Geoscience Reference
In-Depth Information
10.5 Suspension load, bed load and river metamorphosis
River channel patterns have been the focus of considerable study (Fabre,
1797
;Leo-
pold and Wolman,
1957
; Leopold et al.,
1964
; Gregory and Walling,
1973
; Gregory,
1977
; Schumm,
1977
; Anderson et al.,
1996
). Three main patterns were identified
early on - meandering, braided and straight. To these we may add
anabranching
,
anastamosing
and
distributary
channels. Meandering channels are sinuous in plan
form, relatively deep and narrow, with cohesive beds and banks and gentle gradients.
They carry most of their load in suspension. Braided channels, on the other hand,
tend to be wide and shallow, with non-cohesive beds and banks, frequent mid-channel
bars, and are relatively steep. In contrast to the meandering suspension-load channels,
braided channels typically transport most of their sediment load along the channel bed
and are therefore termed bed-load, or traction-load, channels. Mixed-load channels,
as the name implies, transport roughly equal proportions of bed-load and suspended
load. An anabranching channel is one that leaves the parent channel at some point
along its lower course to rejoin it tens of kilometres further downstream (Nanson
and Knighton,
1996
). Such channels are common in the lower White Nile Valley of
central Sudan and in the lower Darling River in semi-arid New South Wales, Aus-
tralia. Anastamosis is an extreme form of anabranching, involving multiple channels.
The Channel Country of south-west Queensland is a classic example of anastamosing
channels during times of extreme flooding; so too are the Sudd Swamps of South
Sudan and the Okavango 'inland delta' during flood. The 'inland delta' of the Niger
is another example, complicated further by partially submerged dunes. Distributary
channel patterns are characteristic of both deltas and alluvial fans and are diagnostic
of all
distributary fluvial systems
(Weissmann,
2010
), regardless of regional climate.
The early empirical studies of small American rivers (Leopold and Wolman,
1957
;
Leopold et al.,
1964
; Schumm,
1977
) demonstrated that river channel pattern and form
were closely related to the type of sediment transported, and changed in response to
changes in sediment supply, sediment calibre and rainfall regime. Changes in the
amount and particle size of sediment transported are controlled by events upstream,
especially changes in vegetation type and cover, as noted by Bull (
1997
). Such changes
in plant cover and precipitation will in turn control the ratio of load to discharge within
the stream channel, as well as the calibre of material ferried from hill slope to river
channel. Where the vegetation cover in the upper reaches of desert rivers is relatively
sparse and the rainfall regime is prone to sporadic, highly intense and often very
local downpours, the rivers will most commonly display a braided channel pattern
and carry a sizeable traction load of coarse and non-cohesive cobbles and even quite
large boulders. Because stream power is roughly proportional to velocity cubed and
particle size carried in traction is proportional to about the sixth power of stream
velocity, desert flash floods are highly efficient agents of erosion. The ephemeral
stream channels change course frequently once they have left the confines of narrow
