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* The flood-frequency relation for Box Elder Creek near 1-7o-(FEMA, 1993) also is shown for comparative purposes (fig. 9). This relation plots above the large!st paleofloods and historical floods. Will Thomas (Michael Baker, Jr., written commun., 1997) developed preliminary regional-regression equations for the 100-year flood using streamflow-gaging station data to assist FEMA in improving flood-frequency relations for eastern Co orado. Thomas found that incorporating a basin-shape factor, which helps account for narrow (elongated) basins. reduces the standard error of estimate to about 25 percent in his regression model. The 100- year flood estimates of Thomas are shown for Cherry and Box Elder Creeks on figures 6 to 9. Thomas' 100-year flood estimates are very similar to results fre m this study for Cherry Creek (figs. 6 and 7) and Box Elder Creek at Elizabeth (fig. 8). However, Thomas' lOO-year flood estimate of about 16,800 ft3/s for Box Elder Creek near 1-70 (fig. 9) plots more than twice the 100-year flood of the U.S. Army Corps of Engineers (1990) and CH2MHlll, 1995). Thomas' 100-year flood is about three times the flood of record of about 6,000 ft3/s in about 100 years, about 1.9 times larger than a paleoflood of 8,700 ft3/s in 100 to 500 years, and about equal to the largest paleoflood in several hundred to several :housand years. The paleoflood data support the flood-frequency relations developed from rainfall-runoff modeling for Box Elder Creek (figs. 8 and 9). . The good agreement (+/-1) percent) between rainfall- runoff modeling flood-frequency relations (U.S. Army Corps of Engineers, 1990; CH2MHlll. 1995) and historical and paleoflood data probably reflects the ability to better assess unique hydrologic characteristics (e.g., narrow basin, high infiltration in soils and into streambeds, and availability of rainfall-runoff data for Denver area streams tel calibrate the model) than presently available estimates from existing regression equations for Box Elder Creek. For comparative purposes, PMF estimates for Cherry Creek are shown on figures 6 and 7 and for Box Elder Creek are shown on figures 8 and 9. Although extrapolations of flood-frequency relations have uncertainties, they can be used to estimate the probability of extremely large floods when paleoflood data are available (Jarrett and Costa, 1938; Jarrett, in review a, levish and others, 1994; Ostenaa and levish, 1995). Floods the magnitude of PMF values for Cherry and Box Elder Creeks have recurrence intervals far in excess of a 10,000 years based on the flood-frequency analyses incorporating the paleoflood data. DISCUSSION AND SUMMARY Although paleoflood estimates involve uncertainties, they are bas.,d on interpretations of physical data preserved in channels and on floodplains for at least several thousand years. These uncertainties primarily are related to post-flood changes in channel geometry and flood heights interpreted from PSis. Where possible, paleoflood sites in this study were located in bedrock controlled channels that minimize potential changes in channel geometry that may occur in alluvial channels; there is little evidence that major changes in channel geometry have occurred in alluvial channels in the study area in many thousandB of years. HWM-PSI relations developed from recent, extreme floods in the western United Statlls clearly demonstrate that the top of flood-deposited sediments (PSis) are equal to the elevation of flood HWMs, thus, helps to reduce the uncertainty of paleodischarge estimates (Jarrett, in review b). Paleoflood estimates for Colorado rivers are believed to have uncertainties of about 25 percent (Jarrett and Way thomas, in press; Jarrett, in review a and b; Jarrett and others, in review). Paleoflood data for 134 sites on the Palmer Divide provide additional data for large floods in the past several thousand years not available by other sources, provide a higher level of confidence in 10 Il I I I I I I I I I I I I I I I I I I