<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
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<br />5
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