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ERDC/CRREL TR-02-14
As with any river in cold regions, the riverbanks of the lower Missouri River
are subject to freezethaw and freezesublimation weakening. Freezethaw
weakening potentially influences riverbank erosion significantly and perhaps
channel morphology as well. Freezesublimation weakening is likely at work
deteriorating the exposed faces of riverbanks. The soils comprising the river-
banks contain enough silts and fine sands to make them prone to freezethaw and
sublimation weakening. The eastwest orientation of the average axis of the
reach may create differences in erosion rates, because riverbanks facing south
receive greater insolation than do north-facing banks. The extent to which the
ice-related weakening influences rates of riverbank recession has yet to be
determined, however.
Flow characteristics
The flow rate through the reach is monitored continuously by several stage
gages operated by the USACE and U.S. Geological Survey (USGS). The provi-
sional gage data referred to in this report are from the Fort Peck Dam (USGS
station number 06132000), near Wolf Point (USGS station number 06177000),
and near Culbertson, Montana (USGS station number 06185500). The Fort Peck
gage was immediately downstream of the dam, and the warmer water released
from the reservoir keeps the gage ice-free; the open water stagedischarge cor-
relation was valid. As the water progressed downstream, it cooled, and the other
gages may have been affected by ice.
Winter flow rates typically have varied within the range of about 2,500
22,500 cfs since the closure of Fort Peck Dam. The maximum flow into the
reservoir is reported to be 135,000 cfs (USACE-Omaha 1990), which occurred in
June 1953. As extensively documented in several reports (e.g., USACE-Omaha
1986, Simon et al. 1999), the dam significantly altered the annual distribution of
flow through the reach. Yet to be investigated are the effects on river ice forma-
tion on the flow released from the dam.
Before the dam was built, flows through the reach were at their lowest stage
throughout the winter, the lowest stage usually occurring early in the winter
(Haydon 1931). It was usual for the reach to convey two annual flood flows: one
in April attributable to snowmelt augmented by rainfall and ice cover breakup
throughout the lower elevations of the river's watershed, and a second, called a
"June rise," caused by snowmelt in the mountains. A further flow increase, of
lesser magnitude, regularly occurred during October or November.
Once the dam began significantly regulating flow releases in 1941, it reduced
the temporal variability of flows in the reach, and it altered the distribution of
peak or flood flows in the reach. For the period 19701995, peak flows usually
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