Large structural estuaries: Their global distribution and morphology

Abstract

Estuaries are marine-flooded topographic depressions that receive sediment from both marine and fluvial sources. The morphology of the underlying drainage basin acts as a boundary condition for the spatial distribution of hydrodynamic energy within an estuary, and by extension the resultant depositional environment. In some large estuaries, bedrock structures at the mouth restrict oceanographic forcing, which has impacts on sedimentary infill. Such estuaries are termed here as Large Structural Estuaries (LSEs). For LSEs, the shape of the basin and associated bedrock morphology is fixed on the millennial scale having been formed by geological processes operating over timescales greater than that of the Holocene. However, the distribution and abundance of these LSEs are poorly understood. In this study, a global survey of large estuaries was conducted to identify and characterize LSEs from the worldwide population to understand regional drivers of coastal geomorphology. A total of 271 large (>50 km2) estuaries were identified, and these were clustered into 5 classes using Gaussian Mixture Modelling. These clusters were interpreted as Barrier Estuaries, Rocky Bays, Sandy Bays, Funnelled Estuaries, and LSEs. LSEs comprised 28.04 % of all large estuaries identified and were spatially heterogenous, associated with collision coastlines, and predominantly located at higher latitudes except for the Malay Archipelago. The global distribution of LSEs suggests that regional distribution of tectonic processes strongly influences their formation. LSEs are important global features comprising many of the major ports in the world and this work indicates they are unique landforms and traditional approaches to understanding sedimentation in these settings require adjustment to their unique geomorphology.