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<br />INTRODUCTION
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<br />"The interrelationships of water, soil and vegetation is intricate and highly complex and has
<br />thus far resisted 'all attempts to describe or identify it precisely, let alone to reproduce it in an
<br />artificial model.' Lanz (1995; p, 16-17),
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<br />There is substantial uncertainty and controversy associated with estimating flood magnitude and
<br />frequency, particularly those of extreme floods, in the Rocky Mountain region, Uncertainties
<br />and controversy result from complex hydrometeorological processes and a lack of data on
<br />extreme rainstorms and flooding, Hansen and others (1988), Jarrett and Costa (1988), Henz
<br />(1991), Tomlinson and Solak (1997), and McKee and Doesken (1997) have recognized the
<br />difficulty estimating extreme rainfall and flooding in the Rocky Mountain region, A long-
<br />standing issue is whether the Palmer Divide, the ridge that separates the South Platte and
<br />Arkansas River basins, has any orographic effect on rainfall and flooding in eastern Colorado
<br />(Jarrett and others, in review), Major floods in eastern Colorado result from a southeasterly
<br />flow of low-level moist air from the Gulf of Mexico, which often crosses over the Palmer
<br />Divide (Collins and others, 1991), The Palmer Divide has a topographic relief ranging from
<br />about 2,000 ft near the Colorado-Kansas border to about 3,400 ft at the headwaters of Bijou-
<br />Big Sandy Creeks over a distance of about 140 miles; thus, the general slope ranges from about
<br />15 to 24 fVmi. Existing hydrometeorologic methods assume that topography in eastern
<br />Colorado including the Palmer Divide has a minimal effect on extreme rainfall amounts and
<br />flooding (Miller and others, 1973; McCain and Jarrett, 1976; Livingston and Minges, 1987;
<br />Hansen and others, 1988),
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<br />A regional paleoflood study was conducted for streams draining from the Palmer Divide in
<br />eastern Colorado to help assess the flood hydrology, The objective of the paleoflood study was to
<br />estimate prior maximum flooding from evidence preserved in the floodplain, Because of the
<br />large variability of the flooding, a hypothesis was tested to assess the effects of topography on
<br />flooding in streams draining from the Palmer Divide, If the Palmer Divide is a major
<br />topographic barrier to the flow of moist air masses from the Gulf of Mexico, then the lee side
<br />(northerly draining) Palmer Divide streams may have smaller extreme rainfall and flooding
<br />than southerly (upslope) draining Palmer Divide streams, Paleoflood data can be used to
<br />complement meteorologic, hydrologic, and engineering methods to improve estimation of the
<br />magnitude and frequency of floods (Jarrett and Costa, 1998; Jarrett, 1990, in review a), The
<br />results of this stuely also are being used to help improve flood-regression relations being
<br />developed for eastern Colorado (Vaill, in review).
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<br />The paleoflood study also provides information that can be used to help resolve flood-hydrology
<br />issues for Cherry and Box Elder Creeks near Denver, Colorado (fig. 1), In the 1950s, the
<br />emergency spillway for Cherry Creek Reservoir was designed for a flood of 181,000 ft3/S,
<br />which was computed from the May 1935 rainstorm (Follansbee and Spiegel, 1937), the
<br />largest known rainstorm in Colorado, and increased by a factor of 25 percent (U,S, Army Corps
<br />of Engineers, written commun., 1997), Recently, methods used to estimate the probable
<br />maximum precipitation (PMP) values were developed for the Rocky Mountain region including
<br />eastern Colorado (Hansen and others, 1988), From these PMP values, the probable maximum
<br />flood (PMF) for Cherry Creek Reservoir was estimated in 1993 by the Corps of Engineers to be
<br />662,000 ft3/S, thus, raising questions about the safety of Cherry Creek Dam, Using older PMP
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