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Age estimates for paleoflood deposits in this study were based on relative-age criteria as described by Waythomas and Jarrett (1994) and Jarrett and Wa:rthomas (in press). Relative- age techniques used for this study were based on the degree of soil development, age of trees located on flood-deposited sediments, and amount of decay of woody-flood debris. Complete decay of woody-flood debris in semiarid and arid climates such as in Colorado can take over 100 years (Jarrett, in review a), and in some streams recognizable woody-flood debris has been 14C dated up to 2,300 years B.P. old (D. Levish, Bureau of Reclamation, pers. commun., 1997). The strategy of a paleoflood investigation is to visit the most likely places where evidence of out-of-bank flooding, if any, might be preserved. Aerial photograohs for much of Box Elder Creek provided by CH2MHILL and Fowler and Associates in Denv,~r were useful in identifying paleoflood sites and PSis. Onsite paleoflood data were collected for Cherry Creek and Box Elder Creek basins and numerous other streams draining from the Palmer Divide. Regional Analyses of Flood Data It is essential to ascertain the flood history for other basins in thl, region, which is broadly defined as streams draining from the Palmer Divide (fig. 1). Re~:ional analysis provides improved estimates of streamflow characteristics for gaged sites by decreasing time-sampling errors of short gage records by substituting space (many gages in a region) for time (short gage record). Predicting the upper limits to the magnitudes of floods in a specific region has been a long standing challenge in flood hydrology. Envelope curves encompassing maximum floods in a relatively homogeneous hydrometeorologic region have long been used in flood hydrology (Crippen and Bue, 1977; Costa, 1987a; Jarrett, 1990). Use of envelope curves for a hydrometeorologic region assumes that the maximum flood in a basin is likely to be experienced in a nearby basin. Not all basins in the region are ex~ ected to have had the maximum flood, but no basin has yet had a flood that exceeds the envelope curve for the specific region. The primary limiting factors for extreme floods are amcunt, intensity, duration, and spatial distribution of rainfall, orographic enhancement of rainfal', and basin slope (Costa, 1987b; Pitlick, 1994). Incorporating paleoflood data for various basins in a region provides an opportunity to add a new level of confidence to envelope curves (Enzel and others, 1993; Jarrett and Waythomas, in press; Jarrett, in review a; Jarrett and others, in review). Flood data for Palmer Divide streams (McKee and Doesken, 1997) were compiled to develop envelope curves of peak discharge versus drainage area. Flood-Frequency Relations Flood-frequency relations were developed from an analysis of annual peak flows through 1996 for Cherry Creek near Franktown (06712000) and near Melvin (06712500), which is located about 2 miles upstream from Cherry Creek Reservoir (fig. 1). Flood-frequency relations were developed using a Log-Pearson Type III (LPIII) frequency distribution (Interagency Advisory Committee on Water Data, 1981). To help facilitate risk assessments of rare floods (e.g, defining the upper end of frequency curves), pale,oflood data (magnitude and ages) were incorporated into the flood-frequency analysis to extend the gaged record. 4 r /I /I II I I I I I I I I I I I I I I I