Aqa Geography AS Rivers

7/28/2019 Aqa Geography AS Rivers

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C/W The Drainage basin

Tributaries at the beginning of the drainage basin, small channels that lead into themain channel

Source were the water begins and starts tributaries there are many sources but themain source is the source at the highest point

Plateau flat area of land in the mountains Watershed defines the boundary between two different drainage basins that lead

to different rivers, these are often steep areas

River mouth - were the river joins the sea Flood plain a flat area next to the river in the lower and middle courses of the river

were there are meanders and sediment is deposited on the inside of the meander

creating the flood plain when the river floods

V shaped valleys often found in the upper course of the river where the watererodes strait down to create a v shaped valley.

Springs similar to sources Confidence were a tributary joins onto the main river body Lake a accumulation of surface water Stream order when two tributaries rated 1 join to become a stream order 2 riverThe hydrological cycle

The hydrological cycle is the cycle/flow of water in this case in a river basin


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H/W Human impact upon the drainage basin and how it effects water storage and transfer

Deforestation deforestation effects the transfer of water in the drainage basin because it

vegetation increases interception of water, this means that when removed there isnt much

interception taking place meaning the water isnt stopped but instead falls to the ground

allowing it to build up and increase ground surface run of so water gets into the river

channel quicker.

Building urban towns in building more urban towns we introduce more concrete which is

impermeable so more surface run of goes into drains which drain the water strait into rivers

this speeds up the waters journey to a river but can also lead to flooding downstream were

the water is dumped

Building levys Levys collect all the water I one place and introduce it back into the river,

in doing this they speed up the waters journey the channel because Levys channel waterstrait to the river, this speeds up the water as the water does not have to go through any

geology which would slow it down.

Building damns by building damns humans speed up the waters path to a river channel

because they channel the rain water all one way strait into rivers so it has a direct path

which increases ground water flow instead of having to go through percolation and

infiltration which would usually slow water down.

Planting crops By planting crops humans slow down waters path to the river channel

because crops increase interception which slows water down as it does not allow any

surface run of into the river.

Mining - Mining can slow water down because there are often huge holes in the earth left

from it, this leads to water collecting in the holes, which means there is to much water in

one place so the ground will become saturated lowering the infiltration rate in this area and

slowing down waters path to the river channel.


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C/W What are the factors which could affect the speed at which water gets into the river

Heavy rainfall- lowers the infiltration rate of the soil which causes increased surface water

leading to faster ground surface run off.

Icy/ frozen ground increases the speed of water flowing into the river as the ground isimpermeable so the infiltration rate is lower and there is therefore more surface water and

more ground surface run off so a faster channel flow.

Geology

Geology

1. If the rock was permeable the water would take longer to get into the river. Therewould be increased percolation into the rocks which increases ground water flow

and overall reduces channel flow as the rock slows the water down.2. Impermeable rock is the exact opposite3. Sediment if the rock is jointed there is increased percolation, increased ground

water flow which increases the time for the water to get into the river

Urban areas- more concrete= more impermeable so more surface run of into drains drain

the water strait into rivers so this speeds up the waters journey to a river

Steep slopes steep slope angle = less infiltration due to the effect of gravitational pull on

the water therefore increases surface run on and this means water gets into rivers quicker.


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C/W How discharge changes downstream

Discharge (Q) the amount (volume) of water passing a point in a channel over a period of

time. Measured in cumecs, m/s-3

It is calculated by Q= CSA (cross sectional area) X Velocity

The Bradshaw model shows discharge increases downstream due to:

More water added through the tributaries CSA increased through erosion Velocity increases with proportionately less friction

Discharge can also change over time in the same place (temporally). By measuring this we

can understand flooding. This is shown on a hydrograph.

Hydrograph

Exercise ( on figure 3.1)

Graph 1 (flash flooding) Graph B (non flash flooding)

a short torrential downpour a steep sided valley high drainage density area of granite area of heathland in summer urban area area with soil saturated a mature deciduous forest in winter circular drainage baisin cold period with snow

A long period of steady rain A gently sloping valley Low drainage density An area of chalk An area of mature deciduous trees Rural area Area with non saturated soil Deciduous forest in summer Elongated drainage baisin Mild period

(hrs)

Base flow

Peak discharge

Riseing limb

Recessional limb

Lag time

Bank fall above this is a floodAntecedent

conditions.

(represent

what the

drainage

baisin was

like beforethe rain

Peak rainfallBase flow is what contributes to

the river underground


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Extra reading The water balance

The water balance shows a state of equilibrium in the drainage between inputs and outputs

and can be expressed as

: soil moisture = P E +/- changes in storage

P= precipitations (measured by rain gauges)

Q= runoff (measured by discharge flumes in the river channel

E= evapotranspiration (hard to measure)

The main input = precipitation

The main output = interception

You must also account for changes in storage

In Britain the annual precipitation almost always exceeds evapotranspiration though this is

not always the case as there have been times ie, summer 1975, 76 and 84 when the

situation is reversed. A reversal of the situation is fare more likely in south and east of

England than in other areas. Should this happen then there will be a soil moisture deficiency

as any surplus soil moisture is used up. There will then be soil moisture recharge periods

until the soil reaches field capacity again.

Rivers: water budgets work sheet

1. The precipitation pattern for Birmingham is that there is allot of rail during the early andlate months in the year so there is a water surplus from January to april and then the soil

recharges from September to December due to a lack of rain throughout summer

months and allot of evaporation causing a water deficiency.

3) the difference between the two models is that throughout the year there is allot more

precipitation in Athens than in Birmingham, further more there is also allot less

evapotranspiration again throughout the year in Birmingham compared to in Athens. Due to

this there is a water deficiency for allot more of the year in Athens than in Birmingham and

there is never a water surplus in Athens whereas in Birmingham there is a surplus from

January though to April. Another difference is that soil moisture is only having to be used

from April to September in Birmingham and in Athens it is used for a far longer time from

march though to October.

1. From July to November there is a soil moisture deficiency occurring and from December

to March the soil moisture is recharging.


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2. Two different soil moisture conditions occur due to two different sets of conditions, From

July to November there Is a soil moisture deficiency because there is a falling rate of

evapotranspiration and a rising rate of precipitation which causes there to be more water in

the soil than is needed. This is different in December to March because at this time the soil

is recharging because the rate of rain is rising so more water is available and the soilrecharges due to low rates of evapotranspiration.

3. The process of soil moisture utilization is when the water in the soil is the only water

around because it is not raining as much so plants get water from the soil which uses up

water causing the water table to shrink and this is known as soil moisture utilisation.

1. The precipitation pattern for Birmingham throughout the year stays relativelyconstant although there are fluctuations with the lowest amount of precipitation at

around 50 mm and the highest up to 70. The highest amount of precipitation

happens between august and December with the lowest from January to April.

3) the two models are very different for example there is a relatively constant precipitation

rate in Birmingham but in Athens there are massive fluctuations from 5mm in July to 70mm

in December compared with Birminghams minimum being just 50 and having a similar

maximum of around 70mm. Both graphs show a spike in evapotranspiration in around July

although Birminghams is only up to around 110mm whereas Athens reaches almost

180mm. You can see from the graphs that Birmingham has water surplus from January to

April and Athens doesnt have any water surplus in the year. Athens has a water deficiency

from march to November whereas the uk only has a very slight water deficiency in July and

April. Similarly both graphs show soil moisture use from around April to October.

1. From July to November the soil moisture is in a deficient stage and from Decemberto march soil moisture is being recharged

2. These two different soil moisture conditions occur because of different amounts ofrain. There is a deficiency in soil moisture from July to November because there is a

high mean temperature and potential evapotranspiration rate combined with a low

precipitation rate. From December to march soil moisture is being rechargedbecause there is a high amount of precipitation and low potential

evapotranspiration.

3. Soil moisture utilisation is when there is not much precipitation so the water from inthe soils water table is used instead of precipitation.


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River processes

Types of erosion

1. Abrasion/ Corrasion rocks rub against bed and banks, suspended load/saltation.Happens in the middle to lower course.

2. Attrition rocks grind against each other, becomes smaller and more rounded. Thishappens in the middle to lower course.

3. Hydraulic action force of water smashing up rock against bank and beds, This occurseverywhere

4. Solution/corrosion minerals dissolved, acidic water, lower course of the river.5. Cavitation turbulent flow causes bubbles (rapids) these bubbles collapse and this

collapse creates a force due to hydraulic action. Happens more in upper to middlecourse of the river.

Types of transportation

1. Traction large boulders to heavy to be lifted up role along the bed of the river.2. Saltation eddy currents lift up heavy sediments which then fall down, this causes the

rocks to bounce along the bed of the river.

3. Suspended load small rocks are suspended in the body of the water and carrieddownstream.

4. Dissolved soluble grit and salts dissolve in the water and are carried within the water5. Floatation low density materials are carried on top of the water.


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Calculating river characteristics

Equations

Discharge = V X CSA = Velocity X cross sectional area

Cross sectional area = Width x average depth

Hydraulic Radius = Efficiency of river = Cross sectional area / Wetted perimeter

Gradient = Height change / distance

Hydraulic radius

- Determines efficiency the amount of energy left over after friction for use of riverswork ( processes)

- This is determined by channel size and channel roughness

River 1 = 6/7 = 0.86 river 2 = 15/11 = 1.4

The higher the value the more efficient the river is

The higher the hydraulic radius there is proportionately less water is in contact with the bedand banks therefore less river energy is used on friction and more is available to undertake

erosion and transportation.

Spearmans rank correlation co efficient

This is a statistical calculation to measure correlation. Correlation looks at the relationship

between two variables. There are three times, positive, negative, and no correlation.

Positive as one variable increases so does the other perfect positive = 1

Negative as one variable increases the other decreases. perfect negative = -1

No correlationthe variables dont effect each other, no obvious pattern. no correlation =

0

How to calculate spearmans rank

1. Set null hypothesis negative hypothesis ( has to say their isnt a relationship) iethere is no statistically significant relationship between .. and ..

2. Set out a data table3. Complete data table using values

3

2 2

5

3 3


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4. Complete rank values highest = 1, if are the same then take average5. Calculate the difference in ranks6. Total the square ranks (Ed2)

Site Gradient Rank velocity Rank Differencebetween

the rank

D2

1 8 1 0.22 15 -14 196

2 7 2 0.25 14 -12 144

3 4 4.5 0.64 11 -6.5 42.25

4 5 3 0.43 13 -10 100

5 3 6.5 0.44 12 -5.5 30.25

6 3 6.5 0.78 9 -2.5 6.25

7 2 9 0.95 7 2 4

8 2 9 0.69 10 -1 19 1 12.5 0.82 8 4.5 20.25

10 2 9 1.10 5 4 16

11 1 12.5 1.02 6 5.5 42.25

12 4 4.5 1.13 4 0.5 0.25

13 1 12.5 1.16 3 9.5 90.25

14 1 12.5 1.3 2 10.5 110.25

15 0 15 1.6 1 14 196

Spearmans ranks = 1 (6Ed2

/ n3-n)

- Were Ed2 = sum of d2- N = number of data sets- Spearmans rank = 1 (5594/3360) = 1 1.66 = -0.66= the critical value- Statistical significance: = above 95% for 15 sets of data = 0.55 (+or-)

Above 99% for 15 sets of data = 0.65 (+or-)

Above 99.9% for 15 sets of data = 0.8 (+or-)

- This explains how sure we can be about a critical value- 95% is the minimum level of significance needed and if this is not met you except the

null hypothesis and say that there is no relationship.

- Our value is above 0.55 and 0.65 so we can be 99% certain that our value is correct andwe reject the null hypothesis as the value is over 95% significance. Although there is a

1% chance of error in our calculation it is above 95%significance so I can reject the null

hypothesis so the correlation is statistically significant.

- You must always comment on the level of significance, comment on the level or errorand accept or reject the null hypothesis.


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Gorges

Where are they formed?

Gorges are formed at the boundary between the upper and middle course of a river. They are a characteristic of waterfalls.

How are they formed?

Stages of the waterfall; A river flows over soft and hard rock, the soft rock erodes more quickly River erodes downwards, and bed becomes steeper Soft rock is eroded and a steep stem is created Water falls over hard rock ledge, soft rock below erodes Waterfall erodes beneath and a plunge pool is created Hard rock is undercut and the ledge collapses into the plunge pool These continue and the waterfall retreats upstream creating a GORGE

What erosion causes gorges to form?

Abrasion-Bits of rock and sand in waves grind down cliff surfaces like sandpaper. Attrition-When the waves causes rocks and pebbles to bump into each other and

break

Hydraulic Action-the pounding effect of water, air is compressed into joints andcracks and weakens the rock.

Characteristics of Gorges

Rapids due to boulders Steep sides Sides are higher than the width Decreasing height up stream

Links to Long River Profile

Rivers have a natural tendency to reach a baseline elevation, which is the sameelevation as the body of water it will eventually drain into.

Water flows between boulders causing turbulence and load to bump against the bed.Gorges due to Geological Uplift

Geologic uplift is the upward movement of the Earths surface. Geologic uplift is often associated with earthquakes and orogeny (formation of

mountains).

During geologic uplift, rock layers beneath the Earths surface bump against thesurface layers. Erosion and geologic uplift often work together to create gorges.

Parts of streams or rivers can be elevated, along with land, during the process ofgeologic uplift.


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Distance

Height (m)

Upper

course

Middle course Lower course

Cross section of the

river from source tomouth

This increases the gradient causing the rate of vertical erosion to increase.

Gullfoss

Gullfoss is 32 meters high It is 2.5 km in length It has a huge fault guided waterfall with two main sections.

C/W The long river profile

21/9/12

Upper Couse features

Steep incline Vertical erosion abrasion, hydraulic action, corrosion Smaller area Less water in channel Water falls Low discharge Large load size Small amount of load


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Middle course features

Meanders Ox-bow lakes Greater discharge than upper course Broad, flat flood plain, bordered by gentle slopesLower course features

Wide channel Deep, efficient shaped channel

River processes and the hjulstrom curve

Types of erosion

6. Abrasion/ Corrasion rocks rub against bed and banks, suspended load/saltation.Happens in the middle to lower course.

7. Attrition rocks grind against each other, becomes smaller and more rounded. Thishappens in the middle to lower course.

8. Hydraulic action force of water smashing up rock against bank and beds, This occurseverywhere

9. Solution/corrosion minerals dissolved, acidic water, lower course of the river.10.Cavitation turbulent flow causes bubbles (rapids) these bubbles collapse and this

collapse creates a force due to hydraulic action. Happens more in upper to middle

course of the river.

Types of transportation

6. Traction large boulders to heavy to be lifted up role along the bed of the river.7. Saltation eddy currents lift up heavy sediments which then fall down, this causes the

rocks to bounce along the bed of the river.

8. Suspended load small rocks are suspended in the body of the water and carrieddownstream.

9. Dissolved soluble grit and salts dissolve in the water and are carried within the water10.Floatation low density materials are carried on top of the water.Energy in a river.

For much of the time low flow conditions dominate in a river. However occasionally after periods of heavy rain much larger quantities of water flow

down a rivers channel.

Its under these conditions that the river becomes so powerful that it can transport hugeamounts of sediment including large rocks.


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Armed with these large rocks and boulders rivers are capable of carrying out severeerosion of the landscape forming features such as V-shaped valleys, waterfalls and

gorges

The amount of energy available in a river depends on:

- The height the water has to descend (and its steepness) (gravity)- The amount or mass of water available. A still body of water at any point above sea level has a certain amount of stored energy.

This is called potential energy

When water starts to move downhill the potential energy is converted into Kineticenergy.

Some energy is lost in overcoming friction so the amount of kinetic energy generated isless than the amount of potential energy.

If the channel is steep and has lots of water there will be allot of energy which is whyfast rivers transport move and carry out more erosion.

Rivers with wide shallow banks and allot of rocks use up a lot of energy overcomingfriction.

When a river has surplus energy this is when it can transport material and erode thechannel.

River transportation

The sediment carried by a river is called its load. Load can be identified as three main types:

Dissolved load: this is the invisible transport of chemicals dissolved in water, eg. Calciumcarbonate which dissolves when water flows over limewater or chalk.

Suspended load: is usually very fine grained mud and silt carried within the main bodyof water. It makes rivers look dark and murky

Bedload: This is larger sediment which is transported along the riverbed, it is usually toheavy to be picked up and carried as suspended load. Material is either rolled along the

river bed (traction) Or will move in a series of bounces (Saltation)

The type and amount of sediment transported by a river depends on:

o The flow of the river, most transportation happens during high flow conditionso The nature of the river bed and banks. If the river is flowing through loose material it is

more likely to pick it up and carry it than when it is flowing over solid rock.

o Human intervention, such as lining riverbanks with concrete which reduces the amountof available sediment.


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River erosion

5Erosion is the picking up and removal of material. It can involve picking up individual rocks

from the river bed of wearing away a rocky outcrop. Erosion only occurs when a river has

surplus energy available.

There are three main types of erosion:

Corrasion is where particles of rock carried by the river scrape away at the riverbed andbanks. This dislodges rock particles adding to the rivers load. If a river flows over bare

rocks a sandpapering effect can occur called Abrasion which results in a smooth rock

surface.

Hydraulic action: is where the power of moving water dislodges loose particles from theriverbed or banks. It is most effective at high flow times when water power is very

strong, particularly on the edge of meanders. It is also active in waterfalls when thewater plunges onto rocks bellow.

Solution is the dissolving of chemicals when a river flows over rocks like limestone orchalk.

How the characteristics of a river change downstream

As a river moves downstream man of its features change from how they were earlier up the

channel. As many of these factors change they effect each other and most of them effect

the rivers speed. These factors are shown on the Bradshaw model which categorises them

as to whether the factor increases or decreases as you move down the river.

The velocity of water itself increases as you move down stream as more and more wateris added to the channel from tributaries and rainfall, furthermore the water picks up

momentum from moving down the hills we assume were in the upper course.

The gradient of a rivers channel decreases as you move downstream because the lowercourse of the river is generally closer to the sea which means the river flattens out from

the steam channel that there was in the upper course of the river caused by

mountainous terrain.

The amount of water also increases in the lower cause of the river because moretributaries join onto the river adding more and more water, another effect of this is that

this increases the channel size due to more erosion so further downstream we expect to

see a smoother, wider and deeper river channel due to the factor of more water which

increases erosion. This wider channel means there is also a larger wetted perimeter the

further downstream you go which again leads to a larger cross sectional area of the

rivers channel.

The roughness of the rivers channel decreases downstream. In the upper course of theriver boulders and rocks create a rough channel which leads to turbulent flow. Once you

move downstream fine sediments are deposited to form a smoother lining to the riverbed. This causes a faster river flow.


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Hydraulic radius of a river also increases in the lower cause because the river becomesmore efficient at carrying water due to there being a smother channel and less load.

There is less load in the lower cause because the river drops its load as it moves

downstream, this leads to a smoother channel which and the water isnt using its energy

on carrying a load so it becomes more efficient which is an increase in hydraulic radius.

1.6 River landforms

Overview

Rivers are constantly shaping the landscapes as they carry water and sediment alongtheir courses.

Rivers are responsible for some of the worlds most spectacular landforms such asNiagara Falls and the Grand Canyon.

When a river has excess energy it will erode its channel carving gorges and creatingwaterfalls.

When energy levels fall deposition will occur forming floodplains. A mix of these processes can combine to form meanders.Potholes

Potholes are small circular depressions in the riverbedcarved out of solid rock.

They are commonly found in upland areas close to thesource of a river.

Potholes are formed when rock fragments are drilled intoholes and cavities by the turbulent water.

This erosion is called corrosion Potholes are commonly found in the lame locations as rapids and waterfalls.Rapids and waterfalls

Rivers often flow over a variety of different rocktypes.

Tough rocks are harder to erode whereas weak rockwill erode easily to form steps in the long river

profile.

On a small scale this causes turbulent stretches of theriver known as rapids.

A waterfall is a single more pronounced step in thelong river profile.

It is commonly formed when a river flows over a relatively tough section of rock withweak layers of rock bellow.


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The weak rock is eroded by hydraulic action and corrosion to form a plunge pool. Erosion causes the waterfall to become undercut. And if this

erosion continues then a gauge is formed.

Meanders

Meanders are sweeping bends made by a river commonlyfound in the lowland middle course of a river.

They are formed when erosion and deposition takes place onopposite sides of a river channel

They are formed from horizontal erosion. It occurs when the actual and long river profile are the same. The fast flow of the river swings from side to side eroding the

sides causing the meanders to migrate across the valley floor.

One of the main features of meanders is shallow fast sections called riffles and slowdeep sections called pools.

Over time meanders migrate as opposite meander bends erode towards each other untilthe neck of the meander is broken of to form an ox box lake.

Meanders form from pool and riffle sequences on a strait river. This causes the water towind round the riffles in the pools. As the water comes out the pool it winds round the

riffle in a corkscrew motion that causes erosion of one bank. This is known as helicoidal

flow.

Braiding

A river is described as braided when it becomes dividedinto many separate channels.

They tend to occur when the river is chocked withsediment

They occur often in the middle (upper middle) course ofthe river.

The rivers they occur on often have a large sediment input They occur in rivers downstream from a glacier They form in periods of glacial melting This is often where there is a change in gradient and often width. As the river widens the river becomes shallower loosing energy and increasing friction

and deposition.

This is when the Bradshaw model is being contradicted. This causes deposition in areas which causes a build-up of sediment in places which cuts

the river forming braiding. These areas are known as eyots.

This occurs if a river becomes overloaded resulting in sediment being dumped andforming islands which cut of sub channels from the main channel.


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Braiding is common in rivers which experience large variations in discharge and wherethe load is relatively large.

Braided channels are seasonal features linked to melt water and deposition of sediment.Explanation

1. Glacial erosion leads to transportation of sediment.2. This is washed downstream, to areas in the upper/ middle course were there is a

change in gradient causing a change in width

3. River widened with change in gradient results in decrease in depth and efficiency4. Results in more friction leads to deposition of sediment forming islands the river

flows round.

5. These are seasonally washed away.

Floodplains and levees

Floodplains are extensive flat areas of land on either side of a river which sometimesflood. They are larges in lowland area.

They are not always flat as they sometimes have depressions in the land that can besaturated.

They are often found beyond the levee, and go up to a bluff zone. The silt deposits that form floodplains are fertile so are often used for farming. Floods deposit layers of silt on the floodplain which is how they are created. They are formed from deposition of silt on the inside of a meander forming land. The

meander then migrates leading to the floodplain being formed. Deposits of silt are

deposited on the inside of a bend of a meander as this is the slowest part of the river

with less energy and more friction.

During high flow conditions when the river cant cope with the water load. Wateroverflows and spill onto the floodplain, depositing larger heavier sediment on top of the

banks. This is because the water velocity is much less than in the main river channel.

The deposition is sequential because as you get further away from the channel thedeposition is finer and early on in the flood plain the levee is formed from larger

sediment due to the theory stated by the hjoulstrum curve

This raises the height of the banks. These raised banks are known as levees. levees are raised ground above the channel/floodplain, with a slightly curved profile Naturally formed due to deposition, and are adjacent to river channel. They are formed of larger sediments such as silt sand gravel and pebbles.


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They are sometimes proportionate to the size of the channel and are layered.Deltas

A river loses energy very quickly when it enters the sea. Thisresults in large amounts of silt and clay being deposited forming

a fan shape where the two meet.

Over time the build-up of deposited materials breaks through thewater surface to form new land called a delta.

Often formed from when there is sequential deposition out fromthe river towards the sea, forming a slope

Clay is deposited near to the sea edge due to the negativelycharged clay reacting with the salt in the river causing it to be

deposited ( this process is known as flocculation)

Deltas are generally flat They have a high density of channels The delta often splits into spate channels called distributes. There are two types of deltas:

1. An accurate delta is gently curved2. A birds foot delta represents deposition along several distributes.Explanation

1. Found in the lower course with rivers full of sediment2. They are often found neat the mouth of a river, but sometimes found at the entranceto lakes3. At this point the river meets a slower body of water.4. As the velocity of the river slows the river loses energy and this leads to deposition5. There is sequential deposition with larger particles deposited first and river particles

carried offshore

6. This creates a layered formation of sedimentary beds, with course particles at the topin beds called top set beds, medium in the middle known as fore set beds and fine

at the bottom known as bottom set beds.

7. The top set and fore set beds can form islands in the delta around which the riverpasses, the river separates into distributaries on route to the sea


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River rejuvenation

What is rejuvenation?

Over a long period a river assumes a generallysmooth long profile.

This is the most efficient profile for a river to have inorder to transport water and sediment.

It represents a state of equilibrium with theenvironment.

A river is always trying to achieve this smoothconcave profile

Occasionally an event occurs that de-stabilises thesituation, and causes the river to actively erode its

channel in order to re-establish its smooth long

profile. This renewed period of erosion is called

rejuvenation.

How does a river become rejuvenated?

The sudden and rapid increase in erosion is due to afall in base level.

Base level is the height or altitude to which the riverflows before it joint the sea.

If the base level falls a kink or step called a knickpoint is formed in the rivers long profile.

The increase in the gradient caused by the fall in base level means the river now hasmore energy available. This is used to erode the irregularity, so that the river once again

achieves a state of equilibrium by creating a smooth long river profile.

Erosion causes a nick point to retreat upstream, decreasing in height until it reaches theold graded profile

With rejuvenation you often get vertical erosion in the lower course of the river. Once the river has reached the long river profile and is on it, horizontal erosion occurs

forming a valley and floodplains. These are formed from meanders.

The bluff zones of a rejuvenated valley is often very steep.There are two main causes of rejuvenation:


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- Sea level change. If seal levels falls faster than the rate of vertical river erosion aknick point will form close to the coast.

- River capture. Over hundreds of years, rivers gradually cut backwards at theirsource. This is called headward erosion. Occasionally as a river cuts back it can break

into an adjacent valley and capture the tributaries of a nearby river. There is often a

considerable height difference at the point of river capture and a waterfall is formed.

This is a knick point and as the river cuts into its former valley features of

rejuvenation are formed.

Causes of a falling Sea level

1. Eustatic sea level fall this represents a change in the height of sea level, this iscaused by a fall in temperature and glaciation. This causes more water to be added

to the sea causing a rise.

2. isotactic sea level fall this represents a change in height of level caused by bothtectonic convergence which causes fold mountains to form, pushing the land up,

and also through removal of glacial ice, when the ice is removed the land rebounds

and starts to push up cause in the land to rise. These cause a relative change in the

sea level comparatively to the height of the land.


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River rejuvenation landforms

River Terraces

A river terrace is a remnant or remains of a past floodplain,

which is now left at a higher level after a river has been rejuvenated.

- The process of river rejuvenation gives the channel more

gravitational potential energy which allows more vertical erosion.

This allows the river to cut its channel down through the

former floodplain alluvial deposits (sands and gravels) and leaves the

river surrounded by the old floodplain above.

- The terraces are cut back as the new river valley is widened

by lateral erosion.-This process can be repeated forming a series of terraces known as

paired terraces in the river valley.

- River terraces are useful as they allow shelter from floods on

the floodplain as they stand proud of the new river level.

Incised Meanders

- If a mature river occupies a valley with well-developedmeanders and river rejuvenation occurs in this valley then the

meanders may become incised or deepened.

- Rejuvenation provides more energy for vertical erosion. It isthis increased vertical erosion which cuts large scars into the

landscape.

-

A type of incised meander known as an ingrown meanderoccurs when the vertical erosion or incision is occurring at a

slow rate and when it is accompanied by some lateral erosion.

Here, the outer bends of the meander become deeper and have steeper cliffs, where as

the inner bends (where the water velocity is lower) have a more gentle 'slip off slope'

due to lower rates of erosion.


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Causes of flooding

Physical causes

Lots of rainfall average of 1337 mm in UK in 2012 High in England of 1095 - Old high in 2000 of 1093 mm Leads to more saturation of the ground and an increase in the water table Wet weather caused by a buckle in the jet stream, more weather systems bringing

frontal weather to the UK especially the west and south west

Almost a meter of soil moisture deficit in the summer Now all the soil is completely saturated as the ground water table is near the surface of

the ground so the ground cannot take any more water.

Changeable weather patterns and regular downpours Flash flooding caused by 49mm of rain in 12 hoursHuman causes

- Global warming possibly responsible- Only 1 million spent on flood defences due to economic recession- Lack of flood defences in certain areas- Defences not built for such extreme floods- Farming and ploughing of farm land, causes flat flood plains, deducing interception- Reduced infiltration capacity of farm land often near to rivers and on flood plains- Urban land with no infiltration due to concrete and a lack of drainage and lots of

impermeable surfaces

- Drains not got a good enough capacity so causes blocked or overflowing drains- Urban land with no infiltration due to concrete and a lack of drainage and lots of

impermeable surfaces

Responsible for flooding in the UK

- Met office- Government- Environmental agency- National rivers agency


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Cockermouth floods case study

In exams they can ask you about case studies

1. Assess the causes of a flood both human and physical- Which was the most significant may not be the rainfall but soils moisture, antecedent

conditions

2. Impact and response of the event on an area and contrast itCumbria floods November 2009 Cocker mouth

- Very high hills fells found in the lake district- Is located in the northwest of England, in the most northerly town before Scotland

(Carlisle )

- Cocker mouth is southwest of Carlisle- Cocker mouth is at a confluence point between the river cocker and the river derwent- The area is relatively free of trees so there is a lack of interceptionCauses of the cocker mouth floods

- The flood was not caused due to normal rain because this area is very used to continualrain

- Its location means that areas of high and low pressure are coming across the gulf stream- In this case there was a large depression with a pressure of 958mb which is almost

hurricane conditions

- There was intense rising air combined with relief over the mountains- In 24 hours 314mm of rain fell which is just under a half of what London gets in year- the soil was already saturated by previous events- the mountains had impermeable rock- cocker mouth is located at the foot of the fells and at the confluence of the two river- the event was a high magnitude low frequency event causing a flash flood


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Case study 2 Haiti floods

- Haiti is part of the Caribbean- The Caribbean is a series of islands within the Atlantic ocean between north and

south America

- The Caribbean is located on a convergent plate boundary between north Americanand Caribbean plate

- Both plates are oceanic plates although the north American plate is denser so issubducted under the Caribbean plate and the Caribbean plate folds up forming the

islands

- The islands are very steep sided- People build on the flat land at the base of the mountains on the flood plains of the

rivers

-

The Caribbean is located within hurricane alley- The northern area of Haiti was the affected area due to the path of the hurricane- The area of Gonaives that was affected is surrounded by 4 main rivers- The area is quite mountainous, so is at the base of the mountains by the sea- Mud was washed down from the mountains and covered the town- The amount of forested areas in 1950 was 25% but this was decreased to 1.4%

before the event which reduced the interception

- The upland areas were deforested due to the government moving subsistencefarmers inland to accommodate international incorporations

- Was a city of 250,000 people- Rainfall caused landslides due to the saturation of the ground- 330 mm of rain fell


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The effects of Haiti and cockermouth floods

Cockermouth Haiti

Social - Keswick main street was flooded and evacuations-including that of a hospital were required

- As the floodwater hit cocker mouth water began to flowalong main street through homes and business

- Floodwater hit Workington at night and many people didnot realise what was happening before they were cut off

- North side bridge in Workington collapsed, killing apolicemen

- Power and communication lines were cut of hamperingrescue and escape efforts

- Flooding of roads further hampered rescue efforts andmade accessing hospitals difficult

- The action of floodwater and large debris damaged thestructural integrity of many bridges. Several collapsed

and all 1,800 were closed for inspection taking 6 months

in total.

- There was disruption to day to day life of localbusinesses, for instance people had to take 70 mile

detours to get from one side of town to the other.

- Bus and train services were disrupted as were schools- Where houses and buildings were impacted people had

to take up temporary shelter- 1,700 homes flooded- 1,500 homes without electricity

- Over 3,000 deaths- Massive destruction of

livestock and crops

- Outbreaks of malaria,typhoid, pneumonia and

dengue fever, and

unprecedented outbreak of

west Nile virus

- Main road in Gonaivesflooded so rescue

operations were hampered

- Over 200,000 people weredisplaced by the flooding

- Lack of clean drinking andfood shortages

- Power cut off-

Economic - The floodwater and carried debris damaged the dockgates in the port of Workington

- The floodwater deposited large amounts of debris ontofarmland, rendering it unusable.

- 100 million insurance claims- 70 mile detours costing money- Bridge had to be rebuilt- 1,800 bridges closed some for 6 months-

- Massive destruction oflivestock and crops

- Over 200,000 people weredisplaced by the flooding

-

Environme

ntal

- The floodwater deposited large amounts of debris ontofarmland, rendering it unusable.

-- Massive destruction of

livestock and crops

- Outbreaks of malaria,typhoid, pneumonia and

dengue fever, and

unprecedented outbreak of

west Nile virus

- Water levels reached 3m-


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Flood management definitions soft and hard engineering

Hard engineering: using manmade structures to go against natural processes of the river or

to alter the rivers processes

Soft engineering: working with the river processes using natural methods, which are

sustainable to alter the rivers processes

Channelization: Involves widening & deepening channel to increase speed of flow

Channel straitening: A river may be straightened or re-sectioned. This may occur especially

along a pronounced meander through the creation of artificial cut-offs. Re-sectioning will

lead to the shortening of the river thereby increasing the speed of flow.

Flood relief channels: A channel constructed to carry flood water in excess of the quantity

that can be carried safely in the stream. Also called bypass channel.

Levees: A dike or embankment, generally constructed on or parallel to the banks of a

stream, lake, or other body of water, intended to protect the land side from inundation by

flood waters or to confine the stream flow to its regular channel.

Damns: A barrier constructed across a watercourse for the purpose of creating a reservoir,

diverting water into a conduit or channel, creating a head which can be used to generate

power, preventing gully erosion, retaining soil, sediment or other debris, and/or improvingriver navigability.

Flood warning: flood warning systems decide whether to issue public warnings, they also

dictate whether or not to change the alertness

Topographic manipulation: changing the line that follows the ridges or summits forming the

exterior boundary of a drainage basin and that separates one drainage basin from another.

Contour ploughing: Contour ploughing (or contour ploughing) or contour farming is the

farming practice ofploughing across a slope following its elevation contour lines. The rowsform slow water run-off during rainstorms

River restoration: River restoration is the act of working with a degraded river or stream in

order to return it to a pre-disturbed condition. A disturbance is anything that disrupts a

stream and knocks it out of equilibrium.

River conservation: revalidate and stabilise river banks to reduce erosion

Afforestation: planting trees within the water basin to increase interception and reduce

surface run of
http://en.wikipedia.org/wiki/Farminghttp://en.wikipedia.org/wiki/Ploughinghttp://en.wikipedia.org/wiki/Contour_linehttp://en.wikipedia.org/wiki/Contour_linehttp://en.wikipedia.org/wiki/Ploughinghttp://en.wikipedia.org/wiki/Farming


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Aqa Geography AS Rivers

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