GEOMORPHIC, HYDROLOGIC, AND BIOTIC FACTORS AFFECTING ABUNDANCE OF COHO SALMON IN CENTRAL OREGON COAST RANGE STREAMS
At the watershed scale, relative abundance depends on the geomorphic landtype, with abundance generally greater in watersheds of moderate relief underlain by marine sandstones and siltstones than in watersheds with high relief underlain by extrusive basalts. Coho abundance also depends on position in the watershed, and highly significant relationships were observed with drainage area, kilometers downstream from the headwaters, stream link magnitude, and stream order. At the reach scale, coho abundance was related to low flow hydraulic conditions, when the field data were collected, more than on bankfull conditions, which control channel dimensions. Channel morphology (entrenchment, channel slope, roughness, percent of transect in pools, effective pool cover) and water temperature were more important than food supply. Substrate and competition from other fish species were not significant. Substrate does affect the abundance of various macroinvertebrates.
The models developed in this study are realistic (based on physical relations between independent variables and coho abundance), accurate (account for a large percent of the variation in coho abundance), precise (use variables that can be measured with consistency), and robust (applicable to well-defined but broad geographic limits). The models can be used to assess the impact of timber harvest activities over time on physical habitat, water quality, and biotic factors.