Pronounced differences were found in the macroinvertebrate riffle fauna upstream and downstream of a flood control impoundment. Downstream differences were comparable with those occurring after mild organic enrichment. Plecoptera were absent, but numbers of Baetis and Caenis (Ephemeroptera) increased, and the abundance and number of species of Stenonema were considerably reduced. Numbers of Chironomidae, Simuliidae, Optioservus (Coleoptera), Hydropsychidae, and Hyalella azteca (Amphipoda) increased downstream. These changes are associated with downstream increase in the availability of detritus, a lag of about 4 weeks in the early summer rise in water temperature and a maximum temperature more than 6 degrees C lower than upstream, and alteration of other environmental factors.