Go with the flow: Minnesota towns see success replacing aging dams with rock rapids

Rock rapids are shown in Pine River.
In Pine River, a 2022 project involving the city and the Minnesota Department of Natural Resources replaced an aging dam with rock arch rapids, a series of boulders that stretch across the water in descending rows for several hundred feet. It's one of around 75 locations in Minnesota where dams were replaced with rock rapids, also known as riffles.
Kirsti Marohn | MPR News

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The Minnesota Department of Natural Resources has been helping some communities replace old or outdated dams with a series of boulders that benefit fish and wildlife and pose less of a safety risk.

Last week’s partial failure of the Rapidan Dam near Mankato has renewed concerns about the safety of aging dams across Minnesota.

In some communities, old dams have been removed and replaced with a series of boulders known as rock arch rapids or riffles. They’re designed to allow the river to flow more freely and fish to swim upstream. They also help avoid costly dam repairs — or worse, a collapse.

More and more communities are having to decide what to do with outdated dams, said Amy Childers, river ecologist and outreach coordinator with the Minnesota Department of Natural Resources.

“A lot of these are designed for a 50-year lifespan, and they’re way beyond 50 years,” Childers said. “Structurally they’re having issues, and a lot of them aren’t serving their original purpose.”

dam with debris
Debris lines the top of the Rapidan Dam as seen from upstream on June 25. While the Blue Earth River carved a new channel around the western side of the dam, local officials say the dam structure itself is solid.
Mark Zdechlik | MPR News

Many dams no longer produce electricity or control floods. Many are full of debris and sediment that’s built up for years — the kind that plugged the Rapidan Dam and caused water to flow around it.

The nonprofit group American Rivers supports removing dams for ecological benefits. But safety is becoming an even bigger issue, said Brian Graber, senior director of river restoration.

“We’re seeing far too many dam failures, far too many incidents like the Rapidan Dam,” Graber said. “Often, dam safety is an incentive. It’s a reason why dam owners choose to remove dams before they become problems.”

The DNR is closely watching rivers in southern Minnesota where dams were replaced with rock rapids, including Windom, Oronoco and Jackson, but there’s not much concern.

The rapids are built to handle a 100- to 500-year flood, said Paul LeClaire, a DNR stream restoration engineer. They’re also designed to concentrate the flow in the center of the channel, reducing the risk of the river overflowing and eroding its banks. And debris tends to flow over the top of the rocks instead of getting caught, LeClaire said.

Rock rapids are shown in Pine River.
Pine River's rock arch rapids are designed to handle higher flows.
Kirsti Marohn | MPR News

A gated dam that had held back the waters of Norway Lake in downtown Pine River for more than a century was removed in 2022. The water now tumbles over staircase-like boulders that stretch over several hundred feet, with pools in between.

Pine River hasn’t had as much rain this summer as some parts of Minnesota. But even if it had, public works director Mike Hansen thinks the rapids would be just fine.

“When we built this structure, it was built to handle a heavy, heavy rainfall," Hansen said. 

Pine River is one of about 75 places around Minnesota where rock rapids have replaced dams. The first project was built in 1994 in Fergus Falls. One of the most recent was last year in Pelican Rapids.

A map that shows the locations of dams that were replaced with rock rapids
This Minnesota Department of Natural Resources map shows all the locations where dams were removed and replaced with rock rapids to reconnect water bodies and improve fish passage.
Courtesy of the Minnesota Department of Natural Resources

The DNR has helped cities design and pay for the projects to improve safety and habitat for fish and other aquatic species, such as freshwater mussels. There’s typically funding for two or three projects a year through the state’s Outdoor Heritage Fund, Childers said.

In Willow River in eastern Minnesota, the DNR removed a dam after it collapsed in 2016 and replaced it with rapids, which were completed in 2022.

Mayor Brent Switzer said there’s more water rushing over the rocks this year, but it hasn’t caused problems.

“Especially looking at some of the damages, the videos that I’ve seen out in western Minnesota, I definitely feel that this is a safer option for sure,” Switzer said. “The water just goes over it faster, and nothing downstream was having any issues with it.”

Not all dams are good candidates for removal. Some generate power or help control large rivers for navigation. Also, removing a dam can be controversial, especially when it's been a centerpiece of the community for a century or longer. 

But replacing a dam is often far less expensive than repairing it, especially when a city receives financial help from the state. And once the rock rapids are built, people tend to like them.

A scene of water moving down and over rocks.
The Willow River dam in Pine County, near its confluence with the Kettle River, failed in 2016 and was replaced with rock arch rapids, completed in 2022.
Courtesy of Minnesota Department of Natural Resources

Pine River expanded a city park next to its rapids. It’s now a gathering place where people eat lunch and kids play in the water.

“It seems like every day when I come by here, I see people taking pictures and whatnot,” Hansen said. “It’s definitely unique to have right in the center of town.”

And the rapids need almost no maintenance compared to the old dam, which required a major reconstruction every 40 years or so.

“It’s built for a forever-type of standard,” Hansen said. “So we think it’s going to serve its purpose for many, many years to come.”

Kids are playing
Young people step from stone to stone across Norway Brook in Pine River, 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.
Kirsti Marohn | MPR News

According to the National Inventory of Dams, the average age of Minnesota dams is 60 years. A quarter of dams are listed in fair, poor or unsatisfactory condition.

The 114-year-old Rapidan Dam was found to be in poor condition in April 2023, according to the inventory.

Blue Earth County officials were considering whether to remove the dam at a cost of more than $82 million, or repair it for about $15 million. Both options had drawbacks. One of the concerns was the decades of sediment built up behind the dam, which is now washing downstream.

Childers said the DNR did draft a plan for a series of rock arch rapids to replace the Rapidan Dam, but it stalled due to high costs. Had it been carried out, “they would have slowly lowered the crest bit by bit, and stabilized those sediments in place,” she said. “It wouldn’t have been all-at-once kind of situation that we’re seeing now.”

A heron looks down at water rushing down rock rapids.
A great blue heron rests on a rock amid the rapids in Pine River on June 26. The DNR says rock riffles benefit fish and wildlife.
Kirsti Marohn | MPR News

As climate change causes heavier rain events and rivers carry higher flows, more communities likely will experience flooding and costly infrastructure problems. The high-profile Rapidan Dam failure could spark more interest from cities in seeking state funding to remove old dams, Childers said.

“I would hope that communities would be more receptive to this idea,” she said. “They’re just not serving their original function anymore, and they’re not helping with flood control.”

Tom Scheck, deputy managing editor of investigations for APM Reports, contributed to this report.