EERSTE RIVER: Hydrology and geohydrology

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Map of the EERSTE River

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HYDROLOGY

The Eerste River and its tributaries lie within the winter rainfall region.  yearly rainfall over its catchment ranges from 3 000mm on the mountains to 700mm or less on the coastal plain.  About 80% of the rain falls in a series of winter downpours which bring the river down in spate.   (Ref 1)

GEOHYDROLOGY

The Eerste River has a steep gradient and forms rapids in the top seven km in the Jonkershoek valley, where it is a mountain stream, 5 to 7 m wide, with an average gradient of 24 m/km.  The substrate consists of boulders, stones and bedrock.  (Ref 1)

This is followed by a 5km stretch through the foothills, where the average gradient is 12 m/km and the width ranges from between 7 and 11 m.  The substrate remains rocky and the flow is swift.   (Ref 1)

The lower river flows sluggishly through agricultural lands where the substratum consists of stones and pebbles on coarse sand.  Here the width increases and varies between 8 and 18 m and the average gradient drops to 2 m/km.  The town of Stellenbosch is situated where the upper and the lower sections of the Eerste River meets.   (Ref 1)

The nature of the geology of the upper catchments of the Eerste and Kuils Rivers is such that run-off is relatively high while subsurface flow will be small.  In the predominantly sandy areas of the lower catchment, surfaces drainage is limited particularly in the sand dune areas while subsurface drainage is significant.  Only 7% of the annual precipitation ranges from about 800mm (Ref 1)

References

Ref 1:  Estuaries of the Cape REPORT NO. 16  EERSTE (CSW)  CSIR RESEARCH REPORT 415

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