8119

• UCREFRP Catalog Number: 8119
• Author: Valdez, R. A. and B. R. Cowdell.
• Title: Effects of Flow Regulation and Ice Formation on Overwinter Nursery Habitat and Survival of Age-0 Colorado Squawfish in the Green River Below Flaming Gorge Dam - Draft.
• USFW Year: 1996.
• USFW - Doc Type: Logan, Utah.
• Copyright Material: NO

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tlannelmouth sucker are candidate species. Opportunities for recovery of these native fishes are
related to understanding their life history requirements for all seasons of the year. However, the
majority of studies on these species have been conducted in spring, summer, and fall. The only direct
winter investigations are of movement and habitat use of adult Colorado squawfish and razorback
suckers in the Green River (Valdez and Masslich 1989) and in the Yampa River (Wick and Hawkins
1989). A synthesis of winter investigations in the Green River and an annotated bibliography of
related winter conditions for fishes from other basins was developed by Valdez (1995). These studies
helped to establish winter flow recommendations for adults of these species in the Green River
downstream of Flaming Gorge Dam, but direct information on winter conditions of nursery habitats
and survival of age-O fish is lacking.

The Green River in Utah has the highest densities of age-O Colorado squawfish of any region of the
Colorado River Basin (McAda et al. 1994). These fish are spawned in late summer in principally two
areas, the lower reaches of the Yampa River and in Desolation/Gray Canyon of the Green River.
Newly-hatched larvae from the Yampa River drift to nursery backwaters between Split Mountain and
Sand Wash, and larvae from Desolation/Gray Canyon drift to backwaters between Green River and
the Colorado River confluence (Tyus 1991, Figure 1). The densities of these young fish are often
high in fall, but these decrease by an average of about 60 percent by spring (Valdez and Cowdell
1995), indicating a substantial overwinter mortality, which we believe is abnormally high and
contributes to reduced recruitment and continued endangerment of the species. The causes for this
decrease are unknown, but may be related to predation by non-native fishes, unstable nursery
backwaters from fluctuating dam releases, infiingement of backwaters by ice processes, and natural
downstream dispersal, since Colorado squawfish are potomodrous or long-distance migrators (Tyus
1984).

2.0 STUDY AREA

The study was conducted on the Green River in Utah. The study area extended from Island Park
(RM 333), within Dinosaur National Monument, to the Ouray Bridge (RM 248) at Ouray, UT
(Figure 1). All river miles (RM) cited denote the distance in miles upstream from the confluence of
the Green and Colorado rivers.

The study area was divided into the following three reaches, based on channel morphology and
projected influence offluctuating flows on ice formation (Valdez and Masslich 1989):

1. Reach 1 began 76 miles downstream of Flaming Gorge Dam and extended from Island
Park (RM 333) to Rainbow Park (RM 326). This reach was most affected by fluctuating
releases and had a relatively steep gradient with a gravel-boulder substrate.

2. Reach 2 began 90 miles downstream of the dam and extended from the Split Mountain
Campground (RM 319) to Bonanza Bridge (RM 290). This reach was moderately affected
by fluctuating releases and had a moderate gradient with gravel-cobble substrate in the upper
regions and sand substrate in the lower regions.