FLOOD09180

• County: Denver
• County: Adams
• Stream Name: Cherry Creek Basin
• Basin: South Platte
• Title: Paleoflood Investigations for Cherry Creek Basin
• Date: 8/1/2000
• Prepared For: Eastern Colorado
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

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Scoured colluvium & NI-surfaces

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~PSI

(1,954-2,266

m3/s)

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1965 PSI

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n=0.035 n=0.035
5=0.005 m/m
Colluvium
Sand and gravel
50 100 150 200
Stationing, m

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Figure 3, Channel cross section for Cherry Creek near Melvin streamflow-gaging station
(06712500), which is located about 3 km upstream from Cherry Creek Reservoir, The highest
paleostage indicator is constrained by uneroded, old colluvium (non-inundation, NI, surfaces),

PMF estimates for Cherry Creek also are shown on figures 5 and 6, Floods the magnitude of the
design flood and PMF values for Cherry Creek have recurrence intervals far in excess of a 10,000
years (10.4 annual exceedence probability) based on the flood-frequency analyses incorporating the
paleoflood data (figs, 5 and 6),

Discussion and Summary

Although paleoflood estimates involve uncertainties, they are based on interpretations of physical
data preserved in channels and on floodplains for at least several thousand years, A sensitivity
analysis of factors affecting paleoflood estimales was made, Where possible, paleoflood sites in this
study were located in bedrock channels that minimize potential changes in channel geometry, For
example, numerOUS headwater slream sites in Cherry Creek basin and near Castlewood Canyon have a
bedrock streambed and undisturbed coluvial streambank material, that provide a stable-channel
geometry for flood estimation, For completely alluvial sites, maximum.channel width is constrained
by undisturbed colluvial soils along the main channel (fig, 3) that are at least I,(JO(J to 5,000 years old
(Hunt, 1954). Because Cherry Creek is constrained by bedrock on the bed and walls, the 1933
paleoflood deposits probably represent lhe largest flood in at least 10,000 years, It was assumed that
for the sites in relatively straight, uniform alluvial reaches, sediments were transport. like on a giant
conveyor belt. Thus, although Some streambed erosion occurs during a flood, there probably is little
net change in bed elevation; there is little evidence 10 suggest major changes in channel geometry for
the paleoflood record length,

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