WSP11633

• File Number: 8220.101.09
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 3/1/1994
• Title: Comments regarding the Draft Biological Opinion on Operations of Glen Canyon Dam
• Water Supply Pro - Doc Type: Biological Opinion

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GRAND CANYON NATIVE FISH MANAGEMENT

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These conditions have occasionaIIy occurred in the post-dam era. During high water years
of 1984-1986, numerous young-of-year humpback chub were obsetved in the confluence
zone of the LCR (Maddux et aI. I987, W.R. Persons, AGFD, personal communication).
RelativeIy few early life stage fishes were found there at Iower regulated main stem flows
during 1991-1993 (AGFD data).

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It is unknown if historically there were separate genetic components among pre-dam
mainstem native spawners and tributary spawners. Mainstem spawning may have been an
adaptation that setved to maximize the probability of successful reproduction of individuals
by spreading the outlay of sexual products across both environments, i.e. fishes may have
released a portion of their eggs in tributaries (LCR) earIy and released the remainder later
in the mainstem. Alternatively, distinct mainstem and tributary-spawning stocks may have
existed.

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Adaptations by larvae of some species (e.g. CoIorado squawfish) to drift from hatching sites'
to more suitabIe downstream rearing sites probabIy served to enhance ,survival and growth
(Tyus 1986). The timing of mainstem reproduction also takes advantage of the presence of
newly formed, relatively productive (Grabowski and Hiebert 1989, KubIy 1990) bacJtwater
habitatS for use by deveIoping native fish larvae and young-of-year juveniles of most species
(Tyus 1987, Tyus and Karp 1989, 1991, Tyus and Haines I991,VaIdez 1990). .,
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771t Post-Dam Environment-Magnitudes; ranges, and seasonal patterns of the, hydrograph
were changed significantly from the historic situation following closure of Glen Canyo~Dam
(Figure 4). Except during years of exceptional runoff, maximum discharges normally did
hot' exceed 31 ,SQ9cfs,. the maximum pc:lwerpIant capacity. of the dam prior to generator
uprating. The annual range in discharge typically did not exceed "30,000 cfs, .but daily
diSCharge fluctuations approaching that range were possible under routine powerpIant
'operations. With the recentestabIishment of "interim" flows, this range has contracted
someWhat, but the pattern of daily flow fluctuation remains.

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The persistent pattern of a large daily _range in discharge is a phenomenon unseen in pre-dam
days. One of the most noticeable effects of this pattern is on backwater and shoreline
habitats, which are inundated and desiccated on a daily basis as a reSult of changes in river
'stilge. " Such temporal instability precIudes the level of biological productivity that. is
characteristic of backwaters in the absence of daily changes in stage. In addition, weak-
swimming larval and post-larval fishes are forced from these refugia and exposed to higher
: mairistem current velocities and often colder temperatures, where they' are subjected to
: inCreased physioIogicai stress, lowered growth rates, and presumably elevated mortality rates
(Kaeding and Osmundson 1988; Thompson et al. 1991, Lupher and Clarkson 1994). Fishes
. may be isolated and desiccated as hab~ts are dewatered and dri~, or subject to lethal