A LITTLE RIVER WITH A BIG IMPACT

Tiny as rivers go - measured from its source in the Outeniqua Mountains to the Knysna Heads it's just 64 km long - the Knysna River is nevertheless an important and defining feature of the Knysna Lagoon.

The River's catchment - which is about 330 km2 in size - is the main source of fresh water for the Estuary and for the town of Knysna, which draws its supplies from the Akkerkloof Dam.

Together with its tributaries - the Swartkops, Steenbras, Gouna, Rooiels, Leiievlei, Witels, Palmiet, Dwars, Kruis, Oubos and Lawnwood Rivers - the Knysna River delivers an estimated 110 to 133 million m3 of water into the Knysna Lagoon every year.
In general, the water in the Knysna River is clean and pollution-free. The reddish-brown colour which stains most of the fresh water in the Southern Cape is caused by humic acid, a natural by-product of the process of leaf decay.

Although the River does occasionally carry silt into the Lagoon -especially after heavy rains and flooding (which generally occurs about once every ten to twelve years) - serious, damaging siltation is preventable through careful land management in the catchment area.

ESTUARY OR LAGOON?

Much confusion has arisen over the correct name for the body of water in the Knysna Basin. Is it the Knysna Lagoon? Or is it the Knysna Estuary'

In general, estuaries remain permanently open to the sea, whilst lagoons are characterised by sand bars which may build up to prevent fresh water from entering the ocean during certain seasons. According to this argument, and because there's no sand bar at The Heads, the Knysna Lagoon should perhaps correctly be called the Knysna Estuary.

But a report byA.E.F. Heydoorn and J.R. Grindley (1985) limits the "strictly estuarine" sections of the Knysna Basin to the water area north of the national road bridge and calls the rest of the system "a marine embayment" with estuarine characteristics which support typically estuarine life forms.

So maybe we should call it the Knysna Marine Embayment? Well, perhaps - but most historical sources refer to it as "the Knysna Lagoon," and although it's sentimental, this is perhaps the main reason why this is the most commonly accepted name today.

ABOUT THE KNYSNA LAGOON

The 1985 CSIR report noted that the Knysna lagoon "is biologically the richest estuary in the Cape ... and one of the largest. Since it is permanently open, and the volume of influent fresh water relatively small, salinities are stable and near to that of sea water. This stable environment accounts for the remarkable diversity of species recorded here, the highest of any South African estuary. As a result, residential and recreational developments are spreading rapidly and changing the natural and rural character of the area. This rapid development must not be allowed to affect the natural ecological processes that maintain the functioning of the lagoon, while the rural character and features which are so attractive should be maintained by carefully controlling any future development."

The Knysna Lagoon covers about 1,827 ha and stretches from the mouth (The Knysna Heads) to the rapids at Charlesford on the Knysna River. It measures about 3 km at its widest. The navigable channel is about 19 km long, although only the first 5 km (from The Heads to the Ferry Terminal) are deep enough for yachts and small ships. The l4 km-long channel above the railway bridge is only suitable for small, shallow-draft craft.

Channel depth averages between 1.2 and 1.5 metres below mean sea level, with a maximum depth of about 16 metres. The tidal influence (tidal reach) stretches 17 km upstream from The Heads. The time-lag between tides at The Heads and at Old Drift is about two hours at spring tide.

The Lagoon floor is covered with material which ranges from pebbly sand at the Charlesford Weir, to soft, black mud in the area of Crab's Creek and clean, loose sand in the main channel between Leisure Isle and the Western Head.

The Lagoon has three islands - Thesen's Island (84 ha). Leisure Island (82 ha) and Rex Island (or Braamekraal, the marshy area bordering George Rex Drive) - and has been crossed by various bridges and causeways. These impediments have a pronounced influence as they slow down the speed of flow of the water and allow siltation to occur.

THE RIVER MOUTH - THE KNYSNA HEADS

Known as The Heads ('head' is short for headland), the Knysna River mouth is about 230 metres wide and, in the navigable channel, about 3.9 metres deep. The tidal rise and fall - which is maintained as far upstream as Belvedere - is about 1.8 metres at spring tide.

The Mouth is characterised by inner and outer rock bars as well as submerged rocks and dangerous currents which flow at up to 76.2 metres per minute - of 4.572 km per hour (depending on the tide, this translates to a flow of between l,OO0 and 2,000 cubic metres of water per second). This, together with the unpredictable nature of the wind in the mouth, makes the mouth particularly dangerous for shipping, and only registered skippers of licensed craft are allowed to put to sea via The Knysna Heads.

SANDBANKS AND SALT MARSHES

The Knysna Lagoon is characterised by sand banks and salt marshes which are generally exposed at low tide. These biologically rich salt marshes - the "pantries of the sea."[read on]