Linking patterns of geographic distribution, abundance, and species traits to community characteristics (e.g. richness) is a principal goal in ecology. We examined these relationships for macroinvertebrates from 435 least-disturbed sites (USEPA EMAP and WSA for the 48 contiguous United States). We quantified 15 traits (69 categories) for 407 genera, and sites were grouped hierarchically by geography (West, Central, East). Regional endemicity was linked to site occurrence, species richness of genera, and local abundance. Widespread genera were smaller, had >1 generation/year, and longer-lived adults than endemic genera. Endemic genera richness was highest in West (24% cf. East = 13%, Central =0.5%). Local and regional genus richness and diversity were similar between the East and West, but lower in the Central region. In contrast, all traits were widespread, and regional differences in richness were small. Trait diversity was highest in the Central region, and lowest in the West. Surprisingly, trait diversity decreased with increasing genus richness resulting from the dominance of functionally similar endemic genera (esp. Plecoptera and Trichoptera in the West). The difference in community characteristics between Central and Western/Eastern streams suggests that the Central region is a geographic and ecological transition zone with different community assembly processes.