Natural Springs contribute their water to rivers and often form the source of rivers. Image credit

Natural Springs contribute their water to rivers and often form the source of rivers. Image credit

ZIMSEC O Level Geography Notes: Surface Water Flow and the Origin of rivers

Surface water flow and origin of rivers

  • Rain falling down on land flows down the slope as sheet flow, rill flow and gully flow all of which contribute to stream discharge.
  • Underground water oozes at certain points called springs and also contributes stream discharge.

Sheet flow

Sheet flow. Image credit

Sheet flow. Image credit

  • It is a type of overland flow or downslope movement of water which takes the form of a thin, continuous film over relatively smooth soil or rock surfaces
  • is generated when rain falling onto the earth’s surface flows over the whole surface as a thin layer of water.
  • It commonly occurs at the head of the watershed where the slope is gentle and the surface flat e.g. artificial surfaces, rocks etc.

Rill flow

Rill Flow. Image credit MediaWiki.

Rill Flow. Image credit MediaWiki.

  • Rills are shallow channels (no more than a few tens of centimetres deep) cut into soil by the erosive action of flowing water.
  • As the slope steepens,the amount of water increases and sheet flow encounters surface irregularities sheet flow turns into small shallow channels or rivulets known as rills.

Gulley flow

A gulley. Image credit

A gulley. Image credit

  • Rills in turn join up with other rills and form gullies.
  • A gully is a landform created by running water, eroding sharply into soil, typically on a hillside.
  • Gullies resemble large ditches or small valleys, but are metres to tens of metres in depth and width.
  • The process by which gullies are formed is called gullying.
  • A gully may grow in length by means of headward erosion at a knickpoint.
  • Gullies are sometimes known as dongas.
  • Gullies empty into streams which are perennial rivers.

The results of water erosion

  • Sheet flow results in sheet erosion
  • This results in the washing away of fertile top soils and shallow soils.
  • Rock surfaces and plant roots are also exposed by sheet wash.
  • Rill flow results in rill erosion.
  • Gully flow results in gullies also known as dongas.
  • Both Rill and gully erosions results in the formation of dongas and ravines.

The problems of dongas.

  • Can lead to some areas becoming inaccessible as they are difficult to cross especially when it comes to carts and motor vehicles.
  • Disrupts communication lines such as roads.
  • Reduces the area available for crops pastures and settlements.
  • Can lead to the uprooting of trees.
  • Contribute to siltation.
  • Humans and animals can fall into these ravines leading to injuries.

To access more topics go to the Geography Notes page.

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