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What is an Estuary


Project Duration: Long Term

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Estuaries are productive, coastal systems where river and seawater mix along a continuum from little mixing (salt wedge) to fully mixed homogenous estuaries. Estuary definitions may differ according to ecosystem (functional) and legal (boundaries) predispositions. A global, encompassing characterisation is that an estuary “is a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage”1. This does not fully address every South African estuary, in that the majority are small, have limited access to the sea, close for long-periods and back flood into supporting habitats and when open, are influenced by only micro-tidal regimes. A better definition for the South African context is “a partially enclosed coastal body of water which is either permanently or periodically open to the sea and within which there is measurable variation of salinity due to the mixture of seawater with fresh water derived from land drainage”2. The classification of an estuary as laid out in the ICM Act (24 of 2008, amended by Act 36 of 2014) very closely follows this; “a body of surface water that is permanently or periodically open to the sea, in which a rise and fall of the water level as a result of the tides is measurable at spring tides when the water course is open to the sea, or in respect of which the salinity is higher than freshwater as a result of the influence of the sea, and where there is a salinity gradient between the tidal reach and the mouth of the body of surface water”.

What is omitted from this definition is the inclusion of a functional zone comprising the adjacent areas of the floodplain associated with the water body, which support the physical and biological processes and habitats necessary for estuarine function and health3. The estuarine functional zone (EFZ) is nonetheless recognised in Notice 3 (GN R 546) under the National Environmental Management Act (NEMA), Environmental Impact Assessment (EIA) Regulations (2010) as a spatial delineation and sensitive area requiring environmental authorisation for developments in this zone. A study investigating whether the current functional zone (EFZ) includes all estuarine habitats shows that the delineation being used (to the 5m topographic contour) does not include all estuarine-associated vegetation in KZN4. Additionally, some critical habitats (e.g. swamp forests) may be under threat due to under-reporting of the EFZ. This has been addressed for all 76 systems and a new, updated EFZ is available for KZN estuaries based on updated habitat information and better topographic data mostly using light detection and ranging (LIDAR) methods.




1 Cameron WM, Pritchard DW. 1963, Estuaries In: The Sea (Ed. MN Hill), Vol. 2, Wiley, New York: 306-324.

2 Day JH. 1980. What is an estuary? South African Journal of Science 76: 198.

3 Van Niekerk L. Turpie, JK (eds). 2012. National Biodiversity Assessment 2011: Technical Report. Vol 3: Estuary Component. CSIR Report No. CSIR/NRE/ECOS/ER/2011/0045/B. Stellenbosch: Council for Scientific and Industrial Research.

4 Veldkornet DA, Adams JB, van Niekerk L. 2015. Characteristics of landcover of estuarine boundaries: Implications for the delineation of the South African functional zone. African Journal of marine Science 37(3): 313-323.



Research Funders


Fiona MacKay

Bronwyn Goble

Steven Weerts

Omar Parak

Santosh Bachoo

Marilyn Bodasing

Marilyn Bodasing