The Botriviervlei and Kleinmond estuary lie between the coastal towns of Kleinmond and Onrus on the south-west coast of the Western Cape. The 42-km-long Bot river and its main tributary, the 48-km-long Swart river, drain the Houhoek, Groenland, Swart, Shaw’s and Babilonstoring mountains, covering a catchment area of c.1,000 km². Although the Bot drains a relatively small catchment, it forms one of the largest coastal, open-water lagoons in the Western Cape.The Bot river flows into the Botriviervlei, which is a shallow triangular coastal lake (lagoon), situated in a broad valley flanked by mountains. The water-levels of the lake vary considerably and it can be up to 7 km long by 2 km wide. The lake is separated from the sea by a 100–200-m-wide coastal dune belt, 3–6 m high, through which washover from the sea occasionally spills. At the seaward western end of the main lagoon there is a shallow sidearm, Rooisand, which is connected to the main vlei by a narrow bottleneck called die Keel.When water-levels in the main lagoon are low, Rooisand is dry, but when the water-levels rise sufficiently, the Botriviervlei overflows through this area and into the Kleinmond estuary, allowing excess floodwater to escape without breaking open the dune barrier to the sea. Occasionally, the dunes have been artificially breached, with the aim of re-establishing estuarine conditions (particularly suitable for angling fish), but the gaps have been closed relatively quickly by wave-driven sand movements. The aquatic vegetation is dominated by Ruppia, which occurs throughout the vlei in water shallower than 3 m. Algae, such as Chara, seem to be present throughout the vlei. Potamogeton occurs in dense isolated patches in the upper reaches. Dense reed-swamps of Phragmites and Scirpus occur in some marginal waterlogged areas.
See Box for key species. The site supports over 163 bird species, at least 62 of which are waterbirds. The wetland regularly supports an average of 25,000 individual birds and on occasion it can hold over 40,000. The wetland is important as a summer refuge for waterbirds, when ephemeral water-bodies dry up and birds are forced to seek out permanent water. The system regularly supports extremely large numbers of duck (Anatidae), including Anas undulata, Netta erythrophthalma and Tadorna cana. Fulica cristata is the dominant waterbird and numbers often exceed 20,000 individuals when conditions are favourable (probably the largest regular congregation in South Africa). The system also supports important numbers of Anas smithii, Podiceps nigricollis and P. cristatus, which breed here. Haematopus moquini occurs near the estuary mouth and along sandy beaches on the seaward side of the coastal dune-fields where it has been recorded breeding. In summer this wetland regularly supports over 4,000 waders of at least 11 different species, with Calidris ferruginea being dominant. Terns, which are at times very abundant, use the estuary largely as a roosting area, from where they move to marine environments to feed.
Non-bird biodiversity: Hyperolius horstockii, Microbatrachella capensis (EN) and Bradypodion pumilum (CR) are threatened herptiles that occur in the IBA. The South-African-endemic amphibians Bufo angusticeps, Breviceps rosei and Tomopterna delalandii have been found nearby and may well be present in the IBA. The lizard Scelotes bipes occurs in the nearby strandveld.
Pressure/threats to key biodiversity
The Botriviervlei and its entire catchment fall within the jurisdiction of the Caledon Divisional Council. The State-owned land on the Houhoek and Groenland mountains is administered by the Department of Water Affairs and Forestry (DWAF). The remainder of the catchment consists of privately owned farmland. About 60% of the drainage basin is used for agriculture, primarily wheat, while the remainder of the farmland is used for sheep pasturage. Soil in the catchment is poor, and fertilizers are used extensively; there is run-off into rivers which poses a major problem. As much as 150 tons of sediment per square kilometre of catchment may be eroded annually, and siltation is a major problem for the estuary. An annual sediment load of 122,000 tons can be expected to reach the vlei from the catchment; this can severely impede river flow and alter the system’s estuarine dynamics. Measures such as switching from wheat to pasturage, or alternating between wheat and pasturage, would reduce erosion in the catchment area considerably.There is controversy concerning management of the system, with regard to the artificial breaching of the dune barrier, which allows estuarine conditions to develop in Botriviervlei. If the dune barrier is breached every 3–5 years, juvenile fish enter the vlei, resulting in good angling in subsequent years. Conversely, angling is poor in years in which breaching does not occur. It has been shown, however, that artificial breaching has detrimental ecological consequences. When considering management options, a range of values should be considered, especially the ecological integrity of the system, as well as interests of the local community, tourism and general aesthetic value. The lowering of water-levels exposes the vlei bottom, resulting in massive macrophyte dieback, consequent eutrophication, and the destruction of much of the aquatic flora and fauna, as well as a reduction in the recreational potential of the area for a considerable period. Any artificial opening should only be made after consultation with Cape Nature Conservation and other concerned biologists.The major obstruction to the Bot river has been the construction of the road-bridge, which cuts across the extensive Phragmites flood-plains at the head of the estuary where the river enters the vlei; it is wholly inadequate in terms of allowing water to pass between the two sections of the flood-plain. Modifications should be made to obstruct the watercourse as little as possible.
BirdLife International (2021) Important Bird Areas factsheet: Cape Whale Coast. Downloaded from
http://www.birdlife.org on 10/05/2021.