The meandering river consists of pools and crossings (Figure 5.14). The thalweg, or main
current of the channel, flows from the pool through the crossing to the next pool forming the
typical S-curve. In the pools, the channel cross-section is somewhat triangular. Point bars
form on the inside of the bends. In the crossings, the channel cross-section is more
rectangular and depths are smaller. At low flows, local slope is steeper and velocities are
larger in the crossing than in the pool. At low stages, thalweg is located very close to the
outside of the bend. At higher stages, thalweg tends to straighten, that is, thalweg moves away
from the outside of the bend encroaching on the point bar to some degree. In the extreme
case, the shifting of the current causes chute channels to develop across the point bar at high
stages. In Figure 5.14, one can observe the position of the thalweg, the location of the point
bars, alternate bars and the location of the pools and crossings. Note that in the crossing the
channel is shallow compared to pools and the banks may be more subject to erosion.
Figure 5.14 illustrates the change in water surface profile from low to high water discharge. At
low flow the water surface slope is steep in the crossing and flatter in the pool. The reverse is
true at higher discharges. At higher discharges the thalweg straightens, shortening the path of
travel and increasing the local friction slope. In the extreme case, river slope approaches the
valley slope at flood stage. It is during high floods that the flow often cuts across the point
bars, developing chute channels and a steeper channel prevails under this condition.
In general, bends are formed by the process of erosion and deposition. Erosion without
deposition to assist in bend formation would result only in scalloped banks. Under these
conditions the channel would simply widen until it becomes so large that the erosion would
terminate. The material eroded from the bank is normally deposited over a period of time on
the point bars that are formed downstream. The point bars constrict the bend and enable
erosion in the bend to continue, accounting for the lateral and longitudinal migration of the
meandering stream. Erosion is greatest across the channel from the point bar. As the point
bars build out from the downstream sides of the bars, the bends gradually migrate down the
valley. The point bars formed in the bendways clearly define the direction of flow. The bar is
generally streamlined and its largest portion is oriented downstream. If there is very rapid
caving in the bendways upstream, the sediment load may be sufficiently large to cause middle
bars to form in the crossing.
As a meandering river system moves laterally and longitudinally, the meander limbs move at an
unequal rate because of the unequal erodibility of the banks. This causes the channel to
appear as a bulb form, generally skewed in a downvalley direction. The channel geometry
depends upon the local slope, the bank material, and the geometry of the adjacent bends.
Over time the local steep slope caused by the cutoff is distributed both upstream and
downstream. Years may be required before a configuration characteristic of average
conditions in the river is attained.
When a cutoff occurs, an oxbow lake is formed (Figure 5.12a). Oxbow lakes may persist for
long periods of time before filling. Usually the upstream end of the lake fills quickly to bank
height. Overflow during floods carries fine materials into the oxbow lake area. The lower end
of the oxbow remains open and the drainage and overland flow entering the system can flow
out from the lower end. The oxbow gradually fills with fine silts and clays. Fine material that
ultimately fills the bendway is plastic and cohesive. As the river channel meanders it
encounters old bendways filled with cohesive materials (referred to as clay plugs). These plugs
are sufficiently resistant to erosion to serve as semipermanent geologic controls and can
drastically affect river geometry. The variability of bank materials, and the fact that the river
encounters such features as clay plugs, cause a wide variety of river forms in a meandering
river.
5.18
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