Steenbras River
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Through Geology
Catchment
The main catchment has an area of approximately 70 km2, including the dams, and lies between the Hottentots Holland Mountains to the northwest and the Kogelberg to the southeast, in the Cape Fold Belt. The valley upstream of the lower dam was a fairly broad, open valley. Downstream of the dam, the river flows through a steep-sided rocky valley to the narrow coastal plain on the eastern shore of False Bay at Steenbras River mouth, 34°11.671′S 18°49.194′E / 34.194517°S 18.819900°E. The river mouth is a small valley cut into the narrow coastal strip, with steep rock shores near the mouth and wave-rounded boulder beaches further inshore, and waves reach all the way up the tidal part of the river, which faces directly into the prevailing south-westerly swell. This section of coastline is considered high-energy coast. The length of the main stream including the dams is about 17 km. The dams are now fed mainly by short, steeply graded mountain streams with waterfalls and gorges.
The geology of the catchment is mostly quartzitic sandstone with thin layers of shale and conglomerate of the Table Mountain Sandstone.
The long-term average runoff of the catchment was estimated as 50 x 106m3 per annum
Most of the catchment is within the City of Cape Town boundaries
Mean annual rainfall for the dam between 1963 and 1981 was reported as 984 mm, but is higher, in the order of 2000 mm p.a., in the higher reaches.
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Water quality
The water is generally slightly acid and naturally stained brown by tannins from the fynbos. It has good taste and low suspended solids, but is treated to neutralise the pH and remove the colour, partly for cosmetic reasons and partly to reduce corrosion in pipes.
Steenbras Nature Reserve
Ecology
Terrestrial vegetation types recorded from the lower reaches include:
- Riverine forest, generally close to the river,
- Wet mountain fynbos
- Riverine scrub,
- Drier mountain fynbos
Kogelberg Sandstone Fynbos is also found in the catchment.
Roads
The R44 provincial coastal highway from Gordon's Bay to Hermanus crosses the river about 200 m inshore of the mouth. The N2 national highway crosses the river a few kilometres east of Sir Lowry's Pass,
History
First crossed by European explorers from the Cape settlement circa mid 17th century. Original Dutch name was Steenbrazans rivier, assumed to be named after the red steenbras Petrus rupestris which could be caught from the rocky coastline near the river mouth.
The river was dammed in the early 20th century as a water supply for Cape Town. The Steenbras Dam originally had a capacity of 2.9 x 106m3 and was completes in 1921. It was extended by raiding the wall in 1928 and 1952, to a capacity of 34.3 x 106m3
In 1977 the Steenbras Upper Dam was constructed directly upstream. It is used for the Steenbras pumped-storage hydroelectricity scheme which supplements Cape Town's electricity supply during periods of peak demand.
The dams are part of the Western Cape Water Supply System (WCWSS), which is system of interconnected system of six main dams, pipelines, tunnels and distribution networks, and several minor dams in the Western Cape. Some are owned and operated by the Department of Water and Sanitation and some by the City of Cape Town.