Salinity and oxygen in central South West Arm of Port Hacking, Sydney, during 1974 - 1977. The top figure shows rainfall at a meteorological station close to the catchment area of South West Arm creek.

The salinity of the lower layer remains close to 35 throughout the entire period. The upper layer responds quickly to freshwater input from rain, when the salinity can drop to less than 30. During these periods the estuary develops a sharp interface between the two layers and displays the structure of a salt wedge or highly stratified estuary. Because the estuary has a sill of only 3 m depth, the circulation in the two layer structure occurs mainly in the upper few metres. This leads to frequent oxygen depletion at depth.

During periods of prolonged absence of rain the estuary turns into an appendix of the ocean, with salinities close to 35 throughout the water column. The reduced stratification allows vertical water renewal and leads to occasional rises of oxygen levels at depth.

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