Flood and drought impacts on the opening regime of a wave-dominated estuary

Wave-dominated estuaries are affected by floods generated in their hinterland catchments as well as marine tide and wave processes. The interaction of these terrestrial and marine influences controls the opening regime of the estuary. Despite the well documented environmental pressures on estuaries including elevated nutrient loadings and abstraction of river flows, little research concerning the variability and controls on estuary mouth opening regimes exists. From water level observations of Tuross Lake estuary in south-eastern Australia, the estuary's recent opening regime is reconstructed and shown to vary significantly over time. Floodwaters fill the estuary and scour the estuary mouth, enhancing the exchange of marine water between the estuary and the ocean, which manifests as an increased tidal range within the estuary. Between floods, tide and wave activity caused aggradation of the estuary mouth such that the tidal range within the estuary declined by -0.5 to -0.7mm per day as the mouth became more constricted. Under conditions of low river flow, high evaporative losses and seasonal reductions in ocean wave height, the estuary mouth can close completely. Using twentieth century streamflow estimates, it is shown that hydrologic variability is likely to have resulted in large variations in the estuary's opening regime. Since 2000, there have been relatively few flood-driven scour events and this explains the relatively congested state of the current estuary mouth. Predicted hydrologic changes under enhanced atmospheric carbon dioxide concentrations are also likely to enhance the variability in the estuary's opening regime.