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<br />RESULTS <br /> <br />Paleoflood Investigations <br /> <br />Paleoflood data were obtained at 134 sites for streams draining from the Palmer Divide (table <br />1). PSI data are readily identifiable onsite by coarse-grained flood deposits (figs. 2a and 3a). <br />Flood deposits typically are coarse to very-coarse, sandy gravel and often have little or no soil- <br />profile development. In contrast, thick, clay-rich, fine-grained, well-developed colluvial and <br />alluvial soil (labelled colluvium on figs, 2b and 3b) generally occur in non-flooded areas in the <br />valley, Because these colluvial soils have few particle sizes larger than sand, they were not <br />deposited by main stream flooding; rather, the sediments primarily originated from hillslope <br />(sheetflow) runoff and aeolian (wind-blown) sediments (R, Madole, USGS, pers, commun" <br />1997; E, Wohl, Colorado State University, pers. commun" 1997), These colluvial organic- <br />rich soils, termed Piney Creek alluvium by Hunt, were dated from a minimum of 1,000 years <br />to greater than 5,000 years (Hunt, 1954), thus, providing a minimum age since floods have <br />inundated these surfaces, These colluvial soils are easily eroded by flood waters, thus, they also <br />provide physical constraints on the present channel geometry (e,g" figs, 2b and 3b), Because <br />streams in the study area are unglaciated, particularly those on bedrock, the age of paleofloods <br />could be many tens of thousands of years old (Jarrett and Way thomas, in press), <br /> <br />In addition, numerous large (3-5 ft diameter) cottonwood and ponderosa pine trees are growing <br />on floodplains of many Palmer Divide streams, Although coring trees to determine their age <br />was beyond the scope of this study, general relations of tree age and diameter for different <br />species (Waythomas and Jarrett, 1994; Jarrett and others, in review) suggest tree ages in <br />streams in this study range from about 50 to 150 years old, Flood-abrasion scars and <br />accumulations of woody debris on trees are very common when flooding occurs (Matthai, 1969; <br />Osterkamp and Costa, 1987; Hupp, 1988), Flood scars are notably absent along Cherry Creek, <br />Box Elder Creek and their tributaries, except for trees located on the main channel banks, The <br />lack of an accumulation of woody-flood debris around trees located on the floodplain of Cherry <br />and Box Elder Creeks indicates flood waters have not reached those trees since they germinated <br />(-50 to 150 years ago), Conversely, extensive stands of cottonwood trees generally germinate <br />only within a few years after a flood has deposited sediments on a flood-plain surface <br />(Waythomas and Jarrett, 1994; Friedman and others, 1996; Jarrett and others, in review); <br />thus, flood waters have been at least as high as the margin of cottonwood trees in the valley <br />bottom shortly before the trees germinated, Generally, there is very good agreement defining <br />maximum PSI elevation for streams in the study area interpreted from the maximum height of <br />flood-deposited sediments and margins of cottonwood forests, <br /> <br />A sensitivity analysis of factors (Manning's n value, channel-bed and width changes) affecting <br />paleoflood estimates was made. Manning's n values were varied by +/-25 percent, which is <br />considered a reasonable range of uncertainty in flow-resistance coefficients (Jarrell and <br />Waythomas (in press), Many streams draining from the Palmer Divide, notably Cherry Creek <br />near the Franktown streamflow-gaging station (06712000) in Castle wood Canyon State Park <br />and the headwaters of Box Elder Creek (Running Creek), have bedrock streambeds that provide <br />a stable-channel geometry for flood estimation, Alluvial-channel geometry can change with <br />time, thus, estimating paleoflood discharge can be affected by channel change, Onsite, <br />maximum-channel width is constrained by undisturbed colluvial soils along the main channel <br /> <br />5 <br />