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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thicker in reach 3, but backwaters often had multiple ice layers indicating a greater frequency of ice
jams. Ice lensing was riot observed in reach 3 because of the lack of flow fluctuation and the more
constant water temperature.

Average free water depth in backwaters varied from 0.7 to 2.9 feet in year 1 and from 0.7 to 3.2 feet
in year 2. Although ice formed solid to the substrate in shallow areas (<0.5 feet) of most backwaters,
we did not observed ice development extensive enough to substantially reduce free water in any
backwater.

Average water temperature in backwaters varied from 0.06 to 3.660 C in year 1 and from 0.10 to
4.780 C in year 2. Despite measurement of supercooled water temperatures in the mainstem (<00 C),
supercooled conditions were not recorded in backwaters during this study . We believe that frazil ice
development was not extensive enough to jam backwaters beneath surface ice and create the
supercooled condition.

Average dissolved oxygen varied from 6.0 to 13.6 mgll in year 1 and from 5.1 to 16.3 mgll in year
2. Most recorded concentrations were at or just below saturation for the given temperature and
elevation. Low concentrations of 5.1 and 5.3 mgll in two backwaters in year 2 were attributed to a
local biological oxygen demand from a dense layer of dead leaves and vegetation, as well as fecal
pellets, associated with a beaver pond. The high concentration of 16.3 mgll was recorded in a
backwater that was isolated from the main channel by an ice plug formed at the mouth. A substantial
amount of periphyton was present on the cobble substrate and the ice was clear and absent of snow
cover, suggesting active photosynthesis and oxygen production in the closed system.

In summary, of the 23 backwaters observed and measured in the winters of 1993-94 and 1994-95,
the only factors that made these nursery habitats unsuitable for young fish were natural geomorphic
changes and ice jams that caused flow through the backwaters and negated their value as sheltered,
low-velocity habitats (Tables 9, 10). While these changes were caused indirectly by ice conditons,
no detrimental conditions were seen in backWaters as a direct result of ice conditons. Those
backwaters that persisted retained greater than the minimum area of30 m2, water depth of9.0 em
or greater, water temperature remained above -0.50C, and dissolved oxygen remained above 5.0
mgIl.

7.0 DISCUSSION

Winter provides one of the most stable environments for riverine fishes of any season of the year
(Hynes 1969). Natural flows are relatively stable and an ice cap often dampens fluctuations (Wick
and Hawkins 1989) and maintains above-freezing, isothermal conditions. Lowered water
temperatures reduce community metabolism and fish respiration, sending fish into a state of reduced
activity (Schmidt et al. 1987).

Since Flaming Gorge Dam was built on the Green River in 1963, winter flows are higher by an
average of over 1,000 cfs for each month. Hypolimnetic releases of 8-140 C in summer have kept the
river relatively colder, and releases of 4-80C in winter have kept the river relatively warmer than pre-
dam conditions. These thermal effects are greatly ameliorated by distance and by the Yampa River,

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