Upper Secondary Geography_Rivers

Upper Secondary Geography

Chapter 6: Rivers

Types of Questions

Explanation

1. Describe the landform. (4m)2. Identify the river landform shown in Figure ____ and explain its formation. (6m)3. Explain the formation of the landform. (3m)

Essays

1. Discuss the impact of channelization. (8m)2. Evaluate which river management strategy is most effective. (8m)3. With reference to a named example(s), assess the extent to which methods used to

conserve rivers have been successful. (8m)

©Elaine Pang March 2011. Do not reproduce without permission.

River Processes

Energy of River


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Landforms

-Answer questions related to landforms by the format:

Definition

Explanation

Drawing (with caption & annotation)

Example


-Waterfalls

Definition Sudden, steep, vertical flows of fast-flowing water falling from great heights

Explanation Found in the upper course of a river where the gradient is steep, and may be formed through the erosion of rocks of different resistance or through faulting

Drawing (Practice drawing this multiple times before your exam)

A river flows across rocks of different resistance.

The river erodes the less resistant rock more rapidly and this causes a change in the gradient of the river course.

Over time, the river plunges from a great height to hit the river bed below with tremendous force. A waterfall forms as a result. Repeated pounding of the river bed may leave a depression at the base of the waterfall. This depression which is deepened as rocks and boulders swirl around, is known as a plunge pool.

Fig 1. Formation of a waterfall and a plunge pool due to the unequal resistance of rocks.

Example Niagara Falls, USA. Limestone [resistant] & shale [less resistant]

*If you have extra time, you may mention the method of faulting.

Explanation Faulting is the process where tectonic plates move in response to the convection currents in the mantle, causing a break or fracture in the rocks, with one block being uplifted relative to the adjacent block. As water flows from the uplifted rock, it plunges downwards over a great height rapidly to the adjacent block that could be many meters lower.

Repeated pounding of the riverbed creates a depression which is deepened by hydraulic action, creating a plunge pool

Example Victoria Waterfalls, Zimbabwe, Africa


-Gorges

Definition A deep, narrow and steep-sided valley

Explanation Found in the upper course of a river where the gradient is steep, and may be formed through the erosion of a channel vertically, when a river flows through an area with resistant rocks by its sides


A river flows over an area of very resistant rocks. The river erodes vertically faster than the sides of the valley can be worn away.

Vertical erosion continues to produce a narrow, steep-sided valley called a gorge.

Fig 2. Formation of a gorge

Example Katherin Gorge in Australia, which is located 340km southeast of Darwin


-Valleys

Definition Long and narrow depression between two areas of higher land, and usually has a river flowing through.

Explanation Found in the upper course of a river where the gradient is steep. The river has sufficient energy to erode the rocks in its path and cut deep into the channel, causing vertical erosion to occur. Hence, V-shaped, narrow and steep-sided valleys are formed.

Found in the middle course of a river where the gradient is not as steep. The river flows with less energy and cuts less deeply. Lateral erosion occurs more than vertical erosion. As a result, the sides of the valley in the middle course tend to be wider than in the upper course.

Found in the lower course of a river, the valleys are exceptionally wide, due to the very gentle gradient.


A river flows across rocks of different resistance

The river erodes the less resistant rock more rapidly and this causes a change in the gradient of the river course.

Fig 3. Formation of a valley

Example Rhine Valley in Germany


-Floodplains and levees

Definition Area of lowland adjacent to a river, which is built up by the deposition of alluvium.

Explanation When there is prolonged rain, there is an increased volume of river discharge. The river overflows its banks and floods surround the valley. Once the water is out of the channel, it will encounter more friction due to an even larger wetted perimeter from the surrounding low-lying, flat plains.

The increased friction causes a reduction in energy and speed, and the deposition of larger, coarser and heavier materials (alluvium) at the river banks. Smaller and finer particles (silt and clay) are carried further away from the river and deposited on the plains when the river has lost its energy.

Repeated floodings will cause the alluvial deposits on the banks and plains to accumulate, materials left at the banks will become raised and embankments called levees and materials left on the plains will form the fertile floodplains.


River before flooding

During a flood

After subsequent floods

Fig 4. Formation of a floodplain and levees

Example Ganges Plain in India, Indus Floodplains in the South Asian continent


-Meanders

Definition Curves or bends along a river, usually found in the middle or lower course of a river, or where the floor of a valley is broad and the gradient is gentle.

Explanation Erosion occurs on the outer bank of a river, and the eroded sediments are transported to the inner bank where they are deposited. Along the outer bank, the speed of the river is high because there is less friction at the outer bank. Currents from the concave bank will descent downwards, undercutting and eroding materials, eventually depositing the eroded materials at the convex bank. Over time, the outer bank of the river gets undercut and a steep-sided bank, known as a river cliff, is formed.

The sediments removed from the outer bank are carried by the flow of the water and deposited along the inner bank. This occurs at the inner bank because the friction between the inner bank and the water is greater than at the outer bank. Over time, a gently-sloping slip-off slope is formed by the deposition of sediments along the inner bank.


Fig 5. Formation of a meander

Example Middle and lower course of Mississippi River, USA


-Braided Stream

Definition A channel that consists of a network of small channels separated by small and temporary islands called braid bars.

Conditions required: Abundant supply of sediment

Rapid and frequent variations in water discharge

Erodible Banks

Explanation The volume of water is insufficient to carry the entire load. Excess load is deposited. The river splits into two or more channels which twist and turn to avoid the deposited materials and join again as one river when there is no obstruction.


Fig 6. Formation of a Braided Stream

Example Zambezi River (near Victoria Falls), Zimbabwe, Africa

-Oxbow Lakes (Meanders -> Ox-bow)


Definition A lake formed as a result of river deposition at both ends of the cut-off of a meander.

Explanation With constant erosion along the outer banks of two consecutive meanders, the two meanders move closer to each other, forming a loop. Over time, the loop becomes more distinct and is separated by a narrow neck of land, which becomes narrower with continual erosion and deposition.

As deposition and erosion continue to take place along the inner and outer banks respectively, the two meanders eventually meet. The sediments deposited will eventually dam up the water in the cut-off, forming an oxbow lake. The river will now flow in a straight path for a short distance before it begins to meander again.


Fig 6. Formation of an ox-bow

Example Cuckmere ox-bow lake, Sussex, England


-Deltas and Distributions

Definition An accumulation of alluvium and sediments at the mouth of a river where it enters a sea or lake.

Conditions required: Sediment load must be substantial

Tidal currents along the coast must not be too strong

Depth of coastal water should not be too great.

Explanation When a river enters a body of water like a lake or the sea, its speed of flow is reduced. This results in a decrease in the energy of the river and deposition of sediment takes place. At the river’s mouth, sand is deposited close to the shore, while fine silt and clay are carried farther out before being deposited. Over a long period of time, layers of sediment build up and eventually form an extensive platform at the river mouth. This depositional feature is called a delta.

As the river flows through a delta, the deposited sediment may become an obstacle to the path of the river. This forces the river to divide into several distributaries to find a way around the obstruction.


The river begins to deposit its load at the mouth. The deposits at the river mouth cause the river to split into several distributaries.

The accumulation of sediment over time forms an extensive platform jutting out into the sea. A delta is formed.

Plants may eventually grow on the delta, thus stabilizing it.

Fig 7. Formation of a delta

Example Ganges Plain in India, Indus Floodplains in the South Asian continent


Uses of Rivers

1. Hydro-electric powerEg. Chenderoh Station along Sungai Perak, Peninsular Malaysia

2. IrrigationE.g. Chenderah Dam, India

3. Transport (Passageways across mountains)E.g. Chang Jiang, China

4. Fertile alluvial soil for farmingE.g. Ganges Delta, India

5. Tourist attractionsE.g. Niagara Falls, Niagara River, AfricaE.g. Victoria Falls, Zambezi River, Africa

6. Domestic Use7. Industrial Use8. Natural boundaries between countries

Therefore, it is important for people to be able to control and modify rivers, to obtain greater benefits from rivers and minimize flooding which results in negative impacts on the environment.

Management of Rivers


-Channelisation

-Bank Protection & Planting Vegetation


Documents

Upper Secondary Geography_Rivers