8112

• UCREFRP Catalog Number: 8112
• Author: Osmundson, D. B. and B. K. Scheer.
• Title: Monitoring Cobble-Gravel Embeddedness in the Streambed of the Upper Colorado River, 1996-1997.
• USFW Year: 1998.
• USFW - Doc Type: Grand Junction, CO.
• Copyright Material: NO

lucius) also appears to experience limitations in food availability and reproductive success.
Despite extensive research on various aspects of the life history of this fish, the effects of
sedimentation on reproductive success and on food base productivity have been largely
ignored. However, the Colorado River is highly susceptible to sedimentation problems.
Primarily because of considerable water development in the headwaters, the frequency of
flows capable of mobilizing the armour layer in reaches occupied by the Colorado squawfish
has been significantly reduced (Osmundson and Kaeding 1991, Van Steeter and Pitlick 1998).
In the 15-mile reach upstream of the Colorado River's confluence with the Gunnison River,
the frequency of years with peak flows capable of fully mobilizing the bed (- 566 cros
[20,000 cfs]; Van Steeter 1996) have declined from 77 percent to 34 percent from the early
part of the century to more recent times (Fig. 2). Conversely, the frequency of years with
peak flows incapable of initiating bed movement (< 283 cros [10,000 cfs]; Van Steeter 1996)
has increased from zero to 23 percent. Mlhous (1998) found that the frequency of flushing
events in the Gunnison River, a major tributary of the Colorado River, has also declined
dramatically since major water development projects in the headwaters were put on line: the
frequency of years with flows capable of flushing fines from riffles has decreased from 68
percent to 25 percent.
While peak flows have declined in both rivers, sediment inputs probably have not. Due to
grazing and irrigation practices, sediment inputs are likely to be at least as high today than
during presettlement times. When storage projects are located in the headwaters above major
sources of sediment, as in the Colorado River, they have little effect on reducing sediment
delivery to downstream segments. Coupled with a reduced flow regime, sediment input rates
are likely to exceed transport rates and depositional problems (aggradation) are likely to
occur with time (Reiser et al. 1989, Van Steeter and Pitlick 1998). In years of exceptionally
high runoff, enormous quantities of accumulated fine sediment is released from the bed and is
deposited on the tops of bars, islands and the adjacent floodplain. After such times cobble-
bed habitats become exceptionally clean and free of fine sediment and are widespread in
occurrence (Fig. 3). How often such flushing flows must occur to maintain these relatively
clean bed conditions is as yet unknown. Because Colorado squawfish spawn in localized,
geomorphically anomalous sites (Harvey et al. 1993), this study does not attempt to monitor
or address the issue of sedimentation of spawning substrates; rather, our objective is to
characterize more widespread substrate conditions primarily relevant to maintaining
productivity of riverine food webs.
METHODS
The general study area was located in the Grand Valley; this area was selected for three
reasons: 1) its importance as a concentration area for adult Colorado squawfish, 2) its
abundance of cobble/gravel based habitats, 3) its close proximity to our field station.
Sampling sites were divided among the 15-mile reach (upstream of the Gunnison River
confluence) and the 18-mile reach (immediately downstream of the confluence). This allowed
for comparisons between reaches to determine if embeddedness might be influenced by the
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