drainage systems

• Drainage basins in South Africa

  • Emphasis on drainage basins, concepts associated with drainage basins, types of rivers, underlying rock structure, drainage patterns, drainage density, stream order, discharge of a river
• Catchment area


  Area over which rain falls and the rainwater is captured by the drainage basin
• River system

  All rivers or stream channels in a drainage basin
• Confluence


  Place where two or more rivers join
• Surface runoff

  Water flowing on the surface
• Source of a river


  Starting point of a river, found in a high lying area where the river begins
• Watershed

  High lying area separating two river systems
• Drainage basin


  Area drained by a river and its tributaries
• Interflow


  High lying area separating two streams of the same river system
• Tributary


  Smaller river that joins the larger river
• Groundwater


  Water found underground
• River mouth


  Where the river ends, for example, at the sea
• Drainage basin processes

  1. Precipitation occurring

  2. Some precipitation gets intercepted by trees

  3. Surface storage

  4. Infiltration

  5. Soil moisture

  6. Percolation

  7. Groundwater flow

  8. Through flow

  9. Surface runoff

  10. Evaporation

  11. Transpiration

  12. Water table
• Episodic river flows only after heavy rainfall and flooding
• Periodic river

  Cuts the wet season water table, receiving water during the rainy season, but does not cut the dry season water table, explaining its flow only during the rainy season
• Perennial river flows throughout the year
• Episodic river flows only after heavy rainfall and flooding, as it does not cut any water table
• Water table


  The upper level of the underground saturated rock where water has reached, varying between wet and dry seasons
• Permanent river flows throughout the year by cutting both the wet season and dry season water tables
• Periodic river flows during the rainy season
• Types of rivers

  • Perennial river
  • Periodic river
  • Episodic river
• Perennial river


  Cuts the wet season water table and the dry season water table, which contains water throughout the year, leading to its continuous flow
• Wet season water table contains water during the rainy season, while the dry season water table contains water throughout the year
• Periodic river flows during the rainy season by cutting the wet season water table
• Episodic river


  Flows only after heavy rainfall and flooding, as it does not cut any water table
• Parallel pattern of rivers caused by steep slopes

  Caused by steep slopes with some relief
• Not all rainfall gets infiltrated

  Resulting in flooding and the river starting to flow
• Exotic river flows from wetter regions through dry regions


  It is considered exotic due to its existence in an otherwise arid area
• Main river in rectangular drainage pattern joins at 90-degree angles

  Underlying rock structure is jointed igneous rocks
• Episodic river flows only after heavy rainfall


  It cuts no water table so heavy rainfall is required for it to flow
• Main streams in trellis drainage pattern are parallel

  Underlying rock structure is folded rocks or rocks with alternate hard and soft layers
• Permanent river flows through drier areas throughout the year

  Source of water allows it to flow continuously
• Exotic river starts out in a humid area


  Flows through regions with enough water allowing it to flow through dry areas
• Radial drainage pattern

  • Dormal features
• Drainage density is the total length of streams per unit area
• Parallel drainage pattern

  • Ridges
• Centripetal drainage pattern


  Pattern where rivers discharge their waters from all directions into a lake, depression, or crater, moving inwards
• Deranged pattern


  • Small streams with no specific pattern, found in flat areas that may have experienced glaciation
• More vegetation


  Lower drainage density
• More precipitation

  Higher drainage density