Landslides in Minnesota: Where Glaciers and Rivers Left Steep, Unstable Slopes

The four regions where landslides were mapped, on a map of Minnesota's elevation. U.S. Geological Survey.

Why Minnesota has landslides

Landslides in Minnesota have killed people, damaged infrastructure and harmed natural resources. They also add sediment to lakes and rivers, which hurts water quality and aquatic habitat. Most landslide-prone ground is on steep slopes next to rivers, lakes and roads and railways, and how prone a place is depends on its geology and glacial history:

• At some point in repeated glaciations over the last 2 million years, glaciers covered every part of the state, leaving loose sediment whose strength varies widely.
• Meltwater cut valleys into that sediment, bounded by steep, potentially unstable slopes.
• The southeast escaped the most recent glaciation, which ended 13,000 years ago; there, rivers cut through older glacial sediment and bedrock, and steep bedrock slopes fail with rock debris.

What kinds

• Shallow slope failures in heavy rain are the most common — often less than 1 meter (about 3 feet) deep, but able to strip a whole slope.
• Rockfalls happen around cliffs, where cracks let big blocks drop, sometimes in freeze-thaw cycles.
• Deep landslides, debris flows and mudflows are possible, but far rarer than in mountainous places.
• Slow earthflows and soil creep can occur when soil moisture and shallow groundwater saturate the ground.

Triggers: rain that soaks soil, raises groundwater, floods streams and runs off the land. Climate change is expected to bring longer, more intense rain, more intense floods and wetter soils — and perhaps more landslides. Land use plays a part too: undercutting slopes, artificial fill, and ravines eroded by extra stormwater from development or farming.

Four regions

Recent mapping of four regions — each with its own geology, glacial history and landslide risk — used lidar topography, satellite and aerial imagery, and fieldwork to find almost 10,000 identifiable landslides.

• Red River (northwest): the valley was the bed of a vast glacial lake, filled with fine sand, silt and clay. The Red River and its tributaries have cut into the flat lake plain, leaving steep slopes by the channels that big floods can submerge — and that are prone to slide.
• Southeast ("bluff country"): Mississippi tributaries cut into flat-lying, layered sedimentary bedrock, mostly spared by the last ice. Strong rock — limestone, dolostone, well-cemented sandstone — forms overhangs that fall when weaker siltstone, shale or poorly cemented sandstone erodes beneath them. Carbonate rock dissolves into caves and sinkholes that feed springs at the foot of slopes. Slides hit the Mississippi bluffs, roadcuts and river bends; small rockfalls come all year, especially in freeze-thaw and heavy rain.
• Minnesota River (south-central): the wide, deep valley was carved mainly by glacial meltwater beginning about 13,400 years ago. Tributaries still cut steep bluffs into thick glacial sediment, clay-rich or sandy, or fractured bedrock. Some big slides — arcuate scarps and lobes of sediment — may be prehistoric, yet heavy rain, floods, springs or slope changes can reactivate them. Runoff from land-use change and farm drainage may add to the activity.
• Northeast (Lake Superior): slides hit the steep Lake Superior shore and stream valleys, where north-shore streams cut into weak glacial sediment over hard bedrock. At the lake's western end, channels cut into fine glacial-lake sediment laid down when the lake stood higher — easily set sliding by storm runoff, river erosion and seeping groundwater. Rockfalls come off fractured cliffs, loosened by freeze-thaw and roots.
• Metro (Minneapolis–Saint Paul): meltwater carved the Minnesota and Mississippi valleys more than 10,000 years ago, and erosion goes on. Steep bluffs of layered bedrock are capped in places by loose sand and other sediment that slides in big rainstorms, and springs can weaken and saturate the rock.

The Duluth storm of 2012

In June 2012, a remarkable rainstorm centered on Duluth caused widespread landslides and flooding. Lidar flown before and after showed thousands of slides along stream valleys, especially in clay-rich glacial and glacial-lake sediment. Together with flood erosion of valley floors, they moved millions of cubic meters of sediment — enough to seriously harm aquatic habitat.

Lidar reveals landslides near Duluth: mapped slides (orange) on shaded relief, and elevation change across the 2012 storm — orange and red are erosion, green is deposition. U.S. Geological Survey.

Cities make it worse

Urban activity affects slopes by:

1. concentrating runoff from buildings, parking lots and roads;
2. steepening slopes beside roads and railways;
3. adding weight with buildings and artificial fill;
4. weakening slopes with soaked-in water or by digging material away.

What makes a Minnesota slope fail

• steep valley sides, cut by meltwater and by streams still cutting down and sideways;
• floods eating into the base of slopes;
• clay-rich glacial sediment soaked by rain and groundwater;
• sandy, loose meltwater sediment, which is weak;
• layered bedrock, where weak layers erode beneath strong ones;
• people: poor stormwater management, undercutting, fill, urban growth and farming practices;
• climate change, with bigger rainstorms coming more often.

More information

• The USGS Landslide Hazards Program, landslides.usgs.gov, or toll-free 1–800–654–4966.
• On the 2012 Duluth storm: USGS reports 2012–5283 and 2016–5104.
• For a specific property, hire a state-licensed geotechnical engineer or engineering geologist, found through the American Society of Civil Engineers or the Association of Engineering Geologists; state or county planning and engineering staff can often suggest one.

Sources

Based on Landslides in Minnesota, U.S. Geological Survey Fact Sheet 2022–3007, by Stephen B. DeLong (USGS), Carrie E. Jennings (Freshwater Society) and Karen B. Gran (University of Minnesota Duluth), prepared with Minnesota and regional universities and the Freshwater Society, USGS Publications Warehouse; published by the U.S. Geological Survey and in the public domain; rewritten in hubnx's own words. Figures recovered from the fact sheet's PDF. Photographs credited to universities, a private individual, Maxar and Minnesota Public Radio are not reproduced.