<br />Running/Box Elder Creek basin is about 4 miles (fig, 1), This narrow width (or basin
<br />elongation), the small areal extent of extreme rainstorms in eastern Colorado (Follansbee and
<br />Spiegel, 1937; Follansbee and Sawyer, 1948; Matthai, 1969; Snipes and others, 1974), and
<br />small likelihood that a storm would move down the entire Box Elder Creek probably contribute
<br />to the relatively small flooding, Conversely, note that Jimmy Camp Creek (fig, 1) also is
<br />elongated, but is a high TR basin south of the Palmer Divide and had a large flood of 124,000
<br />ft3/s in the 54,3 mi2 basin in June 1965,
<br />
<br />5, The envelope curve (fig, 4) of maximum flooding can be used to estimate the hypothetical
<br />maximum flood for streams in Cherry Creek basin (and Box Elder Creek), For Cherry Creek at
<br />the Reservoir (fig, 4), the corresponding maximum flood is about 80,000 ft3/S, This
<br />discharge compares favorably with the maximum paleoflood estimate of 74,000 .ft3/s
<br />determined for Cherry Creek near Melvin (06712500), Even using the envelope curve for
<br />other Palmer Divide streams (fig, 4; these high-topographic relief stream are not believed to
<br />be the same hydrologic region as Cherry Creek basin) for Cherry Creek basin, the maximum
<br />potential flood is about 300,000 ft3/s (fig, 4), PMF values for selected streams in eastern
<br />Colorado (Bullard, 1986) with an enveloping curve also are shown on figure 4, Generally,
<br />PMF values exceed the envelope curve of maximum floods and paleofloods for Palmer Divide
<br />streams by a factor of about 2,2, PMF values exceed the envelope curve for Cherry Creek by a
<br />factor of about 8 at Cherry Creek Reservoir to a factor of about 15 near Franktown. The
<br />maximum paleoflood of 74,000 ft3/s for Cherry Creek near Cherry Creek Reservoir is about
<br />11 percent of the PMF of 662,000 ft3/s.
<br />
<br />Flood-Frequency Analysis
<br />
<br />Flood-frequency relations for Cherry Creek developed using the recorded annual peak-flow data
<br />near Franktown (06712000) and near Melvin (06712500) and incorporating the paleoflood
<br />data into the frequency analyses are shown in figures 6 and 7, Rectangles (figs, 6 and 7)
<br />bracket the estimated ranges of uncertainty of discharge and relative age of a paleoflood, The
<br />historic-record length used in the LPIII analysis for the ages of paleofloods for Cherry Creek
<br />ranged from 1,000 to 10,000 years based on relative-age dating criteria, Flood-frequency
<br />relations for Cherry Creek (figs, 6 and 7), which use the shorter (conservative) historic-
<br />record length of 1,000 years, indicate the 10,000-year flood ranges from about 44,000 ft3/s
<br />(near Franktown) to about 75,000 ft3/S (near Melvin), Because paleoflood data document
<br />maximum flooding in thousands of years, flood-frequency relations reflect any climatic or
<br />land-use (agriculture or wildfires) changes during that time span,
<br />
<br />Because no streamflow-gaging stations have been operated on Box Elder Creek, flood-frequency
<br />relations (figs, 8 and 9) were developed by rainfall-runoff modeling (Water Resources
<br />Consultants, 1979; U,S, Army Corps of Engineers, 1990; CH2MHILL, 1995); the CH2MHILL
<br />modeling was done for the UDFCD, Paleoflood data can be used to help assess the reliability of
<br />regional flood-frequency relations from rainfall-runoff modeling and regional-regression
<br />equations, The age of the largest paleofloods along Box Elder Creek ranges from about 100 to
<br />500 years near Watkins (1-70) and from about 500 to 2,000 years near Elizabeth; more
<br />recent paleoflood data also are shown,
<br />
<br />9
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