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By Hannah Yang, Minnesota Public Radio News

More than a century’s worth of sediment was unleashed downstream into the Blue Earth River after the partial failure of the Rapidan Dam last week. The impacts to water quality and wildlife may not be known for another couple years, experts say.

There was an estimated 11.6 million cubic yards of sediment behind the dam. And it was high in phosphorus and nitrogen because of nearby agricultural runoff, said Minnesota Department of Natural Resources ecologist Neil Haugerud.

“When you release that kind of sediment in that big of an event, you can fill up downstream pools, you can cover up critical habitat for fish and macroinvertebrates,” Haugerud said. “It has an effect on the ecosystem downstream, and how that effect will go on into the future will be yet to be seen.”

That concern is one of the reasons Rapidan Dam didn’t come down after it stopped generating hydropower in 2019 because of frequent flood damage.

Initially, Blue Earth County wanted it removed. But demolishing the dam in a way that mitigated the effects of unleashing all the sediment — by integrating rapids or placing sediment onto the adjacent riverbanks — would have taken a decade and cost millions.

Now county and state agencies are dealing with the repercussions, most urgent of which may be the County Road 9 bridge next to the dam. The impounded sediment held the bridge piers on top of the sandstone.

Now those are exposed, threatening the bridge’s stability.

“Obviously, we’ve lost that in an uncontrolled manner,” said Blue Earth County Public Works Director Ryan Thilges.

“We’ve been in constant conversation with the Army Corps of Engineers and others trying to see what we can do to mitigate further erosion, and I’ll be perfectly honest, all the solutions we came up with had almost as bad or worse adverse impacts that could affect the dam stability farther or could result in damage to the bridge or additional erosion. We’ve had limited success in finding a solution due to rapid flow rates and the volume of water going through.”

For now, the bridge remains closed to cars and pedestrians as officials monitor its safety.

Across the country, private dam owners and governments are grappling with what to do about aging dams. There’s a growing push to take them down, driven by concerns over treaty rights and wildlife, but also because most weren’t built to deal with the kind of rain events our modern climate is producing. The average age of the nation’s dams is 57, and 70 percent of them will be over 50 years old by 2030, according to the American Society of Civil Engineers.

In Minnesota, some dams have already been replaced with rapids. Many in the state are also working to limit the ecological risk of potential dam failures by limiting farm runoff that becomes sediment.

“I think everybody has seen videos, maybe drone imagery, of how powerful that river has become,” said Shane Bugeja, an educator for the University of Minnesota Extension based in Mankato, Minn. “One of the things we deal with in agriculture is we want to keep the soil in its place and then we have these disasters that happen. That soil’s got to go somewhere. Obviously, it has consequences for water quality downstream.”

Experts and agencies are exploring restoring floodplains and planting cover crops. Many farmers are also changing how they fertilize their fields because of escalating costs and incentives, making what does go into waterways lower in phosphorus and nitrogen.

But those are long-range efforts. In Blue Earth County right now, the immediate next step is simply to wait for river levels to go down so the DNR can assess the impacts of Rapidan Dam’s partial failure, Haugerud said, adding that it might even take several years to fully understand what those effects might be.