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<br />. <br /> <br />recorded flows according to the ratio of the drainage areas <br />results in an estimated average annual flow at the project site <br />of 3,438 cfs. <br /> <br />The distribution of the gaged flows is shown on the flow- <br />duration curves, Figure 1-2. The estimated average annual flow <br />at the project site is equal to the 23% exceedance value (see <br />Section V). <br /> <br />. <br /> <br />4. Nature and Frequency of Flooding <br />The principal cause of flooding along the main stem of the <br />Colorado River is the rapid melting of the mountain snowpack <br />from late May through early July. Because the drainage basin is <br />so large, areal rainstorms and convective thunderstorms have <br />little effect on river flows. However, the runoff from snowmelt <br />can be greatly increased should the peak melting period coincide <br />with heavy areal rainstorms. When a large portion of the <br />watershed is covered by melting snow, which is satisfying soil <br />retention losses under the covered area, the runoff from rain <br />fall ing on the snow-covered area may be equal to the rainfall. <br />In addition, the rainfall can significantly accelerate the rate <br />of snowmelt. <br /> <br />There is a long history of snowmelt flooding on the Colorado <br />River, but little detailed or reliable information is available <br />for the period prior to the installation of gaging stations in <br />the early 1940s. <br /> <br />I. <br /> <br />A study by R.W. Beck and Associates (1982) prepared for The <br />Colorado Water Conservation Board on the "Una Reservoir <br />Project," to be located on the Colorado River at DeBeque, noted <br />that the estimated flood of record, based solely on high water <br />marks, occurred in June-July, 1884. Flow was estimated at <br />125,000 cfs below the confluence of the Colorado and Gunnison <br />Rivers at Fruita. The prorated flow for this event at <br /> <br />3686-a/5 <br /> <br />1-7 <br />