Basin infill increases seaward sediment delivery in small, tide-dominated estuaries

Estuaries are sinks for sediment and evolve through a series of infill stages. In the early stages of infill, sediment is stored in the central basin due to ample accommodation space. In the late stages of infill, accommodation space decreases and a higher proportion of the sediment load is routed seaward. To predict changes in the seaward sediment delivery of estuaries, we must first understand how they evolve and how their storage capacity changes over time. In this study, we present an analysis of the current infill stage and relative importance of 15 small, tide-dominated estuaries draining into semi-enclosed Moreton Bay (MB), Australia. Our work shows that these estuaries all exist in a mature (i.e. late) stage of infill regardless of their catchment or antecedent basin size. Due to their high degree of infill and densely urbanised catchments, these estuaries are likely to be contributing a large amount of sediment into MB relative to their size. Using existing infill models from the literature, and Eprapah Creek estuary as a case study, we mapped vegetation and facies unit changes from 1955 to 2019 and collected sediment cores to develop a conceptual infill model for these small estuaries. We show there has been an increase in mangrove and alluvial units and a decrease in open water and saltmarsh habitats. These changes are likely a result of increased sediment bypassing due to the infilling of the estuary. Over 50 m of mangrove progradation at the shoreline has occurred on average since 1955, with progradation rates being highest adjacent to the estuary mouth. Our work illustrates that estuary morphology and infill stage can impact the seaward sediment delivery potential for small tide-dominated estuaries. This is of important for semi-enclosed shorelines globally as sediment is not as easily removed seaward compared to shorelines along the open ocean.