Geology Reference
In-Depth Information
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a
()
b
()
c
()
d
Pool
Riffle
Direction of flow
Point bar
Erosion
Figure 9.6
Pool-and-riffle sequences in river channels. (a) Alternating zones of channel erosion and accretion in
response to faster and slower flow. (b) Pool spacing influencing the evolution of a straight channel into a meandering
channel. (c) Additional pools form as the meandering channel lengthens. (d) Development of meandering channel
with pools and riffles.
Source:
Adapted from Dury (1969)
alluvial flats rise gently to the valley sides. The concave
form is encouraged by a small floodplain area that is liable
to continual reworking by the stream. Most medium-
sized rivers, and many major rivers, have flat floodplains
formed chiefly by lateral accretion (sedimentation on the
inside of meander bends). Flat floodplains may also form
by alluviation in braided streams.
thought that deposition was induced by a break of slope
in the stream profile at the fan apex, but it has been
shown that only rarely is there a break of slope at that
point. The steepness of the fan slope depends on the size
of the stream and the coarseness of the load, with the
steepest alluvial fans being associated with small streams
and coarse loads. Fans are common in arid and semi-arid
areas but occur in all climatic zones. They range greatly
in size. Some in Queensland, Australia, are plain to see
on topographic maps or satellite images, but cannot be
recognized on the ground because they have radii of about
100 km and are so flat.
Alluvial fans
An
alluvial fan
is a cone-shaped body that forms where a
stream flowing out of mountains debouches on to a plain
(Plate 9.7). The alluvial deposits radiate from the
fan
apex
, which is the point at which the stream emerges
from the mountains. Radiating channels cut into the fan.
These are at their deepest near the apex and shallow with
increasing distance from the apex, eventually converging
with the
fan surface
. The zone of deposition on the fan
runs back from the break of slope between the fan surface
and the flat land in front of the
fan toe
. It was once
Playas
Playas
are the flattest and the smoothest landforms on
the Earth (Plate 9.8). A prime example is the Bonneville
salt flats in Utah, USA, which is ideal for high-speed
car racing, although some playas contain large desicca-
tion cracks so caution is advised. Playas are known as
