7801

• UCREFRP Catalog Number: 7801
• Author: Valdez, R. A.
• Title: Synthesis of Winter Investigations of Endangered Fish in the Green River Below Flaming Gorge Dam.
• USFW Year: 1995.
• USFW - Doc Type: \
• Copyright Material: NO

Frazil crystals beneath ice cover will accelerate thickening of the ice cover, because of a
reduction in effective latent heat of solidification. Frazil ice deposits beneath solid ice cover can
dramatically increase surface ice thickness by up to 50 to 100 percent (Calkins, 1979). Further
thickening of a stabilized ice cover formed from frazil is regulated by heat loss from the ice surface
along with heat conduction through the ice from warm water below. Ice growth measurements taken
by Calkins (1979), showed greatest thickening in large pools of up to 0.5 m in a 10-day period (5
cmlday).
Ashton (1979) identified that surface velocities greater than 0.6 m/sec prevent formation of
an intact ice sheet from frazil crystal accumulation. As frazil is formed and carried downstream, it
either forms an ice sheet or, if flow velocities are sufficient, is swept beneath downstream ice cover.
These deposits of frazil can be massive, often extending to the bottom of the channel, and effectively
blocking the flow of water to form a hanging dam (Ashton 1980). There is little flow through a frazil
ice mass, and one or more tunnels of free water may form.
Thickening of the ice cover is also influenced by other factors besides accumulation of frazil
crystals, including air temperature, thermal water masses, overflow of water atop an existing ice sheet,
and an accumulation of solid ice flows. Ashton (1979) also identified thickening of ice cover by the
formation of "snow ice", the subsequent accumulation, submergence, flooding, and freezing of the
surface snow layer.
Thermal effluents have a marked impact on ice cover formation, particularly if the river water
originates entirely from a relatively warm source, such as a deep reservoir. Since water is densest at
4°C, reservoirs with ice cover exhibit a thermal gradient of 0°C near the surface to 4°C near the
bottom. Donchenko (1978) found that the ice front in rivers below reservoirs may oscillate many
kilometers, with variations in meteorological and hydrological conditions. The location and stability
of the upstream ice edge can be determined by the amount and duration of the discharge, as well as
the zone of fluctuating influence (Donchenko 1978). Conversely, the length of open water upstream
of the ice edge is inverse to ice thickness.
River Ice Deterioration and Decay
Deterioration and decay of river ice is a complex process that varies by location, season, and
simultaneous environmental influences (Ashton (1979, 1980). In early winter, water temperature
beneath the ice cover is very close to 0°C, and the ice thickening process results in a smooth, planar
undersurface. As winter progresses, relatively warm water may be introduced from tributaries,
groundwater, rain or snowmelt, or residential and municipal wastes, raising the river temperature
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