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