12
A river corridor can have more than one active channel where braided or
anabranching planforms exist, or where secondary channels are present. Braided
and anabranching planforms, generically referred to as multithread channel patterns, consist of multiple, subparallel channels that branch and rejoin downstream
(Fig. 2.2). Braided channels typically have minimally vegetated bars between subchannels. Individual sub-channels and bars continually change shape and position
during high flows. Mature forest can cover the floodplain between individual
anabranches, which commonly have much slower rates of lateral movement within
individual sub-channels than do braided channels.
Secondary channels are those that contain flowing water only during higher discharges. These channels are distinct from tributaries, which original elsewhere and
terminate in the main channel, and from distributaries on a delta or alluvial fan,
which originate in the main channel and then terminate on the delta or fan.
The main channel and any secondary channels present also determine the channel migration zone (Fig. 2.3), which is the width of the valley bottom across which
channels can migrate and have migrated under the contemporary flow regime.
Although the channel migration zone typically lies within the boundaries of the
floodplain, exceptionally large floods can cause the active channel to move beyond
the floodplain. Explicitly delineating a channel migration zone is a helpful reminder
that natural channels continually experience erosion along some portions of the
bank and deposition on point or alternate bars along other portions of the bank, as
well as abrupt lateral movements known as avulsions.
Fig. 2.1 Downstream view of mature Acacia trees in the active channel of the ephemeral Kuiseb
River, Namibia. Channel is approximately 90 m wide
2 Rivers as Ecosystems
A river corridor can have more than one active channel where braided or
anabranching planforms exist, or where secondary channels are present. Braided
and anabranching planforms, generically referred to as multithread channel patterns, consist of multiple, subparallel channels that branch and rejoin downstream
(Fig. 2.2). Braided channels typically have minimally vegetated bars between subchannels. Individual sub-channels and bars continually change shape and position
during high flows. Mature forest can cover the floodplain between individual
anabranches, which commonly have much slower rates of lateral movement within
individual sub-channels than do braided channels.
Secondary channels are those that contain flowing water only during higher discharges. These channels are distinct from tributaries, which original elsewhere and
terminate in the main channel, and from distributaries on a delta or alluvial fan,
which originate in the main channel and then terminate on the delta or fan.
The main channel and any secondary channels present also determine the channel migration zone (Fig. 2.3), which is the width of the valley bottom across which
channels can migrate and have migrated under the contemporary flow regime.
Although the channel migration zone typically lies within the boundaries of the
floodplain, exceptionally large floods can cause the active channel to move beyond
the floodplain. Explicitly delineating a channel migration zone is a helpful reminder
that natural channels continually experience erosion along some portions of the
bank and deposition on point or alternate bars along other portions of the bank, as
well as abrupt lateral movements known as avulsions.
Fig. 2.1 Downstream view of mature Acacia trees in the active channel of the ephemeral Kuiseb
River, Namibia. Channel is approximately 90 m wide
2 Rivers as Ecosystems