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<br />. physical upper bound on maximum peak discharge, for any combination of rainfall- or snowmelt-runoff in <br />the basin, since glaciation. <br /> <br />Two conclusions can be made about the 1921 flood from a review of annual peak-flow data and the <br />hydrographs for the stations shown in figures 16a to 161, Rrst, in northwestem Colorado, the rainstorm of <br />June 14, 1921 occurred coincident with maximum snowmelt runoff for 1921, The 1921 rainfall-runoff <br />contribution is subtracted from the 1921 peak and then all peaks were ranked for each station. Peak flows <br />(only snowmelt) for 1921 still would have been the third to fifth highest in about the last 100 years. <br />Second, although rainfall did contribute to runoff in 1921, the available rainfall, streamflow and paleoflood <br />data indicates that the rainstorm must have been fairly limited in areal extent Thus, past flood estimates <br />derived from the assumption that large rainfall or rain-on-snowmelt floods are questionable in northwestem <br />Colorado. <br /> <br />Magnitude-Frequency Relations for streams in Northwestern Colorado <br /> <br />. <br /> <br />Magnitude-frequency relations for several streams in northwestem Colorado were developed using the <br />recorded annual peak-flow data and including the paleoflood data, There is some regulation of flow <br />(irrigation diversions) ,and storage effE;lcts from several dam for the gages on the Yampa and White Rivers, <br />but these effects are minor for the streams analyzed. Magnitude-frequency relations were developed <br />using the Log-Pearson Type III distribution (Interagency Advisory Committee on Water Data, 1981). <br />Although there are many combinations of flood-frequency analysis with the paleoflood data (for ranges of <br />maximum flow and estimated ages of paleofloods), selected analysis were made that encompass the <br />range of flood-frequency CUlVes. ThE! historic period of the paleofloods used in the analysis ranged from a <br />maximum of 10,000 years (since glacial times) to 100 years (which reflects an extremely conservative or <br />short-historic period) to provide upper and lower bounds on magnitude-frequency relations at each gaged <br />site, <br /> <br />Magnitude-frequency relations and associated 95 percent confidence limits are shown on figures 17a to <br />, <br />17g. A graph of flood-frequency relations for the four Yampa River gages is presented on figure 17f, <br />which demonstrates the consistency as the size of basin increases. Because results using a historic <br />period of 100 years and 10,000 years only differ by a small amount for sites in northwestem Colorado <br />sites, the interpretations presented are based of results using a historic period of 100 years (a <br />conservative estimate). Regional relations developed for northwestem Colorado (Kircher and others, <br />1985) also were used to estimate flood-frequency curves for Elkhead Creek at Elkhead Reservoir and <br />Fortification Creek at Craig where there are no gaged data. Superimposed on figures 17a through 17g is <br />the value of the maximum paleoflood at each site. The 1 OO-yr flood estimate for Elkhead Creek is about <br />3,000 ft31s at Elkhead Reservoir. The, magnitude-frequency relation for Elkhead Creek at Elkhead <br />Reservoir developed by Ayres Incorporated (written commun., 1996), which was determined by <br />combining the rainfall and snowmelt flood-frequency relations (see Jarrett, 1987 for procedure), also is <br />shown on figure 17d, These relations essentially are the same up to about the 10o-year event The <br /> <br />. <br /> <br />27 <br />