The Work of Running Water and Underground Water: Objectives, Three Functions of a River

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In the previous lessons we have learnt that the ultimate result of gradation is to reduce the uneven surface of the earth to a smooth and level surface. These agents produce various relief features over the course of time. Amongst all the agents of gradation, the work of running water or rivers is by far the most widespread.


The major objectives of this chapter are:

  • To explain the three functions of running water viz. erosion, transportation, and deposition in the different parts of the river’s course

  • To explain with the help of diagrams the formation of various erosional and depositional features produced by the action of running water

  • To explain the cause of fluctuating water table from place to place, and season to season

  • To explain with the help of diagrams the formation of various relief features formed by underground water

  • To distinguish between stalactites and stalagmites; wells and artesian wells; springs and geysers

The Three Functions of a River

Running water or a river affects the land in three different ways. These are known as the three functions of a river. Throughout its course a river displays all the three activities to some extent. They are:

  • Erosion: Erosion occurs when overland flow moves soil particles downslope. Weathering and erosion supply this rock material or the load of the river. This load acts as the grinding tool. It helps in cutting the bottom and sides of the riverbed, resulting in deepening and widening of the river channel. Both the cutting and removal of rock debris by the river is called river erosion. The work of river erosion is accomplished in four different ways, all of which operate together. These four ways are:

  • Corrasion or Abrasion: As the rock particles bounce, scrape and drag along the bottom and sides of the river, they break off additional rock fragments. This form of erosion is known as corrasion. This is the mechanical grinding of the rivers against the banks and bed of the river. Corrasion takes place in two different ways:

    • Lateral Corrasion: This is sideways erosion which widens the river valley.

    • Vertical Corrasion: This is the downward erosion which deepens the river valley.

  • Corrosion or Solution: This is the chemical or solvent action of water on soluble or partly soluble rocks with which the river water comes in contact. For example, limestone or calcium carbonate, when it comes in contact with water, it is easily dissolved and removed in solution.

  • Hydraulic Action: This is the mechanical loosening and sweeping away of material by the sheer force of river water itself. Some of the water splashes against the riverbanks and enters into cracks and crevices. This undermines the soft rocks with which it comes in contact.

  • Attrition: This is the wear and tear of the transported materials themselves when they roll and collide with one another. In the process the coarser boulders are broken down into smaller pieces. The angular edges are smoothened and rounded to form pebbles.

  • Transportation: River carries rock particles from one place to another. This activity is known as transportation of load by a river. The load is transported in following four ways:

  • Traction: The heavier and larger rock fragments like gravel and pebbles are forced by the flow of river to roll along its bed. These fragments can be seen rolling, slipping, bumping, or being dragged. This process is known as traction and the load is called the traction load.

  • Saltation: Some of the fragments of the rocks move along the bed of a stream by jumping or bouncing continuously. This process is termed as saltation.

  • Suspension: The holding-up of small particles like sand, silt, and mud by the water as the stream flows is known as suspension.

  • Solution: Some parts of rock fragments are dissolved in the river water and, are thus transported.

Suspension, Saltation and Traction

Suspension, Saltation and Traction

  • Deposition: When the stream comes down from hills to plain area, its slope becomes gentle. This reduces the energy of the stream. The decrease in energy hampers transportation. As a result, part of its load starts settling down. This activity is termed as deposition. Deposition takes place either due to decrease in slope or due to fall in the volume or velocity of river water. It takes place usually in plains and low-lying areas. When the river joins a lake or sea, the whole of its load is deposited.

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