Graduation date: 1987Timber management of coastal watersheds in southwest Oregon has\ud been complicated by the need to protect anadromous fish habitat from\ud accelerated stream sedimentation resulting from management activity.\ud The rugged terrain of the Elk and Sixes River basins is underlain by\ud the complex geological province of the Klamath Mountains, in which\ud landslides are a common, natural, and important process of sediment\ud production.\ud A landslide investigation, using sequential aerial photographs\ud which covered a time period of 37 years, was used to determine\ud relationships between mass-wasting, geologic types, and timber harvest\ud practices. Averaged over all rock types, harvested areas showed an\ud increase in failure rate of 7 times, and roaded areas an increase of\ud 48 times that of forested terrain. Terrane underlain by dioritic\ud intrusions was the most sensitive to road-related activity, with an\ud increase in failure rate of up to 108 times that of comparable\ud unmanaged land.\ud The complexity of lithologies and deformational history in the\ud area strongly influence slope morphology, and produces characteristic\ud soil types which experience predictable modes and rates of slope\ud failure. Debris slides and torrents are the dominant form of\ud mass-wasting in dioritic and Cretaceous sedimentary terrane. Areas\ud underlain by more clay-rich metamorphic bedrock are prone to slumps\ud and planar streambank failures.\ud Stream morphology is profoundly influenced by both rock type and\ud geologic structure. Within an area characterized by steep, deeply\ud incised streams, several persistent low-gradient reaches were\ud delineated. These low-gradient stream reaches occur where (1) large\ud landslides have locally raised channel bed elevation and (2)\ud valley-floor widening has occurred in sheared rocks along fault zones\ud or in more readily eroded rock types upstream of rock types resistant\ud to fluvial erosion
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