9569

• UCREFRP Catalog Number: 9569
• Author: Valdez, R. A.
• Title: Synthesis of Winter Investigations of Endangered Fish in the Green River Below Flaming Gorge Dam.
• USFW Year: 1994.
• USFW - Doc Type: 18-11,
• Copyright Material: NO

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1988 at a discharge of 340 cfs (9.6 cms). On April 6, just after ice-out, an average daily
discharge of 1,450 cfs (41.1 cms) resulted in the same stage elevation of 92.14 feet.

Ice Conditions

Surface ice covered the Yampa River from December through early March in both

winters investigated. Ice cover in large streams, like the Yampa River, is normally in

floatation (Rantz 1982). As stage increases or ice thickens, increased upward force by the

water causes tension, or coastal, cracks in the ice, usually near the banks. The ice floats to

a position of equilibrium, and water fills the tension cracks and refreezes to form a solid ice

cover. This same phenomenon occurs with a drop in stage, where weight of the unsupported

ice causes stress cracks along the banks and the ice falls to an equilibrium position with the

water. Under extremely cold conditions, heavy surface ice in contact with the stream may

resist this state of equilibrium, causing flow conditions to more closely resemble a closed

conduit. In mid-February, small increases in discharge (197 to 229 cfs; 5.6 to 6.5 cms) of the

Yampa River were not sufficient to raise the level of the ice. The cross sectional area

remained fixed, like a closed conduit, resulting in an increase in water velocity.

Just as the process of ice formation throughout winter effectively increased stage at

a given discharge on the Yampa River, spring breakup had the opposite effect. In cross

sectional measurements taken March 3 and 15, 1988, a 10 cis decrease in discharge resulted

in a stage reduction of 0.4 feet (12.2 cm). This was caused by warm air temperatures in

March, and loss of shoreline ice at anchor points. This resulted in an increase in water

flowing along the river's edge, loss of the damming effect of the shore ice, and reduction of

stage at a given flow.