THE RELATIONSHIP OF HABITAT CHARACTERISTICS TO THE DISTRIBUTION OF CHIRONOMIDAE (DIPTERA) AS MEASURED BY PUPAL EXUVIAE COLLECTIONS IN A LARGE RIVER SYSTEM

Floating chironomid pupal exuviae were collected throughout the Yakima River (Washington, USA) and tributaries in 1989 and 1990. Al samples were taken during the October-November low-now season and most sites w ere visited once with a sampling effort of approximately 10 minutes. S ampled stream segments ranged from 3rd to 7th order and elevation rang ed from 140-1200 m. The total number of taxa distinguished in 77 sampl es was 150 with a mean of 24 per site. Species richness was not highly correlated with stream order or elevation. An ecological gradient def ined by ordinating the taxa by site data was highly correlated with si te elevation (r=.93). Comparison with published attributes of chironom id genera suggested that temperature regime was a dominant environment al variable controlling chironomid distribution in this basin. However , since most measured habitat variables were associated with altitude, their effects could not be separated. Cluster analysis of taxa by sit e data resulted in groupings that distinguished montane from valley si tes and agricultural drains from other valley sites. Differences in sp ecies composition between moderately enriched agricultural drains and less-impacted sites were minor compared with the underlying elevation gradient.