Geoscience Reference
In-Depth Information
Flow subsides into a series of distributary channels with
lower total roughness, and individual bars migrate
downstream. Rivers draining the Himalayas and New
Zealand's Southern Alps may spread 5-20 km wide with
width-to-depth ratios of 250-300 : 1 or more. Where
vegetation stabilizes bars exposed for long intervals
between submergences, redefining them as islands, the
braided channel is said to be
anabranching
.
indicative of lateral accretion. They form a level floor as
meandering channels cut into older deposits and
rework
them as back-fill into abandoned channels, mostly as bed
load. New channels form more dramatically through
avulsion
, where rivers break through old channel seg-
ments. Vertical aggradation buries and thereby envelops
older deposits, mostly by suspended sediment, and
develops a convex floor
falling
away from the channel.
Indeed, raised banks or levées may form as coarser debris
is deposited alongside the channel, rendering low-lying
areas poorly drained. Levées 10
1-3
cm high raise the thres-
hold of the next overbank flood. Water cuts
crevasses
at
low or weak points to regain the channel, restarting the
process with
crevasse splays
. The composite, three-
dimensional floodplain landsystem is shown in
Figure
sea via an estuary or delta (see
Chapter 17).
The flood plain
The third and largest scale of dynamic sedimentary
environments is the
flood plain
, which loosely describes
the valley floor prone to episodic overbank discharge. In
another departure from classic fluvial landsystems, narrow
flood plains with chemically and texturally raw sediments
occur in pockets within mountain catchments. Lowland
flood plains of more mature sediments are far more
horizontally extensive, developed by lateral accretion in
meandering or braided rivers and vertical accretion
through overbank discharge. Both forms usually occur
together, and developmental history is seen in the
array of abandoned channel forms on flood plain surfaces
and stratigraphic exposures 10
1-3
CONCLUSION
Flowing water and its geomorphic activity have been
central to human life since prehistoric hunters exploited
their prey concentrations around riparian watering holes.
Alluvial sediments are important archaeological sources
m thick in palaeo-
environmental sediments.
structures are
Cut-and-fill
(a)
River terrace
or valley berm
Anabranching
(braided)
channels
Avulsion
Levée
Crevasse
splay
Oxbow lake
Buried
channel
Channel fill
Flood-plain
sediments
Backswamp
(b)
Levée
Flood sand
and gravel
Flood silt
and clay
Bars
Terrace
Cut-and-fill
deposits
Figure 14.31
Floodplain (a) morphology and
(b) stratigraphy.
Buried channels
