Distinctiveness and diversity of arid zone river systems - Arid Zone Geomorphology - page 272

bedrock-confined reaches have been used for more de-
tailed palaeoflood reconstructions, including palaeoflow
hydraulics, changes in flood magnitude and frequency,
and the relationship with atmospheric and hydrological
controls (e.g. Thorndycraft et al. , 2005; Greenbaum et al. ,
2006; Benito et al. , 2008) (Figure 12.4(b)). Many studies
have documented the impacts of historical moderate-to-
high energy floods on channels in the region (e.g. Harvey,
1984; Poesen and Hooke, 1997; Lopez-Bermudez,
Conesa-Garcıa and Alonso-Sarrıa, 2002; Greenbaum and
Bergman, 2006), with some channels having undergone
dramatic widening and/or deepening, impacts that in many
cases last for years or decades.
Overall, Mediterranean rivers exemplify many of the
'textbook' generalisations regarding dryland river char-
acteristics. Across much of the region, a legacy of tec-
tonic activity, lithological variations, eustatic fluctuations
and palaeoclimatic changes has created a rugged phys-
iography consisting of short, steep catchments or basin-
and range-type settings, with coastal lowlands relatively
restricted in extent. Strong slope-channel coupling pro-
motes abundant sediment supply to high-energy rivers.
River styles thus tend to be dominated by bedrock chan-
nels or coarse-grained alluvial channels (Macklin, Lewin
and Woodward, 1995). Many valleys preserve terrace se-
quences, and although floodplain development tends to
be limited, subtle changes in the nature of boundary resis-
tance and the calibre of sediment supply can be important
determinants of channel process, form and change (see
Box 12.1). Even with flow regulation and various forms
of channel modification, many rivers are subject to ma-
jor, long-lasting change during erratic, short-lived flash
floods, commonly exhibiting nonequilibrium behaviour
(e.g. Poesen and Hooke, 1997).

dissected piedmont and coastal lowland from the low re-
lief, elevated interior, much of which lies above 1000 m
a.s.l. (Summerfield, 1991). Several types of river system
can be identified, including: (1) moderate-size, moderate-
gradient rivers that drain seaward from the Great Escarp-
ment or coast-parallel ranges to the ocean; (2) longer,
moderate-gradient rivers associated with the extensive
(

2350 km long) Orange River system; and (3) moderate-
to low-gradient rivers of the endoreic Kalahari basin. En-
dogenous intermittent and ephemeral rivers are common-
place, but there are also large exogenous perennial rivers
(e.g. Orange and Okavango Rivers).
Various tectonic, structural, lithological and palaeo-
climatic factors have influenced the characteristics of
southern African dryland fluvial landscapes. Following
Late Mesozoic break-up of the Gondwana supercontinent,
large-scale uplift has driven long-term net river incision
and landscape denudation. Substantial thicknesses (

∼

∼

1-2
km) of basalts and sedimentary cover rocks have been re-
moved, leading to widespread river superimposition onto
underlying, typically more resistant lithologies. Along
with temporal and spatial variations in hillslope sediment
supply, many rivers have become closely adjusted to the
variable lithologies that they traverse, leading to a va-
riety of mixed bedrock-alluvial and bedrock styles, in-
cluding anabranching rivers (Figure 12.2(a)), meandering
rivers flanked by seasonally inundated floodplain wetlands
(see Figure 12.2(a) and Box 12.2) and ingrown meanders
(Figure 12.6(a)) (Heritage, van Niekerk and Moon, 1999;
van Niekerk et al. , 1999; Tooth et al ., 2002a; Tooth and
McCarthy, 2004a; McCarthy and Tooth, 2004). Some dry-
land river reaches are bedrock confined, most notably
along the lower Orange River (Wellington, 1958; Jacob,
Bluck and Ward, 1999) and in the headwaters of rivers
draining the Great Escarpment in Namibia (Jacobson, Ja-
cobson and Seeley, 1995; Codilean et al. , 2008). During
the Cenozoic, there has been a long-term trend towards
drier and more variable climates, and while the direct
effects of Quaternary glaciations and eustasy on most
dryland rivers has been minimal, palaeoclimatic fluctu-
ations involving conditions both drier and wetter than
present have impacted on the region. This has led to
the contraction of formerly more extensive drainage net-
works (e.g. Goudie and Wells, 1995; Tooth and McCarthy,
2007), particularly in western Namibia, where many for-
mer exoreic river systems now terminate in aeolian dune-
fields far from the coastline, forming saline pans and
playas (e.g. Seeley and Sandelowsky, 1974; Teller, Rut-
ter and Lancaster, 1990; Jacobson, Jacobson and Seeley,
1995). Sedimentary records along many of these dryland
rivers typically reveal phases of channel aggradation that

12.3.2 Moderate and lower energy dryland
rivers: southern Africa

Southern Africa can be defined as a region encompass-
ing all or part of the countries of South Africa, Lesotho,
Swaziland, Namibia, Botswana, Zimbabwe and Mozam-
bique. With the exception of a typically narrow (

150
km) humid zone around the southern and eastern margins
of the subcontinent, drylands are extensive. The interior is
characterised by warm, wet summers and cool, dry win-
ters and the western margins by hot, dry summers and
cooler, wetter winters, and the degree of aridity tends to
increase along an east-west gradient, from subhumid and
semi-arid in the eastern interior to arid and hyper-arid
in the west. The Great Escarpment roughly parallels the

<