Page 251 - Fundamentals of Geomorphology

234 PROCESS AND FORM

a
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b

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c

d
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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 thought that deposition was induced by a break of slope
form is encouraged by a small floodplain area that is liable in the stream profile at the fan apex, but it has been
to continual reworking by the stream. Most medium- shown that only rarely is there a break of slope at that
sized rivers, and many major rivers, have flat floodplains point. The steepness of the fan slope depends on the size
formed chiefly by lateral accretion (sedimentation on the of the stream and the coarseness of the load, with the
inside of meander bends). Flat floodplains may also form steepest alluvial fans being associated with small streams
by alluviation in braided 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
Alluvial fans on topographic maps or satellite images, but cannot be
An alluvial fan is a cone-shaped body that forms where a recognizedonthegroundbecausetheyhaveradiiofabout
stream flowing out of mountains debouches on to a plain 100 km and are so flat.
(Plate 9.7). The alluvial deposits radiate from the fan
apex, which is the point at which the stream emerges Playas
from the mountains. Radiating channels cut into the fan.
These are at their deepest near the apex and shallow with Playas are the flattest and the smoothest landforms on
increasing distance from the apex, eventually converging the Earth (Plate 9.8). A prime example is the Bonneville
with the fan surface. The zone of deposition on the fan salt flats in Utah, USA, which is ideal for high-speed
runs back from the break of slope between the fan surface car racing, although some playas contain large desicca-
and the flat land in front of the fan toe. It was once tion cracks so caution is advised. Playas are known as