As part of a hierarchical approach to classifying watersheds and stream habitats
based on geomorphic and geologic criteria, we defined ten classes of fluvial and
lacustrine habitats at the scale of valley segments. Valley segments are landscape units
which encompass surface waters and the adjacent floodplains and hillslopes with which
they interact over time frames of thousands of years. They form a large-scale template
that constrains the character of aquatic habitat, controls the effects of disturbances in
riparian areas, and mediates responses of streams to upland and upstream events. The
regional distribution of valley segment types in southwest Oregon reflects bedrock
geology and tectonic history of the landscape. Fluvial segment types differ in stream adjacent
landforms, slope erosion processes, floodplain and valley morphology, channel
slope, riparian vegetation, streambank texture, gravel bar morphology, and pool-forming
features. Studies that do not carefully account for inherent differences between valley
segment types could fail to detect critical changes in stream habitat caused by human
disturbance. Alluvial valley and alluviated canyon segment types, which have extensive
floodplains, low channel slopes, abundant woody debris, and ample gravel beds, are of
greatest direct importance for salmon and other native fishes. Virtually all alluvial valleys
in the study area have been heavily disturbed by logging, agriculture, and residential
development. Alluviated canyon segments located in the few drainage basins where
human activity has been limited probably serve as habitat refugia for the last diverse
assemblages and productive populations of salmon in the region. Alluviated canyons in
extensively-fogged basins exhibit increased abundance of large woody debris, fewer
cross-channel debris jams, more extensive bank erosion, reduced pool area and
increased riffle area, shallower riffles, and increased surface concentration of fine
sediments in pools and other habitats, compared to similar segments in lesser-disturbed
basins. These changes in channel morphology and stability appear to be driven by
increased sediment load, caused by logging-related landslides and other erosion
sources. Field studies in Sixes River basin indicated that abundance and diversity of
salmonid fishes declines as maximum stream temperature increases. Changes in
summer distribution of juvenile chinook and coho salmon since 1970 are related to
changes In water temperature. Although some tributaries have cooled, a decline in
rearing distribution in mainstem areas could be caused by long-term loss of channel
complexity and associated coolwater refugia. Analysis of fish habitat structures
constructed by federal and state agencies indicated that failure rates are high. Recovery
of anadromous fish runs in southwest Oregon will require protection of remaining habitat
refugia and reduction of sediment yield from disturbed watersheds. / Graduation date: 1992
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/37238 |
| Date | 30 April 1992 |
| Creators | Frissell, Christopher Andrew, 1960- |
| Contributors | Liss, William J. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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