Timber management of coastal watersheds in southwest Oregon has
been complicated by the need to protect anadromous fish habitat from
accelerated stream sedimentation resulting from management activity.
The rugged terrain of the Elk and Sixes River basins is underlain by
the complex geological province of the Klamath Mountains, in which
landslides are a common, natural, and important process of sediment
production.
A landslide investigation, using sequential aerial photographs
which covered a time period of 37 years, was used to determine
relationships between mass-wasting, geologic types, and timber harvest
practices. Averaged over all rock types, harvested areas showed an
increase in failure rate of 7 times, and roaded areas an increase of
48 times that of forested terrain. Terrane underlain by dioritic
intrusions was the most sensitive to road-related activity, with an
increase in failure rate of up to 108 times that of comparable
unmanaged land.
The complexity of lithologies and deformational history in the
area strongly influence slope morphology, and produces characteristic
soil types which experience predictable modes and rates of slope
failure. Debris slides and torrents are the dominant form of
mass-wasting in dioritic and Cretaceous sedimentary terrane. Areas
underlain by more clay-rich metamorphic bedrock are prone to slumps
and planar streambank failures.
Stream morphology is profoundly influenced by both rock type and
geologic structure. Within an area characterized by steep, deeply
incised streams, several persistent low-gradient reaches were
delineated. These low-gradient stream reaches occur where (1) large
landslides have locally raised channel bed elevation and (2)
valley-floor widening has occurred in sheared rocks along fault zones
or in more readily eroded rock types upstream of rock types resistant
to fluvial erosion. / Graduation date: 1987
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/37213 |
| Date | 10 December 1986 |
| Creators | McHugh, Margaret H. |
| Contributors | Swanson, Frederick J. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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