River System and Processes (Revised)

8/3/2019 River System and Processes (Revised)

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RIVER SYSTEM AND PROCESSES

A RIVER SYSTEM

Importance of Rivers:

Shaping landformsThe Grand Canyon

Transportation routestrade and communicationEg. Mekong River


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Table- Countries in the Mekong River basin

Nations Area (km2) Basin in nation (km2) Ratio

The People's Republic of China 9,597,000 165,000 1.7%

The Union of Myanmar 678,030 24,000 3.5%

The Lao Peoples Democratic

Republic 236,725 202,400 85.5%The kingdom of Thailand 513,115 184,240 35.9%

Cambodia 181,100 154,730 85.4%

Social Republic of Vietnam 331,700 65,170 19.6%


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Political Boundaries Eg. Danube

Fresh water supplyEg, Gangesfresh waters from Himalayas Mountains


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Uses of river

Hydroelectric power

Irrigation

Fertile alluvial soil

Tourist attraction

Hydro-electric power

Dams are often build across rivers to tap on the river energy to generatehydroelectric power.

Hydroelectric power plants convert the kinetic energy contained in falling waterinto electricity.

Hydropower is currently the world's largest renewable source of electricity,accounting for 6% of worldwide energy supply or about 15% of the world'selectricity. In Canada, hydroelectric power is abundant and supplies 60% of our

electrical needs.

Hydroelectric power plants capture the energy released by water falling through avertical distance, and transform this energy into useful electricity.

In general, falling water is channeled through a turbine, which converts thewater's energy into mechanical power.

The rotation of the water turbines is transferred to a generator, which produceselectricity.

The amount of electricity, which can be generated at a hydroelectric plant, isdependent upon two factors.


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- the vertical distance through which the water falls, called the "head", and- the flow rate, measured as volume per unit time.

Irrigation

Channeling of water from nearby rivers to places that are in need of water Irrigation is very important to areas where there is long period of drought, areas

with seasonal rainfall.

Irrigation has helped farmers to have double or even up to triple cropping peryear.

Fertile alluvial soil

Floodplain and delta are good agricultural land, which have fertile alluvial soil.

The alluvial is increased when the river floods.

The flat and low-lying floodplain and delta makes it easier for people to buildsettlement and farm. Many urban settlements start in the fertile floodplain.

Bangkok in the Menam Chao Phraya Delta of Thailand is one good example.

Tourist Attraction

Famous rivers are popular tourist attraction. Tourist comes to see the spectacular

view of the various landforms in the river. Many

people flock to North America to see the Niagara fall

on the Niagara River. The Singapore River is also one

of the tourist attraction in Singapore.

Recreational Purposes

Rivers are also places that provide a very good

environment for whitewater rafting and various other

water sport.


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Formation of Rivers

Continuous recycling of water between the atmosphere and the land surface

Heat Evaporate water vapour mixed with other gases in the atmosphere air masses containing water vapour gets heated the become lighter risesdecrease in pressure air masses cool condensation tiny droplets of clouds

droplets gets bigger heavier and larger droplets falls as rain

In colder places water vapour changes into light ice crystals snow

Therefore, the 2 types of precipitation = rain and snow


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RAIN / MELTED SNOW

Usually falls

onto the ground

unless trappedby trees or

plants

maybe absorbed bythe plants or be

evaporated

Those on

the ground

willinfiltrate the

soil

flowdownwardstill it

reaches the

bedrockmove

laterally

exit from

openings(springs)

enter

streams,rivers, lakes

However,

some may

remainedabove the

bedrock

layers ofwater-

saturated

soil or sand

If soil is

impermeable

water iscollected in

undergroundpools or lakes

When the soil

has reached its

capacity tocontain water

water flowson the land as

surface runoff

streams,

rivers & lakes


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Parts of a River

River System OR Drainage System = Inter-connected streams of different sizesand lengths

Like a tree

SourceMountainous areas Tributariesthe starting source of small streams that are connected to the main

river

Distributariessmaller channels usually found at the mouth of the river whichhave been branched out of the main river

Drainage Basinthe area drained by a river system

Watershed or water dividethe highlands that divide one drainage basin fromanother


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Course of a River

The gradient of the river changes as the water flows downwards

Sourceusu steeper

Mouthusu Gentler

Course Features

Upper River begins Many small streams and channels joined to form

larger ones form a river

Middle River meanders

Many tributaries join the river

Lower Meanders are common

Many distributaries

Flows towards the mouth into the sea

Delta may form at the mouth

ENERGY OF A RIVER

Functions of a river = erosion + transportation and deposition

Without energy no work can be done

Hence, energy = volume + speed


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Volume of Flow

Measured by River Discharge (the volume of water that flows through a point

along the river channel within a given time) Factors that affect the amount of discharge

Factor Amount of Discharge

Amount of

Rainfall

More rainfall greater vol. of flow greater discharge (V.V.)

Duration of

rainfall

Longer duration of rainfall greater vol. of flow greater

discharge (V.V.)

Size of Drainage

Basin

Larger drainage basin greater no. of tributaries greater vol. of

flow greater discharge

Presence of

Vegetation

More vegetation greater amount of rain intercepted lower vol.

of flow less discharge (V.V.)Type of Soil More porous soil more water passes through the soil less

discharge (V.V.)Impermeable rock little infiltration more surface runoffmore discharge (V.V.)

Therefore, higher volume of flow greater energy

This energy is stored energy and must be transformed into kinetic energy bygravitational pull (determined by speed of flow)

Speed of Flow Greater speed = greater amount of energy more erosion + less deposition

Speed of flow is determined by:

Factor Speed of flow

Gradient of

River

Steeper slope faster speed of flow (V.V.)

Wetted Perimeter Greater wetted perimeter Greater friction slower speed of flow

(V.V.)

Texture of River

Bed and Bank

Rougher river bed and river banks Greater friction slower speed

of flow (V.V.)

Depth ofChannel

Greater the depth of channel lesser friction from the channelboundaries faster speed of flow (V.V.)


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Wetted perimeter = 10m + 80m + 10m = 100m

RIVER PROCESSES

River Erosion

1. River Erosionwearing down of the banks and bed of a river and theremoval of the eroded materials by the action of gravity and flowing water

2. 4 ways of eroding materials

Hydraulic Action Abrasion

(Corrasion)

Attrition Solution

(Corrosion)

Flowing watersstrikes againstthe banks and

bed of a river Loosened

materials (soil,

sand and single

rocks)

Only requires ashort time

Moving waterscan even widen

cracks in rocks

and break them

down intosmaller

fragments

Rocks in theriver aredragged along

by the swiftflowing water

This scraped andgrind against the

channels walls

and beds

Rock fragmentsare carried bythe water

knocking againsteach other

Breakdown intosmaller

fragments

Changed intorounder andsmoother

fragments

Chemicalreaction of theriver water with

the minerals inthe rocks anddissolves them

into a solution

10m 10m

80m

Cross-section area = 10m x

80m = 800m


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Types of ErosionVolume of flow Gradient Erosion

Upper Course Small Steep Vertical Erosion

(River bed)Middle Course Increasing

(combinedtributaries)

Increasing gentler

Amount of Siltincreases replacing

pebbles and

boulders on the river

beds channel areless rough

smoother flow of

load

Lateral Erosion

(River Banks)

Lower Course Large Gentle Lateral Erosion

(River Banks)

River Transportation


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When eroded materials are moved down the river course

River loadsthe transported materials

The type of transportation depends on the size of the particles, and the speed andvolume of the flow

Traction Saltation Suspension Solution

Large bouldersare too heavy to

be lifted

Roll and slidealong the riverbed

Vertical Erosion

Gravels bouncedand hopped

along the river

To heavy to besuspended

Fine particlesare moved by

the turbulent

flow of thewater

Does not settle

on the beds Esp during

floods

Soluble rocks(eg those in

limestones) are

dissolved in thewater and are

transported

downstream Esp in vegetated

areas

River Deposition

Deposit the load when:o the volume of the flow is reduced ORo the speed of the flow is decreased

there isnt enough energy to move the load (Energy of river < the weight of load)

The coarser and heavier particles will be deposited first

The finer and dissolved particles will be moved downstream and be depositedthroughout along the river course

The amount of sediments deposited depends on the amount of load carried by theriver

Deposition is more common at the lower course where its gradient is gentler


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River Processes at Different Parts of the River Course

Upper coursevertical erosion - erosion

Lower courselateral erosion - deposition

The amount of load transported increases progressively towards the mouth of the

river\ Note: If the gradient of the slope is steeper, the velocity of the river is higher.

Hence, there will be more vertical erosion at the river bed because of gravitational

pull.


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LANDFORMS AND FEATURES

A. Erosional Landforms

1. Valleys and Gorges

Both are usually found in the upper coursevertical erosion

River Erosion typeAbrasion, Hydraulic action and solution

Valleys

Upper course - Vertical erosion steep-sided valley V-shaped valleys

Middle coursevertical and lateral erosion wide V-shaped valleys


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Waterfall is . When river falls over from a high steep gradient. It is formed whenerosion takes place over rocks of different resistance. More resistant rocks will have less

erosion while less resistant rocks will experience more erosion.

Sudden fall in gradient

water plunge

The impact of the water plunge is very greatThe water plunge and the swirling rocks erode the river bed to form a pool plunge


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Victoria Falls (border of Zimbabwe and Zambia)

3. Rapids

Rapids occur when it passed a series of alternating soft and hard rock.


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There would be sudden fall of water along parts of the river and the speed ofthe river is usually fast due to the change in gradient.

The more resistant hard rock would not be eroded as fast as the soft rock andtherefore, there is a difference in level.

B. Depositional Landforms

1. Floodplains and Levees

Floodplains

This is an area of flat land found on either side of a river. This usuallybecomes wider as the river nears its mouth.


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Mississippi River levee at Gretna, Louisiana.

2. Delta As river water enters into the sea or lake (lower course), velocity decreases

energy of the river flow decreases erosion decreases + deposition (fine siltand clay) increase

Over a long period of time layers of sediments at the mouth of the river Delta

As a river flows through a delta deposited sediments obstruct the path ofthe river river divides into several distributaries
http://commons.wikimedia.org/wiki/Mississippi_Riverhttp://commons.wikimedia.org/w/index.php?title=Levee&action=edithttp://commons.wikimedia.org/w/index.php?title=Gretna%2C_Louisiana&action=edithttp://commons.wikimedia.org/w/index.php?title=Gretna%2C_Louisiana&action=edithttp://commons.wikimedia.org/w/index.php?title=Levee&action=edithttp://commons.wikimedia.org/wiki/Mississippi_Riverhttp://upload.wikimedia.org/wikipedia/commons/5/5f/GretnaLevee.jpg


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Formation of Delta depends on:o Lots of sediments must be carried upon entering sea or lake

Many tributaries to increase load Active erosion along the course of the river flows through less

resistant rock

o Tidal waves must not be strong (absence of erosion) (deposited in thecoast exceeds the amount transported away)

o Coastal waters must not be too deep as deposition will not occur on

the sea floor but will be dispersed into the sea instead Physical conditions for deltas to form:

o Gentle or almost flat gradiento Velocity of the river must be lowo Waves and tides must be absento The coast must not be too deep

Deltas are classified according to its shape:1. Arcuate


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"arcuate" delta (arc-shaped), and resembles a triangle or lotusflower

Nile Delta (Northern Egypt)

Nile Delta
http://upload.wikimedia.org/wikipedia/commons/c/cc/Nile_delta_landsat_false_color.jpg


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2. Birdfoot a seaward extension of the river's levee system. The Mississippi River Delta

Deltas are very fertile areas and are important for agriculture; for example,the Nile delta contains 90% of Egypt's farmland.


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C. Erosional and Depositional Landforms3. Meanders and Oxbow lake

Meander

o a river that is twisted and turning and forms hoop-like bends o Lower course - usually found in the lower course (but can occur

anywhere)

o Upper courselittle water low energy rivers bends and turns to

avoid obstructions but no depositional landforms are formed as

there is little load + the flow is too fasto Middle Course less steepvelocity decreases may begin to meander

o River flows around the bend the current is fast hitting theconcave bank erosion by undercutting the water slows down

deposition at the convex bank

Oxbow lakeo Meandering continueso Meander becomes more pronounced till it is separated by a narrow

neck erosion continues the neck breaks and water flows

through a straight channel meander is cut off and separated fromthe main river oxbow lake


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o An ox-bow lake is a horseshoe shaped or crescent shaped lake.


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RIVER CHANNEL MANAGEMENT

Strategies Why manage strategies?

o Prevent destruction of properties and lives

How?o Facilitate navigationo Regulating the speed of the rivero Protecting the river bankso Controlling river floods

HARD ENGINEERING

Re- Channelisation

River Re-sectioning River Realignment

o The widening and deepening of the riverchannel. This increases the capacity of

the water at a specific section of the

river.

How??

o clearing of obstacleso deepening the rivero widening the rivero banks enforcements reinforced b

embankments

o Straightening of a reach of a river, andis often achieved by removing bars or

cutting off meanders.

o Straightening a meandering streamincreases channel gradient byproviding a shorter path, which

increases flow velocities and transport

capacity

o Altering the river channel may lead toa greater risk of flooding downstream,

as the water is carried there faster.

How??

o Building artificial cut-offs at meanders

This may lead to the shortening of the river

increase of water velocity

SOFT ENGINEERING

Afforestation Bank

Protection

Managed/

Ecological

flooding

Planning Bank

Protection

o Trees areplanted nearto the river.

This means

greater

interceptionof rainwater

and lower

o Buildingartificiallevees or

embankme

nts

preventcollapse of

banks

o The river isallowed toflood

naturally in

places, to

preventflooding in

other areas -

o Policies tocontrolurban

development

close to or

on thefloodplain.

This reduces

o Groynesand

Revetment

s


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riverdischarge.

o Plantingvegetation the roots,

plants and tallgrasses bindthe soil

togeth.er

stabilized thebanks or beds

o De-plantingvegetation

trimming orremoval

- increase

rivervelocitystabilize

river

channels

+- remove

stress

causedby the

weight of

the tress

andvegetatio

n

prevent woody

and leafy debristo the river

slow down of

water flow andchoking flood

duringheavy rain

Planting of

vegetationalong the

banks or onlevees

for example,near

settlements.

the chanceof flooding

and the risk

of damage toproperty.

Are Strategies Effective?

Measurement = ability to withstand the test of time

Eg. Embankments may be effective in the short term BUT not in the long termo Usually building of defenses along one side of the bank will lead to

erosion in the other


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Documents

River System and Processes (Revised)