Morrison, Peter Hallock. 1975. Ecological and geomorphological consequences of mass movements in the Alder Creek watershed and implications for forest land management. Eugene, OR: University of Oregon. 102 p. B.A. thesis.
Ecological and geomorphological consequences of mass movement activity were investigated in the Alder Creek basin in the Western Cascade Mountains of Oregon. The 4,300 acre study area is characteristic of areas in the low elevation Cascades with steep, dissected hillslopes underlain by weathered volcaniclastic rocks. Both slow deep-seated mass movements and shallow soil mass movements are active in the watershed.
One hundred and fourteen shallow soil mass movements occurring over the past 100 years were identified, measured, mapped and described. Landslides occur periodically in undisturbed, forested areas of the watershed but are much more frequently associated with roads and clearcuts. Landslide erosion rates in road right-of-way areas were 309 times that in forested areas. The great difference in landslide activity in road right-of-way areas as compared to forested areas is due to the particular geomorphic characteristics of the watershed; and the siting, design, construction and maintenance of roads. Suggestions for reducing the effect of roads on landslide activity are discussed.
Landslide activity was also significantly correlated to slope, aspect and bedrock characteristics of the study area. Most landslides occurred on slopes between 38º and 42º. Aspect profoundly influenced the frequency and magnitude of landslide activity. Nearly 100 times more landslide erosion occurred on north aspect sites than on south aspect sites. The average volume per slide was also significantly greater on north aspect sites.
Slow deep-seated earthflows are common in the northern portion of the watershed. They influence at least 12% of the study area. Slumps and debris torrents were common along earthflow margins but the total amount of sediment contributed by earthflow processes is unknown.
Secondary erosion processes were observed on landslide scars during the fall, winter and spring months. Sediment collected from a ten year old, unvegetated landslide averaged 126 cubic inches per square foot of surface area over a seven month period. During this period erosion transects on an unvegetated ten year old landslide experienced an average depth increase of 0.33 inches due to surface erosion. Established vegetation on landslide scars effectively reduces erosion. Surface erosion impedes revegetation of landslide scars by creating an unstable substrate where establishment of vegetation is difficult.
Plant succession and conifer regeneration was observed on a 90 year chronosequency of landslide scars. Establishments of plant cover took many years on most landslides studied. Successional development on landslides studied. Successional development on landslides appears to have some similarity to seval stages following other disturbances such as fire or logging but is unique in some respects. conifer growth rates were markedly reduced on all landslides studied and a stunted coniferous forest appears to develop on old landslide scars.
Environmental impacts of landslide activity on aquatic and terrestrial ecosystems are briefly discussed. Management implications of this study and suggestions for rehabilitation of landslide scars are mentioned in conclusion.