RIVER ICE INFLUENCES ON FORT PECK REACH, MISSOURI RIVER
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8
FINDINGS AT SURVEY SITES
Culbertson site, RM 16201622
Ice cover formation, as well as differences in ice cover thickness and rough-
ness, dictated which of the two subchannels at this site became the primary
thalweg during the winter season and possibly for the ensuing year. This process
of thalweg switching appears from the survey to be a common occurrence. It
likely contributes to the recurrent problem of sedimentation at the Culbertson
water-supply pump site and at other pump sites along the Fort Peck reach (e.g.,
the Mattelin and Whitmer sites). The pump sites most troubled by sedimentation
largely are situated in sites with two or more subchannels. We introduce the term
"ice-triggered avulsion" to describe the process
Observations
The ice cover formed from pieces of drifting skim ice together with frazil ice
pans and slush that floated downstream, collected, and froze as an accumulation
cover during 20 December 1998. The air temperature on that day dropped to
about 10oC. By 21 December the cover extended well upstream of the site and
encompassed all the sites, although a few small open water patches remained.
They eventually froze over several days later. As is evident from the USGS gage
at Culbertson, the river's stage rose 3 ft once the cover had formed.
The most evident influence of ice cover formation was a shift in the align-
ment of the river's main thalweg from the south subchannel to the north sub-
channel. Drifting ice accumulated first at, and then bridged across, the upstream
end of the north subchannel. Over time a relatively smooth ice cover formed
thermally on the remaining length of the north subchannel. The initial accumula-
tion of ice at the upstream end of the north subchannel deflected a subsequent
heavier drift of ice, which moved into and packed thicker in the south subchan-
nel. Figure 45 was taken as the ice was thickening in the south channel and the
ice was starting to arch across the entrance to the north channel. The sequence of
photos in Figure 46 shows the site before ice cover formation, once the ice cover
had formed, in mid-winter, during breakup, and after the ice cover had broken up
and departed the site. The photographs show that, once the ice cover had
departed, the north subchannel had enlarged at the expense of the south subchan-
nel. A small, vegetated medial bar in the north subchannel was completely
eroded, and a new medial bar had formed in the south subchannel.
The flow resistance of the south subchannel at the Culbertson site had appar-
ently increased much more than did the flow resistance of the north subchannel.
Consequently, as the regulated flow increased during winter, more flow passed
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