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

are ice jams, which typically form in specific locations, rather than appearing throughout the length
of the river. Ice jams typically start at diverging river reaches, or places where the river slope changes
from steep to mild, and surface transport rate decreases as flow slows and the river deepens. Ice jams
often accumulate in narrow, constricted regions, giving the false impression that they originate in
these areas.
Many types of ice jams have been classified, with considerable literature describing the subject,
but there are few quantitative measurements detailing their development. Fundamentally, ice floating
downstream arrives at an upstream ice edge or a constricted channel region, and either accumulates
on the surface or, at higher velocities (>0.6 m/s), submerges and overturns beneath the ice edge
(Ashton 1979). Ashton (1980) described two of the more common types of ice jams. The first occurs
when a flow discharge wave fractures, displaces, and moves the ice cover, which then clogs a
downstream section of river. The resulting rise in water level may dislodge the ice jam, releasing
large blocks of ice further downstream to repeat the jamming process. The second common type of
jam forms when a discharge of ice encounters either an intact ice cover or a change to a lesser river
slope. As transport capacity decreases with reduction in river slope, the ice piles and may block the
stream.
Ice masses below reservoir tailwaters undergo several phases of formation, jamming, and
destruction, depending on flow volume, magnitude, and variation in discharge. Increased discharges
initially thicken shoreline ice cover, which results in increased current velocity (Donchenko 1978).
Further rise in water level invokes deformation and a dynamic destruction of the ice field.
Fluctuations in discharge, such as releases from large hydropower projects, will break the ice cover
and deteriorate associated ice jams when water level fluctuations are three to four times greater than
ice thickness (Donchenko 1978).
Effects of Winter Conditions on Riverine Fishes
Physiological Effects
Fish are cold-blooded or poikilothermic animals. They cannot internally regulate their body
temperature like warm-blooded animals or homeotherms. Body temperature, metabolic rate, activity,
and nutritional requirements of fish are, above all, dependent on temperature of the surrounding
water. Fishes in seasonally warmed and cooled waters of temperate regions undergo lowered
metabolism and growth rates during the coldest months.
Fish exposed to subfreezing conditions can succumb to frozen body fluids and tissues, but the
first effect of severely lowered temperature is inhibited water diffusion pressure that prevents free
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