RIVER ICE INFLUENCES ON FORT PECK REACH, MISSOURI RIVER
77
Strictly speaking, subchannel switching comprises a stochastic dynamic
process that may have a recurrence interval or be narrow-banded about a domi-
nant period (e.g., a certain number of winters). The narrow band may be super-
imposed on a broad band because of the variability of flow conditions during a
year or during ice cover formation, the severity of winter weather, or other
factors, which may have a long-term effect.
A possible complicating factor in interpreting the bed profile changes is the
presence of dunes. The profiles were developed from point measurements across
each cross section. No measurements were made orthogonally to the cross sec-
tions. Such measurements would enable estimation of typical bed form heights.
Nonetheless, the bed profile changes at the cross sections are concordant and do
indicate consistent overall trends.
Tveit-Johnson site, RM 16241626
The Tveit-Johnson site was subject to multiple hydraulic and geomechanical
influences of river ice, which dramatically accelerated erosion along a portion of
the site's north bank and thereby hastened lateral shifting of the channel's single
thalweg through the site. Besides the hydraulic influences of river ice at this site,
the river ice also affected the bank's stability.
Observations
The Missouri River at this site forms a sinuous loop that is migrating down-
stream. The principal hydraulic influence of ice cover formation through the site
is to aggravate the already unstable alignment of the river's thalweg relative to
the north bank. The bank's migration is hastened because the river's thalweg is
not aligned with the bank but sweeps across the riverbed and impinges against
the north bank, as is evident in Figure 18, which shows the site during open water
conditions. Figure 55 shows the site prior to ice cover formation, following
freeze-up, during ice cover formation, and after ice cover breakup.
Observations suggest that river ice formation accelerates the downstream
migration of the meander loop, apparently by concentrating the flow toward the
apex of the loop's eroding outer bank. Flow velocities along the bank's toe are
increased, as is erosion of bank toe material.
Additionally the ice cover structurally loaded the bank and likely influenced
its freezethaw behavior by affecting the elevation of the water table in the bank.
Before the January survey a segment of bank in the vicinity of the bend apex
collapsed through the ice cover (Fig. 56).
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