Long-term isohaline salt balance in an estuary

The salinity budget in a 3-D numerical model of an estuary is analyzed using isohaline surfaces to define the volume of integration. Such surfaces move with the currents, and as a result of turbulent mixing. The isohaline analysis allows us to clarify the processes which create and destroy water in different salinity classes in the estuary. We find that the estuary naturally divides into three salinity classes, with different mechanisms maintaining the volume in each, when averaged over a time long enough to have a steady mean salinity field (two spring-neap cycles in our model). In the low salinity region, where isohalines never leave the estuary, there is an advective–diffusive balance. The river flow tend to increase the volume of low salinity water, while isohaline “drift” tend to decrease it. The “drift” is the non-advective motion of isohalines, caused by turbulent diffusion. In the high salinity region, direct injection of ocean water at the mouth brings in salt, balanced mainly by this drift. There the mean drift of isohalines is toward higher salinity, whereas in the low salinity region the drift is toward lower salinity. Correspondingly, the drift tends to increase the volume of water in the mid-salinity range, consistent with turbulent creation of mixed water out of ocean and river endpoints. This increase is balanced by the direct ejection of that water out the mouth of the estuary, due to restratification during neap tides. The magnitude of this ejection depends upon the location of the estuary mouth, which is somewhat arbitrary. The ability of the estuary to permanently export mixed water at the mouth is a key factor in the overall salinity structure.