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GAMTOOS ESTUARY: Catchment Characteristics |
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Main Rivers and tributaries |
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The Gamtoos has the fourth largest catchment of all the Cape rivers, with an area of 34 438km2 (Heydorn and Tinley 1980) which extends right up into the Great Karoo. Two large tributaries drain the eastern extremity of the Nuweveld Mountain range and the western slopes of the Onder-Sneeuberg mountain ranges of the great escarpment above the Nelspoort and Murraysburg. Major tributaries also drain parts of the Langkloof, Baviaanskloof and Klein Winterhoek mountain ranges. (Ref 1) |
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The stretch of the river known as the Gamtoos, runs from the confluence of the Kouga and Groot Rivers to the sea and is 75km long. the distance from the source of the tributary furthest from the sea to the mouth of the system is approximately 450km. (Ref 1) |
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The
two main tributaries draining the Karoo Region are the Kariega and Sout
Rivers which join at the Beervlei Dam to form the Groot River The
two tributaries draining the southern slopes of the Baviaanskloof
Mountains and the Kouga Mountains are named after these ranges.
'They join above the Paul Sauer Dam after which the Kouga River joins the Groot River to form the Gamtoos. The Loeriespruit on which the Loerie Dam had been built, joins the Gamtoos River 8,5km from its mouth. (Ref 1) |
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Dams |
| Beervlei Dam, Kouga Dam, Loerie Dam | |
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River run-off |
| This is given as 485 x 106m3 by Noble and Hemens (1978). In the publication River flow data of the Division of Hydrology the MAR over the seven year period 1962/63 to 1969/70 was calculated to be 184x106m3 measured at a gauging station on the Gamtoos River at Patensie. This is 50km upstream from the mouth and above the confluence of the Gamtoos River and the Loeriespruit. (Ref 1) | |
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Geomorphology |
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Geology |
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The geology and geomorphological features of the Gamtoos Estuary and adjacent shoreline have been described by the City of Port Elizabeth's Metropolitan Planning Unit in the coastal study undertaken in 1978. According to this publication, shifting windblown dunes cover the fore-shore area. These are fairly wide at the Gamtoos rivermouth and narrow towards Van Stadens rivermouth in the east. Inland of these modern dunes a wide belt of older vegetated dunes occur which cover arenaceous shelly limestone, overlying pre-Cape phyllites, limestone, quartzites and conglomerates. (Ref 1) |
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The dunes in front of the coastal hills between the Gamtoos and Maitland rivermouths have cut off the drainage of a few rivers flowing down a southward dipping dissected coastal plain. This has resulted in the formation of vleis and pans behind the older dune-fields. (Ref 1) |
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As has already been stated the Gamtoos flows through a broad alluvial floodplain. According to Alexander (1976) the two main tributaries, the Kouga and Groot rivers cut through quartzitic material of the Table Mountain Sandstone series which make up the Kouga and Baviaanskloof mountain ranges. "Downstream of the confluence, these rivers become the Gamtoos river, which follows a 90 km long tortuous course through a flat alluvial plain bounded by soft Enon Conglomerate. In the upstream reaches, the conglomerate consists of a homogeneous mixture of hard, well-rounded boulders, cobbles and gravels derived from the T.M.S. in a weakly cemented sandy matrix. Towards the mouth, the conglomerate is predominantly composed of finer gravels and sand which occur in more defined layers". (Ref 1) |
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AL du Toit (1954) describes the cliffs on
the eastern bank of the river upstream of the mouth as "sandstones
with reddish marls and grey sandy clays".
The river bank is composed of a conglomerate of rounded pebbles in a sandy matrix containing a large amount of shell particularly oyster shells. (Ref 1) |
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