Geology Reference
In-Depth Information
However, some rivers, such as many in Africa, flow in
alluvium in their upper reaches and cut into bedrock in
the lower reaches (cf. p. 108). Bedrock channels are not
well researched, with most attention being given to such
small-scale erosional features as scour marks and pot-
holes in the channel bed. The long profiles of bedrock
channels are usually more irregular than the long pro-
files of alluvial channels. The irregularities may result
from the occurrence of more resistant beds, from a down-
stream steepening of gradient below a
knickpoint
caused
by a fall of baselevel, from faulting, or from landslides
and other mass movements dumping a pile of debris
in the channel. Rapids and waterfalls often mark their
position.
Given that many kinds of bedrock are resistant to ero-
sion, it might seem improbable that bedrock channels
would meander. However, incised meanders do form in
horizontally bedded strata. They form when a meander-
ing river on alluvium eats down into the underlying
bedrock.
Intrenched meanders
, such as those in the
San Juan River, Utah, USA, are symmetrical forms and
evolve where downcutting is fast enough to curtail lat-
eral meander migration, a situation that would arise
when a large fall of baselevel induced a knickpoint to
migrate upstream (Colour Plate 10, inserted between
pages 208 and 209.
Ingrown meanders
are asymmet-
rical and result from meanders moving sideways at
the same time as they slowly incise owing to regional
warping. A
natural arch
or
bridge
forms where two lat-
erally migrating meanders cut through a bedrock spur
(p. 207).
Springs sometimes cut into bedrock. Many springs
issue from alcoves, channels, or ravines that have been
excavated by the spring water. The 'box canyons' that
open into the canyon of the Snake River in southern
Idaho, USA, were cut into basalt by the springs that now
rise at the canyon heads.
supply, and other factors change because alluvium is
normally unable to resist erosion to any great extent.
In plan view, alluvial channels display four basic forms
that represent a graded series - straight, meandering,
braided, and anastomosing (Figure 9.1a). Wandering
channels are sometimes recognized as an intermedi-
ate grade between meandering channels and braided
channels. Anabranching channels are another category
(Figure 9.1b).
Straight channels
These are uncommon in the natural world. They are usu-
ally restricted to stretches of V-shaped valleys that are
themselves straight owing to structural control exerted by
faults or joints.
Straight channels
in flat valley-floors are
almost invariably artificial. Even in a straight channel, the
thalweg (the trace of the deepest points along the chan-
nel) usually winds from side to side, and the long profile
usually displays a series of deeper and shallower sections
(pools and riffles, p. 233) much like a meandering stream
or a braided stream.
Meandering channels
Meandering channels
wander snake-like across a flood-
plain (Plate 9.1; see also Plate 9.9). The dividing line
between straight and meandering is arbitrarily defined
by a sinuosity of 1.5, calculated by dividing the chan-
nel length by the valley length. Water flows through
meanders in a characteristic pattern (Figure 9.2). The
flow pattern encourages erosion and undercutting of
banks on the outside of bends and deposition, and the
formation of point bars on the inside of bends. The
position of meanders changes, leading to the alteration
of the course through cut-offs and channel diversion
(avulsions).
Avulsions
are the sudden change in the
course of a river leading to a section of abandoned
channel, a section of new channel, and a segment of
higher land (part of the floodplain) between them.
Meanders may cut down or incise. Colour Plate 10 shows
the famous incised meanders of the San Juan River,
southern Utah, USA.
Cut-off incised meanders
may
also form.
Alluvial channels
Alluvial channels
form in sediment that has been, and is
being, transported by flowing water. They are very diverse
owing to the variability in the predominant grain size of
the alluvium, which ranges from clay to boulders. They
may change form substantially as discharge, sediment
