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HARTENBOS ESTUARY: Catchment Characteristics

Main Rivers and tributaries

Approximately 20 untamed watercourses enter the Hartenbos along its length.  The two tributaries are the Goedemoedrivier and Melkboomrivier in the upper catchment above the Hartebeeskuil Dam. According to Messrs Lamprecht, Kellerman and De Lange, the Matjiesdrif River runs into the Hartebeeskuil Dam.  (Ref 1)

Dams
Hartebeeskuil Dam

River run-off
Midgley and Pitman (1969) give the mean annual precipitation for the Hartenbos catchment as 462 mm to 509 mm.  According to Heydorn and Tinley (1980) the catchment of the Hartenbos falls between the 400mm and 500mm mean annual isohyets.  (Ref 1)

The mean annual run-off is given as 2 072 morgen feet (5,4 x 106m3) (Midgely and Pitman, 1969) and 5 x 106m3  (Noble and Hemens, 1978).  From flow records in the publications Monthly Flow Records (1968) and River Flow Data (1978), the mean annual run-off for the seventeen-year period from October 1953 to September 1970, was calculated to be 5,3 x 106m3 .  The flow measurements were made at a gauging station situated approximately 5 km upstream of the mouth.  (Ref 1)

Flow records from the above mentioned gauging station indicate that before contruction of the Hartebeeskuil Dam, the river flowed intermittently.

Flow measurements for the gauging station were continued after construction of the Hartebeeskuil Dam until September 1973 when the station was closed down due to the effects of the dam.  During this period, run-off occurred only from July to November 1971 and from April to July 1973.  (Ref 1)  

Geomorphology
Downstream of the open floodplains of the upper estuary, the river enters the mouth area by breaking through a massive 20 to 30 m high barrier dune belt which carries the ATKV Holiday development on the south-western bank, and dune vegetation on the north-eastern bank of the river.

Here the estuary forms a funnel-shaped valley which opens asymetrically out to sea in a north-easterly direction. The estuary mouth is characterised by a large flat sandbar, elevated 1 to 2m above mean sea level, which fits into the valley of the Hartenbos River mouth, in this way blocking the inlet most of the time. (Ref 1)

Geology
The bottom material in the estuary mouth consists chiefly of fine to medium grained marine sand.   Terrestrial sediments (black mud and silt) are found upstream of the railway bridge in the meanders and channel system of the upper estuary. (Ref 1)
The Hartenbos riverbed, from the mouth to about 10km upstream of the mouth, lies on Tertiary to Quaternary alluvial valley deposits.  To the north and south of these deposits, Cretaceous to Tertiary deposits of conglomerate, sandstone, siltstone and clay outcrops are found.

Just downstream of the Hartebeeskuil Dam wall the river runs through an area of whitish-weathering quartz sandstone described as medium to coarse grained quartzitic and massive, of the Table Mountain Series.

Immediately to the south of the estuary lies an area dominated by Dwyka tillite and subordinate shale, on which the town of Hartenbos is situated.  To the north and south of the mouth of the estuary, aeolian sands overlie the earlier geological formations in a narrow strip along the coast.  (Ref 1)

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