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~ ~ ~ uQ2547 The BHBF being proposed for 1998 will consist of2-4 days of flows less than or equal to 45,000 cfs sometime during betWeen the months of January through June of 1998. No flows of 8,000 cfs are proposed to immediately precede or follow this 45,000 cfs flow, In addition, it is proposed that the duration of the BHBF be determined based on real-time monitoring of sediment deposition, biological and cultural resources. and that the experimental flow be concluded at the point of diminishing sediment suspension and deposition at critical sites in the sediment-starved Marble Canyon reach. The upramping will begin from the highest point in the existing daily powerplant release hydrograph at a proposed rate of 4,000 cfslhr, The downramping will begin, once maximum sediment deposition has been achieved, at a proposed rate of 1,500 cfslhr until flows rejoin the existing daily powerplant release hydro graph. Adllptive Mllnllvement Objectives. The 1996 BHBF was conducted as an experiment to study planned flooding as a management tool for restoration and preservation of Colorado River ecosystem resources, The primary goals of the 1996 BHBF were to learn about sediment transport under high flows and to mobilize and redeposit channel-stored sand as higher-elevation bars. Additional objectives included preservation of cultural resources, providing more natural flood-disturbance processes to riparian vegetation, rejuvenation of other important channel features such as backwater habitats, and flushing non-native fishes from critical areas in downstream reaches, Of the eight objectives generally listed in the GCDEIS, the 1996 BHBF achieved five that were mostly related to sediment transport and deposition, I. The 1998 BHBF will provide scientific benefit by allowing researchers to test conclusions developed from the results of the 1996 BHBF regarding modes and rates of sediment deposition, 2. The 1998 BHBF is being proposed, initially as a management action to mitigate against the predicted negative effects on sediment resources of high steady releases or unplanned spills which could occur based on early forecasts in WY98, 3, Based on the "State of the Colorado River ecosystem" report, and subject to the acceptance of the recommendations of the "Risk of Spill" workgroup, the 1998 BHBF is also being proposed as a management action that can be designed to benefit natural, cultural, socio-economic, and recreational resources, TIMING AND COMPLIANCE Potential conflicts with the U.S. Fish and Wildlife Service Biological Opinion and National Park Service and Bureau of Reclamation cultural resource management needs may exist and should be addressed by the TWG and the AMWG, Of specific concerns are habitat and incidental take issues related to the endangered Kanab ambersnail, Humpback chub, and Southwestern willow flycatcher, In addition, the effect on other biological resources such as the Lees Ferry trout fishery, the aquatic food base, and riparian vegetation will need to be considered prior to making final decisions regarding the magnitude, duration, and timing of a BHBF, Compliance required under the National Environmental Policy Act, the Endangered Species Act, and the Historic Preservation Act will need to be determined and initiated, as appropriate, 3