The Knysna system fulfils the criteria of an estuary: it is a semi-enclosed water body with a permanent connection to the open sea, and fresh water from land drainage mixes with sea water through tidal action.
The Knysna system also fulfils the criteria of the Dalrymple sedimentological definition: An estuary is “… the seaward portion of a drowned valley system which contains interfaces influenced by tide, wave and fluvial processes. The estuary is considered to extend from the landward limit of tidal interface at its head to the seaward limit of coastal interface at its mouth…”
The lower and middle reaches of the estuary are marine dominated and their structure is determined by the ebb and flow of the large tidal prism (estimated at spring tide at +/-19,000,000 m3!) that enters through the Knysna Heads twice daily.
Based on hydrographical characteristics, the Knysna Estuary can broadly be divided into three distinct regimes:
• Upper estuary regime, which is strongly influenced by the inflow of freshwater from the Knysna River and demonstrates strong vertical stratification in salinity and water temperature;
• Lagoon regime, which is characterised by aged salty water with little direct influence from freshwater. The lagoon is considered as a transition zone between the upper estuary and the marine embayment;
• Large marine embayment regime, which is flushed by tidal flows and exhibits temperatures and salinities similar to the ocean.
The boundaries of these three regimes demonstrate a high degree of temporal variability reflecting amongst others, the magnitude of freshwater inflow from the Knysna and Salt Rivers into the estuary and the inflow of marine waters through the Heads. (Thus, during periods of high rainfall within the catchment area of the Knysna River, the upper estuary may extend as far down as the large marine embayment resulting in the disappearance of the lagoon!)
In the 1940’s, the size and clear waters of the Knysna Estuary made it an obvious choice for UCT’s Professor John Day to begin his research into South African estuaries.
Knysna’s Estuary has the largest tidal volume in South Africa (measured in relation to volume of water between low and high Spring tides). The water fills the estuary, spreading over the salt marshes to replenish nutrients and flush out waste.
Some of the flanking salt marshes have given way to property development, but the Knysna Estuary still has the largest remaining estuarine salt marsh in the country. And salt marshes play a pivotal role in preserving marine ecological balance. They offer shelter and food to a myriad of animals living in the shallows of inter-tidal zones and protect the underlying sediment from erosion.
Without eel-grass, a major component in the protection of fish life is lost because plants release energy through photosynthesis and when they die and decay they release particles which feed other animals.
This estuary is home to the Knysna Sea Horse, the only core estuarine species of sea horse in the world. So named for their horse-like features, these enchanting, armour-plated, upright swimmers are actually fish. But sea horses do not do well in fresh water and are for that reason vulnerable in times of flood.
Parents are inclined to feed on their young and estuarine eel grass offers protection - and therefore survival - for these young.
An important feature of the estuary is the sanctuary it offers to marine fish. Young fish which are hatched in the sea move into its protected waters and grow to a size which allows them to return with comparative safety to the sea.
This is particularly significant because species such as Grunter and Cape Stumpnose are extremely slow growers and if the estuary is depleted of main fish species, it follows that there will be a death of the same species in the ocean as well.
Wetland birds used to be a feature of all estuaries, but this is unfortunately on the decline. Reduction of available space has caused a reduction in their numbers. Diversity still exists in the smaller numbers, however, as migrant birds from Europe and the far north of Russia continue to visit us and feed in an estuary which is also a role player in World ecology.
The Knysna Estuary is also home to the Knysna Basin Project. Directed by Professor Brian Allanson, participants from Rhodes University, UCT, Johannesburg University, Nelson Mandela Bay University and Cambridge University are researching to discover new information about estuaries.
Research conducted by Professor Allanson and his associates has uncovered a species of limpet in the Knysna estuary which, since 1958, had only ever been found in Langebaan. “It’s a special little beast and important in ecology!” the Professor says with a smile.
The Basin Project collaborates with SANParks, and their laboratory provides the means to monitor the water quality of the estuary and facilitates important biological studies.
Some of the original wetland areas have already been lost to urban development, and this is good reason why we should be concerned about the state of the Knysna Estuary, which has been independently voted as “South Africa’s Number One!”
Any uncontrolled, man-made interference will affect the delicate interaction between the sea tide and river, and will therefore adversely affect the ecological balance in the estuary.
Destruction of the wetlands would mean the destruction of most of the estuarine life. We could live with the resultant sterility, but we would be ignoring a fundamental human responsibility - to protect the environment!
Cache Container: Small L’n’L container with pen and log sheet. Please re-seal the log sheet to keep it dry and replace the container where found. The original contents included a wooden elephant, a domino tile and the trackable Masters of Space (TB1GVMY). The latter originated in Japan and had travelled 57 672km up until then!