Geology Reference
In-Depth Information
Meander
wavelength
Mean radius of curvature
Concave bank
Convex
bank
Wave
amplitude
Point of
inflection
Axis of
bend
Meander
belt axis
Figure 9.3 Parameters for describing meanders.
Meanders may be defined by several morphological
parameters (Figure 9.3). Natural meanders are seldom
perfectly symmetrical and regular owing to variations
in the channel bed. Nonetheless, for most meander-
ing rivers, the relationships between the morphometric
parameters give a consistent picture: meander wavelength
is about ten times channel width and about five times the
radius of curvature.
Meandering is favoured where banks resist erosion, so
forming deep and narrow channels. However, why rivers
meander is not entirely clear. Ideas centre on: (1) the
distribution and dissipation of energy within a river;
(2) helical flow; and (3) the interplay of bank erosion,
sediment load, and deposition. A consensus has emerged
that meandering is caused by the intrinsic instabilities of
turbulent water against a movable channel bank.
phases of degradation and aggradation in the channel
and militates against vegetation establishment. Some
braided rivers have twenty or more channels at one
location.
Braided channels tend to form where (1) stream energy
is high; (2) the channel gradient is steep; (3) sediment
supply from hillslopes, tributaries, or glaciers is high and
a big portion of coarse material is transported as bed
load; and (4) bank material is erodible, allowing the
channel to shift sideways with relative ease. They are com-
mon in glaciated mountains, where channel slopes are
steep and the channel bed is very gravelly. They form in
sand-bed and silt-bed streams where the sediment load is
high, as in parts of the Brahmaputra River on the Indian
subcontinent.
Anastomosing channels
Braided channels
Anastomosing channels have a set of distributaries
that branch and rejoin (Plate 9.4). They are suggestive
of braided channels, but braided channels are single-
channel forms in which flow is diverted around obstacles
in the channel, while anastomosing channels are a set
of interconnected channels separated by bedrock or by
stable alluvium. The formation of anastomosing channels
is favoured by an aggradational regime involving a high
suspended-sediment load in sites where lateral expansion
is constrained. Anastomosing channels are rare: River
Feshie, Scotland, is the only example in the UK.
Braided channels (Plates 9.2 and 9.3) are essentially
depositional forms that occur where the flow divides
into a series of braids separated by islands or bars of
accumulated sediment (see Best and Bristow 1993). The
islands support vegetation and last a long time, while
the bars are more impermanent. Once bars form in
braided rivers, they are rapidly colonized by plants, so
stabilizing the bar sediments and forming islands. How-
ever, counteracting the stabilization process is a highly
variable stream discharge, which encourages alternate
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