8028

• UCREFRP Catalog Number: 8028
• Author: Daly, S. F., et al.
• Title: Effect Of Daily Fluctuations From Flaming Gorge Dam On Formation Of Ice Covers On The Green River -Draft.
• USFW Year: 1997.
• USFW - Doc Type: \
• Copyright Material: NO

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FORMATION OF ICE COVERS ON THE GREEN RIVER
Introduction to River Ice Formation with Reference to the Green River
The formation of river ice covers reflect the meteorological and hydrologic conditions of
the region through which the river flows and the hydraulic conditions of the river channel itself.
Ice production in a river begins when the river water reaches a temperature of 32°F (0°C). The
river temperature represents the balance of heat transfer into and out of the river. In the case of
the Green River downstream of Flaming Gorge Dam, the dominant heat exchange is between the
water surface and the atmosphere. The reservoir behind Flaming Gorge Dam represents a large
source of heat throughout the winter. Measurements indicate that the temperature of water
released from Flaming Gorge Dam razely falls below about 39°F (4°C) in the winter (although
Valdez and Masslich (1989) report that the temperature of releases varied from 1.7°C to 6.2°C
[35.1 °F to 42.8°F] during 1987-88). Downstream of Flaming Gorge Dam any heat input into the
river, other than through the water surface, is probably minimal. During cold periods, the Green
River cools in response to heat loss from the water surface to the atmosphere. The distance
downstream of Flaming Gorge Dam where water temperatures drop to 32°F (0°C) depends on
the release temperature, the heat transfer rate, and the volume of water being released.
Wintertime water temperature measurements indicate that the water temperature is often 32°F
(0°C) when it reaches the upstream end of the study reach.
The type of ice formed in the Green River is controlled by the flow conditions in the
channel. In the faster moving reaches frazil ice will form. Frazil ice is formally defined as small
"discoids" of ice formed in turbulent, supercooled water. Soon after they are formed, frazil ice
crystals are a few tenths of a millimeter in diameter. Typically, frazil ice crystals will be
approximately uniformly distributed throughout the depth of the flow, especially in highly
turbulent reaches. -The high-gradient reaches upstream of the study area (e.g., Split Mountain
Canyon) are particulazly favorable areas for frazil ice production. Frazil ice is transported
downstream by the river current and continuously evolves in form as it transported (for a more
complete description of this evolution process, see Daly, 1994). Individual frazil crystals
agglomerate into larger and larger masses of ice known as flocs. Frazil slush is the collection of
frazil flocs and individual frazil crystals on the water's surface in a distinct layer. Frazil slush at
the water surface has a marked tendency to clump together. The initial clumps, if they remain at
the surface long enough, can further clump together and form pans, or small floes. These pans
often grind against one another, causing them to become roughly circular in shape and gain
upturned edges. At this point they are known as pancake iee (photographs and descriptions of
pancake ice in rivers can be seen in Ashton, 1986 and Beltaos, 1995). Frazil slush and floes can
accumulate along or abrade the outer edge of border ice which forms along the banks of the river
channel. It is not uncommon to see parallel lines of raised frazil slush along the outer edge of
border ice marking periods when frazil ice accumulated along the border ice.
In slower moving areas of the river, such as regions behind islands or in the lee of
sandbars, where there is very little or no mixing due to the locally reduced velocity, the surface of
the water will cool sufficiently for ice crystals to farm directly on the water surface. Ice that
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