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At each gaging station, the annual flood peaks caused due to a snowmelt event and due to a rainfall event are separated. Using log-Pearson Type III distribution, the ]0-, 50-, 100-, and 500-year recurrence interval discharges are obtained separately for snowmelt events, as well as for rainfall events. The rainfall events were observed to be negligible in comparison to snowmelt events; Le., the combined probability of occurrence of equal to or exceeding a given discharge for snowmelt and rainfall events was very close to the probability of occurrence of equal to or exceeding that discharge for the snowmelt event alone. The discharge-drainage area relationships developed for the 10-, 50-, 100-, and 500-year recurrence interval discharges for the snowmelt events provided a correlation coefficient of approximately 97 percent. Peak discharge-drainage area relationships for Blue River, Straight Creek, and Willow Creek are shown in Table I, "Summary of Discharges." 3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of flooding from the sources studied were carried out to provide estimates of the elevations of floods of the selected recurrence intervals. Users should be aware that flood elevations shown on the FIRM represent rounded whole-foot elevations and may not exactly reflect the elevations shown on the Flood Proftles or in the F100dway Data tables in the FIS report. Flood elevations shown on the FIRM are primarily intended for flood insurance rating purposes. For construction and/or floodplain management purposes, users are cautioned to use the flood elevation data presented in this PIS in conjunction with the data shown on the FIRM. Water-surface elevations of floods of the selected recurrence intervals for the Blue River were computed through use of the U.S. Army Corps of Engineers (USACE) HEC-2 step-backwater computer program (Reference 12). Water-surface elevations of floods of the selected recurrence intervals of Straight Creek and Willow Creek were computed through use of the USACE HEC-RAS step-backwater computer program (Reference 13). Cross sections for the backwater computations and floodplain delineation for the Blue River, Straight Creek, and Willow Creek were obtained from topographic maps and cross section surveys. The Town of Silverthorne Geographic Information System (Reference 14) coverage for topography was combined with cross sections obtained by field survey for a complete description of the floodplain. All bridges were field checked to obtain elevation data and structural geometry. Channel roughness factors (Manning's "nO values) used in the hydraulic computations were chosen by engineering judgment and based on field observation of the streams and floodplain areas. A roughness value of 0.04 was selected for the Blue River, Straight Creek, and Willow Creek channels, while overbank: roughness values were determined to be 0.10. Starting water-surface elevations for the Blue River, Straight Creek, and Willow Creek were calculated using the slope-area method at the downstream end of each river of tributary . Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway was computed (Section 4.2), selected cross section locations are also shown on the Flood Insurance Rate Map (Exhibit 2). The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the proftles are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. 5