of the Irrigation District has continued at a rate of
tion (from a few hours to a few days for breakup
about 1 ft per 67 years. Total stage increases from
events) make it unlikely that detailed field infor-
1965 to 1988, for a discharge of 40,000 cfs, have
mation will have been gathered at most sites.
been 4.6, 5.2 and 6.6 ft at Gages 5A, 6 and 7, respec-
Even in cases where hydrographic gaging records
tively. For the period from 1988 through 2055, an
exist for a site, ice effects on the gage rating curve,
additional 510 ft of deposition is expected to oc-
the location of the gage relative to the ice accumu-
cur in the reach from the confluence of the Mis-
lation, the potential for gage freezeup because of
souri and Yellowstone Rivers downstream
cold weather, or direct ice action on the gage can
through the Irrigation District to river mile 1530.
reduce its reliability for ice events.
Deposition depths in excess of 30 ft can be expect-
Without prior field observation it is difficult to
ed farther downstream. Stages, for a discharge of
predict where, or even if, ice jams will form along
40,000 cfs, are expected to rise at an average rate of
a river. Because ice jams are site specific, it is gen-
1 ft per 20 years at Gage 5A, 20 years at Gage 6,
erally not possible to extrapolate from stage data
and 17 years at Gage 7. Total stage increases be-
for other sites along the river. Hence, in an analy-
tween 1990 and 2055 would be about 2.7, 3.2 and
sis of ice-related flooding it is often necessary to
3.8 ft, respectively, for Gages 5A, 6 and 7.
resort to other sources of historical data, sources
that are often overlooked or regarded as unrelia-
ble for the analysis of open-water flooding. Dur-
FIELD DATA COLLECTION
ing this study, no significant ice run or jam
The objectives of the field program were to
occurred, and a series of interviews with local
monitor ice, weather and runoff conditions in or-
residents comprised virtually the only source of
der to anticipate potential ice problems during the
information on ice jam processes in the area.
spring of 1992, and to collect additional data nec-
essary to identify potential short- and long-term
Freezeup
flood mitigation measures. The area studied ex-
A primary question asked in regard to freeze-
tends along the Missouri River from Lake Saka-
up was whether the river froze in a manner simi-
kawea to the railroad bridge approximately 3
lar to a lake or a puddle, in which the ice thickens
miles upstream of the Fort Union National Histor-
gradually and smoothly, or whether floes came
ic Site, and along the Yellowstone River upstream
floating down the river and accumulated to form
to Glendive, Montana.
a cover. A common response was that the river
In addition to direct measurements and obser-
froze in place, leaving a smooth cover, but others
vations, a significant component of the data col-
recalled seeing numerous floes accumulating to
lection involved interviews with local residents to
form a rough cover.
obtain their recollections and opinions on ice-re-
lated flooding in the BufordTrenton area. The
Winter ice conditions
first several site visits concentrated on gathering
Ice conditions were reported to be generally
historical information through such discussions,
smooth with some rough areas. Ice thickness esti-
while the remaining site visits focused on docu-
mates were generally on the order of 22.5 ft, al-
menting winter ice conditions.
though some residents reported thicknesses as
high as 3 and 4 ft on occasion. Ice growth calcula-
Historical information
tions based on thermal growth of single-layer ice
Ice-related flooding tends to be local and highly
indicate that the ice thickness in this area might
site specific. While ice jams may be relatively com-
reach 2.5 ft about once in five years, and 3 ft less
mon at a given site, they cannot be predicted with
than once in 50 years. The 50% exceedance ice
certainty in any given year, and they may be total-
thickness would be just over 2 ft. It is possible that
ly absent at other sites nearby. Further, ice jams
these thicker estimates were made in areas where
often occur when flow rates are relatively low,
ice floes have, in some years, accumulated to
perhaps no more than a 0.5 exceedance probabili-
form a multi-layer cover.
ty discharge,* and water levels are normally high
only in the vicinity of the ice and in a backwater
Breakup
zone upstream. Their relatively small geographic
It appears that, in most years, breakup on the
extent (perhaps a few river miles) and short dura-
Missouri River in the vicinity of Williston, North
Dakota, is driven by events on the tributary Yel-
* This is new terminology corresponding to a recurrence in-
lowstone River. In response to warmer weather
terval of two years.
5
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