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Cave Creek below Cave Buttes Dam Reach location: Latitude 33042'55", longitude 112002'44". Reach begins about 500 ft below Cave Buttes Dam. Bed-material size: See channel description. Channel descriution: The constructed reach is straight, and cross sections are trapezoidal in shape. The bottom and sides of the channel are composed of rounded cobbles imbedded in a matrix of cement (approximate mean diameter of the rock projections was 80 mm, about half of which seemed to be exposed to flow). Roughness elements are constant throughout the reach. The channel gradient increases from about 0.002 ftift at cross- section I to about 0.010 ftift at cross-section 8. The stream is ephemeral, and flow is regulated by Cave Buttes Dam. Remarks: Roughness coefficients were determined for both subcritical (Froude number < 1.0) and supercritical (Froude number> 1.0) flow regimes. The Froude number at cross- section 5 approximated 1.0 (critical flow) for both flows and was not incorporated in the analyses. A decrease in Manning's n occurred when flow transitioned from subcritical flow to supercritical flow. This change could be the result of unstable flow conditions as investigated by Koloseus and Davidian (1966). According to their results, roughness depends on the Froude number when unstable flow conditions occur (generally flow is considered unstable if Froude numbers are larger than about 1.6). Although Froude numbers in the supercritical reach averaged about 1.6 or less, the Froude number increased to over 1.8 in cross-section 8. The suspended-sediment concentration was 224 mg/L for the flow of January 9, 1995, and may be indicative of suspended-sediment concentration for the flow on November 2, 1995. This amount of suspended sediment is considered as wash load and to have negligible effects on n values. Table 9. Flow data and computed roughness coefficients, Cave Creek below Cave Buttes Dam Roughness coefficient 01-09-95 11-02-95 Discharge, in cubic feet per second 197 165 Subcritical Supercritical 0.025 .025 0.017 .017 Rating Good Good Date of flow Table 10. Average-reach properties, Cave Creek below Cave Buttes Dam Area, in Top width, Hydraulic Mean velocity, Froude Total length, Total fall, Water-surface Date square feet in feet radius, in feet in feet per second number in feet in feet slope Subcritical flow regime (cross-sections 1-4) 01-09-95 46.6 21.4 2.03 4.36 0.53 311 0.94 0.0030 Supercritical flow regime (cross-sections 6--8) 01-09-95 21.9 19.1 1.12 9.21 1.54 148 2.19 .0148 Subcritical flow regime (cross-sections 1-4) 11-02-95 42.0 20.4 1.91 4.06 .51 311 .95 .0031 Supercritical flow regime (cross-sections 6-8) 11-02-95 18.1 17.9 .99 9.16 1.61 148 1.92 .0130 Cave Creek below Cave Buttes Dam 31