Sediment transfer in an extremely low gradient, low relief, and highly buffered system: Darwin Harbour Catchment, Northern Australia

Abstract

Sediment yields from and denudation rates in catchments of very low relief and gradient, which make up about 50% of Earth’s surface, are poorly documented, and their internal sediment dynamics are poorly known. Sediment sources, their proportionate contributions to valley floors and sediment yield, and storage are estimated using fallout radionuclides (210Pb(ex) and 137Cs) in the catchments that drain into Darwin Harbour, northern Australia; an example of this understudied catchment type that appears to be globally at the extreme end of this category of catchments. Unchannelled grassy valley floors (dambos, or seasonal wetlands) trap ~90% of the sediment delivered from hillslopes by sheet and rill erosion. Further down valley small channels transport ~10% of the sediment that escapes from the dambos, and the remaining sediment come from erosion of the channels. In this case, the fractional sediment storage is very high as a result of the existence of dambos, a landform that depends for its existence on low gradients.