Impact of clear-cutting and road construction on soil erosion by landslides in the western Cascade Range, Oregon

The H. J. Andrews Experimental Forest can be divided into two zones of approximately equal area, each with strikingly different susceptibilities to erosion by rapid soil movements. A stable zone occurs at elevations above 900 to 1,000 m in terrain underlain by lava-flow bed rock. Since logging and road cutting began in 1950, only tow small road-related slides have taken place in the stable zone. In contrast, the unstable zone, located at elevations below 1,000m and underlain by altered volcaniclastic rock, has been the site of 139 slides during the same period.
Slide erosion from clear-cut areas in the unstable zone has totaled 6,030 m3/km2 or 2.8 times the level of activity inforested areas of the unstable zone. Along road rights-of-way, slide erosion has been 30 times greater than on forested sites in the unstable zone; however, only about 8 percent of a typical area of deforested land in the unstable zone is in road right-of-way. At comparable levels of development (8 percent roads, 92 percent clear-cut), road right-of-way and clear-cut areas contribute about equally to the total impact of management activity on erosion by landslides in the unstable zone. the combined management impacts in the unstable zone (assuming 8 percent road right-of- way and 92 percent clear-cut) appear to have increased slide activity on road and clear-cut sites by about 5 times relative to forested areas over a period of about 20 years.