Landslide Susceptibility and Soil Loss Estimates Impacting Streams in the Drift Creek/Siletz Watershed, Lincoln County, Oregon

This study models landslide susceptibility on a watershed scale and estimates soil loss resulting from landslide derived sediment within 30 m of Strahler 3rd order or higher streams in the Drift Creek/Siletz watershed. Landslide derived sediment has been suspected of decreasing water quality, suspending water treatment plant operations, and degrading ecology in the Drift Creek/Siletz watershed by the Oregon Department of Environmental Quality (DEQ). The watershed has been designated as “Impaired by Unknown Stressors” by the MidCoast Watersheds Council Biological Monitoring Results Survey (2013). Logistic regression was used to determine the most significant variables contributing to landslide occurrence and to create a watershed scale landslide susceptibility map with area under the ROC curve of 0.8761. The most significant variables are slope angle, terrain elevation, soil erodibility factor, and plan curvature (curvature perpendicular to slope direction). Soil loss estimates were made by using the U.S. Department of Agriculture RUSLE 2 equation in ArcGIS to accommodate the many different spatial factors in the calculation. The watershed wide average annual soil loss estimate agrees with published findings for watersheds of similar size and physiographical location. Four Upper Drift Creek landslide deposits within 30 m of the stream network showed the highest average annual soil loss estimates (281.5 tons/acre/year combined). These landslide deposits coincide with recent logging activity. Logging activity and the time for forest re-growth appears to substantially influence soil loss in the Drift Creek/Siletz watershed.