Sampling conducted by personnel of two separate laboratories of the U.S. Environmental Protection Agency showed a single amphipod species, Crangonyx pseudogracilis, present in the Ohio River before the early 1970s. Subsequent sampling by personnel of both laboratories, along with invertebrate collections made by the West Virginia Department of Natural Resources and the first author of this paper showed that this species disappeared from the river in approximately 1973. Another amphipod species, Gammarus nr. fasciatus, invaded the Ohio River in 1973. Gammarus nr. fasciatus spread rapidly throughout the Ohio River, and is now one of its most common macroinvertebrate species. Although the evidence is circumstantial, it seems likely that the elimination of C. pseudogracilis from the Ohio River was caused by the damming of the river, especially the use of high-lift dams, completed from 1936 through the present. Experimental evidence indicates that the existing amphipod, G. nr. fasciatus, has a proclivity for slow-water conditions, which have been favored by the installation of the high-lift dams.