THE RELATIONSHIPS AMONG 3 HABITAT SCALES AND STREAM BENTHIC INVERTEBRATE COMMUNITY STRUCTURE

1. The relationships between three habitat scales and lotic invertebra te species composition were investigated for the 15 540 km(2) Yakima R iver basin in south-central Washington, U.S.A. 2. The three spatial sc ales were sample (the sampled riffle), reach (a length of ten-twenty s tream widths) and segment (a length of stream of nearly uniform slope and valley form having no change in stream order). 3. Physical variabl es were highly correlated between scales and expressed a relationship between altitude, basin form and small-scale physical structure. 4. Mu ltiple discriminant function analyses indicated that segment- and reac h-scale variables discriminated among species-defined groups better th an sample-scale variables. 5. Species composition varied along a compl ex altitudinal gradient of changing basin form and resultant land use. 6. There was no clear relationship between species richness and altit ude on a site basis. However, when viewed at the basin scale, maximum richness was observed at the transition between montane and valley sit es.