Population ( Demographic Transition Model) and Q and A on rivers

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Population

Over time the world’s population has changed, due to migration and natural changes. At present there are 7billion people in the world

Population indicators

Fertility: this is the number of live births per 100 women aged 15-49 in 1yr. It is also known as the average number of children a woman in a country population has. A number 2.1 means the population is replacing itself

It is affected by a range of different factors e.g.  In LEDCs such as Niger, Angola children are an economic asset due to dependence on them from their parents in activities such as farming so therefore these countries have a high fertility rate.

Mortality: this simply means death and is measured by indicators such as death rate and infant mortality. Infant mortality is the number of deaths of children under the age of 1 per 1000 births per year. It is very pivotal in understanding socio-economic development of a country as high infant mortality is usually linked to essential services such as healthcare

 Death Rate is the number of death per 1,000 per people in 1yr. In countries such as the UK, Germany and Japan where there is a greater and better provision of healthcare the death rate tends to be low

Population Structures

Demographic Transition Model:  the DTM pictured Fig 1, was developed by Warren Thompson describes how the population of a country changes through time. It outlines 5 stages countries go through population change

Fig 1

Stage 1 (high fluctuation) High birth rate and death rate which both fluctuate. Population is very small, and a slow population increase. Examples include tribes in the Amazon rainforest.  Reason for high BR & DR include no birth control high incidence of disease, children are sign of fertility.

Stage 2(early expanding) high birth rate and falling death rate. The population starts to increase rapidly. Reasons for the falling BR include improved public health, better nutrition.  Counties that fit this stage include Bangladesh, Nigeria, and Kenya.

Stage 3(late expanding) declining birth rates and low death rates. Population growth slows down. Reasons for the falling Br include increased personal wealth, family planning, and compulsory schooling making rearing of children expensive. Countries in this stage include Brazil, China.

Stage 4(low fluctuation) Low BR & DR which fluctuate, fertility also begins to fall. This could be due to increased career opportunities for women, people having leisure interests so children aren’t a priority. Countries include UK, Japan, Italy

Stage 5(decline) this period wasn’t part of the model developed by Thompson, however demographers noticed that in a number of countries their population was decreasing or bound to decrease e.g. Germany . Reasons for this include concern about the impact of the future population on resources, an increase in non-traditional lifestyle e.g. same sex marriage.

The DTM is useful because it is a universal concept, it enables comparisons and prediction to be made to countries or population that fit into the DTM, however critics have argued it is euro-centric and not all countries would follow the European sequence of socio-economic changes, also it doesn’t take into account the role of migration or government policies which can reduce or increase population.

Rivers

1. Briefly describe how a river is formed.
• A river begins with many channels which join up to form a stream.
• It is a part of the hydrologic cycle where different types of precipitation can contribute to the water flowing in its channel.
• Rain and snow are two types of precipitation which fall directly into the river.
• Some of the precipitation falling onto the Earth's surface is intercepted by vegetation, while some infiltrates into the ground.
• It then flows slowly downwards as `
groundwater', collecting in soil spaces forming layers of saturated soil or sand.
• When the soil on the surface can no longer absorb any more rainwater, the excess water will flow over the land as
surface runoff.
• The surface runoff, together with the slow moving groundwater contributes to the formation of a river.

2. Describe what comprises a river system.
• It refers to the main river with tributaries of different lengths and sizes.
• The source of the river system usually begins in the mountains, and may have several sources.
• It flows over different land surfaces before ending at the mouth of the river e.g. at a lake or sea.

3. What is the difference between a drainage basin and a watershed?
• A drainage basin refers to an area drained by a river system.
• A watershed (or the drainage divide) is to the stretch of land that forms the boundary of a drainage basin which separates it from another drainage basin. It is usually located along the crests of hills or mountain ridges.
4. Which conditions will cause a river's flow to be large and fast?
A large and fast river is one which has:
• High rainfall (greater volume of flow - greater river discharge)
• Long duration of rainfall (greater volume of flow - greater river discharge)
• Large drainage basin (greater number of tributaries — greater volume of flow — greater river discharge)
• Small amount of vegetation (less interception — more runoff — greater river discharge)
• Impermeable soil (less infiltration into ground — more surface runoff — greater river discharge) • Steep gradient (river flows faster)
• Small wetted perimeter (less friction — less loss of river's energy)
• Smooth bed and bank texture (less friction — less loss of river's energy)

5. Explain how river discharge affects the energy of a river.
River discharge refers to the volume of water that flows through a point along the river channel within a given time. Therefore, an increase in river discharge increases a river's energy, which is stored energy.

6. Explain how the speed of flow affects the energy of a river.
Gravitational pull transforms the stored energy of a river into kinetic energy, changing its speed of flow as a river flows downstream. An increase in speed of flow will increase a river's energy.

7. Describe the ways in which the river processes are carried out.
Erosion
• Wearing down of the banks, river bed and the removal of eroded materials by the action of gravity and flowing water.
• Vertical erosion results in the deepening of a river channel.
• Lateral erosion causes the river channel to widen.
• Occurs in all three courses
- Upper course is dominated by vertical erosion where volume of flow is small but at a greater speed.
- In the middle course, erosion increases as the river discharge increases when it moves downstream. With fewer big obstructing boulders, its speed slows down.
- Lower course is dominated by lateral erosion as it experiences the largest amount of discharge.
• Four process of Erosion
- Hydraulic action is a process by which loose materials are pushed away and rocks in a river are broken down by the sheer force of water.
- Abrasion is a process by which the load carried by a river grinds against the bed and banks of the river, it is also known as corrasion.
- Attrition is a process by which rock fragments carried in a river rub and knock against each other.
- Solution is a process by which river water dissolves or reacts with the soluble minerals in rocks. It is also known as corrosion.

Transportation
• It is a process whereby eroded materials are carried by moving water down the river course.
• Occurs throughout the three courses.
• Load refers to the materials transported by river.
• Amount of load carried increases progressively towards the river mouth.
• Size of sediments transported decreases from upper to lower course.
• The distance the load is transported depends on the size of particles, the speed of flow and volume of flow.
• Four transportation processes
- Traction is a process whereby large boulders roll or slide along the river bed
- Saltation is a process whereby larger particles in a river bounce and hop along the river bed.
- Suspension is a process whereby flowing water transports insoluble fine particles such as clay and silt in a river while keeping them afloat.
- Solution is a process whereby soluble minerals from rocks such as limestone are carried in the river and transported downstream. In tropical areas with dense vegetation cover, dissolved minerals and organic acids from decayed vegetation are often transported by the river, resulting in the river water becoming brackish.

Deposition
• It occurs when the volume of flow is reduced and/or the speed of flow is decreased.
• Also occurs when the river no longer has enough energy to carry its load e.g. in convex banks of a meandering river or when it flows into a course with gentle gradient.
• Coarser and heavier loads are deposited first followed by finer materials, including dissolved materials. These finer materials are usually deposited at the mouth of the river e.g. the sea or lake.
• It is more prominent in lower course where the speed of flow is slow and the load is at its maximum. E.g. estuaries

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8. Using well-labelled diagrams, explain the formation of the following erosional landforms:
(a) Valleys and gorges
(b) Waterfalls
(a)
• Valleys and gorges are usually found in upper course where vertical erosion is prominent.
• In the upper course, the volume of flow is small but the gradient is steep.
• The swift flowing water exerts a powerful force on the river bed eroding, forming a steep V-shaped valley through the processes of hydraulic action, abrasion and sometimes solution.
• A gorge is an exceptionally deep and narrow valley (Fig 3.14) and it is formed when the river erodes vertically through resistant rocks leaving very steep valley ...

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