Abstract
Minnesota's post-glacial landscape, mantled by thick, discontinuous till and outwash deposits and modified by varied fluvial response to deglaciation and base-level change, is susceptible to landslides that can damage infrastructure, increase sediment flux to nearby streams, and cause loss of life. Following landslide inventory mapping protocols modified from those designed in Oregon and Washington, we investigated past landslide occurrence as a first step in better understanding landslide susceptibility for risk reduction in the metropolitan area of Minneapolis-St Paul. We identified 64 landslides in the region from 1852-2019 based on review of historical documents and mapped an additional 367 previously undocumented landslides using lidar topographic map derivatives. Slope failures were distinguished by features such as headscarps, internal scarps, hummocky terrain, and displaced streams. Measurements of landslide length and width, head scarp length, average slope, area, depth, and volume (estimated from headscarp height and along-slope length) were estimated and recorded in a database. Other remote sensing imagery and field checks of accessible landslides helped to evaluate and confirm the accuracy of the lidar-mapped slides. Landslide morphology and subsurface data were used to classify landslide and failure type, where possible. In the 8,300 km2 study area, 71 slides occurred as failure of exposed erodible rock layers such as the Platteville limestone and St. Peter sandstone, and 296 landslides occurred as failure within surficial units such as glacial till, colluvium, and ice-contact deposits. Rock falls occur on vertical bluffs along the Mississippi River, where fluvial erosion has removed lateral support of underlying soft St. Peter sandstone layers, leaving the well-cemented and overlying Platteville limestone vulnerable to undercutting. In the glacial sediment of the Minnesota River Valley, both deep and shallow slides occur. This work is part of a larger effort to map mass wasting across several landslide-prone regions in Minnesota. The data presented here will support the future production of a state-wide landslide susceptibility map.