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

catchment, as well as numerous small and large settlements, rely on water supplied from these ten dams, or
use local boreholes or direct run-of-river abstraction from perennial rivers and streams. There are also over 500
small farm dams located in this sub-catchment and these trap water for domestic purposes and for limited areas
of small-scale irrigation, as well as livestock watering.
Flow patterns in this portion of the Olifants River are relatively stable as they are controlled by water releases
from the Loskop Dam, as well as from the Mogoma Matlala and Arabie dams. This sub-catchment receives
relatively low rainfalls (average 600 mm per year) because large portions of the sub-catchment are located in a
rain shadow zone. The large volumes of water that are used for irrigation, combined with high rates of
evaporative loss, result in relatively low outflows from this sub-catchment to the Phalaborwa Barrage. In
addition, severe veld degradation over several decades has resulted in extensive areas of soil erosion. As a
consequence of this, the Olifants River carries large loads of suspended sediments. Much of this sediment is
trapped by the larger impoundments, reducing their effective storage volume. A case in point is the relatively
small Phalaborwa Barrage (with a full supply capacity of 11 million cubic metres); this impoundment normally
has 60-70% of its volume taken up by accumulated sediments. During periods of high flows in the Olifants
River, the Phalaborwa Water Board opens the scour gates at the Phalaborwa Barrage in an attempt to flush out
some of the accumulated sediments. This results in extremely high concentrations of suspended sediments
(55,000 – 70,000 mg/litre) further downstream in the Kruger National Park. In turn, the Massingir Dam in
Mozambique traps these sediments and gradually loses its effective storage volume.
5.4.1.2 Geology
The geological features of this sub-catchment consist of a variety of different formations of greatly differing ages
and compositions. The upper (south-western) reaches of this sub-catchment are underlain by the acid and
intermediate intrusive rocks of the Waterberg Group, as well as small areas of mafic and ultramafic formations.
The western portion of the sub-catchment is underlain by extensive Karoo Supergroup formations, principally
basalts of the Lebombo Group and Clarens Formation sandstones, with smaller areas of Ecca Group shales,
siltstones and mudstones. The central and eastern portions of this sub-catchment are underlain by a variety of
rocks of the Bushveld igneous Complex. The most important of these are the Lebowa Granite suite to the
south, with outcrops of the Rashoop Granophyre Suite, followed by rocks of the Rustenburg Layered Suite
further to the north and east. Much of the northern portion of this sub-catchment is underlain by rocks of the
Transvaal Sequence, with the silicified sandstones and quartzites of the Black Reef Quartzite Formation being
very prominent, together with Chuniespoort dolomites and Pretoria group shales, hornfels and quartzites.
These rock formations form the spectacular rock formations of the Drakensberg Mountain range through which
the Olifants River has cut a steep-sided gorge. Quartzitic rocks of the Pretoria Group also outcrop at several
points in the central portions of this sub-catchment.
In the most easterly portion of this sub-catchment, granitic and gneissic rocks of the Basement Complex
underlie most of the area east and north of the Drakensberg Mountains. These formations have been intruded
by rocks of the Gravelotte Group (part of the Murchison Sequence) and Rooiwater Complex. Here, quartzites,
schists, basic lava and granitic rocks dominate the Gravelotte Group lithology, whilst easily erodable felsites and
gabbros dominate the Rooiwater Complex lithology.
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