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S.3 Geography Notes

The Works of a River

THE WORKS OF A RIVER  

As the river flows, it performs three functions/activities: It erodes, it transports and it deposits [erosion, transformation and Deposition]. 

River erosion is done though the following processes: Attrition, Abrasion/Corrasion, Solution/Corrosion and Hydraulic Action. 

River transportation . After erosion, the eroded materials known as the load is transported through the following ways: - Traction, Siltation, Suspension and Solution. 

RIVER DEPOSITION  A river transports its load unit it has insufficient energy to transport it any further. When this happens, the load is deposited. Boulders and pebbles are deposited first because they are heavy, and silt and other fine sediments are deposited last. Rivers can be divided along their course into three fairly distinct parts as the Upper course, Middle course and Lower course.  Some rivers have all three of these stages, while a few have two or only one.  For instance, some small rivers flowing off the plateau edge in South Africa or West Africa, too small, have only upper stage and brief middle stages. Some small rivers flowing in high mountains end their life in a mountain lake and only experience an upper stage. 

RIVER’S LONG PROFILE AND ITS CHARACTERISTRICS  River profile is the outline of the shape of a river from the source to the mouth as illustrated below.

Draw the ideal section along the river from its source to mouth As a river gradually develops its long profile, the cross profile and other general features of the valley also develop, as a result of down and lateral cutting by the river.

THE UPPER VALLEY (YOUTHFUL STAGE) 

In its youthful stage,  

  • The river flows against a very steep gradient
  • The river flow very fast with turbulent flow.
  • It has a V- shape valley  
  • There are interlocking spurs
  • There is vertical erosion  
  • The river has a very narrow deep valley
  • Potholes are common in this stage
  • The river has waterfall and rapids  
  • Erosion is mainly directed to its bed
  • There is no deposition  

Draw the diagram showing the characteristics of the first stage refer to Africa by Minns page 15  

A river in this torrent stage follows a winding course, flowing round obstacles formed by more resistant rocks and led to the formation of the following:  

A gorge is elongated steep sided narrow and deep valley. It forms when water fall retreat up stream.

Formation of a gorge.  Process of formation involves the vertical erosion of the river and by fast flowing water over the soft under laying rock between hard rocks.  

Waterfall and rapids.  Though mainly associated with upper course of rivers, waterfalls and rapids can also be found in the lower courses as well.

Describe the processes responsible for formation of water fall

Plunge pool is a hollow depression formed at the base of a waterfall due to progressive drilling and grinding of the valley floor. Plunge pools are found on rivers with waters in the youth’s stages of rivers in rejuvenated section of the profile. Draw the Illustration of the waterfall and plunge pool refer to Africa by Minns page 16  

Conditions for the formation of a plunge pool 

  • Large volume of water
  • Steep gradient
  • Great erosive energy due to velocity
  • hard rock overlying soft rock 

Processes of formation a plunge pool

Erosion of the underlying soft rock produces a waterfall.   Potholes will be produced at the base on soft rocks.  Undercutting scouring action, eddying and cavitation, swirling and erosion collectively enlarge potholes producing a plunge pool.

THE MIDDLE STAGE

In the middle stage of the river,  

  • There is a reduction in gradient and because of this; the river’s velocity is slowed down  
  • Lateral erosion is dominant in this stage.  
  • This has a wide valley which is deep  
  • There is reduced energy because of the reduced gradient
  • Meanders develops in this stage
  • The river has more water in this stage because of many tributaries.
  • The valley floor is flat (broad valley bounded by a low wall of bluffs – flood plains) because the river has worn back the interlocking spurs.

Draw the diagram showing the characteristics of the second stage refer to Africa by Minns page 15  

THE SENILE LOWER VALLEY/OLD/FLOOD PLAIN STAGE

The senile stage that is when the river is nearing its destination is characterized by  

  • A much-reduced gradient.  
  • In this stage, the river’s speed is greatly reduced and the river flows very sluggishly.  
  • Deposition is far more important than erosion and the load transported mainly include sand, very tiny and light particles and materials carried in suspension & solution.  

Draw the diagram showing the characteristics of the third stage refer to Africa by Minns page 15.

 The main landforms in this stage include ;

Meanders are curved bends in the river channel.  Meanders usually start from the youthful stage as swings in the river but are more developed in the senile stage.  

Oxbow lakes and meander scars . An Oxbow lake is a horse shoe shaped lake formed when pronounced meander is cut off from the main river.   When the water that was trapped in the oxbow lake dries up, a meander scar is formed. 

Flood plain is wide flat plain of alluvium found in the old stage of a river as Illustrated below:

Draw the diagram of the flood plain.

Processes for the formation of a flood plain  Downstream migration of meanders causes the widening of the river valley where the spurs are eroded off and low bluffs formed on either side of the plain.   In this stage the valley filled with alluvium, the river swings freely without touching the valley sides the wide valley filled with sediments at this stage (old) is said to be in its flood plain.

Other features found in the old stage are Levees and Braided channel .

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