ESTUARIES IN SOUTH AFRICA

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BOT RIVER ESTUARY: Catchment Characteristics

	Main Rivers and tributaries
	Apart from a number of small tributaries two larger rivers drain into the Bot River. About 20km from the sea the Bot is joined by the Jakkals River and about 5 km downstream of this junction the major tributary, the Swart River, flows into the Bot. This river also collects waters of the smaller Treintjies River. Several small rivers drain directly into the Botriviervlei. (Ref 2)
	The Hopies River flows into the head of the estuary on the southeastern side while the Afdaks River drains into the same side about halfway between the head of the vlei and the sea. (Ref 2)
	The Lamloch Stream flows both into the shallow western extension of the vlei known as Rooisand and into the Kleinmond swamps. The small Isaacs river enters the Kleinmond estuary near its mouth. (Ref 2)
	Dams

	River run-off
	The MAR (mean annual runoff) from these catchments has been variously estimated as being between 47x106 m3 (Jezewski and Roberts, 1986) and 116x106 m3 (Noble and Hemens, 1978). V3 Consulting Engineers (2000) recently adjusted this figure for virgin MAR to 86x106 m3. It should be noted that the latter figure included the sub-catchments of the Bot, Swart and the Afdaks Rivers only. (Ref 1)
	It excluded runoff from the smaller tributaries and the surrounding mountains directly into the estuary across the lower western and eastern shores. The Water Situation Assessment Model (WSAM) (Schultz and Watson, 2002) provided a similar estimate of 88.54 x106 m3 for virgin MAR and 65.9 x106 m3 for present-day MAR. (Ref 1)
	The run-off is 116x10⁶m³ (Noble and Hemens, 1978). The Botriviervlei and catchment lie within the Winter Rainfall Region with highest precipitation in June/July. The mean annual rainfall varies between 500 - 600mm although this figure may be "1200mm and higher" (Caledon Divisional Council, Memo. no. VB 26/1,1975). (Ref 2)
	Geomorphology
	Although the Bot River drains only a small catchment it forms one of the largest lagoons in the southwestern Cape. The Botriviervlei is a relatively shallow triangular coastal lake, 7 km long and up to 2km wide, situated in a broad valley flanked by the mountains of Kleinmond in the North West and Hawston-Onrust-Hermanus in the South West, both about 450 metres high. (Ref 2)
	The dune is separated from the sea by a 100 - 200 metres wide coastal dune belt consisting of a steep, narrow barrier dune ridge, 3 - 6 m high, and a hinterland of low hummock dunes, both partially vegetated by coastal grasses and shrubs. (Ref 2)
	Geology
	Alien forestation (mainly pine species) in the Bot catchment (nearly 2 500 ha) and alien vegetation infestations (mainly Acacia species, covering over 4 000 ha or 4.5% of the catchment area) in the lower reaches and along the river courses play major roles. (Ref 1)
	At the seaward, northwestern end of the main lagoon there is a shallow side arm, Rooisand, which is connected to the main vlei by a narrow, 80 m wide bottleneck called Die Keel. During periods of low water levels in the main lagoon Rooisand is usually dry, but when the water level rises above MSL + 1,7m, the vlei overflows through this area into the adjacent Kleinmond estuary via the Lamloch swamp area. (Ref 2)
	There is also a distinct overflow channel in the dune hinterland which links the western end of Rooisand with the swamps. (Ref 2)