Landscape Influences on Stream Habitats and Biological Assemblages

<em>Abstract.</em>—Physical habitat degradation has been implicated as a major contributor to the historic decline of salmonids in Pacific Northwest streams. Native aquatic vertebrate assemblages in the Oregon and Washington Coast Range consist primarily of coldwater salmonids, cottids, and amphibians. This region has a dynamic natural disturbance regime, in which mass failures, debris torrents, fire, and tree-fall are driven by weather but are subject to human alteration. The major land uses in the region are logging, dairy farming, and roads, but there is disagreement concerning the effects of those activities on habitat and fish assemblages. To evaluate those effects, we examined associations among physical and chemical habitat, land use, geomorphology, and aquatic vertebrate assemblage data from a regional survey. In general, those data showed that most variation in aquatic vertebrate assemblage composition and habitat characteristics is predetermined by drainage area, channel slope, and basin lithology. To reveal anthropogenic influences, we first modeled the dominant geomorphic influences on aquatic biotic assemblages and physical habitat in the region. Once those geomorphic controls were factored out, associations with human activities were clarified. Streambed instability and excess fines were associated with riparian disturbance and road density, as was a vertebrate assemblage index of biotic integrity (IBI). Low stream IBI values, reflecting lower abundances of salmonids and other sediment-intolerant and coldwater fish and amphibian taxa, were associated with excess streambed fines, bed instability, higher water temperature, higher dissolved nutrient concentrations, and lack of deep pools and cover complexity. Anthropogenic effects were more pronounced in streams draining erodible sedimentary bedrock than in those draining more resistant volcanic terrain. Our findings suggest that the condition of fish and amphibian assemblages in Coast Range streams would be improved by reducing watershed activities that exacerbate erosion and mass-wasting of sediment; protecting and restoring multilayered structure and large, old trees in riparian zones; and managing landscapes so that large wood is delivered along with sediment in both natural and anthropogenic mass-wasting events. These three measures are likely to increase relative bed stability and decrease excess fines by decreasing sediment inputs and increasing energy-dissipating roughness from inchannel large wood and deep residual pools. Reducing sediment supply and transport to sustainable rates should also ensure adequate future supplies of sediment. In addition, these measures would provide more shade, bankside cover, pool volume, colder water, and more complex habitat structure.