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CHAPTERl'l HYDROLOGIC ANALYSIS Figure 2- June 1921 Flood Henson Creek as _ Gunnison A~enue bridge it was washing out o~er Flood Characteristics Most of the annual flood flows on Henson Creek and the Lake Fork of the Gunnison within the study area result from rapid springtime w~lting of the snow pack in the high mountain basins, which comprise the majority of the drainage area for both streams. According to available stream flow records, peak flows occur primarily from May through early July. Snow ~~lt may occasionally be augmented by rain. Stream flows from snowmelt runoff are characterized by sustained periods of high flow and marked daily fluctuation. Periods of thunderstorm activity normally occur during the summer months after the high runoff has receeded. Cloudbursts are normally localized and only occasionally produce high flows in the stream. A re~iew of available runoff records indicate only one summertime peak flow (outside the normal runoff period). .... ". MethodS of Analysis A hydrologic analysis was carried out to establish peak discharge frequency relationships for floods of 10-, 50-, 100-, and 500-yearrecurrence intervals for the Lake Fork of the Gunnison and Henson Creek in the study area. Streamflow records are available for 9 years on Henson Creek and for 14 years on the Lake Fork of the Gunnison (1918 - 1937), station on the lake Fork is located ~ mile upstream of the Henson The gauging station cn Henson Creek is located 1~ miles above its (1918- 1937) The gauging Creek confluence. !routh. 13 Peak flows calculated from the on_stream records were compared to fifteen other gauging anct study sites In nearby similar basins. Thesefifteensites were cnosen for we comparison becau~e or U1<' high corr.:iation bet..e.m basin characteristics such as slope, precipitation, elevation. size and vegetation. Table 1 lists basin characteristics of these streams. Peakflows from six of these stations are from the U.S.G.S. Open File Report 84-137 (reference 2 ) and were determined uSing Log-Pearson Type III analysis as outlined.bytheWaterResourcesCouncit. Peak flows for these six stations are listed In Table 2. Peak flows from the remaining ~:ne s:tes are from recent flood insurance or flood plain reports and may include heavier welg~ting on precipitation analysis th~n the U.S.G.S. flows. Table 3 shows the peak Jbcharg€$ from thes<" streems. Ftgure 3 _ June 1921 Flood . Oen~er & Rio Grande Railroad yards with new channel from flood waters "