7346

• UCREFRP Catalog Number: 7346
• Author: Wick, E. J. and J. A. Hawkins.
• Title: Colorado Squawfish Winter Habitat Study, Yampa River, Colorado 1986-1988.
• USFW Year: 1989.
• USFW - Doc Type: Fort Collins, Colorado.
• Copyright Material: NO

varied only 0.1 foot between December 15 and February 17. In early March, '
effective depth increased 0.3 foot in response to the 340 cfs discharge, then
decreased dramatically by March 15, just prior to ice-out. This decrease in
effective depth was due to a drop in stage and bed changes resulting from
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increased water velocities along shorelines.
These stage-discharge relationships demonstrate several interesting
effects of different ice conditions throughout the winter period. As initial
freeze-up occurs, shoreline ice (Appendix F) forms along the edge of the
river. This has the effect of reducing discharge while increasing water
surface elevation (because cross sectional area is reduced at controls). For
example, the stage height based on the benchmark at RMI 81.1 under ice-free '
conditions was 90.61 feet at a discharge of 236 cfs. Stage height at the same
location under ice conditions on February 7, 1988 was 91.77 feet at 229 cfs
(over one foot higher). Another set of measurements showing ice effect was a
comparison where stage height was identical for two drastically different
discharges. The maximmun stage height measured under winter ice conditions was
92.14 on March 2, 1988, at a discharge of 340cfs. On April 6, just after ice-
out, an average daily discharge of 1450 cfs resulted in the same stage
elevation of 92.14 feet.
As the winter progressed during the second year of the study, both stage
and flaws increased. The increase in discharge thraughout the winter in spite '
of relatively cold temperatures may have been due to heavy snows in the high
country and at lower elevations. Once deep snows build, their insulating
effect allows some runoff and maintenance of subsurface flaws, thus '
maintaining winter discharge levels.
Normally, on larger streams like the Yampa, the ice cover is in
floatation (Rantz 1982). There is no great buildup of pressure as a result of

ice cover. As stage increases or ice thickens, the increased upward force of '
the water causes tension cracks in the ice, usually near the banks. The ice
floats up to a position of equilibrium, and water fills the tension cracks and
freezes, again forming solid ice cover. This same procedure occurs if stage '
drops, the weight of the unsupported ice causes tension 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)
were not sufficient to raise the level of the ice. The cross sectional area
remained fixed like a closed-conduit, resulting only in an increase in '
velocity. Men air temperature warms and discharge fluctuates, ice tensions
may increase enough to cause a return to the state of equilibrium. This was
observed between January 22 and February 3, when stage increased but discharge '
remained the same.
Just as the process of ice formation throughout the winter has the effect
of increasing stage at a given discharge, the ice-Out process can have the .
opposite effect (Appendix F). This was demonstrated in measurements taken ,
between March 3 and March 15 (Figure 18 a-g). With only a 10 cfs decrease in
discharge, stage was reduced by 0.4 feet. This was likely due to loss of
shoreline ice at the controls. As air temperatures warm considerably in March '
and flows increase, ice begins to melt along the banks. This results in an
increase in water flowing along the river's edge, lass of the damming effect
of the sure ice, and reduction of stage at a given flow.

The ice-out period can be a critical time for some fish because their '
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