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MANKATO — The Rapidan Dam, about 12 miles southwest of here, is in "imminent failure condition," Blue Earth County officials warned early Monday.

Hours later, the west bank of the century-old dam had already washed out, along with several nearby buildings, sending construction material and trees downstream, as the Blue Earth River continued to rise after heavy rains fell on the region over the last several days.

"The dam could fail," Eric Weller, Blue Earth County emergency management director, said Monday.

Aaron Lavinsky
Video (00:50) Quick-flowing water erodes the earth around the Rapidan Dam near Mankato on Monday.

The river level at the dam is at 28 feet, with water expected to rise another foot or so over the next 24 to 36 hours before cresting, said Jeff Johnson, director of public works for Mankato. The dam is built to hold 39.5 feet of water, Johnson said. The dam, on the Blue Earth River, hasn't generated power in more than five years and is now owned by the county.

There is no current plan for a mass evacuation and everyone deemed to be in danger has already been notified and many have evacuated, officials said.

Brush and sediment rerouted the pathway of the river, causing water to go around the dam, Rep. Brad Finstad said in a press conference about the flooding Monday afternoon.

"The water is raging, and it's just cutting the shoreline away," Finstad said. "It is the power of Mother Nature, when you see a rain like this, and we have a lot of questions behind that dam."

The Federal Energy Regulatory Commission (FERC) inspected the dam on May 21, and found the plant was in "overall satisfactory condition" and had no major problems that required immediate action.

Still, the dam has long been plagued by problems that posed vexing and expensive questions for county leaders. In 2021, Barr Engineering analyzed restoring or removing the dam after it was badly damaged by flooding and ice in the winters of 2019 and 2020 and has not generated power since.

Workers with the U.S. Army Corps of Engineers discuss the Rapidan Dam failure situation Monday near Mankato.
Workers with the U.S. Army Corps of Engineers discuss the Rapidan Dam failure situation Monday near Mankato.

Aaron Lavinsky, Star Tribune

Ryan Thilges, county engineer and public works director for Blue Earth County, said logs and debris from the river valley ended up plugging five steel gates that were open to allow as much water to flow through as possible.

Without debris, the dam should have been able to clear the water, but instead the water rushed over the top and to the side. Blue Earth didn't have the necessary equipment to clear debris, and a contractor with a critical piece of equipment was hours away, and the high river posed significant safety concerns.

Rising water on the river undercut an Xcel Energy electrical transfer substation.

A crew of about 170 employees traveled to the dam to stack sandbags and set up a replacement substation, said Kevin Coss, spokesman for Xcel Energy. Some 579 customers are without power as of Monday evening, with the energy company hoping for power to be restored by midday Tuesday.

Randy Hanson, who lives about two miles away from the dam, said he lost power early Monday. He drove to the dam at around 8:30 a.m. and took video of the water rushing around the side.

"I was just awed at the power; you can't imagine the power that water has," Hanson said, noting the dam is a landmark in the community.

"It was so heartbreaking," Hanson said. "To see it possibly gone."

A flash flood warning by the National Weather Service for the river basin around the dam went into effect Monday morning and will continue until 10:30 a.m. Tuesday. The city of North Mankato has declared a flood emergency and is building an earthen levee — a dirt wall — "out of an abundance of caution," said Kevin McCann, city administrator.

Authorities are monitoring bridges on county roads 33 and 90 for debris passing downstream and might close them if needed.

Rains over the last week have dumped slightly more than 4 inches of rain on Mankato, the National Weather Service said. The rain flooded basements throughout Mankato and closed numerous roadways along Minnesota River. Mankato estimates almost $5 million in damage to public infrastructure due to storms, city manager Susan Arntz said Monday afternoon.

Melissa Peters of St. Peter watches in disbelief as water rushes over the Rapidan Dam on Monday. “I’m really sad to see it potentially go,” said Peters, who is also concerned about flooding in her hometown.
Melissa Peters of St. Peter watches in disbelief as water rushes over the Rapidan Dam on Monday. “I’m really sad to see it potentially go,” said Peters, who is also concerned about flooding in her hometown.

Aaron Lavinsky, Star Tribune

Repair or remove?

Thilges said the dam was built on erodible sandstone bedrock and is faced with a bevy of problems identified in a series of reports on the infrastructure.

"The overall structure of the dam is in fair condition however there are some structural and other issues with the dam," Thilges said.

The county opted against repairs needed to reopen the plant because the costs were too high for what is a minimal power output, said county board chairman Kevin Paap. The Barr study says the plant could produce up to 5 megawatts of electricity, equivalent to about three wind turbines.

Scrapping the dam isn't cheap either. The 2021 study estimated repairing the plant would cost at least $15 million but removing the dam would cost $81 million.

Thilges said Blue Earth decided to surrender its license with FERC, which would turn oversight of the dam over to the Minnesota Department of Natural Resources. That is still being processed, stalling a decision on whether to remove the dam completely. Blue Earth did not make any major investments in the facility in the meantime but has done routine safety monitoring.

"We're just kind of waiting on FERC is what it amounts to," Paap said.

The Barr report also said a dam breach would be a significant environmental hazard because of "agriculturally impacted sediments that fill the reservoir."

Once the water recedes, county and state officials will have to grapple with tough questions on what to do with aging dams such as the one in Rapidan, U.S. Sen. Amy Klobuchar said in a press conference Monday.

"Is it better to get a new one? Is it better to fix this?" Klobuchar asked. "And those are all questions that we will have to confront, because it's the aging infrastructure that is generally the problem."