Page 242 - Fundamentals of Geomorphology

FLUVIAL LANDSCAPES 225

Meander Mean radius of curvature
wavelength
Concave bank

Convex
bank

Wave
amplitude Point of Axis of Meander
inflection bend belt axis
Figure 9.3 Parameters for describing meanders.

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