River Drainage Systems

The Orange River is South Africa’s largest river with its source in the Lesotho highlands.  Its catchment area covers nearly half of the country.  The large Vaal River is a tributary to the Orange. The Vaal supplies vast volumes of water to Gauteng. The Orange River runs through the Northern Cape area supplying fertile farm lands with water.

In recent years it has been estimated that the volumes of water extracted from the Orange River has resulted in half the water volumes, measured fifty years ago, reaching the river mouth. Pollution through run off from farm lands and sewage spills also threatens the integrity of the river health. 



A catchment area is drained by a river system which is a network of little streams and rivers that funnel into a single main river located in the lowest portion of the valley.  Water always moves along the gravity gradient from a high to a low point.


  • A watershed is the area of high land that separates one catchment area from another.
  • A river network is made up of tributaries. 
  • A confluence is a place where two rivers meet
  • The river mouth is the place where a river flows into another body of water like an ocean.
  • The source of a river is its furthest point from its mouth.


Surface run off and groundwater feeds rivers with water.   The water table is the top layer of ground level.   The water table can vary as the seasons change from wet to dry.  When the water table is higher than the level of the riverbed, groundwater seeps into the river channel.  This is called baseflow.