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AS Geography > Rivers > Flashcards

Rivers Flashcards

1
Q

What is a drainage basin?

A

The area surrounding the river where the rain falling on the land flows into that river

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2
Q

What is the watershed?

A

The boundary of a drainage basin - any precipitation falling beyond the watershed enters a different drainage basin

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3
Q

What are the inputs of a drainage basin?

A

Precipitation

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4
Q

What are the stores of a drainage basin?

A

1) Interception - When some precipitation lands on vegetation or other structures before it reaches the soil. Interception is only temporary as the collected water evaporates quickly
2) Vegetation storage - water taken up by plants
3) Surface storage - water in puddles, ponds, lakes etc.
4) Groundwater storage - water stored in the ground, either in the soil or in rocks.
5) Channel storage - water held in a river or stream channel

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5
Q

What are the flows and processes of a drainage basin?

A

1) Surface run-off - Water flowing over the land. It can flow over the whole surface or in little channels.
2) Throughfall - water dripping from one plant part to another
3) Stemflow - water running down a plant stem or tree trunk
4) Throughflow - Water moving slowly downhill through the soil
5) Infiltration - water soaking into the soil
6) Percolation - water seeping down through the soil into the water table
7) Groundwater flow - Water flowing slowly below the water table through permeable rock
8) Baseflow - Groundwater flow that feeds into rivers
9) Interflow - water flowing downhill through permeable rock above the water table
10) Channel flow - the water flowing in the river or stream itself.

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6
Q

What are the outputs of a drainage basin?

A

1) Evaporation - Water turning into water vapour
2) Transpiration - Evaporation from plant leaves
3) Evapotranspiration - Evaporation and transpiration together
4) River flow

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7
Q

What is river discharge?

A

The volume of water flowing in a river per second

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8
Q

What is river discharge affected by?

A

1) Precipitation - The more precipitation, the higher the discharge
2) Hot weather - The higher the temperature, the lower the discharge because there is more evaporation
3) Removal of water lowers the discharge

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9
Q

What is peak discharge?

A

Highest point on the graph, when the river discharge is at its highest

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10
Q

What is lag time?

A

The delay between peak rainfall and peak discharge. This delay happens because it takes time for the rainwater to flow into the river

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11
Q

What is the rising limb?

A

The part of the graph up to peak discharge. The river discharge increases as rainwater flows into the river

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12
Q

What is the falling limb?

A

The part of the graph after peak discharge. Discharge is decreasing as less water is flowing into the river.

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13
Q

What does a shallow falling limb show?

A

Water is flowing from stores long after it has stopped raining

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14
Q

What physical features affect lag time and peak discharge?

A

1) Larger drainage basins collect more water so have a higher peak discharge, but it takes longer for the precipitation to reach the river so they have longer lag times
2) Steep sided drainage basins have shorter lag times
3) Circular basins - All points on the watershed are roughly the same distance to the point of discharge measurement. This means lots of water will reach the river at the same time, increasing peak discharge
4) Basins with lots of streams have shorter lag times
5) If the ground is waterlogged infiltration is reduced and surface run-off increases, this reduces lag time
6) Rock type - impermeable rocks dont allow infiltration so lag time is reduced
7) Soil type - Sandy soils - lots of infiltration - longer lag time
- Clay soils - low infiltration - shorter lag time
8) Vegetation - more vegetation increases lag time and reduces peak discharge
9) Precipitation - Intense rain will increase peak discharge. Snow can melt in summer giving it a huge lag time
10) Temperature - hot, dry conditions or cold, freezing conditions create hard ground, reduces lag time increases peak discharge
High temperatures can increase evapotranspiration, reducing peak discharge

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15
Q

What is the amount of water already present in a drainage basin called?

A

Antecedent moisture

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16
Q

What human activities affect hydro-graphs?

A

Urban areas often covered with impermeable materials such as concrete. Increasing peak discharge and reducing lag time

Drains increasing peak discharge and reduces lag time as water flows into river before it can evaporate of infiltrate.

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17
Q

What is headward erosion?

A

Makes the river longer, happens near the source when water enters the river channel it erodes the river ‘backwards’

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18
Q

What is vertical erosion?

A

Deepens the river channel. Happens at the upper stages of the river

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19
Q

What is lateral erosion?

A

Makes the river wider. Happens in middle and upper stages.

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20
Q

What are the 5 main ways erosion happens?

A

Hydraulic action - The pressure of the water breaks rock particles away from the bed and banks. Its strongest in rapids and waterfalls, and during floods.
Abrasion - Eroded pieces of rock in the water scrape against the bed and banks, removing material. Most erosion of river beds and bank happen this way
Attrition - Eroded rocks smash into each other and break into smaller fragments. Attrition doesn’t erode the bed and banks - it just makes the particles in the river smaller and more rounded
Cavitation - Air bubbles in turbulent stretches of water implode creating shock waves they break rock off the banks and bed
Corrosion - Dissolving of rick by chemical processes. CO2 dissolves in water to form a weak acid, which reacts with some rocks to break them down.

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21
Q

What are the 4 main ways transportation happens?

A

1) Solution - Substances that can dissolve are carried along in the water
2) Suspension - Very fine material carried in water (suspended) this is how most material is transported
3) Saltation - larger particles are too heavy to be carried in suspension so they bounce along the river bed
4) Traction - very large particles are pushed along the river bed

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22
Q

What is material transported by traction or saltation called?

A

The rivers bedload

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23
Q

Why does a river deposit its load?

A

1) Reduced rainfall causes lower discharge which means that the river slows down
2) Increased evaporation or abstraction (taking water out of a river for human use) reduces discharge
3) Friction e.g. in shallow areas, causes the river to slow down, losing energy
4) When the river is forced to slow down e.g. before a narrow section, the river slows down and loses energy
5) A lot of energy is lost when the river meets the sea

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24
Q

What is the capacity of a river?

A

The total load that a river can carry at a given point

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25
Q

What is meant by a rivers competence?

A

The maximum particle size that a river can transport at a given point

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26
Q

What does a long profile show you?

A

How the gradient of a river changes from source to mouth by showing the height of the river bed above the base level for the whole length of the river.

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27
Q

What is the base level?

A

The lowest point that a river can erode to (usually sea level)

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28
Q

Describe the upper stage of a river

A

The gradient is steep and the river is high above sea level, which gives it lots of potential energy

29
Q

Describe the middle stage of a river

A

The gradient decreases, potential energy is converted to kinetic energy, the river gains velocity

30
Q

Describe the lower stage of a river

A

The river has little potential energy, but lots of kinetic energy - it flows faster

31
Q

What is the hydraulic radius?

A

The channels cross sectional area divided by the length of its wetted perimeter

32
Q

What does it mean if a river has a larger hydraulic radius?

A

A smaller proportion of water is in contact with the wetted perimeter so friction is lower, reducing energy loss, and increasing velocity and discharge.

33
Q

How does channel roughness affect a rivers efficiency?

A

1) Protruding banks and large angular boulders on the river bed increase the wetted perimeter and cause more friction. This reduces efficiency, velocity and discharge
2) As channel roughness increases, so does turbulence, turbulent flow increases erosion
3) Channel roughness is greatest at the upper stage of a river, so the river loses a lot of energy to friction
4) In the lower stages the banks and beds are much smoother so there’s less friction. This means less energy is lost so discharge and velocity are high

34
Q

What is erosion like in the upper stage of a river?

A

Mainly vertical and by abrasion. Erosion occurs when energy is high, e.g. after heavy rain. The rough channel causes turbulent flow which drags the large, angular bedload along the river bed, causing lots of vertical erosion.

35
Q

What is transportation like in the upper stage of a river?

A

Mainly large particles such as boulders carried by traction or saltation during high energy conditions.

36
Q

What is deposition like in the upper stage of a river?

A

Little deposition - mainly largest particles deposited in the river bed as energy levels drop.

37
Q

What is erosion like in the middle stage of a river?

A

Mainly lateral and by abrasion. Attrition means that sediment size decreases.

38
Q

What is transportation like in the middle stage of a river?

A

More material carried in suspension as sediment size decreases.

39
Q

What is deposition like in the middle stages of a river?

A

Sand and gravel are deposited across the flood plain as the river floods.

40
Q

What is erosion like in the lower stage of a river?

A

Although velocity is high here, there is less erosion as turbulence is lower and sediment size is lower (reducing abrasion). Some lateral erosion occurs during the formation of meanders.

41
Q

What is transportation like in the lower stage of a river?

A

Mainly smaller particles such as silt and clay carried by suspension.

42
Q

What is deposition like in the lower stage of a river?

A

Smaller particles such as sand, silt and clay are deposited on the flood plain when the river floods and in the rivers mouth when the sea absorbs the rivers energy.

43
Q

How does a rivers cross profile change downstream?

A

Upper stage - Steep V shaped valleys.
Middle stage - Wider, deposition creates a flood plain
Lower stage - Wide with gently sloping sides. Much wider flood plain causes by deposition.

44
Q

How are waterfalls formed?

A

1) Waterfalls form where a band of hard rock meets a band of soft rock. The soft rock is eroded more than the harder rock, causing a step in the river bed
2) The water flowing over the step speeds up due to a lack of friction, the gives it greater erosive power, meaning further erosion of the soft rock and undercutting of the hard rock
3) As the hard rock is undercut, it can collapse. A deep pool is made by abrasion at the foot of the waterfall as the bits of collapsed rock are swirled around by turbulence
4) Over time, more undercutting will happen causing more collapsing, the waterfall will retreat, leaving behind a steep sided gorge

45
Q

How are potholes formed?

A

By abrasion as turbulence swirls a rivers bedload round in a circular motion, causing it to rub and scrape out holes.

46
Q

How are rapids formed?

A

Rapids are relatively steep sections of a river with turbulent flow. They are formed when there is an area of the river bed made from ‘soft rock, hard rock, soft rock’ the soft rock gets eroded leaving behind only hard rock, this makes the river bed uneven, causing rapids to form.

47
Q

What are pools and riffles?

A

Pools - areas of deep water

Riffles - areas of shallow water

48
Q

How are meanders formed?

A

1) Meanders form where alternating pools and riffles develop at equally spaced intervals along a stretch of a river. The distance between pools is 5-6 times the width of the river
2) Because the river channel is deeper in pools its more efficient, so it has greater erosive power, energy is lost as the river flows over a riffle because of friction
3) The distance between pools and riffles causes the river’s flow to become uneven and maximum flow is concentrated on one side of the river
4) Turbulence increases in and around pools as the water speeds up, so the flow begins to twist and coil
5) This causes corkscrew like currents called helicoidal flow
6) The helicoidal flow causes deepening of the pools, it also causes eroded material to be deposited on the inside of the next bend, where the river loses energy
7) The combination of erosion and deposition exaggerates the bends until large meanders are formed

49
Q

What is a slip-off slope?

A

The deposition on the inside of a meander creates a slip-off slope

50
Q

What is a river cliff?

A

The erosion on the outside of a meander creates a river cliff

51
Q

How are oxbow lakes formed?

A

When the neck of a looping meander is broken through, often during flooding. Deposition then dams off the loop, leaving an oxbow lake

52
Q

Why does braiding occur?

A

1) Braiding occurs when rivers are carrying a lot of sediment
2) If the river’s velocity drops or the rivers load becomes too much for the river, sediment is deposited into the channel
3) This causes the river to divide into small, winding channels so that it can get around the sediment, they they reform into one river

53
Q

How are flood plains formed?

A

When a river overflows its wetted perimeter increases so the hydraulic radius decreases. This increases friction, reducing the velocity of the river, causing fine silt and sand the be deposited on the flood plain

54
Q

How are levees formed?

A

1) Levees are natural, raised embankments formed when a river overflows.
2) During a flood, material is deposited across the flood plain
3) The heaviest material (gravel and sand) is deposited first (nearest the bank)
4) Over time this material builds up to form a levee

55
Q

How are deltas formed?

A

1) When a river reaches the sea, the energy of the river is absorbed by the slower moving water of the sea
2) This causes the river to deposit its load. These deposits build up on the sea bed, until the alluvium rises to above sea level, partially blocking the mouth of the river
3) The river then has to braid into lots if distributaries order to reach the sea, forming a delta.

56
Q

What is rejuvenation?

A

1) A river is said to be rejuvenated if its base level is lowered, this can be caused by either the ground level rising or the sea level dropping
2) The drop in base level gives the river greater potential energy
3) The long profile of a river is extended and a knick point (a sharp change in gradient) will form.

57
Q

What landform can often be seen at a knick point?

A

Waterfall

58
Q

What are the two landforms associated with rejuvenation?

A

River terraces and incised meanders

59
Q

How are river terraces formed?

A

River terraces are formed flood plains which have been left above the level of present day flooding following increased vertical erosion

60
Q

How are incised meanders formed?

A

When a river keeps its meandering course as vertical erosion increases. This results in a deep winding valley with steep sides.

61
Q

What are the two main causes of flooding?

A

Prolonged rainfall - After a long period of rain the ground will become saturated, so infiltration can no longer occur, and so there will be increased surface run-off, causing a higher discharge
Heavy rain - Heavy rain such as thunderstorms can lead to rapid surface run-off if the rainfall is too intense for infiltration to occur. This can lead to a sharp rise in river discharge called flash floods

62
Q

What is another cause of flooding?

A

Melting snow and ice can cause huge increase in river discharge.

63
Q

What is cost-benefit analysis?

A

A method used to choose which places to protect from flooding. For example large settlement with important things are more likely to be protected than small settlements or farmland

64
Q

What is hard engineering?

A

Man made structures that reduce flooding

65
Q

What are the disadvantages of hard engineering?

A

1) Expensive to build and maintain
2) Although floods will happen less often, the can be worse if they do, for example if a dam bursts the flood will be huge
3) Natural processes are disrupted e.g. crops wont get natural fertile silt from the rivers
4) Some people think they are ugly

66
Q

See table on page 16

A

.

67
Q

What is soft engineering?

A

Use of knowledge of the river basin, they work with nature to reduce the impact of flooding.

68
Q

What are the advantages of soft engineering?

A

1) They’re cheaper to maintain than hard engineering
2) Flooding is more predictable, reducing the risk of unexpected disasters
3) They can improve opportunities for recreation such as fishing
4) More attractive than hard engineering

69
Q

See table on page 17

A

.

AS Geography (17 decks)

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