RIVER ICE INFLUENCES ON FORT PECK REACH, MISSOURI RIVER
93
progression through the site affected the flow resistance through the subchannels,
with the principal result that flow in the southernmost subchannel at the site's
upstream end became directed farther along the south bank.
During its initial formation the ice cover froze to the low-lying bars in the
middle of the site. As the flow stage increased at the site in response to increased
flow release from Fort Peck Reservoir and increased flow resistance resulting
from thickening of a fixed ice cover, the ice cover cracked around the bars,
floated above the bars, then refroze over the bars. This change in ice cover eleva-
tion further altered the flow distribution through the site.
Ice cover thickness
The ice cover progressed upstream through the site during 20 and 21 Decem-
ber. It first covered the north subchannel, then eventually covered the other
subchannels. By the time of the January survey, the ice cover varied in thickness
from about 12 to 23 in. It was thickest (about 2023 in.) over the north subchan-
nel between the cross sections at RM 1646.72 and RM 1678. The cover was
thinnest in the wake of a medial bar at the upstream cross section. The cover
thickened between January and February, reaching about 2024 in. thick over
most of the site and 2428 in. thick along the downstream cross section.
Bed profiles
The riverbed through the site actively adjusted up and down in elevation at
each of the cross sections surveyed. However, the bed profiles taken together
reveal only a disjointed image of the changes in subchannel size and alignment
and inadequately reflect the dynamic nature of bed responses to cover formation
and changes in flow rate. The bed profiles at RM 1646.78 (Fig. 67) show a
variety of elevation changes, which, considered at any vertical transect, quickly
become complex. Perhaps the changes are best interpreted as evidence of the
significant amounts of bed sediment moved by flow under the ice cover.
The cross sections at RM 1646.84 were taken once the ice cover had formed
at the site. It was decided that an intermediate cross section was needed to track
the south subchannel. Measurements at RM 1648.84 revealed that the south
subchannel had indeed enlarged subsequent to ice cover formation through the
site. When the cover was present and immediately following breakup, the thal-
weg bed elevation dropped about 5 ft.
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