Geological controls on alluvial river behaviour: a comparative study of three rivers on the South African Highveld

Abstract

Owing to tectonic readjustments following the breakup of Gondwana, many rivers in the subhumid to semi-arid South African Highveld are in a long-term state of incision, with their channel beds positioned on or just above bedrock. Comparison of the upper Klip River, the Schoonspruit and the Venterspruit, three adjacent left-bank tributaries of the Vaal River, demonstrates how dolerite sills and dykes form local base levels in the longitudinal profiles and thus exert strong, but variable, controls on river behaviour in the upstream, dominantly alluvial reaches. The upper Klip River flows largely over weakly cemented Karoo Supergroup sandstones but also crosses highly resistant dolerite dykes and sills. On the sandstones, vertical erosion rates are retarded by the slow erosion of the downstream dolerites, and in the interim the river is laterally planing bedrock and meandering extensively (sinuosity ( P) up to ∼1.75) within broad (up to 1.5 km wide) floodplains marked by oxbow lakes and abandoned channels. The floodplains host extensive wetlands that are in a near-pristine condition. The Schoonspruit flows largely over erodible Karoo Supergroup shales in its upper and middle reaches but also crosses a resistant dolerite sill in its lower reaches. On the shales, vertical erosion previously was retarded by the downstream dolerite, and the river laterally planed bedrock and carved a wide valley. The river formerly meandered ( P∼1.99) within a broad (up to 1 km wide) floodplain marked by oxbows but partial breaching of the dolerite has resulted in headward knickpoint migration, which in turn has led to incision into shale, abandonment and desiccation of the floodplain wetlands, and secondary erosion of the valley fills by dongas (gullies). The Venterspruit also flows largely over erodible Karoo Supergroup shales, apparently having completely breached a dolerite sill in its lower reaches. Vertical erosion has not been retarded to the same degree as on the Klip and the Schoonspruit, and the river follows a much straighter course ( P∼1.53), which is now deeply incised into bedrock, with little surviving evidence of floodplain wetlands, and with numerous dongas having formed in the valley fills. These three river reaches represent a temporal sequence whereby river superimposition first exposes and then breaches dolerite sills and dykes to create a natural cycle of floodplain wetland formation and destruction. Furthermore, depending on their location in the catchment vis-à-vis dolerite barriers, individual reaches of rivers will have responded differently to Quaternary climatic changes, thus resulting in strong diachroneity in the alluvial record. Recognition of the role exerted by dolerite on alluvial river behaviour in the Highveld thus has implications for interpretations of river and landscape response to environmental change, and also may provide key field data for theoretical and numerical models of long-term fluvial landscape development.