Altered Green River Flows Impact Rare Fish Habitat

The last self sustaining riverine population of endangered razorback sucker occurs on the middle Green River, Utah. Since 1962, operations at Flaming Gorge Dam have reduced flow variability and altered timing and duration of flood peaks. These alterations in flow have resulted in poor production of young because of sedimentation at the primary spawning site during high flow years when dam releases are early and high compared to natural inflow. Furthermore, reduced connectivity to floodplain nursery areas results in poor survival of young post-emergence. It is hypothesized that reductions in peak flows have resulted in sediment being stored and transported at much lower flow ranges. Effective discharge has been reduced 44 percent from pre-dam conditions. Results of sediment transport analysis conducted using HEC-6 compared favorably with empirical cross section observations at the site in 1993 and 1996. Predicted sedimentation began at 200 m³s^–1 and resulted in 0.6 meters average deposition of sand in the spawning channel as flows approach 650 m³s^–1. Restoration of spawning and nursery habitat for this floodplain species is achievable through restoration of flow variability and timing at Flaming Gorge Dam and removal or modification of levees at floodplain nursery sites.