How can knowledge of mechanisms and timescales of avulsion and floodout formation in floodplain wetlands inform interpretation of Quaternary fluvial sedimentary records?

River avulsion and floodout formation play crucial roles in the development and dynamics of floodplain wetlands in relatively unconfined, semiarid landscapes. Avulsions laterally redistribute water and sediment, while floodouts form where channels terminate and therefore represent discrete sediment sinks. In the absence of neotectonics or sea-level change, channel-belt aggradation rate related to extrinsic (e.g. climate change) or intrinsic processes (e.g. downstream declines in stream power) is often regarded as a primary control on avulsion frequency. Yet avulsion style (e.g. progradational, incisional or reoccupational) and floodout morphology vary depending on physiographic setting, flood regime and sediment supply. We describe mechanisms and timescales of avulsion and floodout formation in southern African and southeastern Australian floodplain wetlands. In the Blood River wetlands, South Africa, aggradation rates are up to 15 mm a-1 around the channel belt and progradational avulsions occur every 10-30 years on average. In the larger Macquarie Marshes, Australia, aggradation rates are up to ~11 mm a-1 around the channel belt and progradational avulsions occur every 50-60 years on average. Both systems exhibit evidence of a late Holocene transition from sinuous, through-going channels to straighter distributary channels that terminate on floodouts with locally higher aggradation rates (>30 mm a-1) resulting from organo-clastic sedimentation. Positive feedbacks between avulsion, sedimentation and vegetation growth influence contemporary wetland dynamics. Knickpoints form on locally steepened slopes created by organoclastic lobes and potentially reconnect river reaches separated by floodouts. Discontinuous late Holocene sediments are likely to be found atop more laterally continuous older sediments deposited by through-going channels. In these semiarid settings, long-term preservation of organic matter is limited by factors including desiccation and fire, and so many of the subtle surface feedbacks characterising the late Holocene development of these systems will not be recognisable in longerterm sedimentary records.