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<br />(e.g" figs. 2 and 3) that are at least 1,000 to 5,000 years old (Hunt, 1954), Interpretation
<br />of channel and paleochannel location from available aerial photographs and onsite data indicate
<br />lateral-channel migration has been minimal for many thousands of years, Jarrell and others
<br />(in review) noted minimal channel migration during at least the last 150 years, but probably
<br />much longer, in First Creek an alluvial channel adjacent to Box Elder Creek (fig. 1), Channel-
<br />bed changes in relatively straight, uniform reaches (typical of sand-bed channels in the study
<br />area) where paleoflood sites were located were assumed to be minimal. In alluvial channels,
<br />large amounts of bed material are transported fairly uniformly during floods, bed-level
<br />changes likely are small in uniform reaches where cross sections are located as described by
<br />Jarrell and Browning (in review) for 1996 and 1997 floods in sandy channels in Buffalo
<br />Creek, Colorado (fig, 1), In reaches where flood sediments are deposited as flood bars (PSis),
<br />peak discharges were computed with existing channel geometry and without flood deposit (e,g"
<br />figs, 3, area below the lower PSI dashed line), Paleoflood data (corresponding to the range of
<br />the PSis such as shown in figs, 2b and 3b) reflect the larger (conservative) estimate in the
<br />sensitivity analysis of factors affecting discharge,
<br />
<br />Maximum paleoflood estimates are shown on figure 4, To help define the enveloping curve of
<br />maximum flood potential for Palmer Divide streams, flood data from gaged and ungaged sites
<br />(Jarrell, 1990; McKee and Doesken, 1997) also are shown on figure 4, To test the hypothesis
<br />of the effects of the Palmer Divide on flooding, flood and paleoflood data were stratified by major
<br />basin and topographic relief (fig, 4), Basins having less than about 300 It of relief in a
<br />distance of about 3 miles or less, determined from topographic maps, were classified as low TR
<br />basins, High topographic relief basins (high TR) have more than about 300 feet of relief in
<br />about 3 miles or less, The following interpretations can be made about the spatial variability of
<br />flooding of Palmer Divide streams,
<br />
<br />1. The headwaters of Cherry and Box Elder Creeks originate near the highest (-7,700 It on fig,
<br />5) part of the Palmer Divide, Cherry and Box Elder Creek basin streams (fig, 4) have
<br />substantial smaller flood peaks than other streams draining from the Palmer Divide with high
<br />topographic relief (denoted as "other PO high TR" on fig, 4), particularly for increasing
<br />drainage area size, "Other" southerly flowing Palmer Divide streams that have had large floods
<br />include Jimmy Camp, Black Squirrel, Big Sandy, and Monument Creeks (fig, 1). "Other"
<br />northerly-flowing streams draining from the Palmer Divide include Bijou, Plum, and Kiowa
<br />Creeks, The lower elevation of headwaters of Kiowa (-7,400 It) and Plum (-7,200 It) Creeks
<br />at the Palmer Divide allows greater amounts of Gulf moisture over the Divide, and thus, likely
<br />explains larger floods than in Cherry Creek, but smaller than Bijou Creek, which has had the
<br />largest floods in Colorado (e,g" 466,000 1t3ts near Wiggins on June 18, 1965; Mallhai,
<br />1969), [It is important to note that the slope-area site for the Bijou Creek floods of 1935 and
<br />1965 were located downstream from a railroad embankment that failed during both floods, The
<br />estimated 1935 and 1965 flood estimates include the increase in peak discharge from the
<br />embankment failure (described in unpublished flood data, U,S, Geological Survey, 1965),]
<br />Large floods in Bijou Creek basin probably result from a much greater part of its headwaters
<br />near the Palmer Divide (fig, 1) and because the Divide has a much lower elevation (fig. 5). A
<br />substantial saddle in the Palmer Divide at the headwaters of East Plum Creek (fig, 5, -7,200
<br />It), which would allow greater flow of Gulf moisture from the south, likely explains larger
<br />rainfall and floods in this basin than in Cherry and Box Elder Creeks, Follansbee and Spiegel
<br />(1937), Follansbee and Sawyer (1948), Matthai (1969), Snipes and others (1974), Henz
<br />
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