Heavier rainfalls because of a warming climate is accelerating erosion in many parts of Minnesota, leading to more frequent landslides, mudslides and rockfalls.
And those landslides can cause significant damage to homes, businesses, roads and other infrastructure, and have occasionally turned deadly.
The sandstone bluffs and hillsides of the Minnesota River Valley are especially vulnerable.
On a recent foggy afternoon, North Mankato public works director Luke Arnold drives his city truck down narrow Judson Bottom Road. The road squeezes between the Minnesota River on one side and a towering bluff on the other.
Rock falls are quite common here. Arnold says city staff cleans fallen stone and other debris from the roadway every couple of weeks.
The picturesque roadway is popular among dog-walkers and cyclists, and he worries that some day, someone will get hurt.
“All it would take was a little bit larger rock fall at the same time that somebody was driving past,” Arnold said. “Even a relatively small rock fall or tree coming down right at the right time when somebody is either walking their dog or riding past on a bicycle.”
In July 2024, a rockfall caused by heavy rains covered parts of Judson Bottom Road and shut it down for a few weeks. The North Mankato City Council asked residents through a survey on its website about what to do with the road, and has even considered closing it permanently.
But 62-percent of city residents responding to the survey opposed closing the road and urged the city to repair it and keep it open.
Back in 2019, the road experienced similar rock falls and at that time, the council also considered permanently shutting down Judson Bottom Road — but ended up reopening it.
Arnold says North Mankato is currently launching a geotechnical investigation to better understand the potential for landslides on Judson Bottom Road and Lookout Drive on the blufftop above it because of the risks to life and property from such landslides.
The tipping point
In December of 2023 Jack Loso was killed in a landslide. The Minnesota State University, Mankato student was hiking at Minneopa State Park. His family is suing the state Department of Natural Resources, alleging it should have warned hikers that the area is prone to rockfalls.
The DNR declined to comment for this story. In 2013, a landslide also took the lives of two school children in a riverside park in St. Paul.
It’s incidents like those that are adding urgency to the work of geo-scientists as they study areas like the Minnesota River Valley and elsewhere around the state to better understand the causes of rockfalls and landslides.
Phil Larson, Minnesota State University, Mankato earth science director and earth systems laboratory co-director, and Andrew Wickert, professor at the University of Minnesota’s St. Anthony Falls Laboratory and department of earth and environmental sciences, are re-mapping areas where landslides have occurred in the Minnesota River Valley.
At one spot along State Highway 68, the site of a major landslide in July of 2018 that covered the roadway and shut it down for a couple of weeks. Wickert says they’re checking to see what, if anything, changed since then.
“When this originally failed, you could see the bare slope at the bottom, the bare slope at the top,” Wicker said. “The bottom part rotated, so it actually created a lake, a small lake in the middle, half-way up the hill as it broke off.”
Now, huge nets secure over the loosest parts of the hillside to prevent it coming down again. New vegetative growth is helping keep the soil in place and prevent it from eroding even more.
Larson says understanding the soil composition and how it’s held together, and what causes it to fail or come apart when saturated with large amounts of rainfall, is key to their research.
“One of the primary drivers of failure are these high magnitude precipitation events,” he said. “What does it take, and then cause that cohesive strength to fail? What is actually the tipping point that allows these slopes to fail?”
Understanding that, Larson says, is critical to helping local officials understand when hillside failures may happen again and how to minimize the damage from future landslides or rockfalls.
Wickert says providing this data can also involve citizens, decision makers and those who live in the areas where land is especially vulnerable as part of overall management strategies.
“That’s the idea is to go from seeing a bunch of these things happen where an engineer is like, ‘I don’t know what to do’ to then not needing to be confused on the ground, but instead of having a plan before anything even happens,” he said. “And that’s how we save lives. That’s how we prevent damage.”
And for Larson, he says the research is personal.
“We don’t want to see anything get hurt or anybody get hurt from these events,” he said. “I do live in Mankato … what happened at Minneopa State Park was horribly tragic and deeply affected me because it was a Mankato student, and I was like … I need to get off my butt and get back to work on this problem. We can give a lot to understanding how these systems work to help communities move forward.”
