TEMPORAL AND SPATIAL ANALYSIS OF THE BUTTERMILK LANDSLIDE COMPLEX NEAR WEISER, IDAHO

The Buttermilk Landslide near Weiser, Idaho is a historic landslide complex that formed over a century ago on a bluff of lacustrine deposits of the Glenn’s Ferry Formation. Above the slide is a 220 acre irrigated alfalfa field on the same geologic formation as the slide. Below the slide is an irrigation canal that was built in the late 1800’s across the toe of the slide. Historical orthophotos make it clear that the slide began to develop a new headwall by 1994 and continued creeping until there was substantial slope failure on July 5, 2006. This major slide event caused the toe to advance 30-40 meters, and the headwall retreat was 80-100 meters as documented from georeferenced orthophotos. Infiltration of canal water, excess irrigation water from the field, and precipitation into the body and toe of the slide are all possible contributions to the July 2006 slide movement. Annual precipitation from July 2005-June 2006 was 164% of the average for the previous 7 years. March 2006 precipitation was 316% of the average for the previous 7 years at 3.3 inches which is the highest ever recorded. On October 29, 2010 over 650 points were taken using a Leica survey grade GPS rover to outline and document ongoing movement of the headwall, toe, and body of the landslide. Data collected spring 2011 will be used to evaluate possible movement. Topographic profiles created using 1/3 arc second and 1 meter digital elevation models are used to compare pre- and post-slide geometries. GPS and DEM post-slide profiles match up very well when plotted together and validate DEM accuracy for further use in volumetric studies. Test pit and borehole data are gathered into Excel and ArcGIS to analyze the bedding, depth of clay layers, and their effect on groundwater movement in and near the slide. Soil from the slide consists of silty-clay; samples analyzed using the Atterberg limits test have a plasticity index of 46.5% signifying a high volume change potential and high clay content.