Publication Summary

Biological assessment of 20 stream sites was completed in the Columbia Basin and Cascade ecoregions during the summer of 1993. A multi-habitat sampling protocol was used to describe benthic macroinvertebrate assemblages in riffles and pools. Synoptic taxa lists were developed for three ecoregions of Washington State. Biological condition of riffle and pool habitat was determined for each site and ranked in two sets using several biometrics.

Biological assessment of 20 stream sites was completed in the Columbia Basin and Cascade ecoregions during the summer of 1993. A multi-habitat sampling protocol was used to describe benthic macroinvertebrate assemblages in riffles and pools. Synoptic taxa lists were developed for three ecoregions of Washington State. Biological condition of riffle and pool habitat was determined for each site and ranked in two sets using several biometrics.

A disturbance continuum was described from the sites surveyed in each ecoregion. The disturbance may have been naturally influenced or human caused. Protected sites, such as those in National Parks, can appear to have degraded conditions. Ohanapecosh River, in the Mt. Rainier National Park, did not contain a diverse riffle assemblage, but had a species-rich pool condition. Physical features such as a broad channel width and dominance of finer substrate material indicated a capacity to assimilate depositional materials. Additional physical features that explained biological conditions were: proportion of available cobble substrate, flow, average current velocity, wetted width/bankfull width, and proportion of forested land in a montane region. Taxa richness and the EPT Index represented biological condition in regression analysis with stream reach and watershed characteristics. A strong relationship was found between taxa richness and the proportion of forested land (r=0.83). The relationship between taxa richness and wetted width/bankfull width ratio was strong in Cascade (r=0.72) and Columbia Basin (r=0.73) streams.

Variability of replicate samples collected within a stream reach indicated consistent levels of repeatability (coefficient of variation for taxa richness was 6% among four replicate samples). The high level of precision using this biological assessment method provided some indication for sensitivity in applying regional biological metrics. Identification of three biological condition categories (poor, fair, good) were suggested for riffle habitat in the Cascade ecoregion of Washington State.