys Rapids by opening additional gates. Under some circumstances, the flow through the Compensating Works has made up more than 30 percent of the total Lake Superior outflow.
The St. Marys Rapids. Credit: U.S. Army Corps of Engineers.
So with water levels and flows increasing, what can be expected this summer?
Flows and levels have been going up, and many of those who have suffered under low water level conditions for years consider this a good thing. But the International Lake Superior Board of Control recognizes that many people also are concerned about the unusually high flows and levels expected this summer, particularly in the St. Marys Rapids. As a result, the Board received authority from the IJC to adjust outflows over the next several months.
The Board must adjust flows in consideration of interests upstream and downstream by limiting the effects on water levels, and we can’t predict what Mother Nature has in mind. But by adjusting outflows, the Board hopes to limit adverse impacts in the St. Marys River over the next several months.
International Lake Superior Board of Control