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 they 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

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