Caddisflies were collected at 181 wall seep, stream, river, and lake habitats in 7 counties in northwest and north central Washington over a 6-year period. From 17,405 specimens, we identified 164 adult caddisfly species within 62 genera and 16 families. Twenty taxa were new state records, bringing the number of species currently reported from Washington to 230. Species assemblages were compared to altitude, physicochemical factors, aquatic-habitats, and land use (urban, agriculture, and forest) on the west and east sides of the North Cascade Range. Species richness showed significant positive correlations to altitude and pH and showed significant negative correlations to total phosphorus, total nitrogen, and specific conductivity, as well as especially to channel embeddedness. A multilevel hierarchical clustering model separated wall seeps, streams, and rivers into geographic and land-use regions based on adult caddisfly assemblages. We used a multimetric index (caddisfly tolerance index [CTI]) to determine environmental tolerance levels for adult caddisfly species. The index performed well in distinguishing among the effects of total phosphorus, total nitrogen, specific conductance, and channel embeddedness on the distribution of caddisfly species. These CTI values provide baseline information for monitoring changes in ecosystem health in drainages throughout Washington landscapes.
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