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GROEN RIVER ESTUARY: Catchment Characteristics


Main Rivers and tributaries

67 km up to the junction with the Kysrivier and Wilgerhoutsrivier near Loerkop
  Tributaries: Kysrivier, Wilgerhoutsrivier, Swart-Doringrivier, Klipfonteinrivier  (Ref 1)

Dams

River run-off
The Groen falls within a region of episodic rainfall (Hyedorn and Tinley 1980).  The mean annual rainfall in the upper reaches of the catchment ranges between 150mm and 200mm. whilst in the lower reaches down to the coast, it varies from 100 to 150mm (Le Roux and Ramsey 1979; Heydorn and Tinley 1980).  (Ref 1)
The episodic nature of the flow is borne out by flow data for a tributary of the Groen, the Swart-Doringrivier for the period May 1967 to October 1979.  These data were taken from River flow data, monthlly summaries Octover 1960 - September 1970, (1978) and unpublished records obtained from the DWAF (data for the period 1971 to 1979).

From 1967 to 1979 a gauging station, situated at the national road bridge crossing the Swart-Doringrivier about 35km south of Garies and 26km from the confluence with the Groen was in operation.  As this gauging station is the only known one in the Namaqualand coastal region, and also because flow patterns in Namaqualand rivers are so irregular, a fairly detailed flow history is given below.    (Ref 1)

The Swart-Doringriver flowed continuaously, although very irregularly from the end of April 1967 until September 1968 when it dried up.  It did not flow again until March 1972 when there was a brief period of two days of run-off.  The next period of flow occurred from December 1973 to February 1974.  The river flowed sparodically from June to September 1974 during which time a maximum peak flow of 44,33 cumecs occurred on 23 August.  This coincides with the floods in the Groen on 24 August.  From September 1974 to October 1978, no further flow was recorded.    (Ref 1)
Much of the water flowing down the Swart-Doringrivier is probably absorbed by the sandy riverbed between the gauging station and the confluence with the Groen.  Hence it is probably only during extreme floods (such as in August 1974), that surface flow from the Swart-Doringriver actually reaches the Groen.  However, the peramanent surface water in the Groen Estuary is partially maintained by springs situated about 2,5km upstream of the mouth.  Flow from the Swart-Doringriver probably charges the water table in the Groen riverbed which in turn supplies these springs.  (Ref 1)
Geomorphology
Geology
The geology of the riverbed and inland catchment area is predominated by granites of the Namaqualand Natal metamorphic complex.  Along the coast, these bedrock granites are overlain by unconsolidated sands with fluvial and terrestrial gravels, shells, limestone and calcrete cappings.
The Groen, as is the case with other Namaqualand rivers, has been incised into granite bedrock and can be seen as a small water course running dwon a wide shallow valley.  The valley is filled with alluvium and rounded granite boulders.  Granite outcrops can be seen in places in the valley and also along virtually the whole of the adjacent coastline. (Ref 1)
Diamonds have been found in a terrace on the southern side of the Groen River, near its mouth on the farm, Klipkuil.  The terrace lies from 9 to 14 m above the riverbed and is formed of coarse shingle containing water-worn boulders of granite and surface quartzite, up to 1 m in diameter. (Ref 1)

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