FLOOD10348

• County: Statewide
• Stream Name: Colorado River
• Title: Floodplain Information Report, Volume 4, Colorado River from Rifle to Utah Stateline
• Date: 5/1/1995
• Prepared For: Recovery Implementation Program for Endangered Fish Species in the Upper Colo
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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5.0 INTERPRETATION OF THIS REPORT

5.1 Hydrologic Findings

The results of the hydrologic analysis for the Colorado River mainstem are considered accurate.
They are based principally on existing approved data prepared by federal agencies. The
availability of detailed information for almost 70% of the study reach provided accurate
relationships with the unstudied reach fmdings.

5.2 Hydraulic Findings

The hydraulic methods used for this study are similar to those used in more detailed studies.
They are much more detailed than the methods used in typical approximate studies. The 100-
year flood depths for the previously unstudied reaches range from 19.2 feet in a medium reach
of the river to 26.5 feet near the downstream limit of the study (where the drainage area
essentially doubles at the Gunnison River confluence). The majority of the flood depths are
generally consistent throughout the previously unstudied portions of the study reach with
'differences of only a few feet (18.2-26.5 feet for the l00-year flow depths). The variation
between the 50-year and l00-year flood depths is generally less than one foot. To fmd flood
depths for the reaches where there has already been a detailed study, interested parties are
directed to the flood prof1les in those detailed studies.

5.3 Floodplain Delineations

The primary difficulty in presenting precise delineations of the 50-year and l00-year floodplains
for the previously unstudied reaches was the limited quality of the available base mapping for
the unstudied reaches of the Colorado River. Using USGS topographic quadrangle maps at
contour intervals of 20 or 40 feet to portray flood lines in unsurveyed areas provides limited
information. The margin for interpretation is very great, especially in medium to wide reaches
of the river where contours are widely spaced. More detailed topographic information will be
needed in areas of specific interest.

This study can be utilized to prepare more detailed floodplain delineations in such areas of
, specific interest. Those who are interested. in greater detail have three choices for using this
study:

1) They can consult the existing detailed studies, if they pertain to their site, to better
ascertain the flood status of the site in question;
2) They can rely on the hydrologic determinations in this report and use other topographic
information and hydraulic models; or '
3) They can use the depth information in this report with more detailed topographic
information to modify the floodplain delineation.

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