ACTION OF RUNNING WATER
WEEK 2:
DATE: …………………………
SUBJECT: GEOGRAPHY
CLASS: SS 2
TOPIC: ACTION OF RUNNING WATER CONTENT:
(1) Erosional features of a river
(2) Depositional Features of rivers
SUB-TOPIC:
(1) EROSIONAL FEATURES OF A RIVER
The upper course of a river begins at the sources of the river near the watershed. The flow is very swift as it descends the steeps and the action of the river is predominantly vertical erosion. The valley developed is thus deep, narrow and distinctively V-shaped. In some cases where the bed rock is hard and resistant, vertical erosion carves very deep and narrow valleys called GORGES.
GORGES OR RAVINES: The gorges can exist in almost any part of the river course, provided the conditions are suitable. Gorges are narrow, steep sided valleys. They are predominantly found in upper course of a river. Where vertical erosion is greater than lateral erosion. Examples of these features are Shiroro Gorge on River Kaduna in Nigeria, the Volta Gorge in Ghana and the Rhine Gorge in Germany. Gorges provide good sites for building dams across rivers to generate electricity.
A Gorge
V-SHAPED VALLEY: This is formed as a result of vertical erosion of the river. The valley is deepened by vertical corrasion and widened by weathering and mass wasting which causes it to become V-shaped. RIVER CAPTURE: This is also known as river piracy or river beheading. It is a process by which a river acquires the headstreams of another river and thus enlarges its own drainage area at the expense of the other. The weak rivers are deprived of their tributaries; they lose part of their basins and can be totally disconnected to original streams or rivers. Example is Ogunpa streams in Ibadan city of Nigeria. Another is river Tilden Fulani which has cut through the weak link between the Naraguta hills and Shere hills in Jos Plateau State of Nigeria. The rate at which a river erodes its beds depends largely upon its own speed and the nature of the underlying rocks. The development of a sharp almost right-angled bend along the course of the river is called elbow of capture which is common evidence of river capture. Another evidence of river capture is the occurrence of a dry gap in the high ground opposite the elbow of capture. The presence of a misfit in the lower section of a dry gap also strengthens the proof that a river capture has taken place. Misfit is a stream that is too small for its valley.
River capture
RAPID AND CATARACT: These are features formed as a result of sudden increase in gradient due to a resistant rock lying across the river course. Rapids and cataract can occur at any part of the river course, but they are more common in the upper course where changes of the slope are more frequent and abrupt. Many African Rivers have rapid and cataracts. Cataracts can occur when there is a greater jump of water upstream along the course of a river.
WATERFALL: A waterfall is a feature of the upper course of a river. It has a steep slope which forms plunge pools.
Waterfalls develop when a resistant rock overlying a less resistant rock is horizontal or vertical. It may also result when a fault line scarp lies across a river valley, or develops from a rapid; or when water plunges down a plateau edge. Examples include: Kura falls on River Kura near Jos Nigeria, Victoria Falls on River Zambezi, one of the World’s largest falls and the Livingstone falls on River Zaire.
Waterfall
POT-HOLES: Potholes are formed when a fast flowing river erodes the bed rock, circular depressions are created. The depression gradually gets deeper to form potholes. If there is a large pothole at the base of a waterfall, a plunge pool is formed.
RIVER CLIFF AND SLIP-OFF SLOPES: This is a face of a rock standing steeply on the bank of a river. When the flow of a river water AB enter the bend of the river, it dashes straight into B, eroding the outer bank to form a steep river cliff at B. A bottom current CD is set up and is hulled back into midstream at the inner banks. Shingles are deposited at D. Point D is therefore gentle and is referred to as slip off slopes.
River Cliff and Slip-Off Slopes
INTER-LOCKING SPURS: An Inter-locking spur is part of a valley wall projecting from either side of a winding valley. They usually obscure the view of the river valley. When viewed upstream, interlocking spurs overlap, obstructing the view of the valley. As river valleys are widened, their sides are gradually cut back. The sides therefore project to the valley floor in the floor in the form of the alternating slopes. While
downstream, the inter-locking spurs that project from both sides of the valley are cut back by lateral corrosion to form BLUFFS.
Interlocking spur
MEANDERS: This is a feature found in the middle course of a river. They are formed when rivers are unable to flow straight because of the sluggish and low gravity of water movement of heavy loads of sand and other materials. They help to lengthen the course of a river and to widen its valleys. This helps the river to flow in a snake form.
Meanders
EVALUATION:
- List five erosional features of a river.
- Explain any three of the features listed above.
- With the aid of a suitable diagram, explain waterfall.
SUB-TOPIC 3: DEPOSITIONAL FEATURES OF RIVERS
As river flows towards the lower course of the river, the major work is depositional. All the loads carried from the upper course via middle course are deposited at the lower course. This process gives birth to many features which include ox-bow lakes, flood plains and Delta.
- OX-BOW LAKES: An ox-bow lake, otherwise called cut-off or mort lake, is a feature found in the lower course of a river. It is crescent-shaped and has been separated from the main river. As the river meanders, it becomes more noticeable, the neck between two bends of the meanders becomes reduced. Examples can be found in flood plains of Mississippi (U.S.A) and Darling (Australia).
Formation of ox-bow Lake
- FLOODS PLAINS: There is wide, nearly flat or gently undulating plain of Alluvium in the lower course of rivers. Flood plain is made up of sediments deposited on the lowland areas boundary of the river valley. They are formed when river over flows its banks and deposit sediments on the adjacent valley floor continuously for a long period of time. The accumulation of sediment deposited on the surrounding lowland by flood water is called flood plain.
- DELTA: This feature is found in the lower course of a river. The formation of delta is such that as the river moves slowly towards its mouth, it deposits the load it carries. These deposited materials gradually build up, forming a low-lying swampy plain called delta.
As these features form, the river divides into several channels which further divide into smaller channels. All these channels are known as distributaries.
TYPES OF DELTA
There are three types of delta viz; (i) Arcuate (ii) Bird’s foot (iii) Estuary
- ARCUATE DELTA: This delta consists of both coarse and fine sediments and it has the shape of an inverted cone. Delta gradually is crossed by many distributaries. Good examples of Arcuate Delta are the Niger (Nigeria), Nile (Egypt) and the Hwan-Ho.
Arcuate Delta
- BIRD’S FOOT DELTA: This delta consist of very fine materials referred to as ‘silt’ with several long distributaries like the foot of a bird extending into the sea. Good examples are the Mississippi Delta and the Omo River Delta in Ethiopia.
Bird’s foot Delta
- ESTUARINE DELTA: This delta is formed when it is partially submerged in coastal waters and there is a direct flow of water into the sea with little or no sediments. Examples are Amazon, Vistula (Russia) and Seine (France).
Estuarine Delta
CONDITIONS FAVOURABLE FOR THE FORMATION OF A DELTA
- There must be active vertical and lateral erosion in the upper course of the river to provide sediments.
- The coast must be sheltered, preferably tideless.
- The sae adjoining the delta should be shallow to prevent the disappearance of sediments. 4. There should be no large lakes in the river course to remove the sediments.
- There should be no strong current running at right angles to the river mouth to prevent the washing away of the sediments.
- LEVEES: As the river flows, it continues to deposit sediments and materials on its banks when it is in flood, thereby raising the bank upward and higher than usual. These raised banks are called levees.
- Braided Stream: This is a feature of the lower course of a river whereby the river breaks into several channels as a result of deposition of materials along its channel. The river later re-join and split again.
EVALUATION
- List and explain the basic depositional features of a river. 2. What are the importance of Ox-bow Lake?
- Write short notes on i. Arcuate delta ii. Tributaries iii. Distributaries iv. Delta
GENERAL EVALUATION
- With the aid of appropriate diagram, describe any three of the following i. Ox-bow lake
- Meanders iii. Interlocking spur iv. Waterfall
- In what ways is a delta important to man?
WEEKEND ASSIGNMENT OBJECTIVE TEST
- One of the features found in the upper course of a river is (A) gorge (B) u-shaped valley (C) meander (D) levee.
- The bend at which the head waters of a weaker river is diverted into a stronger river is known as (A) wind gap (B) deferred junction (C) elbow of capture (D) beheaded stream.
- Which of the following is a feature of the middle course of a river? (A) Meander (B) Ox-bow lake (C) Gorge (D) River capture.
- Which of the following does not favour formation of deltas? (A) A long upper course of a river(B) A reduction in river velocity at its lower course (C) A lot of sediments in the lower course of a river (D) The existence of large lakes in the middle and lower portions of a river.
- One of the features found in the flood plains of a river is (A) inter-locking spurs (B) tributaries (C) V-shaped valleys (D) Ox-bow lake.
READING ASSIGNMENT: Read the process of wind erosion
REFERENCE TEXT
- Essential Geography for Senior Secondary Schools by O.A. Iwena.
- Comprehensive Geography for Senior Secondary Schools by Peter Oluwafemi.
