Over 250 benthic invertebrate taxa were identified from snags and woody debris in streams and rivers of the Wapsipinicon, Cedar, Iowa, and Skunk River Basins in the Eastern Iowa Basins (EIWA) study unit of the U.S. Geological Survey National Water-Quality Assessment Program. The composition, distribution, and abundance of 74 predominant taxa were related to environmental conditions in the study unit, using habitat, hydrologic, and water-quality data. Four groups of sites were defined, based on the distribution and relative abundance of taxa. Detrended correspondence analysis was used to identify relations in the structure of the invertebrate assemblages, and the correspondence of taxa and sites in the groups was related to habitat, hydrologic, and water-quality information. Responses of invertebrate assemblages were explained by natural factors, such as surficial geology or physical habitat conditions, as well as human influences, such as agriculture or high-density hog-feeding operations.

Mayflies, caddisflies, and true flies were well represented in streams and rivers of the EIWA study unit. The mayflies Tricorythodes and Baetis intercalaris, the net-spinning caddisflies Hydropsyche bidens and H. simulans, and the Chironomidae Glyptotendipes, Polypedilum, and Rheotanytarsus predominated. Spatial variation in benthic invertebrate assemblages within a site was less than that observed among sites. Assemblages from 3 years of sampling generally were grouped by site, with exceptions related to differences in discharge among years.

The benthic invertebrate assemblages associated with the four groups of sites reflected the cumulative effects of agricultural and urban land use, sources of nutrient and organic enrichment, and longitudinal stream succession—the natural sequence of communities in streams from headwaters to large rivers. These factors, especially the natural changes from upstream to downstream, were influential in characterizing the benthic invertebrate assemblages of the site groups.

Stream size, a reflection of basin area, was a principal influence in categorizing the benthic invertebrate assemblages, with sites that have the largest basin areas forming a separate group. Although it is difficult to distinguish among the contributions of large basin area, increased concentrations of nutrients and pesticides, and decreasing instream habitat diversity, the resulting invertebrate assemblage described was distinct. The remaining sites were headwater or tributary streams that reflected conditions more common to smaller streams, such as higher gradients and the potential for more diverse or extensive riparian habitat, but were distinguished by landform. Following basin area in importance, landform contributed to the differences observed among the benthic invertebrate communities at the remaining sites.