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INTRODUCTION "The interrelationships of water, soil and vegetation is intricate and highly complex and has thus far resisted all attempts to describe or identify it precisely, let alone to reproduce it in an artificial model.' Lanz (1995; p. 16-17). There is substantial uncertainty and controversy associated with l!stimating flood magnitude and frequency, particularly those of extreme floods, in the Rocky Mountain region. Uncertainties and controversy result from complex hydro meteorological process~ls and a lack of data on extreme rainstorms and flooding. Hansen and others (1988), Ja.rrell and Costa (1988), Henz (1991), Tomlinson and Solak (1997), and McKee and Doesken (1997) have recognized the difficulty estimating extreme rainfall and flooding in the Rocky Mountain region. .A long- standing issue is whether the Palmer Divide, the ridge that separates the South Platte and Arkansas River basins, has any orographic effect on rainfall and flooding in eastern Colorado (Jarrell and others, in review). Major floods in eastern Colorado result from a southeasterly flow of low-level moist air from the Gulf of Mexico, which often crosses over the Palmer Divide (Collins and others, 1991). The Palmer Divide has a topographic relief ranging from about 2,000 ft near the Colorado-Kansas border to about 3,400 It at the headwaters of Bijou- Big Sandy Creeks over a distance of about 140 miles; thus, the general slope ranges from about 15 to 24 Wmi. Existing hydrometeorologic methods assume that topography in eastern Colorado including the Palmer Divide has a minimal effect on extreme rainfall amounts and flooding (Miller and others, 1973; McCain and Jarrell, 1976; Livingston and Minges. 1987; Hansen and others, 1988). A regional paleoflood study was conducted for streams draining from the Palmer Divide in eastern Colorado to help assess the flood hydrology. The objective of the paleoflood study was to estimate prior maximum flooding from evidence preserved in the .'Ioodplain. Because of the large variability of the flooding, a hypothesis was tested to assess the effects of topography on flooding in streams draining from the Palmer Divide. If the Palmer Divide is a major topographic barrier to the flow of moist air masses from the Gulf Jf Mexico, then the lee side (northerly draining) Palmer Divide streams may have smaller extreme rainfall and flooding than southerly (upslope) draining Palmer Divide streams. Paleoflood data can be used to complement meteorologic, hydrologic, and engineering methods to improve estimation of the magnitude and frequency of floods (Jarrell and Costa. 1998; Jarrell. 1990, in review a). The results of this study also are being used to help improve flood-regression relations being developed for eastern Colorado (Vaill, in review). The paleoflood study also provides information that can be used to help resolve flood-hydrology issues for Cherry and Box Elder Creeks near Denver, Colorado (lig. 1). In the 1950s, the emergency spillway for Cherry Creek Reservoir was designed for a flood of 181,000 ft3/s, which was computed from the May 1935 rainstorm (Follansbee and Spiegel, 1937), the largest known rainstorm in Colorado, and increased by a factor of 25 percent (U.S. Army Corps of Engineers, written commun., 1997). Recently, methods used 10 estimate the probable maximum precipitation (PMP) values were developed for the Rocky Mountain region including eastern Colorado (Hansen and others, 1988). From these PMP values, the probable maximum flood (PMF) for Cherry Creek Reservoir was estimated in 1993 by the Corps of Engineers to be 662,000 ft3/s, thus, raising questions about Ihe safety of Cherry Greek Dam. Using older PMP 2 I I I I I I I I I I I il II II II II II II