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Sources of Water

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Catchment

A catchment is the area of land drained by a single river or body of water.

catchment

A watershed is the line separating two different and adja-cent river systems, which is normally the crest of a hill or mountain.

mountain

Did you know?
The Department of Water & Environmental Affairs (DWEA) 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.

Origin of rivers

origin-of-riversWhen it rains, water starts to flow over the Earth’s surface as sheetwash. That means it 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 tributar-ies 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:

Nile                  6670 km
Amazon             6280 km
Mississippi        6050 km
Irtysh               5150 km
Yangtse            4990 km
Amur                4670 km
Congo               4370 km

The longest river in South Africa is: Orange River 2100 km

Major Drainage Regions in South Africa

The five rivers in the world with the highest discharge:

Amazon                181 000 m3/sec
Congo                   40 000 m3/sec
Yangtse                22 000 m3/sec
Brahmaputra         20 000 m3/sec
Mississippi            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)

Rivers are complex self-regulating 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.

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.

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, 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.

Before removal of alien vegetation              After removalalien-vegetation

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.

wetland-illustration

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.

wetlands are important

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.

Also see:

Wise up to Wetlands
Where does our Water Come From?
Substances Causing Pollution