OVERVIEW OF THE IMPACT OF MINING ON THE ... - IIED pubs

River north of the town of Hoedspruit. Ground water from the Chuniespoort dolomites provides an important
component of the water in the Blyde River.
The Blydepoort Dam is the largest impoundment on the Blyde River and regulates flows in the lower reaches.
Several small farm dams in the upper reaches of the Blyde and Ohrigstad rivers trap water for domestic
purposes and for limited areas of commercial irrigation, as well as livestock watering. The small Ohrigstad Dam
provides water for the town of Ohrigstad.
Flow patterns in the upper reaches of the Blyde and Ohrigstad rivers are relatively stable as these rivers drain
an area that receives some of the highest rainfalls recorded in South Africa (average rainfalls in the headwater
regions of these two rivers exceeds 1,500 mm per year). Both rivers are therefore perennial, though flows
increase during the summer months when rain is received. Smaller tributary streams in the upper reaches are
also perennial, though their flows are more variable. In its lower reaches, the Blyde River receives small
quantities of water from several episodic tributary streams that only contain water during the summer months.
5.6.1.2 Geology
The geological characteristics of the Blyde sub-catchment consist of a relatively complex series of lithological
formations that underlie the area forming the Drakensberg Mountains. In the eastern portion of the upper
reaches of the sub-catchment, the most important features are the deep layers of dolomites of the Chuniespoort
Formation, interspersed with alternating layers of indurated shales and quartzites of the Pretoria Series of the
Transvaal Sequence. In the western portion of this zone, the sub-catchment is underlain by complex
sequences of shales, conglomerates, silicified sandstones and quartzites of the Transvaal Sequence. The hard,
erosion-resistant rocks forming the northward continuation of the Drakensberg Mountains rise steeply from the
Mpumalanga Lowveld, forming conspicuous, steep-sided cliffs.
Water moving through the deep layers of Chuniespoort dolomites in the upper parts of the sub-catchment
becomes saturated with calcium carbonate; this precipitates out to form tufa when the water appears above
ground level. As a consequence, most of the waterfalls in the area are layered with thick, “growing” deposits of
tufa.
In the northern parts of the sub-catchment, downstream of the Blydepoort Dam, crystalline gneissic and granitic
rocks of the Basement Complex underlie the catchment. These coarse- to fine-grained, feldspar-rich rocks
have been intruded by numerous hard, fine-grained syenite “plugs” that are more erosion-resistant; these are
visible as stack-like features across the otherwise undulating terrain in the lower parts of the sub-catchment. In
addition to the intrusive syenite formations, the basement complex has also been intruded by a large number of
dolerite dykes. These dolerites are softer and more easily eroded than the Basement Complex rocks and the
dykes are often visible as troughs in the landscape. These troughs collect rainfall and act as local
watercourses; this water enhances the erosion process.
5.6.1.3 Pedology, agriculture and land use
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