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• Flows equal to the bankfull discharge produce an average boundary shear stress that <br />is about 1.5 times the critical shear stress for bed load transport; this discharge is <br />sufficient to fully mobilize the bed material and thereby maintain the existing <br />bankfull hydraulic geometry. On the basis of data from the 20-year period from 1978 <br />through 1997, we recommend that flows equal to or greater than the bankfull <br />discharge should occur at least 5 days per year, on average. <br />• The single most important thing that can be done to maintain habitats used by the <br />endangered fishes is to assure that sediment supplied to the critical reaches continues <br />to be carried downstream. Sediment that is not carried through will accumulate in <br />low velocity areas, resulting in further channel simplification and narrowing. <br />The recommendations above emphasize physical processes associated with particular <br />flows, and stress the importance of sediment transport in shaping and maintaining habitats used <br />by the endangered fishes. Use of individual habitats within the 15- and 18-mile reaches varies <br />with fish species and life stage [Lagory et al., 2003], but most all habitats are affected by the <br />movement of sediment. Spawning habitats formed by gravel and cobble substrates (riffles, <br />shoals, or bars) require periodic flushing to remove interstitial fine sediment [Pitlick and Van <br />Steeter, 1998; Osmundson et al., 2002]. Low velocity channel-margin habitats, including <br />backwaters and secondary channels, require continued transport of fine sediment to prevent <br />deposition and further channel simplification [Osmundson et al., 1995; Van Steeter and Pitlick, <br />1998]. Disturbance of elevated surfaces by high flows is necessary to limit establishment of <br />vegetation and stabilization of channel bars. <br />4