Planned flooding and Colorado River riparian trade-offs downstream from Glen Canyon Dam, Arizona

Regulated river restoration through planned flooding involves trade-offs be tween aquatic and terrestrial components, between relict pre-dam and novel post-dam resources and processes, and between management of individual reso urces and ecosystem characteristics. We review the terrestrial;(wetland and riparian) impacts of a 1274 m(3)/s test flood conducted by the U.S. Bureau of Reclamation in March/April 1996, which was designed to improve understa nding of sediment transport and management downstream from Glen Canyon Dam in the Colorado River ecosystem. The test flood successfully restored sandb ars throughout the river corridor and was timed to prevent direct impacts t o species of concern. A total of 1275 endangered Kanab ambersnail (Oxyloma haydeni kanabensis) were translocated above the flood zone at Vaseys Paradi se spring, and an estimated 10.7% of the total snail habitat and 7.7% of th e total snail population were lost to the flood. The test flood scoured cha nnel margin wetlands, including potential foraging habitats of endangered S outhwestern Willow Flycatcher (Empidonax traillii extimus). It also buried ground-covering riparian vegetation under >1 m of fine sand but only slight ly altered woody sandbar vegetation and some return-current channel marshes . Pre-flood control efforts and appropriate flood timing limited recruitmen t of four common nonnative perennial plant species. Slight impacts on ethno botanical resources were detected > 430 km downstream, but those plant asse mblages recovered rapidly. Careful design of planned flood hydrograph shape and seasonal timing is required to mitigate terrestrial impacts during eff orts to restore essential fluvial geomorphic and aquatic habitats in regula ted river ecosystems.