formed on a lake. In some areas the ice was clear,
kawea to the confluence with the Yellowstone Riv-
black ice that was apparently formed in place. In
er was essentially clear of ice. Exceptions included
other areas the ice cover was made up of large
some small areas of shore-attached ice in shallow,
slow-moving reaches and some ice floes held in
pans of ice that had formed elsewhere, floated
place by bridge piers. Locations where ice re-
downstream and gently accumulated through jux-
mained bridged across the river on the morning of
taposition to form a single layer of floes, which
7 March were recorded from the air by Kevin
subsequently froze together to form a smooth ice
O'Brien of the Williston Resource Office (Fig. 2).
cover. Near the Fort Union trading post on the
While these bridging locations were remnant ice
Missouri River there was some rough or lumpy
locations rather than ice jams, they do indicate lo-
ice, but it was unclear whether this resulted from
cations of reduced ice transport capacity. Further,
upturned frazil pans or the remnants from beaver
there were remnant ice bridges at two known jam
trapping activity.
sites, the confluence area and near the Bauste
By the end of January, both the Missouri and
Ranch on the Middle Bottom.
Yellowstone Rivers were almost completely ice
Only provisional, uncorrected discharge infor-
covered, with an average thickness of 18 in. and a
mation for the 1991-92 winter is currently avail-
range of measured thickness from 13 to 21 in. On
able from the U.S. Geological Survey (USGS). The
the Yellowstone River at the Route 23 bridge near
Omaha District's Missouri River Bulletin lists the
Sidney, Montana, there had been a small breakup
gage on the Missouri River at Culbertson, Mon-
ice run, and the ice had gouged the left bank of the
tana, as ice affected through 6 March. According
river. Due to subsequent warm, sunny weather
to the USGS records, discharge peaked at 17,500
and a lack of snow, these ice conditions degraded
cfs on the Missouri River at Culbertson at 2300 on
such that by the first of March there were extensive
5 March, while the gage on the Yellowstone River
open-water areas on the Yellowstone, and the ice
at Sidney, Montana, peaked at 18,000 cfs at 1400 on
that remained was significantly thinner and very
the same day. Mean daily discharges for Culbert-
weak due to internal decay by solar radiation. The
son and Sidney on 5 March are listed as 17,200 and
maximum measured ice thickness at this time was
8790 cfs, respectively.
13 in. The reduced thickness and areal extent of the
Since the travel time for water passing each of
ice cover meant that lesser ice volumes would be
these gages to reach the BufordTrenton area is on
available to accumulate in an ice jam on the Mis-
the order of one day, these values can be used to
souri, and what ice remained would be quite
approximate discharge on the Missouri River be-
weak. It was clear that if conditions continued as
low its confluence with the Yellowstone River.
they were, the volume and strength of the ice cover
While not exact, the flow reaching the District can
would make severe ice jamming and flooding un-
be estimated by adding the mean daily values for
likely. Even if the ice cover did not melt out prior to
each of the two upstream gages and adding one
the arrival of the spring runoff, it would have little
day to the date. Recognizing this assumption, and
capacity to form significant jams.
the fact that the flow data are still provisional, we
could estimate that the Missouri River flow
Breakup
peaked sometime during the sixth of March and
In early March, in cooperation with Kevin
that the mean daily value was on the order of
O'Brien of the Corps of Engineers Williston Re-
26,000 cfs. Based on the monthly dischargefre-
source Office (WRO), several local residents were
quency analysis conducted by the Omaha District
enlisted to notify us if the ice began to move. On 3
(USACE 1978), a flow peak of 26,000 cfs in the
March 1992, a small, peaceful ice run was observed
on the Yellowstone River upstream of Sidney,
month of March would be exceeded during
Montana, at the Seven Sisters Boat Landing. Ice
roughly seven out of ten years, making this a rela-
downstream from this point on the Yellowstone
tively common flow event.
and Missouri Rivers was extremely rotted, and
Figure 3 shows the estimated flow record based
numerous open leads had formed with little ap-
on combining the records from the gages at Cul-
parent increase in runoff.
bertson and Sidney, Montana, as described above.
During the morning of Saturday, 7 March 1992,
Since the flow records are provisional and uncor-
the Williston Resource Office relayed a message
rected for ice effects, the sharp reductions in dis-
from one of the river observers, Delbert Dishon,
charge on the sixth and tenth of March may be
that ice had begun to move in the confluence area.
due, at least in part, to the ice cover going out near
By Sunday the Missouri River from Lake Saka-
the gages at Sidney and Culbertson, respectively.
8
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