As we saw in Chapter 2, the input of buoyancy over much of the shelf seas is dominated by heating and cooling through the sea surface. There are additional exchanges of buoyancy at the sea surface through the processes of evaporation and precipitation, both of which modify the salinity of surface layers, but in temperate latitudes their contribution is generally small in comparison with heat exchange. In areas of the shelf adjacent to estuaries, however, freshwater discharge from rivers can make a dominant contribution to buoyancy input (Section 2.3) and maintain strong horizontal gradients of salinity.

In this chapter, we shall consider these Regions Of Freshwater Influence (ROFIs) (Simpson, 1997), the suite of processes which operate within them and the distinctive environment that results. In terms of physical processes, ROFIs have much in common with estuaries, but they differ insofar as estuaries are confined by land barriers and are generally of a smaller horizontal scale than that which is characteristic of ROFIs. In many cases, the circulation in an estuary is predominantly an along-channel flow while transverse motions are relatively small and the effects of the Earth's rotation can be neglected. We shall initially develop the theory of density-driven circulation for such a simplified, non-rotating system before proceeding to consider how rotation modifies the density-driven circulation in ROFIs. We shall next examine the way in which the density-driven circulation is involved in determining the water column structure of ROFIs in competition with stirring and then explore the implications for the biology and environmental health of ROFIs.