The ground is shifting and bluffs are giving way across Minnesota, and researchers want to know exactly where and why.
Geologist Carrie Jennings was driven to lead the statewide effort after being called to a St. Paul bluff that collapsed and buried four children on a fossil-hunting field trip in 2013, killing two of them. Familiar with the terrain in Lilydale Regional Park, Jennings helped emergency workers navigate the steep slopes and was there when the parents were told their children had died.
The emotion of that day haunts her. “It affected me enough that I thought we should be able to predict where these things are going to happen,” Jennings said.
Mapping landslides will be a first for Minnesota as well as a first for the Midwest, she said. She’ll complete a landslide inventory for Hennepin County later this summer and finish a three-year project to map landslide-prone areas across the state by next June.
The information could provide cities and other entities with information to protect people, property and the state’s waterways, said Jennings, a University of Minnesota professor and the Freshwater Society’s research and policy director. “You can’t take all the risk away, but you can design to minimize the risk,” she said.
To pinpoint landslide zones, researchers are digging into old records and newspapers, examining shaded relief maps, tramping along hillsides and steep cliffs and paddling waterways to examine and document where the ground has given way in five areas of the state: along the Mississippi River and its major tributaries in the southeast, the Minnesota River valley from New Ulm to Chaska, the Lake Superior watershed, the Red River Valley and the seven-county metropolitan area.
These areas, formed after the glaciers retreated, are relatively “young,” at least from a geologist’s point of view. “We have a young landscape that rivers are still evolving in,” Jennings said. “Many of these slopes will relax over time.”
But slope failures could occur sooner because of forces such as a wetter climate and developments at the top and bottom of the bluffs. “We’re tempting fate in a lot of these cases,” Jennings said. “Combine that with a slightly different precipitation pattern and the valley walls start to activate.”
For example, a two-day rain in Duluth in June 2012 generated hundreds of landslides, extensively damaging Jay Cooke State Park. In the Red River Valley, weak clay soils frequently fail, undermining homes and roads.
Earlier this year, two grave sites in a Blue Earth County cemetery were moved because erosion had eaten away part of the bluff it sits on, said Dean Otto, board member and treasurer for the Minneopa Cemetery. The cemetery, about 5 miles west of Mankato, overlooks the Minnesota River Valley.
More than a century earlier, caskets washed down the hillside after heavy rains eroded the slope, which was compromised when railroad tracks cut into its base, according to a newsletter of the Friends of Minneopa State Park.
Across the river in North Mankato, a season of heavy rain has taken a toll along some of the city’s bluffs, including six rock slides along Judson Bottom Road, a picturesque byway, according to Mayor Mark Dehen. Rock slides on the road this year have been exceptional. “Limestone chunks of rock the size of cars have come off that hillside,” Dehen said.
City officials closed a stretch of the road earlier this year, and with no feasible way to stabilize the hill or prevent the slides, they’ll decide whether to make the closure permanent. “It’s not the preferred answer for most citizens. Most prefer to have it open,” Dehen said. Maybe, he said, that means posting warning signs about falling rocks.
Meanwhile, in western Minnesota near Granite Falls, the Minnesota Department of Transportation closed a quarter-mile stretch of Hwy. 67 that begins at the Upper Sioux Agency State Park entrance after pavement crumbled and fissures opened. The cause is deep below the surface where the ground has shifted, in part due to erosion that has eaten away at the slope along the Yellow Medicine River and rain that has saturated the soil, explained Cody Brand, a soil engineer with MnDOT.
Fixing the road or building walls to hold up these slopes and others across the state aren’t necessarily permanent fixes, Jennings said. The key, she said, is understanding what’s causing these failures.