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FINAL REPORT, November 2003 High-jlow Requirements for the Duchesne River and after the 2001 runoff. Sandy samples were dry sieved. Silty samples were oven-dried, weighed, then wet sieved to remove the silt and clay fraction. The remaining sample was then oven dried and reweighed to determine the weight of lost fines before dry sieving (Guy 1969). Estimation of the Discharge Necessary for Gravel Entrainment Use ofHEC-RAS modeling produces friction slopes for individual cross sections that can be used with stage and channel geometry data to estimate shear stress at the bed for various discharges. HEC- RAS reports average boundary shear stress for each modeled discharge at each cross section calculated from the Duboys equation: TO = rRS (8) where TO is the average shear stress exerted on the bed, r is the specific weight of water, R is hydraulic radius of the flow at the cross section, and S is friction slope through the cross section. Hydraulic radius, defmed as the area of flow in the cross section divided by the wetted perimeter. The magnitude of shear stress necessary to entrain gravel depends on the sizes of gravel present on the channel bed. We used the Shields equation to calculate the dimensionless critical shear stress through riffle areas at the detailed study sites. This relationship is: T* =Tc"/Dlrs-rw} (9) where Ter is the critical shear stress necessary to initiate movement of bed particles, rs and rw are the specific weights of sediment and water, Dj is a representative sediment particle size, and T* is the Shields parameter expressing the dimensionless critical shear stress for particle entrainment. We evaluated the value of T* for a range of discharges using measured gravel particles sizes and values of shear stress determined from the output of our HEC- RAS models. For purposes of this analysis, we assumed that bed entrainment can be evaluated by entrainment of the median particle size (Dso), even though stream bed sediments are mixtures of a range of particles sizes. . The value of dimensionless critical shear stress for entrainment varies according to differences in the particle size distributions for different mixtures (Parker and Klingeman 1982; Wilcock 1998). Typical experimentally-determined values of T* for uniform mixtures are about 0.04, while the critical value drops to near 0.02 for sand and gravel mixtures (Wilcock 1998). For this analysis, we adopted a value of T* of 0.03. This value of the Shields parameter has been previously applied as the critical value for incipient gravel motion in the development of flow recommendations for the protection of native fish habitat elsewhere in the Colorado River basin 29