Southern Africa is currently a region of remarkable geological stability. The most recent mountain building event culminated in the Cape Fold Belt some 330 Ma (McCarthy and Rubidge 2005), followed by the creation of a massive basaltic outpouring 180 Ma during the break-up of Gondwana. However, since these events, tectonic movement has been limited to mid-ocean ridges that completely enclose the sub-continent. As such, topographic relief over southern Africa is largely related to a series of uplift events 20 and 5Ma that resulted in uplift on the eastern seaboard by 250 and 900m respectively (Partridge and Maud 1987). Uplift on the western seaboard was comparatively less, approximately 150m and 100m at 20 and 5Ma respectively, creating a large westward sloping plateau (Figure 1).
Figure 1: Topographic cross-section of southern Africa, from Port Nolloth (PN) on the west coast, to Durban on the east coast (DBN). Drawn from a 90m resolution Digital Elevation Model provided by NASA (2000).
Glaciation in southern Africa last occurred when the region was located over the South Pole between 286 and 245 Ma (Viljoen and Reimold 1999). However, no topographic features remain of this ancient glacial event as tillite and overlying sediments have subsequently been lithified and eroded. In contrast, many other regions of the globe have recently been shaped by glacial planing of the land surface, resulting in numerous
shallow lakes and a flattened topography that is conducive to wetland formation (e.g.
Brinson 1993, Galatowitsch and van der Valk 1994).
The lack of tectonic activity and recent glaciation on the sub-continent suggests that the most important aspect in current landscape evolution is that of Miocene uplift, which has resulted in the continent entering a period of long-term incision, with the subsequent development of an extremely well integrated drainage network.
Climatically, the development of wetlands in southern Africa may be considered rather unlikely. Mitsch and Gosselink (2000) place a high emphasis on the existence of a positive water balance when describing the global distribution of wetlands. Southern Africa has a low mean annual rainfall of 486mm/a compared to the global mean for continental areas of 900mm/a (Schulze 1997). In addition, potential evapotranspiration is also high, with the result that much of the region experiences a negative water balance (Figure 2). Thus, local rainfall is generally insufficient to sustain wetlands, which therefore must rely on groundwater and/or surface inputs to some extent. In contrast, many wetlands in northern temperate settings occur as a consequence of a water balance in which precipitation far exceeds potential evapotranspiration. In many cases, wetlands of these regions are caused by a combination of the topographic impact of recent glaciation and a positive water balance (e.g. Galatowitsch and van der Valk 1994).
Considering southern Africa’s climate, which generally results in a negative water balance, and its position in a long-term cycle of incision as a result of Miocene uplift, it appears that the region should not be characterized by many wetlands at all. The macro-scale analysis of climate and geomorphology of the region suggests two things about wetland formation on the sub-continent. Firstly, for a wetland to receive sufficient water, it should be located on a drainage network such that it may receive surface and/or groundwater inputs and the water balance may therefore be locally positive for all or part of the year (Tooth and McCarthy 2007). Secondly, it must be located in a local region where incision of the drainage network has been momentarily paused.
Figure 2: Annual water balance for South African quaternary (forth-order) catchments (mean annual precipitation – mean annual potential evaporation), calculated from Schulze (1997).
The aim of this paper is to further our understanding of the geomorphic origin and evolution of palustrine wetlands that are integrated within the southern African fluvial network. Since the emphasis of this paper is on wetlands that are reliant on surface and/or groundwater inputs, the analysis is focused on Brinson’s (1993) valley-bottom and floodplain hydrogeomorphic wetland types, which have been adapted for South Africa by Kotze et al. (2005) and Ewart-Smith et al. (2006). Under these classification systems, both valley-bottom and floodplain wetlands receive the majority of their water from the movement of water down the drainage network, rather than through local rainfall. In the case of floodplains, water inundates the valley through the overtopping of the main channel. The active migration of the main channel in such a setting, combined with occasional overtopping, results in the formation of characteristic features such as levees, oxbow lakes, scroll bars and alluvial fill. In contrast, valley- bottom wetlands may be channelled or unchanneled, suggesting variation in water inputs from surface channels, surface run-off and groundwater. Where there is a
channel in a valley-bottom wetland, features indicative of lateral migration of the channel are absent.
Nevertheless, both floodplain and valley-bottom wetlands are considered zones of deposition characterized by net accumulation of sediment (Kotze et al. 2005). It is this occurrence of deposition on a drainage line that is particular to the formation of valley- bottom and floodplain wetlands in southern Africa. Not only does deposition allow the formation of a particular geohydrological setting that is conducive to the development of wetland systems, but it also causes adjustments to the longitudinal profile of the drainage line on which the wetland happens to occur. As sediment accumulates at a local base level, slope is reduced on the upstream end of the deposition node, while conversely, slope is steepened at the wetland system’s toe. It is the effect of deposition, and its link to wetland formation, that indicates the usefulness of the geomorphic threshold approach in understanding the geomorphic origin and persistence of wetlands.