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Types of Estuaries

According to Reddering and Rust (1990), few southern African systems are true estuaries with only 37 (12.8%) of the 289 river mouths maintaining permanent tidal inlets with the sea. Whitfield (1992) suggested the following classification:

  • Estuarine bays (large tidal prism, tidal mixing process, average salinity 20-35 ppt). Examples: Durban Bay (KwaZulu-Natal), Knysna (Western Cape).
  • Permanently open estuaries (moderate tidal prism, tidal/riverine mixing process, average salinity 10- >35 ppt). Examples: Mlalazi (KwaZulu-Natal), Mzimkhulu, Mngazana, Keiskamma (Eastern Cape), Berg (Western Cape)
  • River mouths (small tidal prism, riverine mixing process, average salinity < 10 ppt). Examples: Mfolozi (KwaZulu-Natal), Orange.
  • Estuarine lakes(negligible tidal prism, mixing process - wind, average salinity 1- >35 ppt). Examples: St Lucia (KwaZulu-Natal), Swartvlei (Western Cape)
  • Temporarily closed estuaries (tidal prism absent, mixing process - wind, salinity 1- >35 ppt). Examples: Mtati, Kasuka (Eastern Cape)