Channel patterns
Straight channels
Straight
stream channels are rare. Where they do occur, the channel
is usually controlled by a linear zone of weakness in
the underlying rock, like a fault or joint system. Even
in straight channel segments water flows in a sinuous
fashion, with the deepest part of the channel changing
from near one bank to near the other. Velocity is highest
in the zone overlying the deepest part of the stream.
In these areas, sediment is transported readily resulting
in pools. Where the velocity of the stream is low, sediment
is deposited to form bars. The bank closest to the zone
of highest velocity is usually eroded and results in
a cutbank.
Meandering channels
Because
of the velocity structure of a stream, and especially
in streams flowing over low gradients with easily eroded
banks, straight channels will eventually erode into meandering
channels. Erosion will take place on the outer parts
of the meander bends where the velocity of the stream
is highest. Sediment deposition will occur along the
inner meander bends where the velocity is low. Such deposition
of sediment results in exposed bars, called point bars.
Because meandering streams are continually eroding on
the outer meander bends and depositing sediment along
the inner meander bends, meandering stream channels tend
to migrate back and forth across their flood plain.
If
erosion on the outside meander bends continues to take
place, eventually a meander bend can become cut off from
the rest of the stream. When this occurs, the cutoff
meander bend, because it is still a depression, will
collect water and form a type of lake called an oxbow
lake.
Braided channels
In
streams having highly variable discharge and easily eroded
banks, sediment gets deposited to form bars and islands
that are exposed during periods of low discharge. In
such a stream the water flows in a braided pattern around
the islands and bars, dividing and reuniting as it flows
downstream. Such a channel is termed a braided channel.
During periods of high discharge, the entire stream channel
may contain water and the islands are covered to become
submerged bars. During such high discharge, some of the
islands could erode, but the sediment would be re-deposited
as the discharge decreases, forming new islands or submerged
bars. Islands may become resistant to erosion if they
become inhabited by vegetation,
