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.
- 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.
- 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.
- 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.
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