LANDSLIDE CHARACTERIZATION IN THE LOWER MINNESOTA RIVER VALLEY

The lower Minnesota River Valley is underlain by unconsolidated glacial till interbedded with fluvial-lacustrine sediment. The valley is flanked by steep bluffs, as are the deeply incised tributary valleys that feed it. The combination of unconsolidated material and steep slopes lead to slope failure, but there are few data on the frequency and distribution of failures, triggering mechanisms, or landscape susceptibility to future failures. The creation of a landslide inventory is the initial step in understanding local landslides and developing a strategy to mitigate future damage to infrastructure and loss of life. As part of a nearly-statewide landslide mapping effort, we mapped landslides in the Minnesota River Valley between St. Peter and Chaska, using lidar topography and a DEM-derived slope map. Several areas were field-verified to validate criteria for landslide identification, to characterize geologic materials, and to make observations about the types of landslide processes at work. The project identified 1774 landslides over a ~2000 km² area with lengths ranging between 1.4 and 792 m. Most of the landslides in the region are on the order of tens to a few hundreds of m² in scarp area. Nearly all the identified landslides lie within glacial till and are not grounded in bedrock. Most landslides are located on slopes over 30 degrees; these may be naturally steep from ongoing stream valley incision or artificially steepened by human activities. This inventory allows preliminary assessment of relationships between geologic material and slope failure in order to gain a broader understanding of Minnesota landslides. These data will be combined with data from other landslide-prone regions in Minnesota and analyzed to create a landslide susceptibility map for the state.