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