SPATIAL DISTRIBUTION AND CHARACTER OF MASS WASTING IN THE MINNESOTA RIVER VALLEY: CASE STUDY OF A REACH FROM NEW ULM TO ST. PETER

A paucity of data about and analysis of mass-wasting processes exists in the relatively low-relief, ice marginal or formerly glaciated landscapes of the last glacial maximum in North America. Many of these landscapes are being actively dissected by drainage networks that continue to develop in response to incision driven by discharges of sediment-poor glacial meltwater post last glacial maximum. Thus, many of the major river corridors contain regionally significant relief, actively incising river systems that accelerate expansion of relief, and rapidly evolving hillslopes. Our need to understand mass-wasting processes in these landscapes has been highlighted by slope failures within the last decade that resulted in economic loss, caused the loss of life and property, and contributed to environmental impairment of surface waters in the state of Minnesota, USA.

One such landscape, the Minnesota River valley (MNRV) of south-central Minnesota, contains abundant evidence of frequent slope failures. We present results from an effort to map, catalog and analyze the spatial distribution and character of mass-wasting processes throughout the MNRV. Mapping efforts focused first on field-based observation and interpretation. Remote-based mapping was conducted on inaccessible sites using LiDAR-derived digital elevation models, hillshades, and aerial imagery. The results reveal ubiquitous mass wasting on slopes throughout the study area, with a total of 916 slides (translational, rotational), 133 flows (debris, earth, mud), 113 falls (topples, rock falls), and 397 areas of complex slope failure that cannot be categorized as a single type. ~80% of failures occur on slopes between 25-45 degrees. The vast majority of failures occur in glacial till, with the exception of rock falls, which occur predominantly in sandstone. Interpretation of sites during field mapping and mass-wasting events that occurred during this study reveal that precipitation and groundwater seeps appear to be major factors that drive mass movement in this landscape.