The warning from the county government was grim. The Rapidan Dam, a feature of the southern Minnesota landscape for more than a century, was “in imminent failure condition.”
“We do not know if it will totally fail or if it will remain in place,” officials said on Monday as dead trees and other debris piled up at the dam and floods overwhelmed the Upper Midwest.
At one point, a support structure alongside the dam partly failed and gushing waters carved out a nearby cliff. Video footage showed a nearby building collapsing on itself and being sucked into the river. But by Tuesday, the main part of the dam was intact, and water flows were beginning to slow. A worst-case scenario, it seemed, might have been averted.
“The Rapidan Dam, we think, is going to continue to hold up,” Bob Jacobson, the commissioner of the Minnesota Department of Public Safety, said late Tuesday afternoon after state officials flew over the dam to survey the damage. “But there are going to be more assessments in the future.”
Experts said the damage and continuing risk in Minnesota underscored the decaying state of the country’s dams and the dangers they could pose when things went wrong. Many catastrophic floods begin with dam failures, and breaches in recent years in states like Michigan and Nebraska have led to widespread destruction.
With climate change making dangerous weather more common, and the average age of an American dam approaching 60, the problems are only expected to get worse.
“It’s the perfect storm, because we are dealing with more severe extreme weather events, and because it’s just the nature of time these dams are getting older,” said Hiba Baroud, an associate professor of civil and environmental engineering at Vanderbilt University.
The problems at the Rapidan Dam, about 90 miles southwest of Minneapolis, came after a period of severe rain that led to widespread flooding in Iowa, Minnesota and South Dakota. At least two deaths were linked to the floods, which destroyed homes, swamped farmland, overtopped levees and caused a major railroad bridge to collapse.
“This, again, was an unprecedented amount of rainfall that came in a very short period of time,” Gov. Kristi Noem of South Dakota said. “Many of these communities, I don’t know how they could have prepared for what they saw.”
Though river levels were dropping on Tuesday in many of the hardest-hit places, some evacuation orders remained in effect, major roads were closed and cities downstream were bracing for potential damage. Moderate flooding was expected in the coming days on portions of the Missouri River in Iowa and Nebraska, and major flooding was expected on parts of the Mississippi River in Iowa and Minnesota.
“We’re not out of the woods yet,” Gov. Tim Walz of Minnesota said. “We have not reached peak water levels in many of these communities yet,” he added, “and so that is going to add to this.”
The Rapidan Dam, a 475-foot-long concrete behemoth, is an age-worn relic of a different era. It was completed in 1910 and, over the generations, became a local landmark on the Blue Earth River that was situated near a park and that supplied power for up to 3,000 homes. Mr. Walz said he had been by the dam “a hundred times on my bike.”
But Rapidan’s problems were well known before this month’s flood, which surpassed a 2019 flood to become the dam’s second-worst on record.
Blue Earth County, which for decades has managed the dam, told residents on its website that “flood events and the toll of time have caused significant damage to the dam’s structure and usability.” A federal inspection this spring noted some cracking and falling concrete, but found that Rapidan was in “overall satisfactory condition.”
A study in 2021 suggested either spending $15 million over four years to repair the dam or $82 million over 10 years to remove it. Most residents who responded to a survey two years ago seemed to favor repairing it.
Rapidan’s problems are far from unique. The American Society of Civil Engineers gave the country’s system of 91,000 dams a D grade in 2021, its latest assessment. With the average dam being 57 years old at the time, the organization said that “the majority of dams will not have been built to current standards, let alone incorporate newer standards that improve their resilience and reduce the risk to downstream areas.”
Despite those well-known problems, Upmanu Lall, the director of the Water Institute at Arizona State University, said too little was being done.
“It costs money to remove it. It costs money to fix it,” Dr. Lall said. “We have to do one of the two. We are not doing either one.”