Environmental News

Impacts of Rapidan Dam breach on Minnesota likely to be long-lasting

A view of the Blue Earth River after it cleared by the Rapidan Dam.
This long-exposure drone photo provided by AW Aerial shows a home as it teeters before partially collapsing into the Blue Earth River at the Rapidan Dam on June 25.
Andrew Weinzierl | AW Aerial via AP

Metal barricades close access to the County Road 9 Bridge on Glacier Road near rural Rapidan. This used to be one of the busiest commuter roads in the area.

Flood waters badly damaged the bridge in late June. Drivers will be redirected for the next several years as Blue Earth County demolishes and replaces the bridge.

County engineer Ryan Thilges said contractors are preparing to take down the old structure.

“They’re in that kind of evaluation and analysis stage of providing the county with a final plan for demolition that will then be reviewed by our structural engineers,” he said. “And once that’s approved, then they would begin removal operations.”

Two images compare the river damage caused by flooding.
This combination of satellite images provided by MaxarTechnologies shows views of the Rapidan Dam on Minnesota’s Blue Earth River, south of Mankato, on Sept. 6, 2011, top, and bottom on June 26, 2024, after floodwaters overcame parts of the structure following days of heavy rainfall.
Satellite image ©2024 Maxar Technologies via AP File

The bridge project is broken down into three parts: removing the structure, a design phase, and then replacement.

Thilges said safety is paramount, but the contractors are working under a deadline.

“At this point, it’s going to be pretty tough for them to hit it, given where we are and the amount of work that needs to be done,” he said. “But our objective is still to get the bridge removed prior to spring flood. That’s really what we’re targeting.”

Blue Earth County also voted to remove the historic Rapidan Dam, which stands next to the County Road 9 Bridge. They’re expected to use a combination of federal, state and county money.

However, there’s another issue affecting a larger area. When the river created a new channel around the Rapidan Dam, it unleashed more than a century’s worth of sediment downstream.

The Minnesota Pollution Control Agency released a report that sediment, phosphorus and organic nitrogen remained elevated into early August and continued to be above normal values into the fall. Water clarity didn’t return to expected values for this time of year.

“My concern overall is what’s gonna happen downstream” said Philip Larson, director of Minnesota State University, Mankato’s earth science programs.

He said he’s worried about what happens when the sediment gets down into the Mississippi River and into Lake Pepin.

To understand what happened this summer, Larson and a group of researchers from Minnesota State University, Mankato and University of Wisconsin-Eau Claire are collecting data to better understand climate change and erosion that may cause dam failures into the future.

“We want to try to understand and quantify that because I think that’ll help us understand when a dam fails what does the erosional process look like and how much sediment is flushed out and where does it come from? How long does it take to adjust,” he said.

Larson says they are working with the Minnesota Department of Natural Resources to plan a drone survey in early January to photograph any changes to the area. They also plan to excavate sediment with students in warmer weather.