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sources of water
catchment
Major Drainage Regions in South Africa
A catchment is the area of land drained by a single river
or body of water.
(Source: Vanishing Waters)
Catchment
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A watershed is the line separating two different and adjacent river systems, which is normally the crest of a hill or
mountain.
origin of rivers
When it rains, water starts to flow over the
Percentages indicate the average contribution of
Earth’s surface as sheetwash. That means it
total runoff from each region
is not yet canalised into streams. Once the
water starts flowing it will soon start to collect
into rivulets which grow into small streams. The small streams flow into each other to form the tributaries. Ultimately the tributaries join into the main stream or river. Streams and rivers extend by eroding headward, that is upstream from the points where the
rivulets join each other. By these processes rivers eventually become very large, with large amounts of water that discharge into
the oceans.
Some interesting facts about rivers of
the world:
The seven longest rivers in the world are:
Did you know?
The Department of Water Affairs and
Forestry (DWAF) has overall responsibility
for the raw (untreated) water that falls in
all our provinces, and even for the water
that flows into South Africa from other
countries. This is because the Department
looks after the water that falls in our
catchment areas. Rand Water buys raw water
from the Department who manage all the public
dams in the country including the Vaal Dam.
Nile
Amazon
Mississippi
Irtysh
Yangtse
Amur
Congo
6670 km
6280 km
6050 km
5150 km
4990 km
4670 km
4370 km
The longest river in South Africa is:
Orange River
2100 km
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The five rivers in the world with the highest discharge:
Amazon
Congo
Yangtse
Brahmaputra
Mississippi
181 000 m3/sec
40 000 m3/sec
22 000 m3/sec
20 000 m3/sec
18 000 m3/sec
The river in South Africa with the highest discharge:
Orange River
22% of total water in South Africa
(including Vaal River as a tributary)
sources of water
Rivers - are complex selfregulating systems supporting
arrays of fascinating and
important communities.
No two parts of the same
river is the same. Various
ecosystems will be present
in one single river. Rivers
are also a reflection of the
“health” of their catchment
area.
Estuaries - this is where the river meets the sea. Estuaries
tend to have wide sand or mudflats that are alternately covered
and uncovered by the tides. Their waters are chemical mixtures
between sea water and fresh water and the biota found here are
adapted to the incoming and retreating tidal conditions.
an interesting fact:
Sea water is denser than fresh water because of the high salt
content. Due to this, one will often find that water entering
estuaries from rivers usually forms a layer of fresh water covering
the more dense sea water. Thus, the deeper waters of estuaries
will be saline, while the upper layers are fresh, or nearly so.
Mangroves - areas on tidal flats,
Desert - an area with a very dry climate and sparse
xerophytic vegetation. Two typical types are sand and
rock deserts. Water is very scarce in deserts and is mainly
found at springs (oasis). Flash floods are also known to
occur in deserts when dry riverbeds suddenly turn into
rivers if rain occurs higher up in the catchment area.
inundated with sea water during
high tides, where the vegetation is
adapted to salt water conditions
in warm to hot climates. Typical
places to find mangroves will
be in estuaries.
Alien vegetation - this
is vegetation that has been
imported into a country from
other countries, eg. Wattle,
Bluegum etc. In South Africa
this vegetation is a threat to
the
indigenous
vegetation,
because it usually uses more
water and tends to compete with
the indigenous vegetation.
Removal of alien vegetation: Before removal
After removal
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Wetland - a permanently waterlogged area that acts as
a natural filter or as a natural dam. Wetlands are therefore
nature’s way of cleaning water by acting as a filter, trapping
sediments, nutrients and even pathogenic (disease-causing)
bacteria. Wetlands store water in summer and release it in
winter, when the vegetation dies off. Vegetation is dominated
by plants adapted to grow in a water environment, eg. reeds,
sedges, etc.
There are different types of wetlands: The smaller wetlands,
pools and ponds, are very often found in mountains. Sponges
are high-altitude wetlands and occur at the sources of rivers
high up in the mountains. Bogs are permanently-wet marshes
dominated by peatmoss causing the water to be dark and acid.
True bogs in South Africa are confined to high altitudes, or
mountain ranges. Swamps are wetlands with trees, their water
being either still or slow-flowing. Marshes are tracts of spongy
land that support low-growing reeds. Most estuaries have tidal
salt marshes on their banks.
Why are wetlands important?
However big or small,
wetlands act as massive hydrological controls of stream flow.
They reduce the force of floods and also store water, releasing
it slowly. Destruction of wetlands in a river’s catchment can lead
to wild and uncontrollable flooding in its lower reaches.