Drone surveys help scientists understand impact of Rapidan Dam ‘avulsive failure’

Researchers are flying drones over the Blue Earth River to study what happened after flooding breached the now-defunct Rapidan Dam last summer. 

Fresh from its flight, a large drone buzzed loudly as it slowly lands. 

During its expedition, it captured photos and data on changes to the Minnesota Valley resulting from the deluge. 

Phil Larson, earth sciences director at Minnesota State University Mankato, is helping lead the project. He hopes to present their findings to Blue Earth County and other public officials who manage similar aging infrastructures. He is also using it as a learning opportunity for his students.

“They’re getting this very rare and unique experience to be able to be part of this work to understand how river systems function, how landslides and hill slopes function,” Larson said. “But, also thinking about how we work and live within river systems with this dam.”

In late June, the flooding Blue Earth River carved a new channel around the dam. It destroyed a family residence and a county storage shed. It also flushed downstream the equivalent of more than 600 Olympic-sized swimming pools of sediment built up behind the dam over more than a century.  

Larson described what happened at the Rapidan Dam as an “avulsive failure.” The Blue Earth River went around the Rapidan Dam and cut through soft sediments, instead of causing the collapse by going through the dam structure. He said there isn’t much documentation in scientific or government literature on avulsive dam failures, though he’s sure it happens. 

“The river just evolved and went over here and said, ‘We don’t want to, I don’t want to deal with the dam,’” he said. “‘I’m going to cut down over here through this landscape, because it’s weaker, softer materials.’ So, the river now has been locked into this new course here, and we have this dam just sitting here doing nothing.”

Documenting and researching this type of dam failure is a way for researchers to help get data out to public officials and respond in case something like this were to happen again in another place. 

There have been more dam failures in the upper Midwest region since Rapidan. The University of Wisconsin-Eau Claire’s Zach Hilgendorf, the drone project’s other leader, said it’s a warning sign. 

“I hope that the conversation looks at this as a cautionary tale of, we need to consider when we have aging infrastructure and when we have the trends that we’re seeing climatologically,” Hilgendorf said. “We maybe need to take these a lot more seriously, to understand and plan for and make our landscapes more resilient, to make our communities more resilient.”

Students from MSU Mankato, University of Minnesota and University of Wisconsin-Eau Claire are doing the drone surveys, funded by a National Science Foundation grant. 

The drone flight studies will continue through summer 2026. Blue Earth County voted to remove the Rapidan Dam, though this can take a few years pending funding sources and different variables. 

A remarkable landscape

The researchers hiked down into what was once the reservoir behind the Rapidan Dam. The scouring of the sediment-laden water transformed it into what Larson described “a pretty remarkable landscape.”

“It looks like we’re on the moon or something,” he added. 

The river carved new cliffs, but also left some sludgy sandbars, dotted with ancient logs and clam shells. Hilgendorf said these are remnants left by glaciers long-gone. 

“That’s all that sediment had been deposited and built up behind the reservoir, and that was what they were concerned about was going to get released and remobilize,” he said. “And that’s the stuff with phosphorus in it that they’re concerned about for ecological perspectives, and that’s the stuff we’re watching fail and fall into the river and get mobilized to move downslope or downstream.”

Downstream eventually means into the Minnesota River, and eventually Lake Pepin on the Mississippi River. Hilgendorf said that will take a long time, maybe even a century. 

“So all that sediment is working like a slug of sediment moving downstream, and that’s just slowly going to work through and churn through that sediment through time,” he said. “And that’s what we’re monitoring, is kind of how that is changing, and the rate that it’s changing with all of these different surveys that we’re doing.”

Jon Lore, the Minnesota Department of Natural Resources clean water legacy specialist, said the agency is watching that impact. He’s already seen the effect of the heavy sediment on native fish species in the area. 

“There used to be really deep pools where people would fish and people really liked to catch flathead catfish, walleye, sauger and now that’s all full of reservoir sediment,” Lore said. “So, the deepest water is like anywhere from one and a half to four feet deep at low flow.”

Lore said the DNR is also tracking fish movements in the Minnesota River to see if they’re finding new habitats and adapting to the new riverscape. It’s possible fish species blocked  from swimming upstream by the dam for more than a century can now make the journey. 

“As this progresses over time, that sediment will continue to slowly move downstream … the river will naturally create those habitats,” he said. “Again, it just…it just took on a lot at one time. So, now it has to take time and flood pulses to move that sediment and to create those habitats again.”

And it’s not just fish. Lore points to new all-terrain vehicle tracks in the sand. People are finding this new habitat too. 

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