Notes on River Studies

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 anymore 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 Erosional processes
- 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
• 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.

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 walls.

(b) Waterfalls are found in the upper course of a river when the gradient of a river bed drops suddenly, the water plunges down from a great height to form a waterfall.
There are two ways a waterfall can form.
- When a river flows over rocks with unequal resistance, the less resistant rocks downstream will get eroded much faster than the resistant layer
• Processes of erosion like hydraulic action, abrasion and solution break down the less resistant rock.
• Over time, this results in a sudden change in gradient and causes the water to plunge.
• As water plunges and hits the bottom of the waterfall, the force of plunging water together with the swirling rocks erode the river bed to form a plunge pool.• An example is Iguazu Falls, at the border of Argentina and Brazil. - By faulting
• During faulting, rocks are uplifted, causing a displacement of rocks where one rock is higher than another.
• When a river flows across an area where faulting has occurred, the gradient of the river bed drops suddenly and a waterfall is formed.
• An example is Victoria Falls along the Zambezi River.

9. With reference to examples, explain the formation of the following landforms:
(a) Floodplains and levees
(b) Deltas
(c) Meanders and oxbow lakes

(a)Floodplains and levees are found in the lower course of a river.
• After a heavy downpour, the volume of flow in a river may increase drastically and the river may no longer be able to hold this sudden increase in volume. The water overflows its banks and a flood occurs.
• As a river overflows its banks, the speed of flow is reduced and it begins to deposit its load especially when the flood starts to subside.
• With its energy reduced, the river deposits the heavier and coarser materials first usually on its immediate banks while the finer and lighter materials e.g. clay and silt are carried further away from the banks before they are deposited.
• Over a series of floods, sediments are deposited layer upon layer forming a flood— Floodplain of River Wyre, England.
• The accumulation of coarser materials on the banks of the river helps to raise the banks higher than the flood plain forming natural embankments called levees.


(b)Deltas are found at river mouths, e.g. Mississippi Delta, USA, Nile Delta, Egypt and Mekong Delta, Vietnam
• When a river enters a body of water e.g. a lake or sea, its velocity is reduced.
• This results in a decrease in energy and deposition takes place.
• Sand is deposited close to the shore while fine silt and clay are carried further out before being deposited.
• Over a period of time, layers of sediments will build up and eventually form an extensive platform of river sediment called a delta at the river mouth.
• Conditions suitable for delta formation include:
- The river must carry sufficient amount of load when entering the sea.
- Weak tidal currents along the coast so that the sediments are not washed out to sea or pushed along the coast much faster than the river can deposit it.
- Shallow coast to ensure that the sediments will not be dispersed into the sea before it is deposited.
• As the river flows over a delta, the deposited sediment may become an obstacle to the path of the river forcing the river to split into several streams to find its way round the obstruction. These streams are called distributaries.

(c) Meanders are formed when a river twists and turns forming hoop-like bends in the river.
• They are most prominent in the lower course but they can be found anywhere along the course of a river.
• In the upper course, a river does not meander because at this stage, the river has little volume and thus little energy so the river bends and turns to avoid obstacles and find the easiest route down the slope.
• In the middle course, where the gradient has become less steep, the river starts to slow down and the river begins to meander, eroding the concave bank by undercutting it and depositing materials on its convex bank where the speed of flow is slow and weak.·

As the water moves downstream and around a bend, it moves in a spiral fashion.
The surface water moves across towards the concave bank and then moves downwards. It then emerges further down along the convex bank.
Thus erosion tends to occur along concave bank and deposition along convex bank.
The concave bank may be eroded to form a steep slope called river cliff.
The deposition of materials on the convex bank results in the formation of a slip-off slope.

Oxbow lake

• In the lower course, erosion and deposition continue on the concave and convex banks respectively and the meanders become more pronounced. Soon a narrow meander neck is formed.
• Eventually the river breaks through the neck and flows through a straight channel. The meander is gradually cut off from the channel forming an oxbow lake.

10. Evaluate the effectiveness of the following river channel management strategies:
(a) River channelisation
(b) River re-sectioning
(c) Vegetation planting and clearance
(d) Bank protection


Re-alignment:
· Refers to the straightening of the river channel
· Method removes meanders and reduces the length of the river channel in order to increase the speed of the river.
· This resulted in the river being able to flow away from the region faster and washes away sediments which have accumulated on the river bed, localized flooding is thus minimized.
· The deepening of the river channel allows the river bed to hold more water and thus reducing the occurrence of flood.

Re-sectioning:
· Refers to the widening and deepening of the river channel, sides of the river bed is also smoothened by replacing soil of the river bed and banks with cement and granite.
· Increases the river’s ability to hold more water, thus reduces the occurrence of flood.
· Re-sectioning also increases the amount of surface run-off from the surrounding areas into the river as more surface runoff can enter the river without causing the place to flood.
· The smoothening of river bed and back by the use of cement and granite reduces the friction between water and the river banks. This increases the speed of the river and hence allowing water to flow away more quickly.

Evaluation of re-alignment and re-sectioning:
· The increased in the speed of river upstream results in an increased in the volume of water downstream. River channel may not be able to hold the excess volume of water resulting in overflowing channel and hence flooding downstream.
It helps to reduce immediate problems of flooding; however the issue of flooding is merely being transferred from one site to another.
· The increased in the speed of the river resulted in increased energy of the river; sediments which were normally deposited on river beds are hence washed off downstream or into the sea or ocean.
· This increased in sediments in the ocean will affect marine life such as seagrasses as there is less sunlight passing reaching the seabed.
· The lining or rivers banks with concrete will also result in the loss of plants and animals as their habitats are destroyed.
· River management strategies will also lead to the loss of wetlands, as with river re-alignment, the shortening of the river course results in the shrinking of wetlands as rivers flows through a smaller area now.

Vegetation planting and clearance
· It is a useful strategy as vegetation helps to slow down the flow of water
· Roots of the vegetation bind the soils and stabilise the river banks.
· It is an effective strategy in the long term and it improves the stability of channel.
· However the weight of trees could cause river banks to collapse and vegetation can contribute to 'accumulation of wood debris in river and thus deflect water flow, accelerate bank erosion, slow down the flow and encourage flooding.

Evaluation of vegetation and clearance:
· This seems to be a more effective way to manage river channels, even though the presence of vegetation could hinder the flow of water.
· Proper maintenance of such vegetation would improve the stability of river channels.
· It is a cheaper and more preferable measure compared to the construction of artificial structures and incurring high cost due to the need to alter the natural course of the river.

Bank protection
· It involves the building of artificial levees, dykes or embankments to prevent river banks from collapsing during heavy rains when volume of rainwater increases tremendously.
· It is an effective strategy in the short term.
· However it is ineffective in the long term.
· It requires a lot of resources (e.g. funds) and manpower.
· It is also aesthetically unappealing.
· The building of defence along one stretch will lead to increased erosion in other parts of the river.

Evaluation of bank protection:
· It is an effective short term measure, but its effectiveness in the long term is doubtful.
· Often, the building of defences along one stretch of the river will lead to an increased erosion rate on other parts of the river.
· Such measures will require high costs in building and maintenance of these structures, which will also interfere with the river's natural course and are considered to be aesthetically unappealing.





Read Users' Comments (13)

13 Response to "Notes on River Studies"

  1. chong, on May 1, 2011 at 3:45 PM said:

    you should state examples for each river channel management

  2. Tomoto Tan, on October 1, 2011 at 6:09 PM said:

    Thanks for the notes! Really helped me :)

  3. SAIYAN GOD, on January 9, 2014 at 10:13 PM said:

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  4. SAIYAN GOD, on January 9, 2014 at 10:14 PM said:

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  5. SAIYAN GOD, on January 9, 2014 at 10:14 PM said:

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  6. SAIYAN GOD, on January 9, 2014 at 10:14 PM said:

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  7. SAIYAN GOD, on January 9, 2014 at 10:14 PM said:

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  8. SAIYAN GOD, on January 9, 2014 at 10:14 PM said:

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  9. SAIYAN GOD, on January 9, 2014 at 10:14 PM said:

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  10. SAIYAN GOD, on January 9, 2014 at 10:14 PM said:

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  11. SAIYAN GOD, on January 9, 2014 at 10:14 PM said:

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  12. SAIYAN GOD, on January 9, 2014 at 10:14 PM said:

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  13. SAIYAN GOD, on January 9, 2014 at 10:15 PM said:

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