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Flood elevations for the 10-, 50-, and 100-year floode on the Cache La Poudre River were computed by the COE, Omaha District (Refer- ence 6). Water-surface elevat~ons for the SOO-year flood were estimated based on rating curv~s prepared for each cross section using the COE information. I Cross section data for Black c.nyonCreek were taken from topographic maps at a scale of 111,200 with a contour interval of 2 feet (Reference 19) and were supple~nted with field-surveyed cross sections. Roughness coefficiepts were estimated by field inspection. The channel value was 0.04 and: the overbanks were 0.06. Water- surface profiles were computed: using the COB HEC-2 computer program (Reference 22). . i Cross section data for the Fal~ River were field-surveyed. A water supply dam located in the uppe~ watershed failed in July 1982 and changed the channel. Roughness coefficients were estimated by field inspection. The channel roughness ranged from 0.04 to 0.05 and the overbank roughness ranged from 0.03 to 0.06. Water-surface profiles were computed using the COB HEC-2 Program (Reference 22). Approximate flood elevations tor the Little Thompson River were determined from a COB Flood Plain Information report (Reference 27). Approximate flood elevations for the Cache La Poudre River were I determined from two COB Flood :Plain Information reports (Refer- ences 6 and 28). Approximate flood elevations for BOxelder Creek and cooper Slough in the vicinity east of the City of Fort Collins were computed using the COB HEC-2 computer program (Reference 22), as reported in the study prepared by Simoqs, Li & Associates, Inc. (Refer- ence 9). LOcations of selected cross s.ctions used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). Por stream segments for which a floodway was computed (Section 4.2), selected cross section locations are also sh~wn on the Flood Boundary and Floodway Map (Exhibit 2). Starting water-surface elevat~ons for the Big Thompson River upstream of Lake Estes and Fish Creek ere the maximum normal operating level of Lake Estes. Startin water-surface elevations for Dry Creek were computed assuming qritical depth. On Fall River, Black Canyon Creek, BOxelder Creek, :and COOper Slough, starting water- surface elevations were computed using the slope~area method. The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the profiles are thus con- sidered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. I I "- 18.