EVENT FREQUENCY AND LANDSCAPE STABILITY ON THE FLOODPLAIN SYSTEMS OF ARID CENTRAL AUSTRALIA

Central Australian floodplains contain landforms suggesting activity at three scales: a set of large sand sheets, sand threads and megaripple-covered channels related to a few enormous floods; the contemporary floodplain which consists of channels and levees, floodouts, unchannelled floodplains and floodbasins; and erosion cell mosaics which result from local redistribution of sediment. Classical floodplain models do not fit these systems which behave more like low-angle alluvial fans because they continue to accumulate sediment. The sedimentary history of the Central Australian floodplains suggests that deposition in the last 10,000 years has been dominated by a few gigantic floods. These events are not represented in the published slackwater deposit record but morphological evidence for them can be found in bedrock gorges as well as on floodplains. Since the last major flood, channel-levee systems have been growing slowly upwards and outwards from the mountain ranges. Away from these systems, sediment supply is limited and the dominant process is local reworking. This has intensified since the arrival of Europeans 100 years ago and also as a result of increased flooding due to variations in rainfall.