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for which a floodway was computed (Section 4.2), selected cross section locations are also shown on the Flood Boundary and Floodway Map (Exhibit 2). Roughness coefficients (Manning's "nlO) used in the hydraulic computations were determined by field observation. Roughness values for the main channel of the Colorado River and Alkali Creek ranged from 0.035 to 0.040, whereas overbank roughness values ranged from 0.060 to 0.070. The coefficients used for the main channel of the Roaring Fork ranged from 0.035 to 0.045 and from 0.050 to 0.090 for the overbank. The Crystal River had coefficients ranging from 0.040 to 0.035 for the main channel and 0.050 to 0.090 for the overbank area. Roughness coefficients for the main channel and overbank areas of Cattle Creek varied from 0.035 to 0.050, and 0.050 to 0.060, respectively. The coefficients used for Fourmile Creek ranged from 0.060 to 0.070 for the main channel and 0.060 to 0.080 for the overbank areas. Threemile Creek had coefficients ranging from 0.045 to 0.050, and 0.080 to 0.090 for the main channel and overbank areas, respectively. Mitchell Creek used coefficients ranging from 0.045 to 0.050 for the main channel and 0.040 to 0.045 for the overbank areas. Roughness values for the main channel of Rifle Creek ranged from 0.040 to 0.050, whereas overbank roughness values ranged from 0.050 to 0.080. Roughness values for the main channel of Government Creek ranged from 0.035 to 0.040, whereas overbank roughness values ranged from 0.035 to 0.045. Roughness values for the main channel of Hubbard Gulch ranged from 0.035 to 0.045, whereas overbank roughness values ranged from 0.035 to 0.050. The starting water-surface elvations for the Colorado River, Government and Alkali Creeks, and Hubbard Gulch were determined by the slope-area method. The starting water-surface elevation for the Roaring Fork River was derived from a previous flood study done by Gingery Associates, Inc. (Reference 12). For the Crystal River, the starting water-surface elevation was based on the flood elevation on the Roaring Fork at the confluence. Cattle Creek, Mitchell Creek, and Threemile Creek derived their starting water- surface elevations from critical depth. Rifle and Fourmile Creeks starting water-surface elevation was taken as the normal depth for the given discharge. Rifle Creek overflows its western bank just upstream of County Highway 291. This overflow is referred to in this Flood Insurance Study as Rifle Creek Splitflow. A separate HEC-2 step-backwater analysis (Reference 6) was performed for the splitflow. Discharge values used in the step-backwater analysis were based on a hydraulic analysis of the overflow from the main channel of Rifle Creek. Cross sections were digitized from aerial photographs (Reference 9). Roughness factors for the channel ranged from 0.050 to 0.080, whereas 0.080 was used for the overbank. The starting water-surface elevations were based upon the computed water-surface elevations On Rifle Creek. A profile base line was used to establish the ] 2