The balance equations of salinity variance are used to study the relationship between exchange flow and mixing of salinity, as well as the influence of river flow and spring neap cycle on them in the Hudson estuary. As river flow increases, mixing of salinity inside the estuary becomes stronger due to more variance input from the open boundaries driven by exchange flow and river flow. During one spring-neap cycle, maximum exchange flow occurs during neap tides, but maximum mixing occurs during the transition from neap to spring tides due to the combined influence of turbulence production and stratification. During most of the transition time from spring to neap tides, advective input of salinity variance from the open boundaries is larger than dissipation, resulting in the net increase of variance, which intensifies the stratification.
The intensified stratification in turn increases dissipation. As dissipation becomes larger than the advective input, i.e., during most of the transition time from neap to spring tides, salinity variance decreases and the stratification is destroyed. Another notable result is that the spatial distribution of salinity variance is found to be very similar to the distribution of water age under steady state conditions. Low salinity variance corresponds to old water. The similarity of the distribution of these quantities clearly reveals the pathway of exchange within the estuary and its relationship to mixing.