Geoscience Reference
In-Depth Information
Prior to the start of European colonisation in the seven-
teenth and eighteenth centuries, human impact on dryland
rivers across southern Africa was minimal, but has greatly
intensified since, particularly over the last 100-150 years.
Land-cover and land-use changes have included reduction
or removal of native fauna (especially grazing ungulates)
and replacement with cattle and sheep, introduction of
exotic invasive riparian vegetation (e.g. willows) and di-
rect channel-floodplain modifications (e.g. drainage, di-
version, channelisation). Unmodified flow regimes tend
to be highly variable on a quasi-decadal basis, alternat-
ing between wet periods with occasional very large, de-
structive floods and periods with below-average flows.
Late Holocene flood records have been reconstructed from
flood deposits (including slackwater sediment sequences)
along a number of rivers (e.g. Zawada, 1997; Srivas-
tava
et al.
, 2006; Eitel
et al.
, 2006), and the geomorphic
impacts of large historical floods have also been docu-
mented from various locations (e.g. Zawada, 1994). Along
mixed bedrock-alluvial anabranching rivers in the Kruger
National Park, northeastern South Africa, several floods
over the past 100 years have periodically stripped alluvium
down to the underlying bedrock in certain reaches, but is-
land and floodplain rebuilding occurs in association with
vegetation establishment and growth during subsequent
lower flow periods (Heritage, van Niekerk and Moon,
1999; Rountree, Rogers and Heritage, 2000; Rountree,
Heritage and Rogers, 2001). Other rivers, however, have
been less sensitive even to large historical floods; for in-
stance, the Orange River near Augrabies Falls, western
South Africa, experienced only modest erosion and de-
position even during a large (8300 m
3
/s) flood in 1988
(Zawada and Smith, 1991) and the channels in the Oka-
vango Delta at present tend to undergo only slow progres-
sive change even during the large seasonal floods (Mc-
Carthy and Ellery, 1998).
Overall, many southern African rivers only partly match
the 'textbook' generalisations regarding dryland rivers.
In western Namibia, moderate-gradient, poorly vegetated
catchments with relatively strong slope-channel coupling
and moderately abundant sediment supply have promoted
a variety of confined bedrock and alluvial braided rivers
(Jacobson, Jacobson and Seeley, 1995) with morpho-
logical and sedimentological similarities to rivers in the
Mediterranean and the American southwest. Many single-
thread, planar-bed, sandy rivers in the Kalahari basin and
adjacent areas (e.g. Shaw, 1989; Hassan, Schick and Shaw,
1999) exhibit morphological characteristics common to
many broad, wide sandy channels described from the
American southwest, Middle East and East Africa (e.g.
Wolman and Gerson, 1978; see also Chapter 13). The
gradient catchments with variable slope-channel cou-
pling and riparian vegetation assemblages, where a vari-
ety of river styles not commonly associated with drylands
has developed, including various mixed bedrock-alluvial
styles (e.g. structurally and lithologically controlled me-
andering and anabranching) as well as extensive flood-
plain wetlands (Figures 12.2(a), (b) and 12.6(a); see also
Box 12.2). Although some rivers are subject to major
(even catastrophic) change during occasional large floods,
channel recovery tends to take place on a timescale of
years to decades, so that rivers may readjust towards and
possibly attain equilibrium before the next large flood
(e.g. Rountree, Rogers and Heritage, 2000). Other rivers
appear to be relatively insensitive to large flood events
and can be viewed as being in short-term equilibrium,
even on landscapes that they are slowly eroding or ag-
grading over longer timescales (e.g. McCarthy and Tooth,
2004).
12.3.3
Lower energy dryland rivers: Australia
With the exception of Antarctica, Australia is regarded
as the driest continent, both in terms of the relative ex-
tent of its arid and semi-arid areas, which cover between
two-thirds and three-quarters of the continent, and its low
mean continental precipitation and runoff (Mabbutt, 1986;
Warner, 1986; Finlayson and McMahon, 1988). Extensive
subhumid areas surround the arid/semi-arid continental
interior, and even the relatively well-watered southern
and eastern margins of the continent are subject to highly
variable climates that include long periods of drought.
In the seasonal (semi-arid) tropics of the north, summer
wet seasons are followed by a 7 to 9 month long, warm,
dry season so that conditions for part of the year are also
similar to many dryland areas. The easterly Great Di-
viding Range passes westward into extensive, low-relief,
low-gradient plains, topographic depressions and exten-
sive aeolian dunefields that are broken only by isolated,
low elevation (mostly
1000 m a.s.l.) ranges. Differ-
ent types of moderate- to low-gradient river system can
be found across the Australian drylands, including: (1)
rivers associated with the exoreic Murray-Darling sys-
tem, southeastern Australia; (2) rivers associated with
the endoreic drainage in the Lake Eyre basin, central
Australia; and (3) rivers associated with isolated ranges
and extensive shield areas such as characterise parts of
South Australia and Western Australia. Endogenous inter-
mittent and ephemeral rivers are commonplace but Aus-
tralia's largest dryland rivers are partly exogenous (e.g.
Cooper Creek, Murray-Darling), receiving regular (sea-
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