1
Q
Facts about the hydrological cycle
A
More than 97% of the world’s water is stored in the oceans. It is saline.
Only 3% of all the available water is freshwater.
Of this 3%:
2% is ice and snow
0.6% is groundwater
0.1% is surface water in rivers, lakes and surface reservoirs
0.001% is held in the atmosphere as water vapour and clouds
2
Q
Is the hydrological cycle an open system?
A
No, the global hydrological cycle is a closed system. This means there is a fixed amount of water on Earth as water neither enters nor leaves
the Earth and its atmosphere.
3
Q
Stores and tranfers
A
Within the system are stores and transfers (flows)
Stores are places where water is held, e.g. in pools and lakes on the surface or in
soil and rocks underground.
Transfers are processes by which water flows, or moves through the system, e.g. infiltration, surface runoff, throughflow
4
Q
Draw the hydrological cycle
A
N/a
5
Q
Describe the water cycle
A
Energy from the Sun heats the surface of the Earth.
Water is evaporated from oceans, rivers, lakes, etc.
The warm, moist air rises because it is less dense.
Condensation occurs when water vapour is turned back into water droplets as it cools down. Clouds are formed.
Precipitation occurs as water droplets get bigger and heavier they begin to fall as rain, snow and sleet, etc.
When the precipitation reaches the surface, some falls directly into the sea but other water falls on land:
Some water is intercepted by vegetation. Some water may then slowly reach the ground. Some will evaporate from the surface of leaves or be taken up by the plant roots, and some of this water will eventually return to the air as vapour through the process of transpiration. This slows down or prevents some water flowing back to the river.
Some water flows across the surface of the ground - surface run-off. This happens when the surface doesn’t allow water to penetrate. Surface run-off is more likely to occur if the ground is
saturated with water or when the rock is impermeable. This water moves quickly to the river.
Some water infiltrates into the soil. This through flow moves more slowly back to the river than surface run-off.
Some water percolates deeper into the ground and is slowly transferred back to the river or sea.
6
Q
What is surface runoff?
A
the flow of water over the ground
surface such as in rivers and streams.
7
Q
What is interception?
A
precipitation that does not reach the soil, instead it is intercepted by plants (leaves and
branches).
8
Q
What is throughflow?
A
Lateral movement of water through top layers of soil
9
Q
What is evaporation?
A
the conversion of water from a liquid to a gas (known as water vapour) due to the heat of the Sun from the surface of the sea and from water surfaces (eg. pond, lake) on land
Evaporation is particularly important in the transfer of water from the sea store into the atmosphere.
10
Q
What is transpiration?
A
the taking up of liquid water from the soil by plants and ‘breathing’ it into the atmosphere as water vapour
11
Q
What is evapotranspiration?
A
the loss of moisture from the ground by direct evaporation from water bodies and the soil, and transpiration from plants
12
Q
What is condensation?
A
The change in the atmosphere when water vapour cools and becomes liquid
13
Q
What is precipitation?
A
The transfer of water in any form (rain, hail or snow) from the atmosphere to the land or sea surface
14
Q
What is overland flow?
A
Most precipitation that hits the ground moves due to gravity and eventually enters a stream, river or lake. This is known as runoff.
15
Q
What is infiltration and percolation?
A
The transfer of water downwards through the soil (infiltration) and rock (percolation, deeper) into the aquifer or groundwater store.
16
Q
What is throughflow?
A
This takes place between the ground surface and the top of the groundwater store. As a result of gravity, water moves slowly through the soil until it reaches a stream or river.
17
Q
What is groundwater flow?
A
This happens in the rocks of the aquifer and is underground transfer of water to rivers, lakes and the sea.
18
Q
Infiltration vs percolation
A
Infiltration is throughflow in the soil, in groundwater stores. Percolation goes to aquifers in the rock and is groundwater flow.
This is all within the crust
19
Q
What are the types of mouths of a river?
A
deltas and estuaries
20
Q
What is a drainage basin?
A
A drainage basin is the area of land that is drained by a river and its tributaries, feeding the river.
All of the precipitation that falls in this area will flow into the river through surface run-off, through flow and groundwater flow, processes we have seen in the water cycle.
There is a divide, usually of high land and mountains, known as the WATERSHED between one river basin and the next.
The watershed is seen on the diagram as a red dotted line.
21
Q
Is a drainage basin a closed system?
A
Unlike the Hydrological Cycle, Drainage Basins are an Open System. This means that the system has inputs and outputs and the amount of water in our drainage basin will vary over time.
21
Q
Draw a diagram of a drainage basin, and label it
A
N/a
22
Q
Inputs of a drainage basin
A
How water is introduced into the drainage basin system. This is known as precipitation. Also energy from the Sun.
23
Q
What are the stores of a drainage basin?
A
How water is stored or held for a period of time within the drainage basin system - interception (by vegetation), soil moisture, surface storage (lakes), groundwater
24
What are the transfers of a drainage basin?
A process or flow of water from one place to another in the drainage basin system -
surface run-off/overland flow, infiltration, percolation (movement of water through rock),
through-flow, groundwater flow (these are the same as in our hydrological cycle)
25
What are the outputs of a drainage basin?
How the water is released either back to the sea or back into the atmosphere - river discharge, evapotranspiration, evaporation, and river discharge.
26
Types of drainage basin
--> Those that simply collect and deliver water directly to the sea
--> Those that are parts of much larger drainage basins. For example, the basin of the River Negro in Brazil is a tributary or part of the huge drainage basin of the River Amazon. The water it collects is conveyed to the Amazon and then eventually to the sea
--> Some drainage basins do not lead, either directly or indirectly to open sea. Rather they lead to 'inland' seas or lakes such as the Caspian Sea located between Europe and Asia, the Aral Sea to the east of it, or Lake Victoria in East Africa
27
What is the channel (or drainage) network?
The system of surface and underground channels that collects and transports the precipitation falling on the drainage basin.
28
How can channel networks be mapped?
using their lengths and densities (number of channels per unit area). This can change over periods of time though. For example, during flooding drainage basins often have many more and longer channels than they do in periods of low or normal rainfall.
29
What unique features does a drainage basin have?
size, shape, rock type, relief and land use
These determine how quickly or slowly water moves through the basin.
30
What land use makes for slow and quick runoff?
Slow - woodland as the water is intercepted
Fast - cities, as concrete is smooth so water doesn't enter the soil/rock which is slower, and drainage systems increase the speed as well
31
What is river discharge?
This is the total volume of water flowing through a river channel at any given point and is measured in cubic metres per second (cumecs).
The discharge from a drainage basin depends on several factors which we will explore in detail today.
32
What is lag time?
Lag time is the time taken for precipitation that has fallen to reach the river.
It is the time between peak precipitation and peak river discharge
33
The factors and characteristics that affect the lag time of the drainage basin
Rock and soil type
Land use/ vegetation cover
Amount of rainfall
Relief
Weather conditions
Water abstraction
Precipitation
Temperature
Wetlands
Deforestation
Dams
Size and shape of drainage basin
Slopes
Drainage density
34
How does rock and soil type affect river discharge?
Permeable rocks and soils (such as sandy soils) absorb water easily,
so surface run-off is rare = longer lag time
Impermeable rock and soils (such as clay soils) are more closely
packed. Rainwater can’t infiltrate, so water reaches the river
more quickly = shorter lag time
More compact surfaces have less slow infiltration and more quick overland flow. Sandy soils dry out rapidly so have less overland flow, while clay soils have very fine particles so fewer air spaces and high cohesion
35
Permeability vs porosity
Porosity - measure of the rock's ability to hold water (chalk)
Permeability - measure of the ease with which water can flow through a rock (carboniferous limestone)
36
How does land use or vegetation cover affect river discharge?
In urban areas, surfaces like roads are impermeable – water can’t soak into the ground. Instead, it runs into drains, gathers speed and joins rainwater from other drains, quickly spilling into the river = shorter lag time. And parks and commons have hard, compact soil, increase surface runoff.
In rural areas, ploughing up and down (instead of across) hillsides creates channels which allow rainwater to reach rivers faster increasing discharge = shorter lag time
Deforestation means less interception, so rain reaches the ground faster. The ground is likely to become saturated and surface run-off will increase = shorter lag time.
Forested areas slow movement of water
37
How does Amount and type of rainfall affect river discharge?
The amount, intensity, duration and type of rainfall will affect a river’s discharge
Rainfall that has already happened can mean that the ground has
become saturated. Further rain will then flow as surface run-off
towards the river = shorter lag time
Heavy continuous rain, or melting snow, means more water
flowing into the river
Snow settles, then melts (increasing discharge quickly), while rain is immediate increase
38
How does relief affect river discharge?
Steep slopes mean that rainwater is likely to run straight over the surface before it can infiltrate.
On more gentle slopes or flatter land infiltration is more likely as water does not run away over the surface as quickly, therefore the lag time will be longer.
39
How does weather conditions affect river discharge?
Hot dry weather can bake the soil, so that when it rains the water can’t soak in. Instead, it will run off the surface, straight into the river.
High temperatures increase evaporation rates from water surfaces, and transpiration from plants – reducing discharge.
Long periods of extreme cold weather can lead to frozen ground, so that water can’t soak in/infiltrate and there can be lots of surface water.
Lots of heavy rainfall will increase the amount of surface water - think of a waterlogged football pitch.
40
How does vegetation cover affect river discharge?
areas of high vegetation cover intercept more rainfall, so water reaches the ground more gradually, reducing overland flow and increasing infiltration rate, increasing lag time
broad-leaved vegetation intercepts more due to high surface area
in winter, deciduous trees lose leaves so less water is intercepted
41
How does temperature affect river discharge?
Higher temp = more evaporation = less water in rivers
Warm air can also hold more water vapour than cold air, so very high precipitation and river discharge exists in hot, moist climates
42
How do wetlands affect river discharge?
Wetlands can hold water and release it slowly into rivers, increasing lag time
43
How does water abstraction affect the lag time/river discharge?
This is the removal of groundwater from water sources on the surface or underground, for consumption, irrigation and other uses, so clearly increases lag time and decreases river discharge as there is less water.
44
How do dams affect river discharge?
They regulate river flow for flood control, navigation, irrigation, hydropower production and water supply.
45
45
Drainage basin vs drainage network
A drainage basin is an area of land drained by a river, defined by the watershed, while the drainage network of linked streams and rivers within the basin.
46
What is the river discharge?
The quantity of water flowing in a river channel at a particular location and time
47
What are cumecs?
A measure of discharge in cubic meters of water per second moving past a particular point of the river's course.
48
What is a river regime?
The seasonal variations in the discharge of a river
49
What is a hydrograph?
A graph showing the discharge of a river over a given period of time
50
What is permeable?
A material that allows liquids to pass through it
51
What is impermeable?
A material that does not allow liquids to pass through it
52
Hydrographs and river regimes
It is important to know how quickly rain falling in a drainage basin will
reach the drainage network.
If rain water reaches the river quickly, the channel may not be able to cope and flooding will occur.
Flood = when discharge (volume of water which flows through it
in a given time) exceeds the channel capacity.
River discharges vary throughout the year, from month to month and day to day. These variations make up what is called the river
regime.
In most rivers, the regime closely reflects local climatic conditions, particularly the rainfall.
53
How to describe a river regime with a hydrograph?
The graph we have just looked at shows the average monthly discharge of the River Ganges as it passes through Bangladesh.
This diagram, showing the discharge of the river over a given period of time is called a
hydrograph.
Clearly, mean (average) discharge is high between June and October. This period of high discharge coincides with the monsoon season, during which total rainfall can exceed 2750mm.
54
Draw a hydrograph
N/a
55
What do storm hydrographs do?
record the changing discharge of a river after a rainstorm
56
Draw a storm hydrograph
N/a
57
Labels of a storm hydrograph
There is a bar chart showing the rainfall in an hour (with its own axis and scale).
The rain stopped after 12 hours but the water is still at the surface.
The rising flood water in the water affected by the rain is shown by the increase in the line graph or the rising limb.
The difference between peak rainfall and peak river flow is the basin lag time.
The discharge is shown in cumecs, as a line graph.
The peak flow is the maximum discharge in the river after the rain.
Falling flood water in the river on the line graph as the recession limb.
The base flow is the normal discharge of river.
58
Storm hydrograph explanation
A storm hydrograph records the changing discharge of a river after a rainstorm.
The bars in the left hand corner of this graph show the input of rain.
After the rain hits the ground, it takes time for rainwater to reach the river and cause river levels to rise.
This delay between the peak rainfall and peak discharge is called the lag time.
The shorter the lag time, the
quicker the water reaches the
river channel.
A short lag time causes the river discharge to rise steeply.
The steeper the rise in discharge, the greater the chances of flooding.
It is possible to mark on the
storm hydrograph the level of
discharge above which the
river will flood.
Once the storm and its peak discharge have passed, the amount of water in the river starts to decrease.
59
'Flashy' drainage basin
In some drainage basins, discharge and river levels rise very quickly
after a storm. They have a "flashy" response to inputs of precipitation.
This can cause frequent, and occasionally serious, flooding.
59
Does impermeable rock have a shorter or longer lag time?
Shorter lag time with high peak flow
60
Does a lot of vegetation cover have a shorter or longer lag time?
Longer lag time with a lower peak flow
61
Does a small drainage basin have a shorter or longer lag time?
Shorter lag time with lower peak flow
62
How does the amount and intensity of rainfall affect lag time?
Heavy rain over a short time will not sink into the ground. Instead, it will become overland flow or run off and quickly reach river, decreasing the lag time.
Light rainfall over a longer time period means water infiltrates the ground and takes longer to reach the river by overland and throughflow.
63
How does the temperature affect the lag time of a drainage basin?
Decrease lag time (higher temperature):
- hot, dry conditions can harden and bake ground, causing faster runoff
- increased evapotranspiration
- faster snowmelt
Increase lag time (lower temperature):
- precipitation is stored as ice or snow which delays water entering the channel
- frozen ground reduces infiltration
64
How does the slope angle affect the lag time of a drainage basin?
Decrease lag time (steep slope):
- rapid overland flow due to gravity
- reduced infiltration: water is too quick
- reaches channel faster (increases speed)
Increase lag time (shallow slope):
- surface runoff is slow, so more infiltration
- takes longer
65
How does the rock type affect lag time of a drainage basin?
Decrease lag time (impermeable rock, eg. granite, slate, clay):
- don't allow for infiltration, so faster runoff, as this is quicker than underground flow
Increase lag time (permeable rock, eg. limestone, chalk):
- allow for infiltration/percolation, so water moves slowly underground as baseflow or throughflow
66
How does vegetation and land use affect the lag time of the drainage basin?
Decrease lag time:
- urbanisation (concrete, drains) transport water to rivers very quickly
- deforestation reduces interception & infiltration, so water moves quicker
- agriculture creates drainage channels
Increase lag time:
- dense vegetation = more interception
- root systems creates soil pores, which throughflow, which is slow
- water moves slow through vegetation
- afforestation increases trees
67
How human intervention affect the lag time of the drainage basin?
Decrease lag time:
- urbanisation
- drainage systems
- deforestation
- agriculture and channels
Increase lag time:
- natural flood management
- dams and resevoirs
68
How does the channel density affect the lag time of the drainage basin?
Decrease lag time (more dense):
- more, closer, efficient channels so water travels shorter distance to a stream
- reduced infiltration
Increase lag time (less dense):
- water has to travel longer to reach stream
69
5 key fluvial processes
1. Erosion
2. Deposition
3. Transportation
4. Weathering
5. Mass Movement
70
What is erosion?
The wearing away and/or removal of rock by force
71
What is deposition?
The laying down of sediment transported by rivers as energy levels decline
72
What is transportation?
The movement of material by river
73
What is weathering?
The in situ breakdown of rocks exposed at, or near, the land surface (physical, chemical and biological)
74
What is mass movement?
The downslope transportation of material under gravity
75
What are the four processes of erosion?
1. Corrosion/Abrasion
2. Attrition
3. Solution/Corrosion
4. Hydraulic Action
Mnemonic CASH
76
What is corrosion/abrasion? Draw a labelled diagram of it.
The wearing away of the riverbed and banks by the river's load that it picked up by the river hitting them repeatedly against other rocks or the cliff, rubbing and smoothing the rock over time.
N/a
77
What is attrition? Draw a labelled diagram of it.
In swirling water, angular rocks and stones picked up by the river collide or knock against each other and with the beds and banks. Over time the sharp edges of the rocks and stones smoothen, and become smaller in size and rounder.
N/a
78
What is solution/corrosion? Draw a labelled diagram of it.
Some rocks, such as limestone (which is soluble in slightly acidic water) dissolve slowly in river water due to salts and chemicals, that creates faults and notches in the rock.
79
What is hydraulic action? Draw a labelled diagram of it.
The sheer force of the river water forces water and air in rock cracks, causing faults and notches to split, and removing all the loose material from the bed and banks of the river.
80
What is vertical erosion?
This takes place in the upper course of the river near the source where the river cuts down into its bed deepening the valley
81
What is lateral erosion?
This takes place in the middle and lower courses of the river and widens the valley
82
Draw vertical erosion
N/a
83
Draw lateral erosion
N/a
84
Transportation types
1. Suspension
2. Solution
3. Traction
4. Saltation
85
What is suspension? Draw a labelled diagram.
The smallest particles (silts and clays) are carried in suspension by moving water.
N/a
86
What is solution? Draw a labelled diagram.
In areas of calcareous rock (limestone), material is carried in solution as a dissolved load.
N/a
87
What is traction? Draw a labelled diagram.
Large pebbles are rolled and shunted along the bed.
N/a
88
What is saltation? Draw a labelled diagram.
Larger particles (sand, gravel, very small stones) are transported in a series of 'hops' or bounces
N/a
89
What is the transportation that requires the most to least energy?
Most = Traction (slow)
Saltation
Suspension
Least = Solution
90
What is deposition?
--> takes place when a river doesn't have enough energy to carry its loas. For example:
--> the gradient decreases
--> discharge falls during a dry period
--> the current slows inside a meander
--> the river enters a lake or sea
When a river loses energy, the large, heavy material known as the bedload is deposited first. Lighter material is carried further downstream.
The gravel, sand and silt which is deposited is called alluvium. This is spread over the floodplain. The load transported by solution is carried out to sea with much of the clay, the lightest suspended particles.
91
What is mass movement?
It is the large-scale movement of unweathered material downhill under the influence of gravity. This occurs continuously on slopes, mostly slowly. It carries weathered material to a river's load so:
- increases erosion in the upper course
- adds to deposition in the middle and lower courses
92
What is soil creep? Draw a labelled diagram
A slow, small-scale movement of individual soil particles downslope, mostly in winter. The rate is 1-3mm/year in temperature areas and up to 10mm/year in tropical rainforest. This often happens on slopes with an angle of more 5 degrees.
N/a
93
What is slumping? Draw a labelled diagram.
This occurs when a significant section of rock moves a short distance down the slope in a rotational movement. This often occurs when softer materials (eg. clay) overlie more resistant impermeable material. The softer material is saturated in rainwater, so slides downwards along a concave surface. This leaves an exposed scarp face below the head of the slump, and a hummocky toe at the foot.
N/a
94
Is slumping or soil creep quicker?
Slumping is quicker
95
What is weathering?
The physical or chemical breakdown or change of objects due to weather conditions (eg. wind, water, rock temperature).
--> Does not involve transport of objects to a new location
96
3 Types of Weathering
1. Physical Weathering
2. Biological Weathering
3. Chemical Weathering
97
Physical Weathering (draw diagrams for each)
1. Freeze Thaw
--> occurs when water in joints and cracks in rock freezes at temperatures below 0 degrees. The water heats and expands by 10%, exerting large pressure on rock, causing larger cracks
2. Exfoliation
--> during the day, the Sun heats up the surface of the rock, so it expands, and then contracts at night. The expanding and contracting causes pieces of rock to flake and fall off
N/a
98
Biological Weathering (draw a diagram)
plants and animals break up rock with rock
on coastal platforms made of chalk, limpets can excavate hollows
N/a
98
Chemical Weathering (draw a diagram)
water dissolves rock, as rainwater and seawater are weak acids, so a bank or bed made of limestone or chalk can become dissolved by the acid in water (carbonation). Basalt is much stronger
99
How does climate affect fluvial processes?
Erosion: increase erosion if there is more precipitation, higher temperature or flood
Transportation: increases it with rain, and if it is warm the river is less viscous with a settled velocity
Deposition: increases it as low rain reduces discharge so more deposition
Weathering: increases as for chemical there is more oxidation or it is moist, for physical, temperature is key in freeze thaw, for biological more plants grow in warm weather
Mass movement: increases as precipitation can trigger landslides, or slumping as it saturates the rock
100
How does steep slope affect fluvial processes?
Erosion: increase as this increases speed with quicker runoff and more vertical erosion, a convex slope also increases it
Transportation: increases it due to increased water velocity and volume, as steep stream have a lot of GPE meaning increased runoff
Deposition: decreases it as the high velocity means that more can be transported and less is deposited
Weathering: increases physical as there is more exposed rock surface, but decreases chemical as water moves quicker over the rock
Mass movement: increases as it moves the mass quicker, due to the gravity and instability of a steep slope
101
How does geology affect fluvial processes?
Erosion: soft rock with joints and faults is easy to erode, permeability is also important
Transportation: soft rack erodes providing more sediment load for the river to carry, the sediment size, and the permeability/impermeability
Deposition: soft rock erodes more so is deposited, hard rock has more energy, valley shapes
Weathering: hard rocks weather slow, minerals in rocks can be chemically affected, solubility of rock
Mass movement: soft rock can slide easily, rock layers (permeable or impermeable), faults
102
How does high altitude affect fluvial processes?
Erosion: vertical erosion increases due to steep gradient
Transportation: steep gradient means high erosion increasing sediment load and the river is fast flowing
Deposition: high energy so minimal deposition
Weathering: more freeze thaw and glacial activity, but less chemical due to low temperature and moisture, less vegetation for biological
Mass movement: when permafrost thaws it has less strength so moves, melting glaciers reduce stability
103
Draw hjulstrom curve roughly
N/a
104
Describe and explain the Hjulstrom Curve
It shows what is happening to sediment at different sizes and speeds (erosion, transportation, deposition).
River velocity is a logarithmic scale.
The smallest particles like clay are never deposited because they are so light, held in suspension
Larger materials are deposited at higher speeds vs small materials as transportation and erosion of large material requires more energy, so for things like boulders the river has to be flowing very fast for it to be transported or eroded.
Very small particles like clay need high velocity for erosion as they’re so small.
Erosion requires the most energy.
With boulders, even the smallest drop in velocity means they are deposited.
105
Draw the long profile of a river
N/a
106
Describe and explain the long profile of a river
The source of the river is the original point from which the river flows (eg. lake, marsh, spring, glacier) which is high altitude.
In the upland section of the river, it is very narrow with V-shaped valleys where there is usually vertical erosion or waterfalls.
The middle course has meanders (bends in the river) where the river loops along flatter ground. There may be ox-bow lakes where the river cuts through a meander.
The lower course is where the river widens due to lateral erosion. The flat land around is called a floodplain, which floods when the river discharge is large. Many people live there. But with more population, people build on floodplain.
Lots of deposition occurs when the river meets the sea, forming deltas and estuaries.
107
What is in the upland section of a river?
Upland: The source of a river is often (but not always) in an upland area. Near the source, a river flows over steep slopes with an uneven surface. It often flows over a series of waterfalls or rapids. Highland areas are usually composed of hard igneous rocks, which are ideal for forming such features.
As the river flows down steep slopes the water performs vertical erosion. This form of erosion cuts down towards the river bed and carves out steep-sided V-shaped valleys.
108
What is in the lowland section of a river?
Lowland: As the river flows towards the mouth, the slopes become less steep. Eventually the river will flow over flat land as it approaches the sea.
The discharge (amount of water flowing in the river) will increase as the river approaches the sea.
109
Draw the Bradshaw Model
N/a
110
What is the Bradshaw model?
It shows the changes in channel characteristics from the upland to lowland sections of a river.
The diagram shows that discharge is lowest upstream at the source of the river but increases as you move further downstream. However the slope angle is much higher upstream and decreases as you move further downstream.
111
Describe and explain the Bradshaw Model
River discharge: quantity of water passing a given point at a given time, in cumecs, increasing downstream, as there is an increase in the cross-sectional area of the river downstream due to tributaries and erosion
Occupied channel width: increases downstream, as it becomes wider and deeper (due to lateral and vertical erosion of the floodplain) increasing cross-sectional area
Channel depth: increased in lower course, as both vertical and lateral erosion is acting on the river more, deepening it
Average velocity: it is higher downstream, as, upstream, with rapids and waterfalls, turbulence is created, creating inefficient water flow as water is thrown up in the air (caused by the friction of the rocks and channel roughness), while downstream, the channel is smoother, wider and deeper, with fewer large rocks, so less friction, and velocity increases creating a smoother channel bed
Load quantity: increases as it moves downstream due to the river being large and more angular downstream so it can carry and transport more sediment and more sediment that has been eroded is transported by the river
Load particle size: decreases as you go downstream, as upstream rocks are sharp and rough, recently being eroded, as they haven’t had time to be smoothened by erosion (eg. attrition or abrasion, the sandpaper effect), while downstream, it is smaller and smoother, and the load is more fine sediments and mud, called alluvium
Channel bed roughness: this decreases downstream, as the particle load and river discharge is higher downstream, eroding the riverbed by abrasion or the sandpaper effect, with more sediment to erode the riverbed and smoothening out the riverbed (less lamina flow)
Slope angle (gradient): this decreases as you go downstream, as, in the upper course, the height difference between altitude and the mouth (sea level) creates high gravitational potential energy, eroding the riverbed vertically in its upper course. In the lower course, it is flatter, as lateral erosion is more predominant, and, steep slopes create mass movement
112
Features of the upper course
- Steeply sloping towards the lower sections of the river
- Steep-sided V-shaped valley, thin river channel, deep in places
- Primarily vertical erosion, through attrition, abrasion and hydraulic action. Large boulders deposited and eroded in situ
- Traction and saltation
- Interlocking spurs, waterfalls, V-shaped valleys, gorges
- Relatively slow-moving, despite areas of fast-flowing water, the large amount of material on the river channel bed means friction will slow down the water
113
Features of the middle course
- shallow slopes towards the mouth of the river
- v-shaped valley remains with a wider valley floor and the river begins to meander across it. The river channel begins to widen and become deeper
- continues to cut vertically. But it also begins to cut laterally as it gets closer to base level. Deposition occurs in the slower moving insides of the meander
- saltation, suspension and solution
- meanders, slip-off slopes, ox-bow lakes
- the water has increased in speed as the channel widens and becomes smoother. Some boulders cause friction to slow it down a little
114
Features of the lower course
- Almost at sea level, very gently sloping towards its mouth
- Wide, shallow valley, with large flood plains and meanders. The river channel is wide, deep and smooth sided
- Primarily cuts laterally as it has almost reached base level. The erosive energy of the river is almost totally concentrated on cutting sideways. Much deposition occurs.
- Mainly suspension and solution.
- Deltas: flood plains: levees: meanders, ox-bow lakes
- The fastest section of the river, as the channel is widest, with very smooth sides, and the greatest volume of water.
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Upland features
The main river landforms in upland areas are:
Steep V-shaped valleys
Interlocking spurs
Waterfalls
Gorges
These features are all formed primarily by river erosion (hydraulic action, abrasion and corrosion). The processes of weathering and mass movement also play a part.
116
Formation of V-shaped valleys + draw a diagram of this
1. A river’s discharge is low in the upper course and there is high gravitational potential energy, and so the river only has enough energy to erode downwards. This is called vertical erosion.
2. The valley’s sides are slowly broken down through weathering
3. The weathered material is transported via gravity and rainfall towards the river channel. This process steepens the valley sides
4. The steep-sided and narrow valley shape that is created is typical of upper course of rivers
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117
Formation of interlocking spurs + draw a diagram of this
Interlocking spurs are formed in the upper course of a river, where the river has relatively low energy and flows through a V-shaped valley.
1. Vertical Erosion – In the upper course, the river has a steep gradient, so most of its energy is used for downcutting (vertical erosion) rather than lateral erosion. This creates a narrow, steep-sided V-shaped valley.
2. Obstacle Avoidance – As the river encounters hard, resistant rock, it is forced to wind and bend around these areas of more resistant rock rather than cutting straight through them.
3. Formation of Spurs – Over time, the river carves out a winding path around the resistant rock, creating alternating projections of land (spurs) that interlock as the river meanders through the valley.
N/a
118
Formation of Waterfalls + draw a diagram for this
Waterfalls occur when a band of hard rock is much more resistant to erosion than the softer rock below.
The softer rock is readily eroded by the force of the water as it falls over the hard cap rock.
Gradually, the falling water excavates a plunge pool at the bottom of the falls.
1. Waterfalls are often formed where a layer of hard rock overlays softer rock
2. As the river passes over the softer rock, it is able to erode it at a faster rate, forming a step in the riverbed
3. The force of hydraulic action does two things:
4. As the notch grows, eventually there isn’t enough support under the harder rock and it collapses into the plunge pool
5. This adds rocks and boulders to the plunge pool, and so the process of corrasion works with hydraulic action to further erode the plunge pool and notch
6. The process of erosion continues, further eroding the notch and plunge pool. Eventually, the harder rock above or the overhang will collapse again, meaning the waterfall will retreat upstream over time
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What is a gorge?
Hydraulic action and abrasion will under-cut the base of the waterfall and eventually the overhanging hard cap rock may collapse.
Therefore the waterfall retreats backwards. As this process continues over many hundreds of years, the waterfall may retreat up the valley towards the source, creating a narrow steep-sided gorge.
120
Lowland river features
- floodplains and levees
- meanders
- ox-bow lake
- delta or estuary
121
What is a floodplain?
A river floodplain is the flat land lying on either side of a river that periodically floods.
122
Draw the valley and channel upland vs lowland
N/a
123
Advantages of floodplains
Floodplains are very fertile due to the rich alluvium (fine sediment deposited by the river) deposited by the floodwaters during a flood.
This makes floodplains a great place for agriculture.
124
Formation of levees + draw a labelled diagram
During a flood, such a large load of material has been picked up that, once the river loses energy, it drops dome of that load on the floodplain. It drops heavy load, such as boulders as they require a lot of energy to transport, while silt or alluvium can be carried further. This is usually mud, stones and other organic matter.
Every time the river leaves its channel, its velocity decreases, due to friction or vegetation. Once this happens, sediment is deposited across the valley floor. A great thickness of sediment builds up. The largest amount of deposition is always on the banks of the channel, which builds up to a great height compared to the rest of the floodplain, forming levees.
There is also lateral erosion creating the floodplain overtime as it meanders.
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125
What are meanders?
Meanders are bends in a river. The study of meanders and ox-bow lakes show how the river both deposits and erodes laterally.
126
Formation of meanders + draw a diagram of slip-off slope vs river cliff
Where there is a weakness in the rock, due to a tree root or animal habitat or another natural cause, the force of the water undercuts the bank, eroding it laterally, creating the outside of a band, forming a steep bank to the channel called a river cliff (or buff). Here there is the fastest current, with much fine material in suspension.
An underwater current with a spiral flow then carries the eroded material to the inside of the bend, where the flow of water is slower, so material, such as sand and shingle, is deposited to form a gentle bank, known as a slip-off slope or point bar.
The bend created is known as a meander.
N/a
127
Formation of ox-bow lakes + draw a diagram
While the current is strongest on the outside of the bend, and sediment is deposited on the inside of bend, rapid erosion of the banks continues on the outside of the bend.
The gap between two arms of the river is narrowed by erosion.
The river then breaks through the narrow gap when flooded, but the river still flows around the meander.
The current along the straighter path then becomes dominant, the old path drying up, to leave an abandoned meander or ox-bow lake.
N/a
128
Difference between a delta and an estuary
A delta forms where there is more **deposition** than erosion.
An estuary forms where there is more **erosion** than deposition.
129
Does the mouth of the river have to be a delta or an estuary?
Yes
130
Formation of a delta
A river delta is a landform produced by the deposition of sediment at the mouth of a large river.
They form when the river deposits sediment faster than the sea can remove it.
They are vast areas of alluvium.
131
Types of deltas + draw diagrams of each
1. Arcuate or fan-shaped - the land around the river mouth arches out into the sea and the river splits many time on the way to the sea, creating a fan effect
2. Cuspate - the land around the mouth of the river juts out in an arrow-like shape into the sea
3. Bird's foot - the river splits on the way to the sea and each part of the river juts out into the sea like a bird's foot
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132
What is an estuary?
An estuary is the wide mouth of the river where it meets the sea. Much less deposition so new land is not being created. Erosion is the dominant process.
Where the freshwater from the river and salty seawater mixes, brackish water is formed. Brackish water is somewhat salty, but not as salty as the ocean.
Water continuously circulates into and out of an estuary and the water level of an estuary will rise and fall with the tides. Rivers may also sometimes flood the estuary with freshwater.
133
Draw an estuary
N/a
134
What type of mouth is the Thames?
an estuary
Geography GCSE
flashcards
Decks in class (16)
# Cards
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Globalisation29
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Outsourcing5
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Geopolitics8
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Migration50
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Intro To Physical Geography9
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Processes Shaping Coastal Landscapes74
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Coastal Deposition9
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Coastal Defence34
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Sea Level Rise Effect25
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Coastal Ecosystems10
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Skills3
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Tourism27
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Hazardous Environments179
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Exam Technique1
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Economic Activity And Energy85
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River Environments138
