8137

• UCREFRP Catalog Number: 8137
• Author: Harvey, M. D., R. A. Mussetter and E. J. Wick.
• Title: A Physical Process-biological Response Model for Spawning Habitat Formation for the Endangered Colorado Squawfish.
• USFW Year: 1993.
• USFW - Doc Type: \
• Copyright Material: YES

- .

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DISCHARGE (ds)

FIGURE 11. Variation in dimensionless grain shear stress (r:) at cross section 2.7 for two values of
dimensionless critical shear stress (T.) at discharges ranging from 500 to 10,000 cis in the right branch
channel, RM 16.5, Yampa River. The median size of the bed material is 68 mm (2.7 in.).

the time of the field visit in 1991, the bed
material was observed to be mobile only
at the tertiary bar at cross section 1.5 (Fig-
ure 3) and the hydraulic analysis indicated
that the bed material should have been mo-

bile (7*' > 1.0) at a discharge of about 1,200
cfs. Because of raft access problems no at-
tempt was made to ascertain the presence
of Colorado squaw fish at the associated
pool.

DISCUSSION

Colorado squaw fish spawn at a limited
number of sites in the lower Yampa Can-
yon on the recessional limb of the annual
snowmelt hydrograph when the water
temperature exceeds about 190C (Figure 1)
(Wick et al. 1983; Tyus and Karp 1989; Tyus
1990). Based on fish-catch data, the phys-
ical requirements for successful spawning
are: (1) a relatively coarse-grained mid-
channel bar with an associated pool or
shoreline eddy for the resting-staging
phase (Wick et al. 1983), and (2) a clean
cobble substrate to permit adhesion of the
eggs during the deposition-fertilization
phase (Patten and Rodman 1969; Hamman
1981) .
The midchannel bar at RM 16.5 has
formed upstream of a sharp bend in the
bedrock-incised meandering channel. Un-
der higher discharges, the bend causes
backwater that flattens the energy gradient

(Figures 7 and 8). The lesser energy gra-
dient in turn reduces the ability of the flows
to transport gravels and cobbles that were
entrained farther upstream in reaches that
are unaffected by backwater. The behavior
of the midchannel bar is similar to that of
a riffle in a pool-riffle sequence. During
high flows, velocity is greater in the pools
and lower in the riffles, resulting in ero-
sion of previously stored sediments in the
pool and deposition of sediment in the rif-
fle. Under low flows, the velocity and sed-
iment transport capacity of the flows are
reversed-the pools become depositional
sites and the previously deposited sedi-
ments on the riffles are eroded (Keller 1971;
Lisle 1979). Scour of sand from the pools
during high flows and redeposition during
recessional flows were observed by O'Brien
(1984) in the lower Yampa Canyon. Hy-
draulic modeling of the RM 16.5 reach

I M. D. Harvey et al.

125 II~