The fish assemblage of the Canadian River in Texas historically was dominated byHybognathus placitusandNotropis girardi. These species represented > 90% of fishes collected from the Canadian River in 1954–1955. Construction of two reservoirs on the Canadian River in the 1960s (Ute Reservoir, New Mexico, 1962; Lake Meredith, Texas, 1965) altered hydrologic conditions in the river and affected the fish assemblage. Downstream from Ute Reservoir, mean annual discharge decreased by about 38% after impoundment, butH. placitus, N. girardi, and other mainstem species still dominate the assemblage. Downstream from Lake Meredith, mean annual discharge decreased by 76% and the historic mainstem fish assemblage has been almost completely replaced by species that formerly were restricted to tributary streams. The magnitude of post-impoundment changes in the fish assemblage of the Canadian River appears to be related to the degree that discharge has declined, especially during the spawning season.