Important Terminology

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Important Terminology

  • Condensation: The cooling of a gas so that it changes into a liquid, for instance as water vapour cools, it condenses to become water droplets, which, when heavy enough, fall as rain.
  • Confluence: Where two rivers meet and join to form one larger river.
  • Delta: A build up of sediment at the point where a river meets a sea or lake, due to the water velocity slowing and the river having less energy to carry the sediment.See later section for details.
  • Drainage Basin:The area of land drained by a river and its tributaries.
  • Estuary: The point at which a river begins to meet the sea. The river will be tidal, meaning that it will have both salt water and fresh water in it.
  • Evaporation: Water that is warmed, usually by the sun, so that it changes into a gas (water vapour).
  • Evapo-transpiration: The combination of evaporation and transpiration.
  • Fluvial: relating to a river, from the Latin for water.
  • Groundwater: see Percolation
  • Hydrology: The study of water
  • Infiltration: The downward movement of water that seeps into the soil or a porous rock.
  • Mouth: The end of the river, where it meets the sea, or a lake.
  • Overland Flow: When water flows over the surface of the ground. This occurs for a number of reasons: the soil may be saturated and therefore be unable to absorb any more water; the underlying rock may be impermeable or the ground may be frozen.
  • Percolation: The movement of water through the soil or underlying porous rock. This water collects as groundwater.
  • Precipitation: Waterfalling to Earth in any form: e.g. rain, sleet, hail, snow, and dew, all are encompassed by the term precipitation.
  • Surface Run-off: see Overland Flow
  • Throughflow: the movement of water with in the soil sideways, towards the river.
  • Transpiration: The water loss from vegetation into the atmosphere.
  • Tributaries: rivers running into the main one, that form part of the same drainage basin system.
  • Velocity: The speed of the flow of the river.
  • Watershed: The imaginary dividing line between neighbouring drainage basins.

The drainage basin of a river forms an integral part of the entire hydrological cycle (also known as the water cycle), which is shown in the diagram below.

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The drainage basin acts as an open system,with a number of inputs, outputs, stores and transfers.

The main input into the system is precipitation, mainly as rainfall, but also as things such as snow, sleet and hail.

This water is then transferred through the system by the processes of infiltration,percolation, overland flow and throughflow.

During the course of its movement between the sky and the river, water can also be stored in a number of places within the system. Vegetation may interceptthe falling precipitation and store it, water may be stored on the ground in lakes, it may be stored within the soil, or it may be stored as groundwater.

Finally the water will reach the river, which is the primary out put to the system. However water will also have already been lost due to the processes of evaporation, transpiration and evapo-transpiration.

These are also shown in the diagram below:

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As water flows downhill into rivers it can createa number of different drainage patterns. These are primarily influenced bythe underlying geology (rock type) of the area


  • The rivers flow inwards towards a point.
  • Occurs due to the underlying rock forming a basin.
  • Examples include the Sea of Galilee


  • The rivers form a tree shape, with the primary river forming the trunk.
  • Occurs in areas where the rock type is uniform (it is all the same)
  • Examples can be found commonly throughout the world, and include the Mississippi, in the United States.


  • The rivers run parallel to each other downhill.
  • The underlying rock is uniform and the surface is flat


  • The rivers flow outwards from a central point.
  • The underlying rock has been uplifted to become a dome, or may be a cone of a volcano.
  • Examples include the uplifted granite dome of Dartmoor, or the perfect volcanic cone of Mt. Taranaki in New Zealand.


  • The river and its tributaries run parallel to each other, before turning at right-angles to meet up.
  • The underlying rock is an alternating structure of resistant and less resistant rock.
  • The main river, which flows in the direction that the underlying rock dips, is called the Consequent River. The tributaries flowing into it are called Subsequent Rivers.