Landslide occurrence in the Blue River drainage, Oregon

Rapid, shallow soil mass movements (landslides) are examined for a 6,000 ha managed forest area in the Oregon Western Cascades. Analysis of landslide occurrence considers the physical characteristics and frequency, the influence of clearcutting and road construction, and some resource impacts. nonparametric statistical methods are employed to test the significance of the observed variations in landslide characteristics.

Landslide size and site characteristics appear highly consistent. Fifty-five to eighty percent of all landslide length, width, depth, area, and volume measurements fall within the lower 15% of their respective dimension ranges. Landslides occur most frequently at slope angles of 30º - 40º, in northern aspects (NW-NNE), and in smooth slope locations. Landslide occurrence does not vary significantly ( = 0.05) with relative hillslope position.

Clearcutting and road construction appear to strongly affect landslide frequency and location. Landslides occur 24 and 253 times more frequently (relative to forest rate) in clearcut and road areas, respectively. Significant variation in landslide geomorphic setting with land use suggests that clearcutting and road construction may increase the landslide susceptibility of hillslope nose and hollow locations. They do not influence landslide size, slope anle, slope aspect, or hillslpe position.

Resource impacts from landslides are varied. Although on-site disruption is generally substantial, total ground area affected by landslides is small (approximately 0.5%). Roads stand out as an important reasource considerationbecause landslide frequency and the number of stream entries by landslides are significantly higher for road-related failures. Road location, drainage, and fill slope constuction methods are the probable causes of this accelerated landslide activity.