discharge was present while the jam was in place.
On 1 March 1986, water was said to have
reached the top of the Borlaug Bridge on the Mid-
The fact that the highest stage occurred while the
dle Bottom, which corresponded to an elevation of
ice was observed to be in motion may either indi-
1870 ft msl. This location coincides approximately
cate that the peak flow was due to the release of
with HEC-2 cross section 1567.44. (A Corps of En-
an upstream ice jam, in which case the flow situa-
gineers survey in the spring of 1992 indicated that
tion was analogous to a dam-break wave, or pos-
the top of road at this site, as defined by a photo-
sibly that there was an unrecorded ice jam down-
graph of the event, was about 1868.5 ft msl).
stream of the gage.
According to Bob Bearce, a resident of the Mid-
1972. An ice jam in March caused water to back
dle Bottom, the jam extended from approximately
into the main drains, flooding most of the Middle
halfway between HEC-2 cross sections 1564.48
Bottom and half of the West Bottom. Thirty-one
and 1565.44 to about cross section 1569.24. This
families had to move out for five days.
would give a jam length of about 4.2 miles. He
1975. An ice jam event occurred at some unre-
estimated that ice floes at this site were about 18
corded time during the spring. Also, in June the
in. thick.
highest open-water flood levels experienced
During June 1990, photos were taken of severe-
since closure of Garrison Dam occurred.
ly scarred trees at the Bauste property, which is
1976. Ice scars found on trees on the Floyd Ry-
der property in the West Bottom (near HEC-2
downstream of the Hurley Bend on the Middle
cross section 1577.15) were thought by local resi-
Bottom. Since that time, most of these trees have
dents to have been made during an older event,
been cut down. The scars were very regular in
which perhaps took place in 1976. The scars were
both their orientation to the river and in the eleva-
approximately 9 ft above the water surface on 26
tions of the top of scarring between numerous
June 1990 (determined using a hand level). Based
trees as determined by hand level. The regularity
on a HEC-2 simulation of the water surface pro-
in scar orientation and elevation suggest damage
file for that day, the tree scars would be at an ap-
by ice (or perhaps debris) rather than by animals.
proximate elevation of 1870 ft msl. However, this
The scars extended from approximately 4 to 6 ft
above the ground surface on a relatively level ter-
elevation would also correspond to computed
race near HEC-2 cross section 1564.48. The top ele-
water levels during the 1986 event, making the
true date of the scars uncertain without a tree ring
vation of the tree scars was approximately 1860 ft
analysis.
msl, which is slightly below the ground elevation
1978. High ice jam flood levels occurred in
at the corral buildings on the property. Local resi-
1978. According to Clarence Johnsrud, "Larsen
dents thought that the scars were from the 1986 ice
lost out." The Larsen property was located on the
breakup event, and water during that event was
Middle Bottom. Clarence Johnsrud also recalled
said to have come near to a corral building at the
that although there was substantial overbank
site.
flooding, most of the ice remained in the channel
in the Middle Bottom reach of river during this
Winter 1992 field observations
event.
The plan for the 1991-92 winter field program
1986. The most recent ice jam event occurred in
included a series of site visits to document ice
1986. During this event, water almost overtopped
thickness and type, ice bridging and jamming
a lateral in the West Bottom near HEC-2 cross sec-
locations, and other site characteristics necessary
tion 1578.03. A local resident recalled that it took
for the formulation of ice-related flooding mitiga-
about 11 hours from the time the initial jam
tion techniques. To anticipate changing ice condi-
tions or possible ice breakup events, we also mon-
formed until water reached the top of the road.
itored weather conditions and forecasts at Willis-
Before overtopping the road on top of the lateral,
ton and several sites in Montana for any indica-
however, the jam released. The ice then passed
tion of an increase in runoff that could initiate
downstream and re-jammed in the Hurley Bend
breakup on either the Yellowstone or Missouri
area in the Middle Bottom. Tree scars found in
Rivers.
this area, which local residents suggested might
have been from the 1976 event, were at an eleva-
Winter ice conditions
tion of approximately 1870 ft msl. The road on top
Observations during 1992 showed that, with
of the lateral in this area has comparable eleva-
rare exceptions, the ice in the study area was a
tions, suggesting that the tree scars may have
smooth, single-layer ice sheet not unlike that
been from the 1986 event.
7
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