RIVER ICE INFLUENCES ON FORT PECK REACH, MISSOURI RIVER
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PHYSICAL SETTING OF THE FORT PECK REACH
Much has been written about the physical setting of the Missouri River,
including the Fort Peck reach of the river. Numerous publications document the
reach's flow and channel characteristics, especially in the years following the
closure of Fort Peck Dam, when flow in the reach began to be regulated by the
dam. Several investigations address concerns about channel bed degradation
along the upper half of the river, subsequent to the river's closure in 1940 by Fort
Peck Dam (notably, USACE-Omaha 1945, 1952, Williams and Wolman 1984,
Pokrefke et al. 1998). Several recent publications (e.g., Simon et al. 1999,
Pokrefke et al. 1998, Wei 1997) describe and discuss channel stability concerns
as well as concerns about riverbank erosion along the reach. To date, no study
has investigated the interactive influences of river ice formation and channel
morphology along the reach, though Simon et al. (1999) include a general review
of river ice concerns for alluvial channels, such as the Fort Peck reach.
Channel morphology
The Fort Peck reach of the Missouri River begins at the tailwaters of Fort
Peck Dam, Montana. It ends at the headwaters of Lake Sakakawea, which is
retained by Oahe Dam, in North Dakota. The river channel itself is approxi-
mately 170 miles long and 8001200 ft wide along the reach. The channel's
average slope over this distance is about 1.6 104 to 1.8 104.
The closure of Fort Peck Dam in 1940 affected peak flow rates, sediment
supply, and channel bed elevations along the reach. It thereby also affected ice
formation along the reach. Studies of changes in bed elevation along the reach
(notably, USACE-Omaha 1945, 1952, Williams and Wolman 1984, Wei 1997,
Pokrefke et al. 1998) show that the channel bed degraded over a distance of
about 70 river miles immediately downstream of the dam. Near the dam the
average bed level dropped about 6 ft, with drops in bed elevation of as much as
11 ft occurring at some locations along the primary channel, or thalweg. The
consensus opinion is that the dam is not likely to further degrade the channel bed
significantly (e.g., Pokrefke et al. 1998, Simon et al. 1999).
The channel morphology of the Missouri River along the Fort Peck reach can
be described as sinuous braided. Its characteristics are transitional between those
of meandering channels and braided channels. The channel's planform is sinu-
ous, but the channel width fluctuates substantially, ranging from 800 to 1200 ft.
At its wider sections the river normally flows in several subchannels. A common
feature throughout the reach begins where the river narrows to a single channel
bend with a point bar, followed by a wider subreach that contains one or more
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