FLOOD01533

• County: Statewide
• Stream Name: Colorado River
• Title: Colorado River Flood Risk Analyses
• Date: 1/1/1995
• Prepared For: Colorado Division of Wildlife
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

Section 2

Flood Hvdrolol!V and Recent Flood Historv

Subsection 2.1

Flood Hvdrolol!V

The determination of the flood threat at the 8 ponds is based on the determination of peak
flood flows for the reach of the Colorado River from Connected Lakes State Park
upstream to Rifle. Two hydrologic analyses were used for that determination. A flood
hydrology analysis for the reach of the river from just below Fruita upstream to just
upstream of Palisade was prepared by J.F. Sato & Associates, Inc. in 1989 as part of the
revisions to the FrS' in the Grand Junction area. A separate flood hydrology analysis had
already been performed by the Corps of Engineers in 1985 for the reach of the Colorado
River from the upstream end of DeBeque Canyon upstream to Glenwood Springs. The
Sato analysis took account of the earlier work by the Corps of Engineers, so the two
reports are consistent. Both reports are based on gage records for gages at Glenwood
Springs, Newcastle, DeBeque, Cameo and the ColoradofUtah stateline. At two particular
hydrologic points the peak flows computed by each study are identical: I) The Cameo
stream gage and 2) A point just downstream of the confluenoe of the river with Plateau
Creek. Although the hydrologic analyses are continuous from Glenwood Springs to
Fruita, there is a gap in floodplain mapping. That gap will be addressed in Section 3 of
this report.

Seven hydrology points on the Colorado River are relevant to the 8 ponds. They are:

I) The gage at Glenwood Springs (below the Roaring Fork River confluence),
2) A point below Mamm Creek (above the Rifle Creek confluence),
3) A point below Rifle Creek (above the Parachute Creek confluence at
Parachute),
4) The gage near DeBeque (above the Roan Creek confluence),
5) The gage near Cameo (above the confluence with Plateau Creek),
6) The gage at Palisade (below the c01ifluence with Plateau Creek), and
7) The Stateline gage (downstream of the confluence with the Gunnison River).

The Corps projections for the 0,1 % chance (10-year), 0.02 % chance (50-year), and 0.01
% chance (IOO-year) flows for each of those hydrology point are listed below in Table
I. In addition, the peak flows measured at the Glenwood Springs, Cameo and Stateline
gages during the floods of 1983, 1984 and 1993 are listed. The Palisade gage was only
operated after 1991, so the 1983 and 1984 values there were estimated by adding the 1983
peak flow on Plateau Creek, 5010 cfs, to the Cameo gage readings. (Unfortunately the
Plateau Creek gage did not. operate in 1984 or 1985, so the 1983 value had to used as an
estimate for 1984,) There were no estimates of peak flows during those floods at either
of the two hydrology points in the Rifle/Parachute area. The historic flows themselves
were included in Table 1 below so they could be compared to computed peak flow values
for the 10-year, 50-year and 100-year flood events. Details on the effects of those historic
events are provided in Subsection 2.2.

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