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

required by many invertebrate species (Osmundson and Scheer 1998, Osmundson et al.
2001). If the tops of rocks become covered with fine sediment, algal production may also
decline. To flush fine sediment from the bed, flows must be high enough to dislodge and
move the larger particles. Gravel river beds are typically composed of surface and
subsurface layers. The surface layer, sometimes called a pavement or mobile armour, is
coarser than the subsurface material. Milhous (1973) found that mobile armour plays an
important role in the deposition and retention of fine sediment. At low flows, the immobile
surface layer acts as a sink for suspended sediment, which deposits at the interface between
the pavement bottom and the subsurface top. This zone acts as a silt reservoir: at high
discharge the pavement is set in motion; the bed then becomes a source of suspended
sediment as fines are winnowed out. Milhous (1973), O'Brien (1987) and Wilcock et al.
(1996) have emphasized the necessity of surface layer mobilization for removing fines
below the surface layer. Pitlick and Van Steeter (1998) found that the minimum flow
necessary to produce widespread movement of the bed in the 15-mile reach, and the two
reaches immediately downstream of the Gunnison River inflow (18-mile and Ruby-
Horsethief Canyon), corresponded with the bankfull flow. As discussed above, the
bankfull flow in De Beque Canyon is approximately 20,500 cfs and in the De Beque-to-
Rifle reach, approximately 22,000 cfs. In the 15-mile reach, the bankfull flow is
approximately 21,500 cfs (Pitlick and Cress 2000).
Thus, bankfull flows not only provide razorback sucker larvae and adults with
critically important off-channel habitats and entrain organic debris from bottomlands into
the main channel, they also serve to mobilize within-channel substrates, flushing fines from
the bed and optimizing living space for macroinvertebrates. Ideally, these high spring flows
should then be followed by base flows with low turbidity to promote algal production.
Colorado Pikeminnow Spawning Cues
Cues that trigger Colorado pikeminnow to undergo spawning migrations and to
initiate spawning appear to be largely controlled by river flows and temperature. Adults
begin migrating to spawning areas as peak runoff declines and water temperature increases
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