Stratification induced by freshwater buoyancy input constitutes a key determinant of the environment both in estuaries and adjacent regions of freshwater influence (ROFIs). Predicting the development and breakdown of estuarine stratification is inherently more difficult than the prediction of thermal stratification because the buoyancy input is localized at the lateral boundaries rather than being uniformly distributed over the surface. The primary processes responsible for controlling water column stability have been identified in a previous paper (Simpson et al., 1990 Estuaries12, 125–132) as (i) the estuarine circulation driven by horizontal density gradients, (ii) stirring by tidal and wind stresses and (iii) straining of the density field by tidal shear. These processes are incorporated in a prescriptive model of the time evolution of stratification which utilizes analytical solutions for the tidal and density current profiles, specifies the mixing in terms of energy conditions previously used in the heating-stirring models and assumes a horizontal density gradient which is invariant with depth.

The behaviour of this simple model is described and compared with recent time series observations of the variability of stratification in a tidally energetic ROFI which indicate the presence of strong semi-diurnal and semi-monthly cycles in water column stability. The model reproduces the main qualitative features of the observations but is restricted as a predictive tool by the constant density gradient assumption and the omission of the interaction between the tidal- and density-driven flows. The inclusion of this latter interaction, which occurs via the frictional stresses and the control of eddy viscosity by stratification and the turbulent energy intensity, requires a dynamically active model which specifies the feedback processes in terms of an appropriate closure scheme.