AN ASSESSMENT OF THE PHYSICAL FACTORS THAT TRIGGER LANDSLIDES AND THE RELATIVE IMPACT OF A 500-YEAR FLOOD WITHIN THE RED RIVER WATERSHED (WISCONSIN/MINNESOTA)

The 7.2 mile long Red River is a tributary of the St. Louis estuary in far southwestern Lake Superior. The river downcuts a clay plain from a former glacial lake making the area highly susceptible to landslides when the clay becomes saturated. Landslides along the valley walls of the Red River result in higher stream turbidity and increased sedimentation within the St. Louis estuary, negatively impacting aquatic organisms and requiring dredging in the harbor. In June 2012 nine inches of rain fell within a 24 hour period, triggering many new landslides within the Red River watershed. By digitizing pre- and post-flood landslides using aerial photographs we assess the relative impact of the June 2012 flood, and use a GIS to determine landscape characteristics that lead to landslides. Prior to the flood there were 170 recently active landslides (barren of vegetation) as well as 184 older landslides that were partially-vegetated. Regardless of age, the landslides share an average slope of 22 degrees and the majority have a south-facing aspect. While post-flood assessment is not yet complete, we are using landscape characteristics that typify landslide areas to create a hazard risk map using a logistic multiple regression technique, and will assess how well the post-flood landslides fit this model.