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S.3 Geography Notes

Deltas

Deltas is a large flat low-lying plain of silt, boulders, pebbles and gravel deposits at the river mouth where it flows into the sea or lake. The deposited material forces the river to divide into channels called distributaries.  These are flanked by swamps, lagoons, spits and bars. The delta is built on the sea and in the senile stage where the river loses competence to carry load.

As the river loses energy, it deposits, heaviest first.  This goes on through the long profile of the river.  However, whenever there is an increase in energy, the river will pick up the deposited material again sweeping it into the sea which is the goal of any river.   As sediments formulate at the mouth, a delta is formed.   

Types of Deltas  

Arcuate is a type of delta which is triangular in shape, with a convex out margin which is rounded and composed of coarse sediments like gravel, sand and slit The Arcuate delta has many distributaries and is the most common on River Nile in Egypt and River Niger in Nigeria.  

It is formed where offshore currents are strong to round the river mouth, sea ward edge with fine and course materials.

Draw the diagram of the arcuate delta 

Lacustrine Delta is an inland delta formed in the lake for example River Omo in Lake Turkana.  

Illustration: Draw the diagram of lacustrine delta 

Bird’s food delta/Digitate resembles the bird’s foot formed as a result of the river carrying a lot of load composed mainly of the fine material or silt and having low wave energy.  

The river channel divides into a few distributaries bounded by levees.  

Illustration  

Estuarine Delta is a type of delta found in submerged river mouth (Rias).   Here a river deposits its load in the submerged river mouth leading to formation of sand banks and Islands.  Around these sand banks and islands, numerous distributaries and currents remove the sediment load as it deposited.  Rufigi has an estuarine delta as it has submerged river mouth and River Zambezi . Associates with delta formation are coastal features like spits and lagoons. Illustration  

Conditions necessary for the formation of a Delta 

  • The river should have a mountainous upper course where erosion is rapid.  The delta building power of a river is greatly increased if it has a number of tributaries which also gather a considerable load of sediments 
  • There should be no strong tidal currents to remove the accumulation of silt from the mouth of the river.  If tidal currents exist, they should be minimal. 
  • Low of flat gradient for accumulation of material
  • Presence of sheltered bay
  • Absence of artificial barriers like walls or dykes at the mouth
  • The river should have a long plain or lower course so that its current will have slackened before it reaches the sea.  Where the gradient of the lower course of the river is shift the strength of the current may be sufficient to carry the sediments well out to the sea beyond the mouth of the river. 
  • There should be no big lakes along the course of the river because the lakes remove sediments from the river channel before reaching the sea. 

Formation (processes for the formation of a delta)

When the fresh rivers meet the saline water of the ocean, the meeting produces an electric charge the causes the clay particles to coagulate/precipitate and settle on the sea bed (a process known as flocculation). The fines particles are carried fastest and deposited as bottom set.  These are covered by slightly course materials which are deposited to form a slope making up the fore set bed.  

The upper part is nearest the; and composed of more coarse sediments form the top set. Increased deposition causes splitting of the river into distributaries.   Levees extend to the sea via distributaries. Spit, bar and lagoons are formed. Lagoons are later filled with sediments becoming swampy places. Plants colonize older parts of the delta.  As the delta takes a sound appearance, swamps disappear forming or leaving dry land. The range which the delta grows varies according to the rate at which silt is deposited.   

Importance or benefits of deltas in Africa 

  • Deltas contain fertile soils so provide man with agricultural land.
  • Deltas attract settlement due to the presence of well drained fertile soils.  The delta of the Nile has for a long time been a home to a large population.  
  • Mining (pottery and petroleum deposits) takes place in delta areas like in Niger delta.
  • Delta are tourist attractions hence tourism.
  • Fishing takes place in the delta, lagoons. The silt that is brought by the river is good food for these planktons.
  • In deltas where the mangrove forests are found the trees are harvested for poles which are good building materials.
  • They have promoted art and craft industry due to the presence papyrus vegetation. 

Problems associated with deltas in Africa 

  • Due to many distributaries a delta has, this impedes communication.
  • Delta’s harbour diseases carry vectors like mosquitoes and snails.
  • Flooding occur especially after exceptionally heavy rains
  • Salinity of the water. 

 

Alluvial fans

 Alluvial fans are similar to deltas (usually referred to as dry deltas) except that they are built on land and not at sea as the delta. An alluvial fan is fan shaped mass of materials (sand and gravel) deposited by a stream with large load as it emerges from a steep narrow valley onto a wide gentle plain.

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