RIVER ICE INFLUENCES ON FORT PECK REACH, MISSOURI RIVER
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south channel, bed elevations had risen locally by as much as 15 ft (cross section
RM 1620.35).
The bed along the north subchannel remained in the lowered state through
the February survey but had partially infilled by the time of the April survey.
Several explanations can be offered for the partial recovery in bed elevation of
the subchannel:
Some sediment scoured from the upstream end of the subchannel may
have deposited along the lower end of the subchannel as the subchannel
widened and the flow velocities in it decreased;
At the lower end of the medial bar separating the two subchannels, the
rise in bed elevation of the south subchannel could have spilled sediment
back into the lower end of the north subchannel;
When open water conditions were re-established, the enlarged north
subchannel may have drawn proportionally more bed load sediment than
water into the south subchannel; the sediment transport capacity of the
flow may not match the imposed bed load;
The overall sediment load entering the river may have increased with the
increase in snowmelt runoff and overall warming of the watershed during
and after ice cover breakup; and
Clumps of bank material from the collapsed riverbanks may have
provided a source of sediment to be transported by the flow, and the open
water velocities may have been insufficient to convey all the sediment
through enlarged areas eroded while the ice cover was in place.
Clearly, evidence confirming these explanations is fragmentary and the fac-
tors themselves still are conjectural, requiring further investigation.
Flow velocities
The January and February velocity profiles shown in Figure 48 are from
about the middle of the north channel at ice station 1+00 with respect to the north
bank. The plotted October and April open water velocity measurements are from
the closest locations, which were 22 and 10 ft respectively away from the loca-
tion for the winter measurements. The excessive bending of the fiberglass rods
used during the January survey prevented measuring the velocity in the unex-
pectedly deeper and faster water.
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