Page 239 - Fundamentals of Geomorphology

222 PROCESS AND FORM

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