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<br />. Magnitude-Frequency Relations for Selected Streams in Northwestern Colorado <br /> <br />Magnitude-frequency relations were developed from an analysis of annual peak flows for selected <br />streamflow-gaging stations in northwestem Colorado where paleoflood studies also are available. Gaged <br />site flood-frequency relation were developed using a Log-Pearson Type III (LPIII) frequency distribution <br />(Interagency Advisory Committee on Water Resources Data, 1981), Paleoflood data (magnitude and <br />relative ages) were incorporated into the LPIII analysis as historical events to extend the systematic <br />record. Regional flood-frequency relations for the northwest region of Colorado (Kircher and others, 1985), <br />which are applicable to the Elkhead Creek Basin, were used to estimate flood characteristics on Elkhead <br />Creek at Elkhead Reservoir. The regional relations developed for the northwest region of Colorado have a <br />mean-standard error of about 63 percent. <br /> <br />PALEOFLOOD STUDY RESULTS <br /> <br />Paleoflood Investigations <br /> <br />No evidence of substantial out-of-bank flooding was found in any investigated stream channel in <br />Elkhead Creek Basin or stream channels above 6,000 ft in northwestem Colorado (Table 1; 93 sites). <br />. The discharges provided in table 1 are estimated to be about +/- 25 percent. Photographs were taken at <br />sites likely to have flood deposits, had there been floods; several representative channel and sediment <br />deposits are shown in figures 5a to 51. Flood bars are small having low relief in most channels and some <br />streams have no bar development. Had flooding occurred in tributaries that were inaccessible, the <br />evidence would be preserved at downstream sites investigated. There is little flood bar fonnation where <br />bridges or culverts fonn channel constrictions (figure 5e). Channel geometry, slope, and sediment size <br />data were collected at the sites listed in table 1 (figures 5a and 6), Flood evidence was found related to a <br />moderately intense rainstonn that occurred on May 30, 1995, in the Fortification Creek Basin at Craig, <br />which is located immediately to the west of Elkhead Creek Basin, This stann is discussed below in the <br />section .Summary of Fortification Creek Rainstonn of May 30, 1995." <br /> <br />No woody-flood debris from large floods was identified in the study area, except associated with the <br />Sage Creek Dam failure flood near Hayden, Colorado (figures 7a and 7b). Local residents indicated that <br />the dam failed in the mid-1980s. They indicated the failure resulted from seepage through the dam and <br />was not related to a meteorologic event. This is supported by the maximum paleoflood on Sage Creek <br />upstream from the dam of about 100 ft31s (table 1), The peak discharge resulting from failure of the dam <br />was estimated to range from 5,000 to 6,500 ft31s. The extensive erosional and deposition features from <br />this relatively small flood provides an excellent analog of the type of evidence remaining after a flood, <br /> <br />. It is particularly noteworthy that more sediments have not been mobilized and deposited as flood bars <br />(table 1). The few in-channel bars that exist are fairly small, low relief, and have particle sizes of cobble or <br /> <br />18 <br />