Implications of channel avulsion for the structure and functioning of semiarid floodplain wetlands: examples from South Africa

Floodplain wetlands in drylands have distinct characteristics that are a function of climatic conditions (low rainfall, high evapotranspiration), variable discharge, and complex processes of erosion and sedimentation that may include avulsion, the process whereby a channel abandons an old course in favour of a new course on the floodplain. Long-term (decadal or longer) perspectives on wetland development – including the frequency of channel avulsion – are crucial for understanding changes in wetland structure and functioning in the past, at present, and in the future, and the links with key drivers such as climate and land use change.
The Tshwane River in semiarid, northern South Africa is characterised by a through-going channel that traverse’s floodplain wetlands, but numerous adjacent palaeochannels at test to a complex avulsion history. Historical aerial photograph analysis and optically stimulated luminescence dating of the lower reaches of the modern river and palaeochannels reveal three reach-scale (1-5 km long) avulsions over the last 650 years, which is equivalent to an avulsion frequency of ~4.6 avulsions ka^-1. These rapid channel-floodplain adjustments have been driven by a mix of catchment and reach-scale controls. A downstream decline in discharge and stream power results in a downstream decrease in channel size, flow conveyance, and sediment transport capacity. Rapid bank erosion drives increase in channel sinuosity, leading to decreases in local channel slope and further reductions in flow and sediment transport. Local sedimentation rates up to ~11 mm yr^-1 occur within and adjacent to the modern channel. This promotes the formation of low levees and an alluvial ridge with a pronounced cross-floodplain gradient, and encourages the erosion of a new channel on the floodplain during periods of overbank flow. This new channel enlarges and grows headward, ultimately capturing the bulk of flow and sediment transport from an older section of the channel, which is then abandoned and left as a topographic high.
Comparison with other South African floodplain wetlands highlights the importance of robust and detailed geochronology for comprehensive analysis of these system dynamics. For instance, the Klip River wetlands in eastern South Africa are similar in morphology to the Tshwane River wetlands but are characterised by a much lower avulsion frequency (~0.3 avulsions ka^-1). Such differences in avulsion frequency may be related to differences in regional aridity. Subhumid to semiarid eastern systems are generally characterised by discharges that are maintained or even slightly increase downstream, which helps to maintain sediment throughput and results only in infrequent avulsions. Semiarid to arid northern and western systems are characterised by downstream decreases in discharge and sediment transport capacity, which drives more frequent avulsions. Avulsions are a key control on the distribution of water, sediment, and nutrients across floodplains wetlands, thereby influencing topography, substrate type, and hydroperiod, all of which have implications for ecological processes and patterns. Significant differences in channel avulsion frequency would not be identified without detailed chronological investigation, yet clearly have important implications for wetland conservation and management, particularly in view of the predicted future decreases in surface water availability.