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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HYPOTHESIS PRESENTA nON

Hypothesis I, Native fish composition and abundance is greater in rivers with higher base
flows,

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Observation: The Yampa River has had very low base flow in the past four years and the
native tish population has been effectively replaced by non-natives, The Dolores River has
had very low base flow for a long period, The native fish population there has very low
biomass and is comprised of small-bod-ied fish. The rivers that have been able to maintain
large and stable populations of blue head sucker, flannelmouth sucker and roundtail chub are
the Colorado River, in both the IS-Mile Reach and near Parachute (Anderson 1997), and the
Gunnison River downstream of Austin, Both of these rivers have high base flows because of
downstream senior water deliveries, Another river that could be used to examine this
hypothesis is the White River, which historically has had high base flows. But, a lack of
recent fish population data means sampling is required to use this river for this type of
companson,

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Hypothesis 2, Bluehead sucker and flannel mouth sucker biomass is highly correlated with
habitat availability and the meso-habitat suitability values reported in
Anderson and Stewart (2003) are transferable between rivers,

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Observation: Data used to generate habitat suitability values were from two Yampa River
sites and two Colorado River sites, These suitability values were used by the habitat model
to predict impacts of flow alternation on native sucker abundance, An independent data set is
required to validate suitability values, If 2-D modeling sites on the Gunnison River confirm
and validate the suitability values presented in Anderson and Stewart (2003) than instream
flow recommendations methods would be greatly simplified.

Hypothesis 3. White sucker abundance and hybridization rates with native sucker are
longitudinally distributed and may be a function of water temperature.

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Observation: White sucker were most common in upstream sites and least common in
downstream sites in the Yampa, Colorado, and Gunnison rivers, This longitudinal
distribution of white sucker in these rivers gives the appearance that their range did not
completely overlap with native sucker, A change in water temperatures in the Gunnison
River, due to altered releases from the Blue Mesa Project, could have an unforeseen impact
on white sucker abundance and distribution, This in turn could alter current hybridizing rates
with nati ve sucker. Efforts to remove white sucker with the goal ofreducing hybridization
rates with native sucker appears warranted in the Yampa and Gunnison rivers.

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Hypothesis 4, A manned low-head permanent or temporary dam could be the most effective
tool for nonnative fish control over the long term, especially for channel
catfish.

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Observation: The Redlands Diversion dam on the Gunnison is apparently responsible for the
lack of channel catfish and may also be a factor for the lack of other nonnative predator

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