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What heavy ice coverage means for Great Lakes shipping and water levels

Sarah Hulett
Michigan Radio

Ice formed on the Great Lakes early this year, thanks to the arctic temperatures we’ve been experiencing.

And that should be good for lake levels, which have plummeted in recent years. Right?

Well, it turns out the answer to that question is a bit complicated.

Lake levels are affected by a number of factors, including temperature, precipitation, evaporation and ice cover.

Heavy ice cover in the winter months acts like a cap on the lakes, preventing water from evaporating.

But a study released this week takes a look at some of the mitigating factors.

Its author, John Lenters, says it’s true that ice cover cuts down on evaporation, which means more water stays in the lakes.

“The only problem with that argument is that to get to where we are already this year, with all this high ice cover, you need a lot of evaporation prior to that.”

The lakes sweat, just like you

Here’s what Lenters means. In order for there to be lots of ice on the lakes, the lake has to cool off. And when it cools off a lot, that means there’s a lot of evaporation. Kind of like when you sweat in the summertime.

“The sweat, when it evaporates, it cools your skin quite rapidly. That’s actually one of the most effective ways your body loses heat. And the lakes are really no different. They’re a giant body of water. And so when they evaporate a lot, that’s actually when they’re most effective at cooling the lakes,” Lenters says.

So the fact that there is all that ice on the lakes suggests the lakes lost a lot of water to evaporation. And Lenters says there’s evidence that did, in fact, happen.

“We actually know this from measurements we have out on an island in Lake Superior,” he says. “This past year we’ve actually had really high evaporation rates through today.”

Lenters says that evaporation should be offset by reduced evaporation because of the ice cover. So lake levels should continue to rebound. Lenters says the wild card is precipitation. And there aren’t great models for predicting that.

“Like living in a rock tumbler”

So what else has this icy winter meant for the Great Lakes? For the shipping industry, it’s meant some major headaches. Great Lakes freighters carry cargo like coal, grain, aggregate (think stone and road salt – kind of important right now), and taconite, which is used to make steel.

Credit Sarah Hulett / Michigan Radio
Michigan Radio
Lt. Kenny Pepper's crew has spent 150 hours this winter freeing vessels stuck in the ice – 27 so far.

It’s the job of the U.S. Coast Guard to keep shipping lanes open in those waterways. And this has been a very busy season for the eight cutters that work the Great Lakes.

“We’ve helped more than 27 vessels stuck in the ice,” said Lt. Kenny Pepper, captain of the USCGC Morro Bay. “It’s been about 150 hours dedicated to those assists, not to mention all the others spent just doing preventative ice breaking.”

Pepper and his crew have been breaking ice since Dec. 16, with only one stop back in home port since.

“A lot of captains say they haven’t seen an ice season like this for at least for 25 years,” said Operations Spec. 1st Class Galen Witham. “With the jet stream that dropped down bringing arctic air, it really became what we call a fast-making ice season.”

Sarah Hulett is Michigan Public's Director of Amplify & Longform, helping reporters to do their best work.
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