RIVER ICE INFLUENCES ON FORT PECK REACH, MISSOURI RIVER
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Significance
The observations and data produced in this study reveal how river ice forma-
tion processes may adversely affect the stability of the Fort Peck reach of the
Missouri River, as well as other flow-regulated, alluvial channel rivers. Prior to
the survey these processes either had not been identified or had not been docu-
mented. The observations and data show how ice cover formation can trigger
shifts in channel location, which can accelerate bank erosion. Active movement
and switching of thalweg position and extensive bank erosion were observed
during the six-month survey.
The observations and data are of direct significance for engineering activities
along the Fort Peck reach of the Missouri River and along similar alluvial rivers
subject to frigid winters. They reveal how ice-induced channel changes may
adversely affect the functioning of channel and bank stabilization structures,
water intakes, and bridge crossings, as well as potentially interfering with
dredging work. In general terms, the observations and data are of use for identi-
fying and mitigating problems caused by regulated flow releases from reservoirs
during frigid winter conditions. Although a small amount of information is
available on the winter conditions along the river between its confluence with the
Yellowstone River and Lake Sakakawea (Wuebben and Gagnon 1995), few
aspects of winter conditions have been documented for the Fort Peck reach.
Since this survey was exploratory, unique, and planned with an imprecise
knowledge of the processes whereby ice influences alluvial channel morphology,
it was not complete. Some key aspects of the processes were not fully detected
during the survey and had to be inferred from survey observations and data.
Those aspects need further investigation.
Background
Several concerns prompted the need for the survey. The Omaha District of
the U. S. Army Corps of Engineers (USACE-Omaha), which has long-term
interests in the overall behavior and stability of the Missouri River, wished to
determine whether and how channels along the Fort Peck reach of the Missouri
River altered during winter. The insights obtained would assist USACE-Omaha
in mitigating possible adverse effects of flow releases from Fort Peck Reservoir
during winter. USACE-Omaha is considering enhancements to the Corps' Cur-
rent Water Control Plan (CWCP) for the river. The preferred alternative to the
CWCP apparently may entail increasing flow releases from Fort Peck Dam
during winter (Pokrefke et al. 1998). River ice formation seems not to have
figured prominently in evaluating the alternative schedule.
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