Floodplain wetland formation and avulsion in the semiarid Tshwane-Pienaars Catchment, South Africa

Controls on floodplain wetland formation and river avulsion (channel relocation on the floodplain) are diverse and can include external (e.g. geology, climate) or internal (e.g. sediment deposition, erosion) drivers. Using a range of techniques including mapping historical channel change, morphological analysis of downstream channel characteristics and optically simulated luminescence dating of recently deposited floodplain sediment, this thesis investigates floodplain wetland formation and avulsion in the Tshwane-Pienaars Catchment in northern South Africa. The broadest scale control on river character and wetland development is catchment lithology, which affects floodplain width and accommodation space for unconfined alluvial reaches and floodplain wetlands. Downstream declines in discharge account for the overall morphology and behaviour of the Tshwane and Pienaars Rivers and their wetlands. Downstream channel diminution due to declines in discharge leads to aggradation of levees and alluvial ridges, and lateral adjustment occurs by channel migration and avulsion on the lower reaches of both rivers. Avulsion of the Tshwane River occurs autogenically during meander belt development in response to reductions in sediment transport capacity. The Tshwane River has a high avulsion frequency ( ̃4.6 avulsions ka⁻¹) and a high vertical overbank sedimentation rate ( ̃11 mm a⁻¹), which provides a well constrained dataset to demonstrate that sedimentation rate and avulsion frequency are correlated.