Rock rapids replace aging dams to let Minnesota rivers flow

rock arch rapids require less maintenance, help connect water bodies and allow fish and mussels to move upstream

Young people step from stone to stone across Norway Brook in Pine River, Minnesota, where a former dam was removed and replaced with rock arch rapids that allow freer flow of water. The rapids are a popular gathering spot for people who want to fish, splash around or listen to the sound of running water. A popular swimming hole near the former dam has been replaced with a safer sandy beach farther upstream.
Kirsti Marohn | MPR News
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PINE RIVER, Minnesota — For more than a century, a gated dam in Pine River, Minn. held back the water that flowed out of Norway Lake into the Pine River and eventually to the Whitefish Chain of Lakes.

The Norway Brook dam was a centerpiece of the small north-central Minnesota city of about 900 residents, a summertime gathering spot for its annual summer festival and for kids itching to cool off.

But like many other dams that hold back water in Minnesota rivers, the gated structure, built in 1910, was aging and in need of repair.

"It was listed as a high-hazard dam,” said Bryan Drown of Bolton & Menk, the city's engineer who helped Pine River decide what to do with the aging structure. "Not necessarily like it was going to fail tomorrow. But they knew something had to be done in the relatively near future."

Bryan Drown, city engineer with the firm Bolton & Menk, talks about the 10-year effort to replace the aging dam in Pine River, Minnesota, with rock arch rapids, which will require less maintenance, help connect water bodies and allow fish and mussels to move upstream.
Kirsti Marohn | MPR News

When the Minnesota Department of Transportation decided to replace Highway 84 that ran along the top of the dam with a separate bridge, city leaders considered several options: Rebuild the dam with some modifications, or remove it and replace it with something that would reconnect the lake and the river, and allow fish to travel upstream.


What they settled on was rock arch rapids, also known as a rock riffle. It’s a series of boulders that stretch across the water in descending rows for several hundred feet, with pools of water in between – sort of a giant staircase for fish.

Owen Baird, fisheries management specialist with the Minnesota Department of Natural Resources in Brainerd, helped with the project, which received a $2.2 million grant from the state’s Outdoor Heritage Fund.

Baird said dams prevent fish, such as walleye, white suckers and redhorse suckers, from swimming upstream.

"They get to the dam, and they hit that and stop,” he said. “Now they have an opportunity to move farther upstream and find new habitats for spawning.”

Many fish species use faster-moving water and rocky rapids to spawn and place their eggs, Baird said. Often, those narrower stretches of river are also where dams were built, he said, “so a lot of habitats have been lost.”

Dams also pose a barrier to native freshwater mussels, whose life cycle depends on their larvae hitching a ride on fish. And they have other drawbacks, including holding back sediment that builds up in lakes and reservoirs instead of naturally flowing downstream.

Dams also pose a risk to the public, Baird said.

"When you hold back a lot of water, there's that safety issue,” he said. “If the dam were to break, what happens and who gets flooded?”


Aging liabilities

Many of Minnesota's dams were built a century or more ago to control lake levels or generate hydropower. But the average lifespan of a dam is about 50 years, and the structures are no longer fulfilling their original purpose, said Amy Childers, outreach specialist for the DNR’s river ecology unit.

“Many of these are reaching the end of their design’s life,” she said

A kayaker paddles down Knutson Dam rapids on Cass Lake in Minnesota. The 2015 project removed the failing Knutson Dam and reconnected the Mississippi River at the outlet of Cass Lake with rock arch rapids, restoring spawning habitat for warm-water species such as walleye and white sucker.
Courtesy of Minnesota DNR

Also, most dams weren’t designed for heavier flood events and more water runoff caused by climate change, so there have been dam failures around the state, Childers said.

"So it comes down to trying to decide if it's worth rebuilding these structures,” she said. “It's very costly, and I think people are starting to realize the impact they're having on the river networks."

Ideally, the best option for a river's health is to completely remove a dam and return the river to its natural form.

But Childers said that's not always feasible, when there are property owners who would be affected by a sudden change in water levels, or a large amount of sediment built up above the dam that would be released downstream.

A rock riffle or rock arch rapids is a solution that allows the river to flow while still maintaining some control, and is being replicated on rivers across Minnesota. The DNR has been involved with about 90 projects so far, Childers said.

"As more and more people see them and see all the benefits, I think they're going to be more and more common,” she said.


The first rock arch rapids in Minnesota were built to replace the Midtown Dam on the Red River of the North in Fargo-Moorhead. The dam had been the site of at least 19 drowning deaths, Childers said. Since the project was completed in 1999, there have been none.

A dam on the Willow River in Pine County, near the confluence of the Kettle River, was breached and damaged during a 2016 rainstorm. The DNR helped replace it with rock rapids to allow fish, including lake sturgeon, to pass through.

After a lot of initial resistance, Childers said Willow River residents are mostly happy with the changes.

“It's really pretty to look at in the waters rushing over the rocks,” she said. “It's really hard for people to think of something different. But a lot of times, after we've built these structures, communities are pretty happy with it.”

The Willow River dam in Pine County, Minnesota, near its confluence with the Kettle River, failed in 2016 and was replaced with rock arch rapids last year.
Courtesy of Minnesota DNR

Adapting to change

In Pine River, residents seem to be adjusting to the changes, which were completed this spring.

On a hot afternoon last week, an angler cast a line while wading through the rocks. A group of teenage girls splashed through the water, then found a cool spot to sit and soak their feet.

The old swimming hole near the dam has been replaced with a larger, sandy beach farther upstream. And kayakers and canoeists can now shoot the rapids.

For recreation, “it’s just absolutely perfect,” said Tim Mahoney, a retired medical technician who spends summers on his property near Pine River.

"I think the fishing has been pretty good here on this river,” he said. “So I'm not sure if it improved the fishery. But it certainly, I think, makes it less stagnant and gets the flow going."

One of the biggest changes is the rushing and gurgling sound of water flowing over the almost 700 boulders and about 10,000 tons of smaller rocks that form the rapids.

Terri Dabill, a Pine River native and the city’s clerk-treasurer for the past 9 years, often spends her lunch break in the city park at the water's edge.

An angler fishes midstream of Norway Brook in Pine River, Minnesota, on Thursday, June 30. The city, with help from the Minnesota Department of Natural Resources, recently replaced a century-old gated dam with a series of boulders – known as a rock riffle or rock arch rapids.
Kirsti Marohn | MPR News

“It's beautiful,” she said. “I love coming down here and just watching what's going on. Just the sound of it is very peaceful to me.”

Dabill said the community's reaction has been mostly positive, aside from a handful of people who have complained that the sound of the rapids is too loud.

“Sometimes change is hard,” she said. “So you just smile."

The city changed the name of its annual Summerfest this year to Dam Days to celebrate the new rock riffle. Dabill said more changes are in the works to make the area around the rapids more enticing.

“We’re hoping to do as many things as we can to enhance it as a draw to our town,” she said.


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