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<br />100000
<br />
<br />. PMF
<br />.. Paleoflood
<br />Contemporary flood
<br />
<br />PMF envelope curve
<br />
<br />(2)~ \.,.",.........~
<br />~.,.,.,.,:.,..,'....,."..,.,
<br />Ch""V( 2C:~ \bas':,.,.,.,.,....~:.~,:"...,..,.,."."..,."'.,
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<br />ill M,~..."
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<br />
<br />Other Eastern Colorado streams
<br />(data not Shown)
<br />
<br />10000
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<br />
<br />2,270 m3/s
<br />
<br />1000
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<br />100
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<br />
<br />10 100
<br />Drainage ar.., km2
<br />
<br />1000
<br />
<br />10000
<br />
<br />Figure 4, Relation of peak discharge for contemporary floods and paleofloods with drainage area
<br />for slreams in Cherry Creek basin, Envelope curves for Cherry Creek basin floods, other eastern
<br />Colomdo streams (data not plotted), and PMF values for eastern Colorado also are shown,
<br />
<br />Analysis of the differences in flood-deposited sediments (new PSIs) and highwater marks (HWMs)
<br />of recent extreme floods for 122 sites in 90 streams of the western United States indicated that the
<br />elevation of the top of flood-deposited sediments (PSIs) generally are within :tQ,2 of flood HWM
<br />elevations (Jarrett et aI., 1996), Therefore, use of the top of flood-deposited sediments as PSIs
<br />provides a reliable estimate of the maximum paleoflood depth, This then helps reduce the
<br />uncertainty of paleodischarge estimates for streams in this study, Manning's n values were varied by
<br />:1:25 percent, which is considered a reasonable range of uncertainty in flow-resistance coefficients,
<br />Alluvial-channel geometry can change with time, thus, estimating paleoflood discharge can be
<br />affected by channel change,
<br />
<br />A large source of uncertainty of flood variability is effects from natural or anthropogenic climate
<br />change, Paleoflood estimates incorporate the effects of climatic changes on hydrology during the
<br />period of the paleoflood record (Jarrett, 1991), Certainly, moderate climate changes (or other
<br />changes such as wildfire effects on flooding or vegetation changes or agricultural land-use changes)
<br />have occurred during the Holocene, however, these effects are reflected in the maximum flood
<br />preserved at a site.
<br />
<br />Paleoflood reconstructions for 99 sites in Cherry Creek basin provide data for large floods not
<br />available by other sources, provide a higher level of confidence in envelope curves of maximum
<br />flooding, and suggest that Cherry Creek basin flooding differs from other basins in eastern Colorado,
<br />Although the study area is relatively small, extreme rainstorms appear to be very localized (fig, 1;
<br />nole, rain cells primarily resulted from different storms, Hansen ct aI., 1988), which help reduce
<br />interstation correlation in the regional analysis.
<br />
<br />7
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