were made during a 20-day steady flow period,
Breakup ice jams often form in reaches that
and repeated for a 4-day period with a daily peak-
change from steeper to milder water surface
ing pattern. The study concluded that the daily
slope, such as river to lake or pool confluences. If
peaking pattern had little effect on the overall
the lake or pool level can be regulated, it may be
annual pattern of ice cover progression on the
possible to influence the ice jam location, but the
river. However, the fluctuating flows did cause a
effect of water level change on ice jamming is
shoved type of cover at locations in the upstream,
often difficult to predict. Operation of river and
steeper portion of the study reach, where a thin-
reservoir dams can affect the timing and severity
ner juxtaposed cover had existed under steady
of breakup as well by storing water to reduce the
flow conditions.
rate of hydrograph rise and the peak flow in
downstream reaches. This strategy may delay
breakup or at least reduce its severity.
Midwinter reservoir levels
It may be possible to control water level at the
and ice jam flooding
Winter reservoir operations may also influence
time of ice cover formation to mitigate breakup
the ice regime at upstream locations. The winter
severity. Donchenko (1978) predicted that the ice
pool level at water storage reservoirs can influ-
cover would release when stage exceeds a level
ence the location and severity of freezeup ice jams
of three to four ice thicknesses above the freezeup
on tributary rivers and streams. On some west-
water level. Based on this, one strategy is to raise
ern reservoirs, winter pool levels have been raised
the freezeup water level, then lower the water
substantially to meet increasing water demands.
level before breakup, creating some in-channel
This change may cause freezeup ice jams on
storage and delaying breakup. In contrast to flow
feeder streams, or displace jams upstream from
control to delay breakup, Ferrick and Mulherin
their historic locations. The South Fork Shoshone
(1989) proposed using reservoir releases to prema-
River that flows into the Buffalo Bill Reservoir
turely break up a section of ice cover to serve as a
near Cody, Wyoming, is an example.
receiving area for ice from the subsequent natu-
In response to increased water storage demand,
ral breakup. Once an ice jam has formed, it may
the Buffalo Bill Reservoir raised its winter pool in
be possible to decrease outflow from upstream
1995 to an elevation 13 m above the average pool
dams to prevent or reduce resultant ice jam flood-
level of the previous 19 years. The winter of 1996
ing. Finally, river structures can be operated dur-
97 saw consistently below-average air tempera-
ing extreme breakup events to prevent damage to
tures accompanied by above-average discharge
the structure while minimizing ice effects and jam
on the South Fork Shoshone River. A serious
flooding in upstream and downstream reaches.
freezeup ice jam flooded properties several miles
upstream of the reservoir where few recent ice jam
Controlling ice jam location
floods had been reported. Because of the short
Breakup ice runs on rivers often stop when
period of record and limited historical data, it was
they encounter an intact sheet ice cover on a pool
difficult to determine the relative significance of
or reservoir. If the pool level can be regulated,
the severe cold and the higher reservoir elevation
raising or lowering the water level will move the
with respect to the ice jam severity in 199697.
head of pool location and the ice jam initiation
However, the higher reservoir elevation probably
point upstream or downstream, respectively. In
did have some influence (Tuthill 1997).
many instances, raising the water level in a reser-
voir will displace the breakup jams on tributaries
upstream. There are also numerous examples
BREAKUP PERIOD
of breakup jam problems moving downstream
Flow control and project operation can affect
following reservoir lowering or the removal of old
the location and reduce the severity of ice prob-
mill dams on small rivers. A strategy for displac-
lems during the breakup period. Although the
ing ice jam location downstream is to lower the
subject of much research, ice processes during the
pool level before breakup takes place. This tech-
breakup period are more complex and less well
nique has the added advantage of providing a
understood than ice formation or those during
storage area for the breakup ice. Lowering pool
the midwinter. Ashton (1986) and Beltaos (1995)
elevation may have unexpected results, however.
describe much of the research to date, as well as
The resulting slope may be steeper and the
the factors that influence the timing, location, and
jam thicker, causing ice jam flood levels in the
nature of river ice breakup.
upstream problem area equivalent to the higher-
15
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