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trout fishery. However, high steady releases may be detrimental to the quantity and quality of sediment resources in the ecosystem, and may indirectly harm cultural, biological, and recreational resources through erosion of existing sandbars and export of stored sand (contributed by tributary inflows). " ; Other resources, such as native fishes, may benefit from higher constant flows (e.g., constant 25,000 cfs), because flooded tributary mouths and backwater may provide slackwater nursery habitat for young-of-year fish. Constant high flows are expected to increase the aquatic foodbase in the Lees Ferry and downstream reaches, a coodition that may benefit both native and non- native fishes. " Runoff Scenario 2: This scenario assumes full reservoirs (Lake Powell and Lake Mead) and an expectation of above-average inflows to Lake Powell. Lake Powell would be expected to fill under this scenario and annual releases would exceed 8,23 Maf. Inflow hydrology and resource conditions may trigger a BHBF, which may best be conducted in late March/early April. These conditions indicate a higher risk of an uncontrolled spill later in the Water Year, This scenario would also require high flows early in the year to increase storage space in Lake Powell in anticipation of a high spring runoff. Under these conditions, higher flow releases might be needed for variable periods, ranging from 25,000 to 44,000 cfs. ," ~. ~ ~ ~" ~ Data from the 1996 BHBF (suspended sediment, grain size/concentration, and reverse grading seen in new bars), suggest that coarsening of channel-stored sediment and deposition of sand or bars occurred rapidly at these high flows (hours to a few days). Very rapid deposition of fine sediment along channel margins has been attributed to a secondary-circulation phenomenon (J.D. Smith, U.S. Geological Survey, personal commun.). Such secondary circulation may result in sediment deposition rates within recirculation eddies and along channel maTgins that exceed downstream transport rates to the Lake Mead delta, but this hypothesis requires furtheT testing. " ;::. . .' :.. " In addition, reworking of aggraded rapids and debris fans also occurred quickly during the 1996 flood, mainly during the Tapidly rising limb of the hydrograph. After the relatively short period in which suspended sediment concentration and grain size declined and sandbars were formed, sediment transport cooditions stabilized during the last four days of the high flow at most of the sites under study, particularly those in the sediment-rich segment downstream from the Little Colorado River confluence. For instance, depending on the quantity and distribution of channel- margin sediment storage, short-durationlhigh-magnitude flows above the 45,000 cfs (BHBF) levels may provide optimal sand storage with minimal sediment export to Lake Mead. y $ " ,; if' I~~ ,,.. :"h ,,' ~.~ f: .~ j~OJ ~ .; ;::J Impacts of high flows on endangered species are a concern to this planning process. At present, the U.S. Fish and Wildlife Biological Opinion on GCD operations directs Reclamation to avoid planned flooding that results in excessive loss of Kanab ambersnail and its habitat at Vaseys Paradise (Mile 31.5) until additional populations of the snail are discovered or established. Southwestern willow flycatcher habitat is also potentially affected by high flows. Other species [WORKING DRAFT 2 - GCMRC: 12/04/98] 5