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Dredging Lake Mitchell 'pointless without wetlands upstream,' say DWU students who conducted lake project

“What they found is that if that wetland is in place and we take out half the sediment, then eventually — with about 10 or so refills of the lake — the remaining sediment would get sufficiently rinsed out if the water coming in is cleaner,” DWU biochemistry professor Paula Mazzer said of the findings from the student-led lake project.

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Public Works Director Joe Schroeder explains the design of the city's wetlands on Friday while pointing out the areas along Firesteel Creek where the wetlands will be constructed. The wetlands will aim to reduce the phosphorus and sediment entering into the lake from Firesteel Creek.
Adam Thury / Mitchell Republic
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MITHCELL — How much impact will the city’s wetlands along Firesteel Creek paired with a potential dredging project have on improving Lake Mitchell’s algae-laden water quality?

That’s a question a group of Dakota Wesleyan University students sought to answer through a project that examined the city’s Lake Mitchell restoration plans. And the findings of the students’ research indicated that creating efficient wetlands along the creek and dredging at least half of the sediment on the lake bottom could reduce the phosphorus that’s known for causing algal blooms by as much as 60%.

As the city gears up to construct four wetlands on a piece of low-lying land along Firesteel Creek while also mulling over a potential multi-million-dollar dredging project, the DWU students concluded that dredging the sediment out of the lake “would be pointless” without the creation of a wetland up stream.

“When we add in this wetland, it’s going to help no matter what. We’re going to see a large reduction,” said Carter Jahnig, a biochemistry student who participated in the lake project. “If we don’t have the wetland it’s not going to work, even if you took all of the sediment out of the lake.”

The students estimate the amount of phosphorus that is in the lake on a typical summer is 704 parts per billion. For the lake to see significantly fewer algal blooms and support healthier wildlife populations, Jahnig said the amount of phosphorus would need to dip below 80 parts per billion.

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When Lake Mitchell contains at least 80 parts per billion of phosphorus, Jahnig said the body of water is in what’s called a “eutrophic” condition. A lake that’s in a eutrophic state contains high amounts of nutrients, namely phosphorus and nitrogen, which supports an abundance of algae — including harmful types — and plant life.

“The idea is to get it below 80 parts per billion for eutrophication to stop and stop impeding the wildlife,” Jahnig said.

Algae in Lake Mitchell. (Matt Gade / Republic)
Algae warning signs are posted at Lake Mitchell in 2019.
Mitchell Republic file photo

With the creation of a wetland along Firesteel Creek, Hunter Shanks said it could reduce the amount of phosphorus in the lake by 25% on the low end and 60% on the high end.

“A low success wetland of about 25% would leave 528 (phosphorus) parts per billion, and a high success wetland would leave 60% or 282 (phosphorus) parts per billion,” said Shanks, a DWU biochemistry student who participated in the lake project.

According to Morgan Oedekoven, a DWU student studying biochemistry, it would take roughly two years for a low-efficiency wetland to lower the phosphorus entering the lake to 520 parts per billion.

“If we remove 50% of the sediment, it would not be useful unless we had a wetland of some efficiency,” Oedekoven said.

To create a more efficient wetland, Shanks pointed to yearly controlled burns as a method that would drastically enhance the effectiveness of the wetland in reducing phosphorus entering the lake.

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Shown here is the area where the city will be constructing wetlands along Firesteel Creek to reduce the phosphorus and sediment flowing into the lake.
Adam Thury / Mitchell Republic

“A controlled burn of a wetland every fall would increase biochar, which essentially is just free carbon that would be placed in the water to attach onto the water that next spring,” Shanks said during the students’ presentation on Monday at the Mitchell City Council meeting. “Free carbon is really good at catching on to loose phosphorus. It binds to it. Instead of having loosely bound phosphorus, we would now have bound phosphorus.”

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The DWU students built their findings from previous studies on the lake, including recent information from Barr Engineering’s preliminary lake dredging design. While previous studies indicated that 53% of the phosphorus and sediment entering the lake comes via Firesteel Creek and 47% is in the lake itself, the students further investigated how much phosphorus is unloaded into the body of water in a wet year and dry year to quantify the impact an upstream wetland would have on nutrient reduction.

In an average year, the students projected the lake experiences about five refills. After five refills of the lake, Shanks said roughly 23,000 kilograms of phosphorus is unloaded into the lake without any wetlands up stream.

According to the students’ findings, a low-efficiency wetland would drastically reduce phosphorus levels entering the lake to 2,620 kilograms in a year that the lake sees five refills. The students claim a high-efficiency wetland that uses controlled burns on an annual basis would outright eliminate the loosely held phosphorus unloading into the lake.

“After five refills, a high-efficiency wetland will eliminate all the loosely held phosphorus and will become strictly dependent on the input concentration to the lake from the creek,” Shanks said. “In 2019 when we had the floods, the lake refilled close to 10 times that year. That’s when you would get a high-efficiency wetland.”

City’s wetland project taking shape

After listening to the DWU students’ presentation during Monday’s Mitchell City Council meeting, it provided a degree of confidence for some city leaders who have been developing the wetland project that crews are set to begin creating this summer on a portion of city-owned land along Firesteel Creek.

Public Works Director Joe Schroeder said the students’ information on the effectiveness wetlands have on improving water quality at nearby lakes was “eye-opening.”

“It’s good to have a second source to backup additional data that we have on the wetland project,” Schroeder said.

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Shown here is the area where the wetlands along the creek will be constructed.
Adam Thury / Mitchell Republic

According to Schroeder, the four adjacent cattail-filled wetlands will cover roughly 30 acres of land along Firesteel Creek near the Kelley house that the city purchased several years ago in order to attain the 371 acres of land surrounding the creek for the wetland project to take shape.

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Ducks Unlimited recently wrapped up the design of the wetland, freeing up the city to open the construction portion of the project for bids this summer. Schroeder estimated a completion date for the wetland project could come as early as June 2023.

“This design does not include a weir structure or dam,” Schroeder said, noting the plan entails excavating dirt to create the wetlands.

Initially, the construction of a dam in the wetland area was discussed when the city purchased the 371 acres in 2019. Instead, crews will excavate dirt in four adjacent areas surrounding Firesteel Creek to form the wetlands. Schroeder said the cattails that will be planted in the wetlands will capture the phosphorus and sediment in such a way that eliminates the need for a dam to be constructed.

Considering the research that has shown the effectiveness of wetlands reducing nutrient loads flowing into lakes, council member Dan Allen said the DWU students’ presentation underscored the need to continue focusing efforts on working upstream Firesteel Creek.

As city leaders have been working with farmers who grow crops and graze cattle along the Firesteel watershed by encouraging producers to practice sustainable farming in attempt to reduce agricultural runoff entering the creek, Allen said some producers upstream have recently shown interest in allowing wetlands to be built on their lands.

“We recently identified two growers who might come on board with providing some additional land for wetlands,” Allen said.

‘We don’t want a lake that’s too clean’

During Barr Engineering’s presentation of the preliminary lake dredging design a few months ago, project leaders unveiled a design that called for dredging roughly 50% of the sediment on the lake bottom, with the brunt of the sediment removal taking place on the west side of the lake where the creek connects to the body of water.

Some city leaders were concerned whether dredging 50% of the lake sediment would result in an effective reduction in algal blooms that have been plaguing Lake Mitchell for decades.

Paula Mazzer, a biochemistry professor at DWU who helped guide the students’ lake project, said answering that question was a big motivator for the students’ research. And the results are promising for the future of Lake Mitchell.

“What they found is that if that wetland is in place and we take out half the sediment, then eventually — with about 10 or so refills of the lake — the remaining sediment would get sufficiently rinsed out if the water coming in is cleaner,” Mazzer said. “In a couple years, we could reset the lake 30 or 50 years, depending on the efficiency of the wetland.”

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An aerial view of Lake Mitchell.
Republic

While Mazzer is confident that the wetland project paired with dredging half of the lake’s sediment will have a profound impact on improving the water quality, she said the reality is that “it’s never going to look like a northern Minnesota lake.” And that’s a good thing, Mazzer says.

“We don’t want a northern Minnesota lake. We want a good intermediate productivity lake that will make way for good fishing, healthy wildlife like waterfowl,” Mazzer said.

The students also found that there are portions of the lake where phosphorus is tightly bound, which Mazzer said isn’t the type of phosphorus that’s primarily causing the algal blooms and high cyanobacteria levels.

“It’s only the loosely held stuff that matters because the rest is so tightly held that it will stay put,” Mazzer said. “It’s that 9,000 kilograms of loosely held stuff that’s causing us problems.”

While the city’s wetland project is fully funded and less than a few months away from kicking off, a dredging project has yet to be approved as Barr Engineering is in the middle of a final design.

Rough cost estimates of a lake dredging project have come in at a little over $15 million. Mayor Bob Everson said he’s committed to dredging and continuing upstream projects like the wetlands.

“We are continuing to look for more places where we could do some wetlands along the creek. We know both an in-lake solution and upstream solution are going to get this lake in a much healthier state,” Everson said.

Sam Fosness joined the Mitchell Republic in May 2018. He was raised in Mitchell, S.D., and graduated from Mitchell High School. He continued his education at the University of South Dakota in Vermillion, where he graduated in 2020 with a bachelor’s degree in journalism and a minor in English. During his time in college, Fosness worked as a news and sports reporter for The Volante newspaper.
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