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backwater method and stage discharge curves at several cross sections on the Cache La Poudre River. Starting water-surface elevations were taken from those determined for the South Platte River. Cross section data for Sheep, Ashcroft, and Eaton Draws, and the Tri-Area Drainageway were obtained from field surveys by the study contractor. Additional cross section data for a part of the Tri- Area Drainageway were obtained from I-foot contour interval, 1:1,200 scale topographic mapping produced as a part of a 1975 drainageway study for Weld County (Reference 16). Additional cross section data for a portion of the overbank area of Sheep Draw were obtained from 2-foot contour interval, 1:2,400, scale photogrammetric maps (Reference 17). All bridges were field surveyed to obtain elevation data and structural geometry. Roughness coefficients were estimated from field inspection. Channel "n" values for Sheep, Ashcroft, and Eaton Draws and the Tri-Area Drainageway, including the Tri-Area Drainageway Overflow Channel, ranged from 0.030 to 0.040, from 0.035 to 0.040, from 0.016 to 0,050, and from 0.045 to 0.060, respectively, and overbank "n" values ranged from 0.040 to 0.070, from 0.040 to 0.060, from 0.040 to 0.070, and from 0.045 to 0.090, respectively. The COE HEC-2 computer program was used to compute water-surface profiles for all four streams (Reference 15). Starting elevations were determined from slope-area computations at the downstream sections. Cross section data fo'r Coal Creek were obtained from topographic maps at a scale of 1:4,800, with contour intervals of 2 and 4 feet (Reference 18). Field measurements were taken by the SCS to obtain elevation data and structural geometry of all bridges and culverts. Roughness coefficients for Coal Creek south of the Union Pacific Railroad were determined by the WRC (Reference 19), These values were reviewed and approved by the CWCB. The Hnt! values range from 0.075 in the channel to 0.055 in the overbank. Topographic mapping for the Big Thompson River was established by photogrammetric methods from flights made in June 1975. 8ridge and roadway survey data were provided by the CWCB. Water-surface profiles were developed using the COE HEC-2 computer program (Reference 15). The computations were based on channel and floodplain conditions as represented by survey data gathered in June 1975 and as supplemented by later field investigations (Reference 20), Locations are shown fl oodway locations of selected cross sections used in the hydraulic analyses on the Flood Profiles. For stream segments for which a is computed (Section 4.2), selected cross section are also shown on the Flood Boundary and Floodway Map. Flood profiles were drawn showing computed water-surface elevations to an accuracy of 0.5 foot for floods of the selected recurrence intervals. 14