EARTH’S EXTERNAL PROCESSES AND LANDFORM DEVELOPMENT
WEEK 1: DATE: …………………
SUBJECT: GEOGRAPHY
CLASS: SS 2
TOPIC:
EARTH’S EXTERNAL PROCESSES AND LANDFORM DEVELOPMENT CONTENT:
(a) Action of Running Water (b) Processes of River Erosion
(c) Processes of River Transportation
SUB-TOPIC 1: WATER AS AN ENERGY SYSTEM
When rain falls on the earth, it is distributed in various ways. Much of the waters from the rain flow directly off slopes to join streets and rivers, eventually reaching the streams and rivers, oceans and seas. This is a run-off. A great proportion of waters received percolates downward into the soil filling up spaces and joint and pore spaces, and forming what is known as ground water. The source of a river may be spring, lake or marsh, generally rivers flow from uplands to lowlands. The run-off carve out channels as they flow and they transport and deposit materials for very great distances along the course. It becomes great agent of denudation.
The amount of erosion, transportation and deposition a river carries depends on the flow of water through the channel and this flow is determined by several reasons.
The water action in a river flows in two ways; 1. Lamina Flow 2. Turbulent Flow
LAMINA FLOW: This form of flow is found in layers parallel to the bed of the river. This occurs as a result of the slope of river on which the river flows.
TURBULENT FLOW: This flow occurs in a circular form. This is due to interruptions along the course like stacks, rapids and cataracts. Sometimes the loads it carries. The energy of the river decreases when its flow is turbulent because energy has to be used to overcome the friction.
FACTORS AFFECTING THE VELOCITY OF A RIVER
- SLOPE OF THE RIVER VALLEY: The slope of the river is been determined by the nature of rock bed course of the river. The steeper the slope or gradient, the higher the speed or velocity of flow of the river.
- THE SHAPE OF THE RIVER CHANNEL OR VALLEY: There are different types of shape in the channel of a river, V-shape, U-shape, and plain shape. The shape determines the amount of energy a river has for erosion and transportation. A river uses more energy to flow through plain shape valley than V-shape and U-shape valley. This is because plain shape has a large surface area.
- THE VOLUME OF RIVER AND RIVER REGIMES: The volume of waters in a river determines the velocity of that river. The greater the volume of water released by a river, the higher the velocity. During dry season, most river decreases in the volume of water in them, and this affects the velocity of flow. The seasonality in most countries also affects the volume of water in the rivers. This variation is known as the regimes. The knowledge of river regimes helps to control possible floods, store water for irrigations and domestic use etc.
- SIZES OF MATERIALS CARRIED (RIVER LOAD): At the source of any river, it erodes the floor and the sides of the river along its course. The river uses its energy to transport its load and overcome frictions along its channels. The greater the velocity of a river, the greater the materials or loads it can carry or move.
THE COURSE OF A RIVER
The entire length of a river from the source to the mouth of the river is divided into three; namely; Upper course, Middle course and Lower course.
These three have different characteristics features;
Stages of a River Development
Characteristics of Upper Course of a River 1. It marks the beginning or source of a river. 2. It is found around the source or highland. 3. It has a steep side, V-shaped valley.
- There is more of down cutting of the river bed than lateral cutting. 5. The river flow swiftly down the steep slope.
- The dominant work of the river is vertical corrosion or erosion.
- The river is characterised by river capture, waterfall, cataracts, gorges and plunge pools. Characteristics of Middle Course of a River
- Lateral erosion is more than vertical erosion resulting to widening of the river valley. 2. Deposition begins to take place at the side and floor.
- There is increase in the load of the river.
- The volume of the river increases due to addition of water from tributaries. 5. The work of the river is mainly transportation.
- It has a gently gradient leading to low velocity of the river.
- It is characterised by bluffs, river cliff, meander, U-shaped valley, and interlocking spur.
Characteristics of Lower Course of a River
- The flow of the river is very sluggish.
- The speed of the river is drastically reduce. 3. It is the last stage of the river.
- The work of the river is mainly depositional. 5. There is equally active lateral erosion.
- It is characterised by flood plains, ox-bow lakes, levees, delta, braided stream.
EVALUATION:
- Write a short note on the following: i. Slope of the river valley, ii. Ground water, iii. Lamina flow iv. Turbulent
- (a) What is a River Regime? (b) List four characteristics of middle course of a river. (c) Explain the word run-off.
SUB-TOPIC 2:
PROCESSES OF RIVER EROSION
The load or materials carried by a river are the main agents of erosion, but the erosive work of a river consist of four interacting processes. This includes;
- HYDRAULIC ACTION: This is a process by which fast flowing water forces itself into cracks and crevices within the valley under pressure and enlarges the cracks. This enables the water to remove loose materials such as gravel, sand and silts and also able to weaken solid rock by surging into cracks in the rock.
- CORRASION OR ABRASION: This is the wearing away of the sides and floor of a river by the loads it carried. This load includes pebbles, boulders, sand and silt which are being transported. This process eventually widens and deepens the valley of the river because of the loads it carried along the course of the river.
- CORROSION OR SOLUTION OR SOLVENT ACTION: This is the chemical action of the water on the materials it comes into contact with while flowing. For instance, calcium carbonate in limestone is easily dissolved and removed in solution.
- ATTRITION: This is the wearing-down of the transported materials when they roll and collide with one another. During this process large rocks are broken down into smaller stones while angular fragments are rounded into shapes.
TRANSPORTATION PROCESS OF A RIVER
The materials transported by a river are called LOAD. The transportation of the load is undertaken in the following four ways:
- SUSPENSION: This takes place when smaller particles such as silt and mud are carried in suspension as the water flowing along the river course.
- SALTATION: This is the process where larger particles are moved in series of hops and jumps along the river bed, touching the river beds and sides at intervals as they are transported.
- TRACTION: This is a process where large fragments of materials are rolled or pushed along the bed of a river.
- SOLUTION:Thisisa process whereby certain minerals dissolved in water are carried in solutions along the course of a river, from the upper course of a river to the lower course. It is especially potent in limestone regions.
EVALUATION:
- Discuss four processes of river erosion.
- List and explain any three transportation processes in a river course. 3. What do you understand by the word ‘load’?
GENERAL EVALUATION:
- (a) With the aid of annotated diagrams, describe the various stages of a river course. (b) Mention four ways in which rivers is important to man. (WASSCE June, 1999)
WEEKEND ASSIGNMENT OBJECTIVE TEST
- A river transports its load through the following processes except (A) solution (B) suspension (C) traction (D) plucking.
- Which of these is a process of river erosion? (A) Suspension (B) Saltation (C) Corrasion (D) Abrasion (E) Traction.
- The wearing away of the sides and bottom of river channel is called (A) corrosion (B) suspension (C) corrasion (D) solution.
- The seasonal variation in the volume of water in a river is known as the (A) regime (B) drainage (C) pattern (D) water divide.
READING ASSIGNMENT:
Read Erosional and Depositional Features of a River. REFERENCE TEXTS:
- Essential Geography for SSS by O.A. Iwena.
