9413

• UCREFRP Catalog Number: 9413
• Author: Osmundson, D. B.
• Title: Flow Regimes for Restoration and Maintenance of Sufficient Habitat to Recover Endangered Razorback Sucker and Colorado Pikeminnow in the Upper Colorado River.
• USFW Year: 2001.
• USFW - Doc Type: Grand Junction.
• Copyright Material: NO

Hiebert 1989, Mabey and Shiozawa 1993), underscoring the importance of larval access to
floodplain habitats.
To appreciably flood bottomlands along the upper Colorado River, flows must
exceed the bankfull level. Pitlick and Cress (2000) estimate the median bankfull discharge
in De Beque Canyon (upstream of the upper diversion structure) is approximately 20,500
cfs (based on field measurements of bankfull characteristics of eight evenly spaced cross- ..
sections). In the alluvial reach upstream (De Beque-to-Rifle), where adjacent bottomlands
are present, the estimated median bankfull discharge is 22,000 cfs (based on 24 cross
sections).
In the Carter et al. (1985) mapping study, those habitats that exhibited predictable
and regular changes with flow were generally associated with physical features of the river
that were inundated when the river flowed beyond its normal channel banks (backwaters,
flooded woodlands and rubble flats) or were manifestations of river hydraulics at various
stages (rapids, runs and eddies increased with discharge; riffles decreased). Total area of
backwaters declined when flows increased from 1,710 to 9,000 cfs, but then increased 50-
fold at discharges over 10,000 cfs. Some flooded bottomlands were present at discharges
of 10,000-21,000 cfs, but were nonexistent at flows less than 10,000 cfs. As discharge
increased above 21,000 cfs, total area of flooded bottomlands began to increase
exponentially, indicating a threshold for significant over-bank flooding somewhere between
21,000 and 23,400 cfs. This inflection point corresponds with the median bankfull
discharge of 22,000 cfs calculated by Pitlick and Cress (2000) for the entire De Beque-to-
Rifle reach.
Successful year classes of razorback sucker largely depend on larvae being placed in
habitats containing adequate densities of forage; such densities are largely restricted to
flooded bottomlands. The necessary duration of flooding may depend on the time required
for larvae to feed during their critical first phase of life. The timing, density, size, and
duration of zooplankton availability must `match' the timing of the swim-up stage of fish
larvae. Razorback sucker larvae must find food of the right size and density within 8-19
days of swim-up or they will exceed the point of irreversible starvation (Wydoski and Wick
1998). After inundation, some time is required for larvae to drift into or hatch within the
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