Summer-dry stream invertebrates in a western Oregon lowland landscape

Seasonal rainfall in western Oregon causes many lowland streams to flow during the wet-season, but they are generally dry during summer and autumn. In the southern Willamette Valley much of the land surrounding summer-dry streams is cultivated for grass seed production; thus watersheds not influenced by agricultural land-use are rare. Stream bottoms are primarily hard clay with varying amounts of rooted vegetation. Our paper summarizes 3 years of sampling in these agriculturally-influenced streams. Invertebrate densities from an initial area-wide survey were positively correlated with the amount of stream bottom vegetation. We subsequently sampled paired vegetated-unvegetated sections within 4 streams; the pattern of invertebrate density increasing with more in-channel rooted vegetation was confirmed. In these streams invertebrate assemblages were comprised primarily of non-insects, but some insects, including EPT taxa, were present. Sites to the east of the Willamette River, where watershed agricultural land-use was higher, had lower taxa richness and greater proportional abundances of oligochaetes. Site west of the Willamette had less intense agricultural land-use, higher taxa richness and generally had several EPT taxa. Reference sites, located in wildlife refuges and conservation units, exhibited similar characteristics to the latter, less disturbed sites.