The Drainage Basin (Edexcel IGCSE Geography)

Spec point — 4GE1-S3.042

What is a drainage basin?

• A drainage basin is the area of land drained by a river and its tributaries

  • This is also known as the catchment area of the river

• Drainage basins are open systems

• As well as stores and transfers drainage basins have inputs and outputs

  • The input is any water entering the system (precipitation)
  • Outputs are where water is lost from the drainage basin (evaporation, transpiration and into the sea/lake)

• When precipitation falls into the drainage basin it will take different paths. These include:

  • direct channel precipitation which occurs when the water falls directly into a river
  • overland flow (surface runoff) when the water cannot infiltrate the soil due to the ground being impermeable
  • throughflow when the water flows through the soil
  • groundwater flow when the water flows through the rocks

• Every drainage basin is unique

• They have different:

  • shapes
  • sizes
  • rock types
  • relief
  • land use

Drainage basin features

• All drainage basins have some features in common:

  • The watershed is the boundary between drainage basins

  • A source is the point of the river which is furthest from the mouth; this is the point at which the river begins and is usually an upland lake, spring or glacier

    • Gravity then causes water to flow downhill, taking the fastest and easiest path

  • A confluence is the place where two or more streams/rivers meet

  • Tributaries are streams or rivers flowing into larger streams or rivers

  • The mouth of a river is where it enters the sea/ocean or sometimes a lake

The main features of a drainage basin

Channel network

• The channel network consists of the main river channel and all of its tributaries

• Every drainage basin is covered by a network of tributaries which connect to the main river channel

• The number of tributaries in a drainage basin is referred to as the drainage density:

  • Drainage basins with lots of tributaries have a high drainage density
  • Drainage basins with few tributaries have a low drainage density

• The drainage density is the result of the soil and rock under the surface

• Where the rock or soil is impermeable this leads to high drainage density because the water cannot infiltrate

  • This means it flows over the surface in tributaries

• Where the rock or soil is permeable water can infiltrate leading to low drainage density

Exam Hint You need to ensure that you are clear about the difference between a closed system such as the hydrological cycle and an open system such as a drainage basin. Remember a closed system has no inputs or outputs whereas an open system has both inputs and outputs.