Landslides after clearcut logging in a coast redwood forest
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Landslides have been mapped at least annually in the 473 ha North Fork Caspar Creek watershed since 1985, allowing evaluation of landslide distribution, characteristics, and rates associated with second-entry partial clearcut logging of 1989 to 1992. Comparison of sliding rates in logged and forested areas shows no appreciable difference for streamside slides (size range: 7.6 to 380 m3). However, the incidence of large landslides, including both streamside and upslope slides of 98 to 4900 m3, varied by treatment. Such slides displaced 12 m3/yr per km2 of unlogged forest but showed rates one and two orders of magnitude higher in logged areas and along roads, respectively. Moreover, the volume rate of sliding from roads in logged areas was more than three times that from forested roads. The largest slides occurred 9 to 14 years after logging, when root cohesion is expected to be near its minimum value; and within a few years of pre-commercial thinning, when hydrologic changes are again evident. Large slides may strongly influence suspended sediment yields both by increasing yields for several years after the slide and by emplacing temporarily stable channel and floodplain deposits, which then provide a sediment source for future gully and bank erosion.