Knysna Estuary

The Knysna Estuary

The Knysna Estuary forms part of The Knysna Hope Spot. Mission Blue defines Hope Spots as special conservation areas that are critical to the health of Earth’s blue heart - our oceans.

According to a report produced by the CSIR (Council for Scientific and Industrial Research) in 1985, the Knysna Estuary “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 Estuary, while the rural character and features which are so attractive should be maintained by carefully controlling any future development.”

The Knysna Estuary 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 14 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 Estuary 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 Estuary has three islands - Thesen's Island (84 ha), Leisure Island (82 ha) and Rex Island (or Braamekraal, the marshy area bordering George Rex Drive). Various bridges and causeways that were built in the late 19th and early 20th centuries have a pronounced influence on the Estuary since they down the speed of flow of the water, which causes siltation.

The Knysna Heads: the river mouth

The Heads ('heads' - short for headlands) define the Knysna River mouth, which is only 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 guarded by an inner bar and an outer bar (both of rock) and by a number of other submerged rocks. The current flows through at speeds of up to 76.2 metres per minute - of 4.572 km per hour (depending on the tide, this translates to a flow of between 1,000 and 2,000 cubic metres of water per second). All of this, together with the unpredictable nature of the wind in the mouth, makes The Heads particularly dangerous for shipping.

Sandbanks and salt marshes

The sand banks and salt marshes of the Knysna Estuary are generally exposed at low tide. Please read about salt marshes - the ‘pantries of the sea - here.

The Knysna River

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 Estuary.

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 River, Steenbras River, Gouna River, Rooiels River, Lelievlei River, Witels River, Palmiet River, Dwars River, Kruis River, Oubos River and Lawnwood River - the Knysna River delivers an estimated 110 million to 133 million m3 of water into the Knysna Estuary 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 comes from humic acid, a natural by-product of the process of leaf decay.

Although the Knysna River does occasionally carry silt into the Estuary - 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.

Bridging the Knysna River

Knysna was a commercial centre from the early 1800s, but overland transport was at first almost impossible because of the many deep crags and rivers of the Garden Route. For travellers during the Colonial period, the most efficient method of transport to or from the Southern Cape was by sea, and construction began on the first road from George via Knysna to Port Elizabeth only in the 1860s - although the work was so slow that it took until 1885 for the road to reach as far east as Humansdorp.

This first road which crossed the Knysna River was built at Old Drift, but it became impassable when the River was in flood. The earliest attempts at bridging the River were unsuccessful - a wooden bridge built in 1895 became unsafe within only ten years, and an iron bridge built in 1915 was washed away within six months. It was therefore only with the opening of the Red Bridge, at the foot of Phantom Pass, that Knysna finally had a safe and permanent land connection to the outside world.

The present day White Bridge became the main crossing point for vehicles when the N2 (or National Road) was built in the 1940s. The White Bridge was widened when the N2 was upgraded in the mid-80s.

An early ship yard and a remarkable discovery

The first shipyard in Knysna was built in 1820 in the general area of what is now the Knysna Waterfront - but two fires razed it to the ground, and the project was abandoned in 1824 before even a single craft was built.

In 1826 George Rex - 'the founder of Knysna' - erected a slipway at Westford on the banks of the Knysna River. Here he completed Knysna's first ship: the 139 ton, 73-foot Knysna, a brig (two-masted square-rigger) which sailed on her maiden voyage in 1831 and continued in service until 1844, when she was wrecked off the coast of Britain.

The slipway was abandoned after the launch, but a chance discovery in 1946 lead to the recovery of some of the stinkwood beams with which it had been built. The timber was still in excellent condition, and some of it was used to build the triangular-shaped table and fifteen chairs which now stand in Knysna's council chambers. Some of the material was used to make two scale models of the brig Knysna - one of which now stands in the Old Gaol Museum. The other is displayed in the Ann Bryant Gallery in East London.

Protect the catchment - protect the estuary

The way in which we use the land in the catchment area has an immediate and forceful impact on water quality in the Knysna River and will ultimately influence the sustainability of the Knysna Estuary.

Of particular concern is the introduction of silt to the estuarine environment. Siltation happens when the land is stripped of its natural cover, which leaves it vulnerable to soil erosion: this may happen as a result of overgrazing, monoculture (like plantation forestry), or the presence of gravel roads.

Silt suffocates the flora on the Estuary floor and increases turbidity (the number of particles in the water), which reduces light penetration, thus decreasing plant production and excluding visual feeders. If siltation is allowed to continue unchecked, the result would be a 'dead' body of water.

Text: Martin Hatchuel www.thistourismweek.co.za