Geoscience Reference
In-Depth Information
incisional trend (e.g. de Wit, Marshall and Partridge,
2000) (Figure 12.6(a)). By contrast, in the more northerly
part of the interior, the seismically active Kalahari Basin
formed in a broad crustal downwarp and during the Ceno-
zoic has been gradually infilling with colluvial, lacus-
trine, fluvial and aeolian sediments (Thomas and Shaw,
1991). In this largely endoreic basin, dry valley networks
('mekgacha') and ephemeral single-thread, sandy allu-
vial rivers are the most widespread fluvial landforms (e.g.
Shaw, 1989; Nash, Thomas and Shaw, 1994). The basin
also hosts the extensive (
Tooth and McCarthy, 2007). The wetlands are sustained
by seasonal floods arising in the exogenous headwater
tributaries of the Okavango River and by local rainfall, and
are traversed by unusual, peat- and vegetation-lined mean-
dering, stable sinuous and straight sand-bed channels that
occur within local anastomosing and more extensive dis-
tributary networks (McCarthy and Ellery, 1998; Tooth and
McCarthy, 2004b). Along these channels, a combination
of Late Quaternary flow regime changes, subtle tectonic
activity (minor faulting and tilting) and autogenic changes
(channel sedimentation and vegetation encroachment; see
Figure 12.6(b)) has resulted in lateral activity (migra-
tion) and frequent avulsion, leading to large-scale shifts
in the distribution of water and sediment across the delta
(McCarthy and Ellery, 1998).
40 000 km
2
) Okavango Delta,
which has formed within a depression related to the south-
westerly extension of the East African Rift Systems and
incorporates prime examples of permanent and seasonal
wetlands in a dryland setting (McCarthy and Ellery, 1998;
∼
Box 12.2
Mixed bedrock-alluvial rivers on the South African Highveld
In subhumid to semi-arid northeastern South Africa, many perennial and intermittent rivers commonly meander
within floodplain wetlands up to 2 km wide. The upper Klip River, eastern Free State (Figure 12.7(a)), exemplifies
how the formation, distribution and character of these meanders and floodplain wetlands are strongly influenced by
local structural and lithological factors that tend to override the dryland climatic influences (Tooth
et al.
, 2002a).
Catchment geology consists of sedimentary rocks (sandstones, mudstones) of the Karoo Supergroup, which have
been extensively intruded by resistant dolerite sills and dykes ('barriers'). Where these barriers crop out in the
river bed, they form local baselevels for the river upstream, with the corollary that vertical erosion in the weaker
sedimentary rocks cannot proceed faster than vertical erosion of the channel bed at the downstream dolerite barrier.
Over short to medium timescales (decades to tens of thousands of years), dolerite erosion rates can be presumed
slow, and erosion in the upstream sandstone/shale valleys is restricted to lateral erosion down to the level of the
dolerite. This takes place as the river migrates across the valley floor, simultaneously planing the sedimentary
rocks underlying the channel bed and reworking floodplain alluvium (Figure 12.7(b)). Over time, this has widened
the valleys and created additional accommodation space for meanders, floodplain alluvium and the associated
wetlands. Floodplains are inundated during summer rainfall and flooding (Figure 12.2(b)), with topographic lows
(e.g. abandoned channels, oxbows, backswamps) retaining water the year round and forming significant areas
of wetlands in an otherwise seasonally dry environment. Luminescence dating of alluvial deposits in abandoned
channels indicates that the Klip River wetlands have formed, at a minimum, over the last 30 000 years, with
regional climatic fluctuations having had remarkably little influence on channel morphology or dynamics (Tooth
et al.
, 2007).
Similarly pronounced transitions from alluvial meandering reaches with floodplain wetlands in sandstone/shale
valleys to bedrock-influenced straighter reaches in dolerite valleys occur along many other dryland rivers in
the region (Tooth
et al.
, 2002a). Over long timescales (>10
5
years), meandering and bedrock planing in the
sandstone/shale valleys has resulted in lowering of the channel gradient in relation to gradient in the dolerite valleys
so that many rivers have strongly stepped long profiles (Figure 12.7(c)). Meanders and floodplain wetlands persist
in the low-gradient reaches as long as the downstream dolerites continue to form stable barriers, but ongoing river
incision means that the dolerites are eventually breached, either partially or fully, which leads to a fall in local
baselevel. When this happens, headward-retreating knickpoints migrate upstream into the sandstone/shale valleys,
leading to river incision, river straightening (Figure 12.7(d)) and channel-floodplain decoupling (Tooth
et al.
, 2004).
With the cessation of regular flooding, the former floodplain wetlands desiccate and become susceptible to erosion
by dongas (gullies), as is demonstrated along a number of rivers in the region (Figure 12.7(d)). These findings
show that in this part of South Africa, dryland river process, form and behaviour depends largely on the interaction
