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

are formed. Deposits of cobble and gravel can also create islands resulting in multi-thread
channels or complex river reaches. Often associated with multi-channel sites are riffles at
the upstream end and sometimes pools at either the downstream end or to one side of the
riffle. Backwaters are formed at the downstream end of some side channels when inflow at
the upstream end ceases or is reduced as runoff flows subside. Eddies form at the interface
of the backwater mouth and the main channel. Thus, lateral movement of the channel
during high flow events is the process responsible for channel complexity and the creation
of preferred mesohabitats (Pitlick et al. 1999).
Adult Colorado pikeminnow in the Grand Valley prefer segments of river that
contain a diversity of habitat types. Partitioning the river into 0.65-km segments from
Loma to Palisade, Osmundson and Kaeding (1991) categorized segments as either simple
(single-thread) or complex (multi-thread or containing backwaters), and found that the river
consisted of about equal proportions of the two segment types. During radiotelemetry
studies, adults were located in complex segments 85% of the time during spring; 71%,
during summer; 62%, during winter. These complex segments are preferred presumably
because they contain more of the preferred mesohabitats (pools, eddies and backwaters)
than do simple segments (primarily consisting of fast and slow runs) and because a variety
of habitat types juxtaposed to one another allows more efficient exploitation of resources,
i.e., feeding and resting habitats are close together (Osmundson and Kaeding 1991).
Osmundson et al. (1995) selected four complex sub-reaches within the 15-mile reach
for habitat mapping during 1990-1991. The reaches selected were those heavily used by
adult Colorado pikeminnow. At a moderate base flow (1,630 cfs), backwaters comprised
5% of the total water surface area of the four sub-reaches; pools comprised 6%; eddies,
0.4%; riffles, 25%; slow runs, 56%; fast runs, 7%; rapids, 0.5%. Thus, even in complex
reaches, a small percentage of the total wetted channel area is comprised of non-run
habitats; the preference by adult Colorado pikeminnow for such sites underscores the
importance of creating and maintaining such features.
When flow is reduced and sediment input remains the same, fine-sediment
-deposition occurs in low velocity sites such as side channels, backwaters and the river
margin. Osmundson et al. (1995) and Van Steeter (1996) documented the deposition of
22