and increasing runoff, the ice on the Yellowstone
between cross sections 1569.24 and 1574.16. One
begins to break up and run several weeks prior to
of the prime locations for ice jams to form is at a
breakup on the Missouri. The breakup of the Yel-
transition from a steeper to a milder-sloped reach.
lowstone River then proceeds downstream in a
Such a cutoff channel would have significantly
series of ice jamming and release events. Eventu-
increased the slope within its limits but led to a
ally the breakup front on the Yellowstone reaches
significantly greater reduction in slope when the
the Missouri and proceeds farther downstream
ice reached the natural channel in the Hurley
through the area of the BufordTrenton Irrigation
Bend area. The cutoff channel could exacerbate
District towards Williston. During an ice run, it
the potential for ice jams in that lower reach.
was reported that very large ice floes pass down
Ice jam events
river. One resident commented that the ice floes
At times the dates of ice jam events estimated
typically appeared to be 2.53 ft thick and
by local residents differed by a year or so. For ex-
"...gym-size in area."
ample, several persons mentioned a relatively
The ice on the Missouri River upstream of the
large event in the spring of 1951. However, Bob
confluence area typically remains in place for ap-
Bearce (a resident of the West Bottom) said that he
proximately two weeks after the Yellowstone Riv-
knew this event had occurred in 1952 because the
er runs, in large part due to the small, steady dis-
river was flooded on 28 March 1952--the day his
charges maintained by the release schedule of
son was born. In a subsequent discussion, Clar-
Fort Peck Dam in Montana. While spring runoff
ence Johnsrud (a resident of the Middle Bottom)
on the Yellowstone River rises to values on the
also recalled that the event was in 1952, not 1951.
order of 20,00040,000 cfs, the Missouri River at
This example shows that some uncertainty is to
Fort Peck is typically held below 10,000 cfs until
be expected, especially for events 3040 years
after the Yellowstone River flood peak has
ago. Therefore, the dates of the ice-related events
passed.
cited below may not be exact, and not all events
Ice jam locations
may have been recorded.
A number of residents commented that ice
There were six reported jams in the last 40
jams in the vicinity of the BufordTrenton Irriga-
years. The earliest was in 1952, the most recent in
tion District form in the same locations year after
1986. Based on six jams in 40 years, the jamming
year but with varying severity. Since the ice nor-
frequency would be 6/40 = 0.15, or about once in
mally starts running (and jamming) on the Yel-
seven years. Since the backwater condition
lowstone River two or three weeks before the ice
caused by the formation of Lake Sakakawea can
run begins on the Missouri River, ice runs from
have a significant impact on ice jam formation
the upstream portion of the Missouri River were
and since some long-past events may not have
felt to be of little consequence to the BufordTren-
been recorded or recalled, we might instead use
ton area.
only the last 20 years of record. In that case, jam
Once the ice run on the Yellowstone River
frequency would be 5/20, or once in four years.
reaches the Missouri River, it often jams in the
Reported years with flooding included the fol-
confluence area. This jam causes few problems in
lowing.
the District. As on the Yellowstone River, howev-
1952. According to Roger Bearce, the ice jam
er, the ice marches downstream in a series of jam
flood in 1952 covered much of the West Bottom.
and release events. Once a jam in the confluence
Prior to breakup the measured ice thickness was
area releases, subsequent jams are likely to occur
approximately 32 in., and there had still been
in the vicinity of Ryder Point on the West Bottom
roughly 10 in. of snow on the ground one week
prior to breakup (USDOC 1953). In a review of the
(between cross sections 1576.38 and 1578.03) and
floods of 1952, the Weather Bureau (USDOC 1953)
the Hurley Bend in the Middle Bottom (cross sec-
indicated that the peak stage of 17.76 ft at the
tion 1569.24). Ice jams were reported to be nor-
Williston Gage occurred at 0820 on 1 April but
mally between 0.5 and 2.5 miles in length, but as
that the ice was still moving. Flood stage for this
long as 4 miles on occasion.
gage was set at 20 ft. The stage had receded to
One resident, Clarence Johnsrud, also men-
15.8 ft by 1400 but then once again rose to 16.9 ft
tioned a former jam site between these two loca-
by 1630 on the same day in response to an ice jam
tions, in the bend immediately upstream of the
below the gage.
Hurley Bend, but he stated that this ceased to be a
USGS records for 1 April 1952 show a mean
problem when this bend was isolated by a man-
daily flow of 124,000 cfs, but it is uncertain what
made channel cutoff in 1958. This cutoff is located
6
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