Downstream fish assemblage response to river impoundment varies with degree of hydrologic alteration

River impoundments alter downstream hydrology and habitat, often resulting in significant changes in stream communities. The degree of impact of a river impoundment on downstream hydrology and biological communities can be dependent on many factors, including underlying natural hydrologic regimes and reservoir operation purpose and scope. We compared pre- and post-impoundment hydrologic patterns and fish assemblage structure at two sites below reservoirs differing in natural hydrologic regimes in Oklahoma. We observed significant shifts in fish assemblage structure downstream after stream impoundment at both sites. The Bird Creek flow regime shifted from a historically intermittent stream to one with stable perennial flows, and changes in fish assemblage structure covaried with changes in all five components of the flow regime; most species that increased in abundance require fluvial habitats and likely benefited from increased flows during historically low flow periods. In contrast, the Kiamichi River flow regime did not change significantly for most flow components despite shifts in fish assemblage structure; however, most species shifts were associated with lentic environments and were more likely related to proximity of reservoirs in the drainage system rather than changes in stream flow. Findings from this study suggest that downstream fish assemblage response to river impoundment can be associated with high levels of hydrologic alteration, but other factors including expansion of lentic species into lotic environments may also influence shifts in assemblage structure, particularly when hydrologic changes related to impoundment are limited.