Ships that sail under Duluth's Aerial Lift Bridge carry many things -- they carry iron ore to steel-making cities on the lower lakes. They haul coal from North Dakota to make electricity in Detroit. They bring windmills from Europe, and haul grain to Africa.
All of that cargo has to pass through not only the locks at Sault St. Marie, but also along the St. Clair River.
That's the link between the upper and the lower Great Lakes. It's near Detroit, and it flows between Lakes Huron and Erie. It's a natural channel, or at least it was -- once.
Mary Muter, who lives in Lake Huron's Georgian Bay, says she was shocked the first time she saw The St. Clair.
"It is the most heavily altered part of the Great Lakes," she says. "The entire shoreline has been hardened. There is no natural shoreline around there at all."
Muter and her neighbors are upset because the level of Lake Huron is so low. Wetlands that normally line the shore are disappearing, and with them the spawning grounds for bass and muskie.
Muter blames the St. Clair River. She says the water is flowing too fast through the river, in effect emptying Lake Huron, and above it Lake Michigan, and above that Lake Superior.
It's like sitting in a bathtub without a plug, the water rushing down the drain, and only a trickle coming in from the tap.
Over the years, people have interfered with the natural system around the St. Clair River. In the 1800s, locals cut through a sandbar that slowed the flow out of Lake Huron. They also mined gravel from the river bottom. And in the 1960s the U.S. Army Corps of Engineers deepened the channel when the St. Lawrence Seaway was built.
"I frankly think we have to pray for a very cold winter, a lot of snow, a rainy fall -- an overall dramatic change in the climate."
Mary Muter says at the time, the Corps planned to put structures in the river bottom to control the flow.
"But then there was unprecedented high levels of precipitation, water levels started to go up, and they simply forgot about doing anything about it," she says.
Muter is the Waterkeeper for the Georgian Bay Association. She's been pushing officials both high and low to act quickly to solve the problem. She wants them to put big rocks in the river bottom where it's eroding the most.
"They would basically be returning the river bed to a more natural condition, and it would actually improve spawning habitat," she says. "It should be a win-win for the fish. So that's a no-brainer, as far as we're concerned, and it's time our governments moved on and just got that work underway."
The governments, of course, are the U.S. and Canada. The International Joint Commission is the body responsible for managing the waters that separate the two countries.
The IJC is launching a major study of water flows between the lakes. It will start with the St. Clair River, but that part of the study alone could take two and a half years.
Kay Felt is co-chair of the public interest advisory group for the study. She lives on Lake St. Clair, where the St. Clair River widens before it reaches Lake Erie. She says the shorelines in her neighborhood are getting clogged up with silt and sand.
But she says lake levels are affected by many things, including variations in precipitation, climate change, and even the fact that the earth is still rebounding after the retreat of the last glacier.
"It would be nice if we could come in with a quick fix," Felt says. "And I would like nothing better than that, because I've been living with my particular problem now for over 10 years, and it's a smelly, messy problem. But there is no point in doing something that doesn't have a sound scientific basis."
She says it's important to avoid unintended consequences, like the ones that have happened already.
And as to whether slowing the flow on the St. Clair River would raise the level of Lake Superior, that's another complicated question the study will try to answer.
"It's hard to say what we can do about Lake Superior," Felt says. "I frankly think what we do is have to pray for a very cold winter, a lot of snow, a rainy fall -- we've already had a rainy few days up here -- and to try to see an overall dramatic change in the climate."
Experts on climate change say the Lake Superior region will experience dramatic changes, but it's far from clear whether that will mean more snow and rain, or less.
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