9671

• UCREFRP Catalog Number: 9671
• Author: Rees, D. E., W. J. Miller and J. A. Ptacek.
• Title: Volume 1 Draft Final Report - Ecological and Physical Processes During Spring Peak Flow and Summer Base Flows in the Colorado River.
• USFW Year: 2003.
• USFW - Doc Type: Fort Collins, CO.
• Copyright Material: NO

14
12
10
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E
i= 8
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C
Q) 6
0
Q5
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4
2
0

/DailY Discharge

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1000

10000

Discharge (m3/s)

250
200
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Q)
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Figure ES-3. Summary of annual sediment loads at the Palisade gage. Qt represents the
total load, Qs represents the suspended load, and Qb represents the bedload (modified from
Pitlick et al. 1999).

periphyton and macro invertebrate habitat. Following mobilization of the bed material during the
snowmelt runoff period, a new armor layer is formed on the bed of the river in both the riffles
and the runs, and provided that the subsequent post-runoff discharges are less than about 4,800
cfs, the armor layer will be stable throughout the site.

Transient responses are primarily related to the fine sediments (mud) (Dso less than 0.06 mm)
that is deposited on, and eroded from, the stable gravel-cobble riverbed at different locations in
the site when discharges in the river are less than about 4,800 cfs (discharge that is equaled or
exceeded about 15 percent of the time). Fine sediment is supplied during post-runoff summer
thunderstorms over the lower elevation portions of the basin upstream of the 15-MR that are
underlain by highly erodible, sedimentary rocks. Discharges in the river during this period
(baseflows) are controlled by upstream reservoir releases and irrigation diversions, and generally
range from about 800 cfs. During events when fine sediment is supplied to the reach, mud is
deposited on the gravel and cobbles in areas where velocity and shear stress are less than about
2.5 ft/s and 0.03 Ib/ft2, respectively. At locations where velocity and shear stresses are higher
than 2.5 ft/s and 0.03 Ib/ft2 respectively, mud is not deposited even when there is a supply of fine
sediment, and previously deposited mud is re-entrained (Chow, 1958; Smerdon and Beasely,
1961; Partheniades, 1965; Graf, 1971; Partheniades and Kennedy, 1973; Haralimpedes et aI.,
2003). Because the shear stress and turbulence are generally higher in riffles than runs, riffles
tend to be more sensitive to small changes in discharge than runs. Removal of previously
deposited mud may occur in the riffles during small increases in discharge associated with the
summer thunderstorms. However, even if it is not deposited, the fine sediments in transport may
still affect periphyton and macro invertebrate communities by abrasion or scouring.

Draft Final Report - 15 Mile Reach Studies
Miller Ecological Consultants, Inc.

Page ES-7
October 24, 2003