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• DRAFT February 25, 1998 connectivity through the development and removal of sandbars, sediment deposition in the channels connecting the bottomlands and the river, sediment deposition in the bottomland areas, changes in channel morphology during the rising and falling limbs of the flood hydrograph and bank erosion. The duration of flooded bottomland connectivity also can be affected by sediment deposition. During the falling limb, the loss of connectivity between flooded bottomlands and the river can result in ponded water which, without drainage, will isolate fish (FLO Engineering, Inc. 1997b). A new floodplain, in apparent equilibrium with reduced floods, has developed in the Uinta Basin especially in the Ouray National Wildlife Refuge. This floodplain is regularly inundated but the area extent of these surfaces is an order of magnitude less than the pre-dam floodplain. It appears that significant inundation of the alluvial valley of the Green River may not have regularly occurred since the 1920s. Two study reaches were selected for the investigation of bottomland flooding and backwater inundation in the Green River (FLO Engineering, Inc., 1996). The first reach was located in the vicinity of Ouray National Wildlife Refuge, near Vernal, Utah (in reach 2 as designated in this report) and the second reach was located in Canyonlands National Park near Anderson Bottom (within reach 3 for this report). The average bank-full discharge at the Ouray reach is now 20,300 ft3/s with a return period of 3.4 years, which is slightly greater than the mean annual peak. The average bankfull discharge in the Canyonlands reach is 39,000 ft3/s with a return period of 15 years. Under pre-1963 discharges, overbank areas were flooded one out of every three years, based on the cross sections measured in 1995. The investigation of bottomland flooding at Ouray and the backwaters in Canyonlands was conducted through the hydraulic prediction of water surface using the Corps of Engineers BEC-2 step backwater model. A total of 36 cross sections in the Ouray study reach and 22 cross sections in the Canyonlands reach were surveyed three times over the course of the 1995 runoff season hydrograph to create a data base for the BEC-2 model. The results of the BEC-2 analysis included the prediction of the bankfull discharge and flood inundation area as a function of discharge. The analysis of flood levels for Canyonlands focused on flooding available side canyon backwaters. The Ouray investigation focused on the potential for inundating historic flood plains with overbank flows. The results of the Ouray inundation analysis indicated that overbank flooding is initiated at discharges ranging from 15,800 ft /s to 22,700 ft /s under existing conditions with levees. The exception is Old Charley Wash, that has a small side channel that allows flooding to occur at approximately 13,000 ft /s. Currently, some application flooding is also available with the use of • existing inlet structures which are operational at discharges on the order of 3,000 ft3/s to 4,000 10