Rapid Cycles of Episodic Adjustment: Understanding the Holocene fluvial archive of the Daintree River of Northeastern Australia

The dramatic decline in the quality of coral reef cover of the Great Barrier Reef (GBR) over recent decades has resulted in targeted research to better understand the dynamics of sedimentary sources within river systems of Northeastern Australia. European land-use practices are thought to have increased sediment yields to the GBR lagoon by 5–10 times, yet there is a poor understanding of the pre-1850 sediment dynamics. This study presents the first detailed alluvial chronology of the spatial and temporal responses of fluvial systems in the Wet Tropics of Northeastern Australia over the late Holocene. Valley fill sequences of the Daintree River, one of the least disturbed catchments draining to the GBR lagoon, are examined to investigate the significance of floodplain dynamics in sedimentary process. An optically stimulated luminescence chronology combined with a detailed sedimentary analysis suggests that floodplain stripping is a major, and hitherto unrecognised, source of sediment. Furthermore, rates of floodplain accretion are far greater than has been previously estimated from sediment modelling for wet tropical catchments. Spatial analysis of the topographical relationships between floodplain morphological units suggests that a total of 178,516 ton/ha of sediment has been stripped from three small confined floodplain reaches between 1038 ± 215 and 99 ± 10 years. Evidence suggests that these erosion events are followed by phases of rapid accretion with an average depositional rate of 3.87 ± 0.92 cm/yr between 572 ± 74 and 51 ± 12 years across the study area. The floodplain appears to be in a constant state of disequilibrium, experiencing spatially discontinuous phases of erosion and aggradation resulting in much higher volumes of sediment being redistributed within the catchment than previously considered. The unpredictable nature of these regimes and the shear volume of sediment mobilised poses significant challenges in managing sediment sources to the GBR lagoon.