WSP12249

• File Number: 8240.200.10.B
• Description: UCRBRIP - Riverine Fish Flow Investigations
• Basin: Colorado Mainstem
• Date: 6/1/2004
• Author: CDOW
• Title: Riverine Fish Flow Investigations 2004
• Water Supply Pro - Doc Type: Publication

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The year with the largest channel catfish numbers in the 1 SoMite Reach was also the year
with the lowest number of Age-! roundtail chub in the sample.

It appeared one year of low flows in the 1 S-Mile Reach was not disruptive to the fish
community structure that had been established over a long period of higher flows, Flows
returned to nonnal (@I,OOOcfs) in the 15-Mile Reach in 2003 so a drought of two or more
consecutive years did not occur. Significant changes in community structure did not appear
on the Yampa River until the second and third years oflow flows. This provided some
support to the speculation that the increased presence of carp, channel catfish, smallmouth
bass and white sucker observed in 2003 were not likely w persist above the baseline levels
lound in 1999, 2000 and 2001.

Geomorphic features including stream width and bed slope were similar for the
Yampa and Colorado rivers (Anderson and Stewart 2003). The 2D modeling found that
habitat availability for bluehead and flannelmouth sucker was similar in both rivers given
similar flows, Therefore the differences in density and biomass of native sucker and the fish
community in general could be a function of differences in their base flow regimes,

Base Ilow hydro graphs were much different between rivers, Base flows in the
Yampa River were approximately 250 cfs in 1998 and 1999, 110 cfs in 2000 and 2001 and
only 12 cfs in 2003, In contrast, base flows in the I S.Mile Reach of the Colorado River were
near 800 to 1,000 cis 2000 and 2001 and dropped to 100 cfs in 2002,

GUNNISON RIVER

Severe low flows were not observed during 2002 or prior years, In 2003, there was a
large population of adult-sized flannelmouth sucker, bluehead sucker and roundtail chub,
Osmundson (1999) and Burdick (1995) reported high abundance of adult flannelmouth
sucker, bluehead sucker and roundtail chub in the Gunnison River.

The Gunnison River did not have nonnative predators in its fish community. Without
predation, population size appeared to be regulated primarily by competition for limited
resources or habitat. On the Yampa River it appeared white sucker and its hybrids were
more resistant to predation by small mouth bass, Hybridization rates between white sucker
and native sucker on the Gunnison River was likely regulated by meso habitat availability,
Hybridization with white sucker was quite common on the Gunnison River with the site
upstream of Austin displaying the highest incidence.

Burdick (1995) reported white sucker catch rates in 1992 and 1993 were three times
higher than white sucker catch rates from Fish and Wildlife surveys in 1981 and 1982 by
Valdez et al. (1982), Species composition for white sucker at Delta and Escalante were
higher in 2003 than found in 1992 and 1993 by Burdick (1995), suggesting that white sucker
have increased during the past 20 years, Also hybridization between white sucker and native
sucker appeared to be increased in the 2003 samples compared to data from prior surveys.
Hybridization with white sucker appeared to have had a significant negative impact to native
suckers in the Gunnison River.

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