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To provide a better evaluation of the hydraulic and incipient motion conditions in the main flow path, which is more closely related to the dynamics of the riffles/tertiary bars, the maximum grain shear stress in each cross-section and associated incipient motion conditions were computed based on the incremental conveyance across the cross-section using the average gradation of the riffles/tertiary bars (050 -84mm). The results of this computation are shown in Figure 3.10. This figure indicates that, in the main flow path, incipient motion conditions are reached at slightly less than 5,000 cfs in the reach between the bend and the riffle (XS 5-10). At the base of the riffle (XS 10), critical conditions are reached at a discharge of approximately 630 cfs. , Analysis of the variations in dimensionless grain shear stress ('t .) with discharge at key locations along the study reach indicates that with two exceptions, the bed shear stress increases with increasing discharge over the range of discharges modeled (Figures 3.10 through 3.14). The exceptions occur at XS 6 and XS 10. At XS 6, the maximum shear in the cross section reaches its largest value at a discharge of 15,000 cfs. Although the shear stress decreases at higher discharges, it generally stays above critical conditions through the highest modeled discharge of 32,300 cfs. (Using 't *c of 0.047 indicates conditions slightly below critical above 22,000 cfs). At XS 10, both the maximum shear (Figure 3.10) and the average shear (Figure 3.9) in the section reach their highest values at about 2,000 cfs. At this cross section, the dimensionless critical shear stress stays relatively constant at discharges above 2,000 cfs, with values of 't" in the range of 1.5 using dimensionless critical shear value of 0.03 and slightly below critical ('t" = 1) using a value of 0.047. Based on the maximum shear stress analysis, transport of the sizes of material characteristic of the riffles/tertiary bars occurs at discharges above about 5,000 cfs in the reach between the bend and the base of the riffle (XS 5-10). This material apparently cannot be transported through the bend at discharges be/ow about 22,000 cfs. The tertiary bar downstream of XS 6 is probably associated with local flow separation where the sandstone wall diverges from the main flow direction at the entrance to the bend. At XS 10 (Figure 3.13), which is indicative of conditions controlling the dynamics of the two riffles/tertiary bars in that vicinity, the gravel and cobble material is mobilized at discharges greater than about 630 cfs. Considering the range of reasonable dimesionless shear stresses, critical conditions are only slightly exceeded over the range of flows above about 630 cfs. The reduction in shear stress that occurs at discharges greater than about 2,000 cfs is related to the backwater effect created by the bend which drowns out the base of the riffle. From this result, it would appear that the different tertiary bars in this area are at, or slightly above, critical conditions over a very large range of discharges. 3.21 Resource Consultants & Engineers, Inc.