Issues Afflicting Reservoir Tailwaters Across the United States

The tailwater is the reach of a stream immediately below an impoundment that is hydrologically, physicochemically, and biologically altered by the presence and operation of a dam. The overall goal of this study was to gain a nationwide awareness of the issues afflicting tailwater reaches. Specific objectives included (1) estimate the percentage of reservoirs that support viable tailwater reaches with environmental conditions suitable for fish assemblages throughout the year; (2) identify and quantify major sources of environmental concerns in those tailwaters that do support fish assemblages; and (3) identify environmental features of tailwater reaches that determine prevalence of key fish taxa. Data were collected through an online survey of fishery managers. Relative to objective 1, nearly 50% of the 1,286 reservoirs included in this study had tailwater reaches with sufficient flow to support a fish assemblage throughout the year. Area of the reservoir and catchment most strongly separated between reservoirs with or without viable tailwaters. Relative to objective 2, major sources of environmental concerns generally reflected flow variables, followed by water quality variables. Relative to objective 3, next to zoogeography, a broad spectrum of variables contributed to discriminating fish taxa prevailing in tailwaters, but principally flow and water quality variables. Results for objectives 1-3 varied greatly among nine geographic ecoregions distributed throughout the U.S. Our results provide large-scale understanding of the effects of reservoirs on tailwater reaches and may help guide research and management needs.