The nature of estuarine circulation under highly stratified conditions is reviewed in the context of southern African estuaries that are small and have a constricted connection to the ocean. While the focus is on the generic hydrodynamic processes and features, attention is given to field observations from a particular estuary as a way of illustrating these processes and features. Notable features are the tidal intrusion front, the bottom density current and the long-residence deep water in the upper estuary. The important hydrodynamic processes, which account for these features, are internal hydraulic control of exchange through the mouth, buoyancy-driven landward intrusion of sea water, upward shear-driven entrainment and ebb tidal outflow. The context in which these processes and features are expected to occur is addressed through a discussion of seasonal variations in the prototype and through a discussion of these features observed in other southern African estuaries. A distinction is made between those systems that are permanently highly stratified and those that display highly stratified phases. The ecological and management implications of strong stratification in an estuary are discussed briefly.

Following the closure of the mouth of the Palmiet estuary in February 1988, the thermohaline structure was recorded at approximate weekly intervals. The evolution of the two-layer structure was studied and the mixing processes causing the changes were determined. Vertical diffusivities were computed and the qualitative understanding of the slow mixing rates in the deep pools was confirmed. The study indicates a probable summer condition of the estuary following upstream impoundment.