Development of macroinvertebrate-based index for bioassessment of Idaho rivers

Theoretical constructs, such as the river continuum concept, predict that t he composition of benthic fauna in rivers will be different from that of he adwater streams. There exists a need to modify, for use on larger rivers, t he bioassessment techniques commonly used on small streams. Using aquatic m acroinvertebrates and the "reference condition" approach, we developed and tested a multimetric index for use on the rivers of Idaho. Reference sites were selected to represent the best current conditions (i.e., least impacte d) among Idaho rivers. The index performed well in distinguishing reference sites from sites displaying some form of anthropogenic impairment. Individ ual metrics used in the index included: number of EPT taxa, total number of taxa, percent dominant taxon, percent Elmidae, and percent predators. The index we developed for Idaho rivers was essentially a modification of a fra mework designed for small streams, suggesting that techniques, including da ta analysis, currently used for streams can be adapted for use on larger ri vers. Adapting these methods for use in rivers is primarily a matter of (1) selecting metrics relevant to the rivers of interest; (2) expanding the fi eld sampling to encompass the greater habitat area and. potentially, hetero geneity of rivers; and (3) selecting an appropriate form of data analysis. The approach we describe here should be applicable to geographic regions ot her than Idaho.