EVALUATING CHANNEL CHANGE AT TAMARISK REMOVAL AND CONTROL REACHES FOLLOWING A PRESCRIBED FLOOD ON THE UPPER GREEN RIVER, LODORE CANYON, DINOSAUR NATIONAL MONUMENT

Stream channels of the upper Colorado River Basin narrowed and simplified in planform during the 20th century. These changes were the result of natural and human induced shifts in the hydrologic and sediment regimes caused by climate change, land use changes, the completion of the Upper Colorado River Storage Project (CRSP) and the establishment and spread of the non-native riparian shrub tamarisk (Tamarix ramossissima). Channel narrowing and simplification resulted in the loss of important aquatic and flood plain habitat features, including in-channel backwaters, which serve as nursery habitat for some endangered endemic fish species and seasonal pools, which provide breeding habitat for some amphibians. Several options for mitigating undesirable environmental conditions associated with channel simplification and widespread establishment of tamarisk in the upper Colorado River basin have been considered. In the Upper Green River of Dinosaur National Monument, paired experimental reaches have been established in the fan-eddy complexes of Lodore Canyon. Each pair includes a reach where tamarisk have been completely removed by teams of volunteers (weed warriors) and a reach where no riparian treatment has been applied. In the summer of 2005, an experimental flow of 192 m³/sec was released through the Canyon of Lodore from Flaming Gorge Dam. The flood was one of a few post CRSP floods on the Upper Green River that have significantly exceeded Flaming Gorge Dam powerplant capacity of 130 m³/sec. We evaluated channel geometry changes and scour and fill resulting from the rare flood at 17 cross-sections within the tamarisk control and treatment reaches. Pre and post-flood channel geometry surveys were performed on cross sections containing the most significant sand storage units. Scour chains were installed and recovered on beaches above and below channel constrictions to evaluate flood scour and fill in backwater and expansion hydraulic environments. Results from this study will assist managers of regulated rivers in assessing the ability of physical removal of tamarisk in combination with controlled floods to reverse undesirable channel simplification and subsequent loss of riparian and aquatic habitats.