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ERDC/CRREL TR-02-14
midchannel bars that divide the flow into several subchannels under normal flow
conditions. At flood flow and sometimes under ice-covered flow, the bars are
under water. The wider subreach is followed by another narrower and curved
subreach. Figure 2 illustrates typical subreaches. The narrower subreaches com-
monly occur where a bluff deflects the flow, as in Figure 2a. In these subreaches
the river is similar in morphology to a sinuous point-bar channel. In other
subreaches (Fig. 2b), the river begins to assume the characteristics of a braided
channel. Figure 3 illustrates how two subreaches typically merge. The river is
constrained at its upstream and downstream ends by bluffs, while over the middle
subreach the river widens and flows in several subchannels around a vegetated
island and bars.
A feature of the wider subreaches is the switching of the thalweg position
between subchannels. Such switches can be triggered by several factors, includ-
ing a large change in flow magnitude, a change in orientation of flow entering the
subreach, and (as revealed in this study) ice cover formation.
The variability of the river's channel morphology between two subreach
types makes channel morphology unstable where the two subreaches merge.
Usually at those locations the river's thalweg alignment does not coincide with
the alignment of riverbanks. Instead, it practically impinges against a riverbank.
Those locations are especially vulnerable to riverbank erosion.
Bed sediment
The sediments comprising the channel bed of the Fort Peck reach are mainly
sands with zones of gravel deposits, exposed clays, and silts. USACE-Omaha
(1986), among other reports, describes the nature and disposition of bed sediment
along the reach.
In the subreach immediately downstream of Fort Peck Dam the bed sedi-
ments are coarser than farther downstream, and they reflect the overall degrada-
tion in the upstream portion of the reach. The subreach evidently exhibits bed
armoring, where the upper surface of the bed is armored with coarser sediment
less readily entrained by flow. For the extent of the Fort Peck reach encompassed
by the survey (RM 1717 to RM 1620), the riverbed is formed mainly of fine to
medium-size sand, with the median particle diameter in the range of 0.20.3 mm.
Occasional patches of coarser sediment occur along the reach. Also, zones of silts
and clay sediments are exposed along the riverbanks. Figure 4 indicates the trend
in median diameter of bed sediments (USACE-Omaha 1986).
The sediment load conveyed by the river comprises bedload sediment whose
source is the eroding riverbanks and shifts in the channel bed. The average
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