The horizontal salinity gradient

Estuaries show a great diversity of size, shape, depth, and forcing characteristics, but a general characteristic of estuaries is the presence of a horizontal salinity gradient (Fig. 2.1). Normally the salinity decreases from the ocean toward the head of the estuary due to freshwater input; in the case of inverse estuaries, the salinity increases in the landward direction due to excess evaporation (seeChapters 1 and 9). This salinity gradient is the key dynamical variable that makes estuaries different from any other marine or lacustrine environment. The horizontal salinity gradient is the key driving force for the estuarine circulation, which in turn plays a key role in maintaining salinity stratification in estuaries. The combined influence of the estuarine circulation and stratification determines the fluxes of salt and freshwater within the estuary, and their intensity varies with the strength of the freshwater inflow. Because of these dynamics, estuaries are often the most strongly stratified aquatic environments, but they also tend to have vigorous water and salt exchange, due to the estuarine circulation.

This chapter explores the coupled equations involving the estuarine circulation, stratification, salt flux and freshwater inflow. A major outcome of this analysis is to reveal the essential importance of horizontal salinity gradient in the estuarine circulation, stratification, and salt balance, but also to find that the horizontal salinity gradient ultimately depends on the strength of the freshwater outflow and the intensity of mixing by tidal currents.