Breede River Estuary
- General Information and Location
- Catchment Characteristics
- Land and Estuary Uses
- Ecology
- Water Quality
- Hydrodynamics and Mouth Dynamics
- Physical Features
- Climate
- References
General
The Breede River Estuary stretches roughly from 30 km above Malgas to the town of Witsand. The estuary has a permanently open mouth and the channel is incised to depths of 3 to 6m or more. Tidal influence extends beyond Malgas to approximately 80 km upstream of the mouth. During summer when river flow is at its lowest, the system becomes more saline, while during the rainy season salinities is usually much lower when stronger river flows limit the intrusion of seawater to the lower reaches. The estuary is a large system covering 455ha and supports six of the possible nine plant community types, and 59 fish species from 30 families, of which nearly 40% are entirely dependent on estuaries. It is also considered to provide a relatively important habitat for waterbirds. For these reasons the estuary is ranked 19th in South Africa for estuarine conservation importance and has the 5th highest botanical importance score. The recreational, nursery and scenic value of the estuary is also high. Currently the overall ecological health of the Breede Estuary is considered to be good, with the largest impact being a reduction of river inflow. It is recommended that the estuary condition be maintained in this state which is critically dependant on maintaining a flow regime to the estuary which includes flood and low flows.
Access to the estuary is through Bredasdorp, Swellendam and Heidelberg. There are no bridges below the National road (N2) and the lowest crossing point is the Malgas pont, 36km upstream from the mouth. This is one of the last ponts in operation in South Africa. The northern side of the estuary (Port Beaufort and Witsand) has access along a tar road whereas access to the southern side (Infanta village) is along a gravel road.
The Breede River estuary is a single sand spit estuary perenially open to the sea. The sand spit, situated on the northern bank, consists of a highly variable intertidal section adjacent to the mouth and a semi-permanent supratidal section adjoining the northern embankment of the estuary. The mouth itself is against the southern bank which is a wave cut terrace of Bokkeved shales. The main channel is maintained by strong tidal scour and extends to a boulder bed.
Location (co-ordinates)
The Breede River estuary is located at 34°24′ S, 20°51 E’; the nearest towns being Swellendam and Heidelberg. Distances by road from these towns are 61 and 46 km respectively.
Dimensions
The area of the estuary, between the mouth and Rooiheuwel (18km upstream) is 1 113 ha. This estimate includes macrophyte and saltmarch areas. The area of open water surface is 835 ha (Devenage, 1983).
Catchment Characteristics
Rivers and Tributaries
The catchment area of the Breede River is 12 625km 2 (Noble and Hemens, 1978) and after the Gouritz is the second largest in the southern Cape. The Breede drains large areas of the Cape Mountains including the Langeberg mountains and, through the Riviersonderend, the Hottentots Holland and Sonderend Mountains.
The Breede River is 257km long measured from the confluence of the Koekedou, Titus, Vals, Skaap and Waboom Rivers, 1,5km south-east of Ceres and about 36km longer when its furthest sources near the Theronsberg Pass are taken into account.
The Breede River has more than 58 tributaries with regular flow. These range from the short Melkhout and Jacobs Rivers draining the Potberg and the Noree River draining the Langeberg to the longer Skaap and Slang Rivers. The major tributary is the 140km long Riviersonderend. This tributary largely to the catchment area as it drains portions of the Hottentots Holland Mountains.
The Lower Breede sub-catchment includes the lower reaches of the Breede River after its confluence with the Riviersonderend River. In this section the river widens, becoming a meandering lowland system. In these lower reaches the surrounding land use changes from vineyards and orchards to wheat fields. The most significant tributary to join the Breede River along its lower reaches (in terms of flow contribution) is the Buffeljags River, which drains the north-western portion of this sub-area. Upstream of Barrydale, the Buffeljags River is known as the Tradouw River and it drains the northern slopes of the Langeberge. Further downstream the Breede River is joined by several smaller saline perennial river systems which drain the alkaline shales of the Overberg region (Napkei, Dipka and Slang Rivers). The head of the estuary is at Malgas nearly 50km from the sea where the river enters the Indian Ocean at Sebastian Bay, approximately 322 km from its source. The Breede River estuary is currently listed as the 19th most important estuary in South Africa.
Dams
Buffeljags Dam on the Buffeljags River has a capacity of 5,7 million m3 and a firm yield of 11 million m3/a. The dam supplies water for irrigation to the Buffeljags Water User Association and is required to make environmental flow releases.
River run-off
The catchment of the Breede lies in the winter rainfall area with the junction between the Riviersonderend and the Breede as well as the Breede downstream of Swellendam, lying in the boundary between the winter and bimodal rainfall regions (Noble and Hemens, 1978; Heydorn and Tinley, 1980).
Geomorphology
Geology
The tributaries of the Breede River drain predominantly Table Mountain Sandstone formations as well as Cape granites, Bokkeveld and Witteberg shale. However, the Breede River flows primarily through Bokkeveld shales with the river valley itself being composed of alluvium. These features lead to the river being turbid due to deflocculation of colloidal material caused by low salinities within the river.
Land and Estuary uses
Residential
Two towns, Port Beaufort and Witsand. These two townships consist mainly of holiday and retirement homes as well as having a hotel and holiday chalets.
Agricultural
Most of the land bordering on the Breede river is privately owned and devoted to agriculture. Due to its length the river flows through a variety of land types with associated variations in farming practice, irrigation water availability, etc.
In the upper reaches of the Breede, i.e above Swellendam, farming concentrates on vineyards and deciduous fruit with smaller efforts directed towards lucerne, tobacco, vegetables and cattle. Other is sheep, buck and ostrich farming.
In the vicinity of Swellendam farming practices are varied. For example in the Langeberg foothills mixed farming is practised; concentrating on grapes, peaches, citrus, loganberries, vegetable, cereals, dairy products, wool and mutton. In the Malgas area efforts are focussed more on cereals, wool, dairy products and wheat. On the vlaktes of the northern bank wheat and wool are dominant. In the Potberg area most of the land is reserve with beef being grazed on natural veld and sheep on pastures.
In the area between Malgas and the mouth, the southern bank has be subdivided into land units too small to farm and holiday cottages are being erected upon them. The northern bank remains farmland with some historic farms, e.g. Renosterfontein.
Recreation
Towards the mouth there are camping grounds.
On the actual banks of the estuary are a slipway, now used primarily for boat angling, and a jetty.
High volumes of boating activity takes place in the estuary during summer when the system is most biologically active. This impacts negatively on the productivity of estuarine biota.
Ecology
Flora
Semi-aquatic vegetation
Semi-aquatic vegetation types are not extensively developed along this estuary because of the steepness of the river banks (Day, 1981). Zostera capensis beds are notable from Port Beaufort to above Karools’ Kraal while Potamogeton pectinatus is found higher up in the river.
The largest saltmarsh is found at Green Point while less extensive marshes are found at Moddergat and in small coves along the river. These marshes are well stratified with Cotula coronopifolia (gansgras) directly above the Zostera zone. Sarcocornia decumbens is found on the lower saltmarshes and Triglochin bulbosa (arrow grass) and Limonium scabrum (sea lavender) at the mid-high neap tide mark. Above this level a zone of Juncus kraussii is ofter present followed by an upper saltmarch vegetation consisting of Sarcocornia pereniis, Chenolea diffusa (soutbossie), Apium graveolens (wild celery) and Plantago carnosa.
On the lower flood plain, Sarcocornia pillansiae and Samolus sp. are found, followed by Salsola sp. and Dysphyma crassifolia. Terrestrial vegetation gradually becomes dominant.
Sarcocornia natalensis is found in areas flooded by fresh water. Spartina capensis (strandkweek) dominates large areas in the lower estuary, especially along the southern shore.
Further upstream, the mud flats are dominated by Cotula coronopifolia and Triglochin spp. Juncus kraussii becomes more prevalent and the flood plain vegetation, where present, is similar to that described above. Approximately 10km from the mouth, Phragmites australis becomes dominant along the river banks with patches of reeds such as Cyperus textilis (kooigoed) and Typhia capensis (bulrush).
Terrestrial Vegetation
South/South-West Coast Renosterveld.
Fauna
Fish
The Breede River Estuary is famous for catches of large dusky kob, a species which has been heavily reduced by fishing. The recent national reduction of the bag limit to one fish per angler per day should help to reduce mortality, but in places such as the Breede River Estuary, even this restriction is not sufficient to prevent loss of spawner-biomass. The aggregation of large, sexually mature fish and their vulnerability to capture suggests that additional restriction is required in the Breede River Estuary.
Mammals
Blesbok and Bontebok
Water Quality
Physio-chemical Characteristics
The lower Breede River is moderately enriched with nutrients (mesotrophic to eutrophic). The salinity levels are also suitable for irrigation and industrial purposes along the entire river, however the Sodium Adsorption Ration is considered unacceptable for irrigation purposes along the entire length of this river. The pH of the water is also occasionally deemed unacceptable for most water uses.
Hydrodynamics and Mouth Dynamics
Mouth dynamics
Swart and Serdyn (1981) demonstrated from 19 years of data, that 64 percent of deep-sea waves on the southern Cape coast are from the south-westerly sector. Further the highest significant wave height was also recorded in this sector. The Median deep sea wave height for the are was 2,4m and the highest waves, on an annual basis, were 7,8 m (Swart and Serdyn., 1981)
The Cape Infanta promontory shelters the Breede River Estuary from these high waves but energy is fed into the estuary area and the lee of the headland by wave diffraction around the promontory. Refraction of waves as they shoal landwards tends to change the approach direction of the waves closer to normal (90°) to the beach. Therefore the wave diffraction and refraction processes work together to both dissipate wave energy and also to change the direction of approach of waves. Thus waves at the Breede River estuary itself are generally small relative to the deep sea waves and approach the beach from a south-easterly direction.
Both wave diffraction and refraction effects decrease north-eastwards of Cape Infanta and consequently waves at Moodie Point and Voëlklip are generally higher than those at the estuary itself. This gradient in wave height leads to a gradient in ‘wave set-up’ which induces longshore surfzone currents flowing from the region of high waves (Moodie Point) to the region of low waves (Breede river estuary).
North-easterly longshore surf zone currents will only occur when the deep sea incident wave direction falls in a narrow section around south-east.
Physical features
Bathymetry
The upper reaches of the region between the mouth and Malgas are generally deep. Some spectacular holes occur. The middle reaches are shallower with depth increasing again towards the mouth. Again some deep holes are apparent in the latter two areas. The overall mean depth of the estuary was 3,04m.
Obstruction
The Breede river estuary has no obstructions or bridges below Malgas.
Climate
Rainfall pattern: Winter to all year.
Mean annual precipitation – 300-600mm.
Average daily temperature – 10°-20° C.
References
Estuaries of the Cape: Report no.21: Breė (CSW22) – CSIR – RA Carter
State of Rivers Report – Rivers of the Breede Water Management Area 2011