Estimation of Holocene bedload sedimentation rate in a paleo-drowned river-valley (Middle Harbour, Sydney estuary, Australia)

Middle Harbour is a drowned-river valley located adjacent to the larger Sydney estuary, Australia. Extensive, high-resolution seismic data were correlated with borehole, land use, topographical, and geological data to calculate the mass of genetically different sediment deposits in Middle Harbour. The Harbour follows a well-defined drowned river-valley structure featuring small fluvial bedload delta deposits in the upper reaches of the embayments, a deep, central extensive mud basin overlying transgressive basal accumulations and a large flood-tide delta at the entrance. Deposits of an estimated 5,094 t of bedload, 21,143 t of suspended sediment and 5,947 t of transgressive basal material located in the estuary provided sedimentation rates of 0.68 t y-1, 1.29 t y-1, and 2.86 t y -1 respectively. These rates, determined from measured accumulations, were surprisingly low and substantially smaller than modelled rates. However, low sedimentation rates for suspended material may be due to fine sediment escaping over the top of the marine tidal delta, which effectively traps all bedload material from exiting the Harbour. Results of this study indicate that Holocene bedload sedimentation in Middle Harbour was slow and regular until a rapid increase after urbanisation commenced in the catchment. Most pre-Holocene material was eroded from Middle Harbour during the Last Glacial period with sediment currently present in the estuary having been deposited since sea-level recovery.

Underwater Sydney

Admired all over the world and loved by locals for its natural beauty, Sydney Harbour is enjoyed by thousands of people every day. But rarely do we look below the surface where, beneath all the hustle and bustle, lively communities go about their business. With underwater forests and gardens, hundreds of species of fish and thousands of invertebrates, Sydney is as colourful and diverse below the water as it is above! Underwater Sydney celebrates Sydney’s incredible harbour and coast through eclectic stories and stunning underwater photography. It also explores the challenges the harbour is facing today after more than 200 years of coastal development and the role that marine science plays in maintaining the harbour’s health. Underwater Sydney will open your eyes to the rich marine life that makes the Sydney estuary and coast so special.