The hydrographic character of the Knysna Estuary

Abstract

The thermohaline character of the Knysna Estuary can be best described by identifying a “bay regime” near the ocean, an “estuary regime” near the river and a “lagoon regime” in between. Each regime is characterised by a different thermohaline character, consistent with different hydrodynamic processes being dominant in each regime. The bay regime is well flushed by tidal flows and exhibits salinities and temperatures similar to the ocean. The estuary regime is characterised by the effects of river inflow and stratification—it is fairly well flushed by density-driven estuarine circulation. In between these two regimes, the lagoon regime is flushed less rapidly, subject only to tidal diffusion effects, and exhibits long residence times. Salinities here are close to ocean salinity, but temperatures may be several degrees warmer. The lagoon regime expands as the estuary regime shrinks in response to decrease in river flow and as the bay regime shrinks in response to decrease in tidal range (i.e. neap tides). The larger the lagoon regime and particularly the further it extends landward, the longer water is retained in the estuary regime of the Knysna Estuary. During floods and droughts, extreme variations on this theme are observed. The lagoon regime disappears during strong inflow (floods) and expands during low inflow (drought). In spite of very low inflow, no hypersalinity was observed.