HOUTBAY RIVER: Topography, geology and soils

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Map of the HOUTBAY River

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TOPOGRAPHY
The total area of the catchment, calculated from maps and aerial photographs, is 33,8 km2.  

The Disa and the Original Disa originate on Table Mountain at an altitude of 720m amsl. The streams join in Orange Kloof and the river runs the length of the valley through farmlands and residential areas to the sea.  Five dams with a total volume of 22 375, 000 MI control the headwaters of these two streams which supply potable water to Cape Town. (Ref 2)

Taking the longer perennial tributary, the Disa Stream, as the main river course, the length of the river is 2 km, and approximately 26km of tributaries join the river on its course to the sea.  Many of the tributaries are wet-weather or intermittent streams which are dry for most of the year, but flow strongly after winter rains. (Ref 2)
    GEOLOGY                                    
Table Mountain Sandstone overlies a granite base with a narrow band of shale at approximately 200 m.  This granite basement gives the valley its gentle lower slopes, although the rock itself is not often exposed.  Sandstones were laid down on the eroded surface of the Cape Granite 320 million years ago and are known as the Table Mountain Group.  (Ref 2)
These sandstone deposits are the dominant rock in the Cape Peninsula giving the mountains their characteristic tabular form.  Sandstone weathers slowly and the narrow kloofs, such as Disa Gorge, have followed the faults which have fractured the horizontal bedding planes.

It is relatively more porous than granite and in the Hout Bay valley, underground springs are located on the contact zone.  Oxides of manganese and iron are sparsely distributed throughout the sandstone of the Table Mountain Group, and several deposits occur within the catchment.  (Ref 2)

    SOILS                                    
Most of the catchment avor the 200m contour is classified on the Soil Map of The Cape Peninsula (1976) as rocky with shallow soil.  In these areas the parent sandstone is exposed and the sandy soils formed by weathering are soon removed by the high rainfall.  Limited areas of shallow to moderately deep grey sandy soils of the Cartref form occur on the upper slope of Vlakkenberg, Constantiaberg and the Twelve Apostles.  (Ref 2)
The red and yellow apedal soils of Orange Kloof have a clay content > 15%, the underlying material being gleyed as a result of the excess water in the area.  An augerhole drilled on the west side of the river opposite the Orange Kloof Forest Station in March 1983 revealed that the water table was only 50cm below the surface. (Ref 2)
Long fingers of shallow non-hydromorphic soils on weather rock or clayey substrate of the Glenrosa and Swartland forms penetrate the apedal soils of Orange Kloof, following the drainage lines.  These soils are also found on the lower slopes of Vlakkenberg, and in two gullies on Constantiaberg.(Ref 2)
Alluvial deposits of Dundee and Oakleigh forms with unconsolidated underlying material form a broad strip running the length of the valley to approximately the 100m contour, and along the main drainage pathways. (Ref 2)
Red and yellow apedal soils with a clay content of >15 percent are found throughout most of the remaining areas of the catchment. These soils are of Avalon, Bainsvlei, Clovelly and Hutton forms some of which occur on gleyed underlying material.  The sand corridor which runs across Leeukoppie Nek consists of acid Fernwood  soil with upper regic sand of medium grade, while Fernwood is found in the flat low-lying areas around the bay and at the foot of the sand corridor.  (Ref 2)
  References                                      

Ref 2:  Estuaries of the Cape, Report No. 29 HOUT BAY (CW 27)CSIR RESEARCH REPORT 428
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