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<br />Paleoflood data also were used to validate flood-frequency relations for an ungaged basin such as <br />Box Elder Creek developed using rainfall-runoff modeling. The 100-year flood for Box Elder <br />Creek ranges from about 5,700 ft3/s at Elizabeth to 7,900 ft3/s near 1-70 in Adams County, <br />Floods the magnitude of PMF values for Cherry and Box Elder Creeks plot far in excess of a <br />10,000-year event. Study results are consistent with flood-frequency analysis for other <br />streams in eastern Colorado, For example, flooding in Plum Creek in 1965 has been shown to <br />have a recurrence interval of at least 1,500 years (Osterkamp and Costa, 1987; Jarrett and <br />Costa, 1988) demonstrating the small frequency of the 1965 flood. Jarrett and Costa (1988) <br />summarize flood-frequency estimates for other extreme floods in eastern Colorado, <br /> <br />The maximum paleoflood of 74,000 ft3/S for Cherry Creek near Cherry Creek Reservoir is <br />about 11 percent of the PMF of 662,000 ft3/S, The maximum paleoflood of 8,700 ft3/S for <br />Box Elder Creek near Watkins is about 2 percent of the PMF of about 410,000 ft3/S, Several <br />recent paleoflood studies, primarily conducted by the U,S, Geological Survey and Bureau of <br />Reclamation (Jarrett and Costa, 1988; Jarrett, in review a, Levish and others, 1994; Ostenaa <br />and Levish, 1995), also demonstrate large differences between maximum paleoflood estimates <br />and PMF estimates in the Rocky Mountain region, Jarrett (in review a) found that maximum <br />paleofloods average about 7 percent of PMF values for streams at higher elevations in the Rocky <br />Mountains, Such large differences in estimates of maximum flooding demonstrate the need to <br />conduct additional hydrometeorologic research to reduce the uncertainty in design-flood <br />hydrology, Maximum rainfall and flooding in eastern Colorado are associated with areas of <br />high-topographic relief, Using extreme rainstorms and floods that have occurred in these areas <br />of high-topographic relief to assess maximum rainfall and flood potential in low-topographic <br />relief areas, which cover about 75 percent of the high plains of eastern Colorado, needs to be <br />assessed, Spatial variability of extreme rainfall affects not only peak discharge but also the <br />volume of flood runoff, which in turn affects flood-control storage requirements, <br /> <br />A statewide regional flood-frequency study for Colorado (Vaill, in review) is incorporating the <br />results of this paleoflood study and results from Thomas' preliminary flood-frequency <br />assessment (writ. commun" 1997) into the regional relations for eastern Colorado streams, <br />Results of the paleoflood study will help explain the spatial variability of flooding (to define <br />hydrometeorologic regions), and help identify additional explanatory variables used in the <br />regression models, <br /> <br />The important issues in flood hydrology related to these study results are: (1) large differences <br />in flood hydrology for eastern Colorado using different methods; (2) the major effects of <br />topography, including local areas of relatively small relief, and basin elongation on flood <br />hydrology in eastern Colorado; and (3) the need to improve the understanding of flood <br />hydrometeorology in eastern Colorado, thereby reducing the uncertainty of estimates of the <br />magnitude and frequency of flooding, Future research in flood hydrometeorology is needed to <br />address these issues. <br /> <br />13 <br />