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The starting water-surface elevation and discharge for the USBR study (Refcrence 3S) where the overflow channel converges with the main channel are not the same as those used in the Simons, Li & Associates. Inc., study. This is due to the very significant reduction in discharges from losses to Cooper Slough. A rating curve was drawn from Simons, Li & Associates, Inc., data, and elevations corresponding to the discharges resulting from the reductions were used. For Dry Creek, the starting water-surface elevations were taken from the existing USACE profiles of the Cache La Poudre River at the confluence. The HEC-2 computer program was used to compute the water-surface profiles (Reference 36). Roughness coefficients ranged from 0.03 to O.OS in the channel and from O.OIS to 0.100 in the overbanks. Cross sections along Dry Creek were taken from topographic maps with a 7-foot contour interval. ... . The approximate flood boundaries for Boxelder Creek were taken from an August 1981 Simons, Li & Associates, Inc., Master Drainageway Planning Study for Boxelder Creek and Cooper Slough (Reference 2). The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the profiles are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. All elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGVD). 4.0 FLOODPLAIN MANAGEMENT APPLICATIONS The NFIP encourages State and local governments to adopt sound floodplain management programs. Therefore, each Flood Insurance Study produces maps designed to assist communities in developing floodplain management measures. 4.1 Floodplain Boundaries To provide a national standard without regional discrimination, the I-percent annual chance (lOO-year) flood has been adopted by FEMA as the base flood for floodplain management purposes. The 0.2-percent annual chance (Soo-year) flood is employed to indicate additional areas of flood risk in the community. For each stream studied by detailed methods, the 100- and SOO-year floodplain boundaries have been delineated using the flood elevations determined at each cross section. For Dry Creek, between cross sections the boundaries were interpolated using topographic maps at a scale of 1 :4,800. with 2-foot contour intervals (Reference 37). The floodplain boundaries for Cooper Slough were delineated using topographic work maps at a scale of I: 1,200, with 2-foot contour intervals (Reference S). Flood boundaries for Spring Creek were interpolated using topographic work maps ranging in scale from 1 :480 to I: 1,200, with contour intervals ranging between 1 and 2 feet (References 38 and 39). 1', . For the Cache La Poudre River, the flood boundaries for the 1oo-year and SOO-year floods were delineated using the flood elevations determined at each cross section. The flood boundaries were plotted on 1:2,4oo-scale orthophoto maps (Reference 40). There were several low-lying areas protected from flooding from the main channel by gravel 18