Rivers - KQ1 (river processes and fluvial landforms) Flashcards
1
Q
What is a catchment area/drainage basin?
A
A land area drained by a river, providing the water source for the main river and tributaries
2
Q
What are sub-aerial processes?
A
A combination of weathering and mass movement processes operating on river slopes
3
Q
What processes occur on river valley slopes?
A
- Mass movements
- Flows of water
- Erosion
- Weathering
4
Q
What are the mass movements occurring on river valley slopes?
A
- Soil creep
- Slides
- Falls
5
Q
What is soil creep?
A
Type of mass movement
- Individual soil particles are pushed to the surface by wetting, heating or freezing of water.
- Move at right angles to the surface as there is least resistance there
- Fall under the influence of gravity after particles dry, cool or the water has thawed
6
Q
What are slides?
A
Type of mass movement
- Sliding material maintains its shape until it impacts at the bottom of a slope and leads to large, slumped terraces
7
Q
What are falls?
A
Type of mass movement
- Occur on steep slopes, often due to weathering (freeze thaw = erosion prising open cracks)
- Once rocks are detached they fall under influence of gravity
8
Q
What are the flows of water on river valley slopes?
A
- Surface wash
- Sheet wash
- Through flow
9
Q
What is surface wash?
A
Type of flow of water
- Occurs when soil’s infiltration capacity is exceeded and gullies may form
- Could occur when water drains across saturated or frozen ground, following previous heavy rainfall
10
Q
What is sheet wash?
A
Type of flow of water
- Unchannelled flow of water over a soil surface
- Material dislodged by rain-slash can be transported
11
Q
What is through flow?
A
Type of flow on water
- Water moving down through the soil
- Channelled into natural pipes in the soil, giving it enough energy to transport material (= amounts to a considerable volume)
12
Q
What are the types of weathering occurring on a river valley slope?
A
- Freeze thaw
- Carbonation
- Oxidation
13
Q
What is freeze thaw?
A
Type of weathering
- Occurs when water in joints and cracks freeze at 0*C and expands by 10% and then exerts pressure
- Makes rocks crack and break away
14
Q
What is carbonation?
A
Type of weathering
- Occurs on rocks with calcium carbonate (eg chalk and limestone)
- Rainfall and dissolved carbon dioxide forms a weak carbonic acid
- Calcium carbonate reacts with the acid water and forms calcium bicarbonate which is soluble and removed by percolating water
15
Q
What is oxidation?
A
Type of weathering
- Occurs when iron compounds react with oxygen to produce a reddish brown coating
16
Q
What are the factors affecting slope processes?
A
- Sea level change
- Weathering rate
- Rock type (hard and soft rock)
- Climate (temperate environments have rounder slopes due to chemical weathering)
- Rock structure
- Aspect (north-facing slopes remain in the shade and temperatures rarely rise above freezing, however south-facing slopes experience freeze-thaw)
17
Q
What are factors affecting the rate of mass movement?
A
- Slope angle
- Amount of regolith (loose material created by weathering)
- Amount of water present
- Amount of vegetation
18
Q
What are the processes of fluvial erosion?
A
- Abrasion
- Attrition
- Hydraulic action
- Corrosion/solution
- Cavitation
19
Q
What is abrasion?
A
Sediment scours the channel, undermining the banks and valley slopes
20
Q
What is attrition?
A
Sediment collides with other sediment and erodes
21
Q
What is corrosion/attrition?
A
Chemical action of stream water, which dissolves carbonate rocks such as chalk and limestone
22
Q
What is hydraulic action?
A
Occurs when tiny bubbles of air implode in fissures and cracks in channel banks
- Tiny shock waves that result weaken the banks and eventually lead to collapsing
23
Q
What is fluvial erosion?
A
Erosion = a particle becomes part of the streams load when it is in motion (entrained)
- Sediment eroded by the stream is acquired in vertical, lateral and headward erosion
- Erosion decreases downstream as the gradient is shallower downstream
24
Q
What is vertical erosion?
A
When the channel is deepened
25
What is lateral erosion?
When the channel is widened
26
What is headward erosion?
When the channel length increases
27
What factors affect the rate of fluvial erosion?
- Rock type = strength of bonded particles and presence of joints and bedding planes. Weak rocks such as sand and gravel are more easily eroded than consolidated rocks
- Rock structure = orientation of beds, faults and weaknesses
- River velocity = Greater the discharge, the greater the potential for erosion
- Rainfall regime = seasonal rains on the Nile
- Sea level change = downcutting results when seal level falls or land undergoes uplift
- Bed roughness = Rougher bed = more friction = more energy lost from river flow = less energy for erosion
- Channel shape = a more semi-circular river bed = less friction so more energy for erosion
- pH = rates of solution increase when water is more acidic = more carbonation can take place
28
How are fluvial deposits transported?
- Stream carries sediment as its sediment load
- Suspension
- Solution
- Traction
- Saltation
29
What is the name of the size of the particles of a load?
Callibre of the load
30
What is suspension?
The smallest particles (silts and clays) are carried in suspension as the suspended load
31
What is solution?
In areas of calcareous rock, material is carried in solution as the dissolved load
32
What is traction?
Pebbles are rolled along the bed
33
What is saltation?
Larger particles (sand, gravels, very small stones) are bounced along the bed
34
What is the order of transportation from smallest to biggest?
- Solution
- Suspension
- Saltation
- Traction
35
Why might sediment load on different rivers vary?
- Rainfall total (climate)
- Erodible sediments (eg loess, glacial deposits and volcanic ash)
- Lack of vegetation
- Deforestation
- Uplift (leading to downcutting and more erosion)
- Intense human activity, removing vegetation and exposing group surfaces
36
What causes fluvial deposition?
- Shallowing of gradient decreases velocity and energy
- Decrease in volume of water
- Increase in friction between water and channel
- Human obstructions such as dams
37
What factors affect the amount of deposition?
- Velocity
- Availability of sediment
- Human intervention (eg dams and weirs)
38
What are the landforms of fluvial erosion?
- V-shaped valley
- Waterfalls and rapids
- Meanders (outward bends)
39
What are the landforms of fluvial deposition?
- Meanders (inside bend)
- Oxbow lake
- Floodplains
- Distributaries
- Levees and flood banks
40
How are v-shaped valleys formed?
Fluvial erosion
- Weathering and mass movements occur on the valley sides, while the river erodes the base of the slopes
- V-shape depends on rate of downward erosion by the river, rock resistance, climate, location along river's course (rivers in upper course tends to have steeper v-shaped valleys than those in lower course)
41
How are waterfalls and rapids formed?
Fluvial erosion
- Occur on the margins of horizontally bedded rocks
- Softer rock underneath harder rock is eroded quicker = overhanging
- Eventually harder rock collapses into plunge pool = worn away by waterfall
Main process = erosion
- Hydraulic action and abrasion = plunge pool
- Kinetic energy due to change in gradient = encourages formation of plunge pool and overhanging = provides rocks to become tools for abrasion within plunge pool
- Repeated undercutting and collapsing = waterfall retreats and gorge is formed
42
Give examples of waterfalls
- EXAMPLE = Severn Breaks its Neck, Wales, Plynlimon Hills
- EXAMPLE = High Force Waterfall, River Tees, near Middleton-in-Teesdale, Tees Valley, England:
1 - at High Force river runs over whinstone (hard igneous rock) which is above carboniferous limestone (weak rock) = eroded quicker
2 - creates overhanging of whinstone
3 - eventually collapses into plunge pool, which is worn away by waterfall (hydraulic action, abrasion and solution)
4 - overtime waterfall retreats as the overhanging collapses and develops again = currently formed 700m gorge in valley and waterfall is 20m high
43
How are meanders formed? (outside bend)
Fluvial erosion
- In lower land, river goes side to side on valley floor
- Water approaches slight bend = continues to flow straight ahead on outside of rivers bend
- Flow is strong and fast and has high amount of energy = erodes bend faster
- Outer riverbank becomes undercut = collapses and retreats = small river cliff is formed
- Line of fastest flow = thawleg
Main processes = lateral erosion, transportation, deposition and evaporation
44
How are meanders formed? (inside bend)
Fluvial deposition
- Inside bend = current is slowest with least energy = deposition = creates gently sloping slip-off slope
- Below slip-off slope = point bar (deposited material (sand and gravel) that accumulates inside bend)
45
How are oxbow lakes formed?
Fluvial deposition
- Meander neck gets narrower as inside bend is eroded more and more = eventually river cuts through neck and shortens course
- Deposition occurring in old neck of old meander makes it blocked off and forms oxbow lake = dries up due to evaporation
EXAMPLE = Welshpool
46
How are floodplains formed?
Fluvial deposition
- Valley floor between SHREWSBURY and IRONBRIDGE is often flooded by River Severn after heavy rainfall further upstream
- Fine silt and mud is deposited onto valley floor = part of valley floor is covered by river when it floods called flood plain
Main process = deposition
47
How are distributaries formed?
Fluvial deposition
- Form when river's flow is blocked by deposition (branch of river that does not return to main body after leaving it)
- Silt is deposited by each of distributaries and they spread out over wide area
- More and more silt is deposited into sea until sufficient thickness accumulates to form new land
48
How are levees and flood banks formed?
Fluvial deposition
- Deposited material on rivers bed and banks builds up over time
- Deposits of alluvium on either side of river are levees/flood banks
49
What are the main factors affecting the development of fluvial landforms?
- Availability of load for transport
- Energy of river
- Channel roughness
- Channel shape
- Velocity
- Rock type
- Rock structure
- Beach size
- Sea level change
- Human intervention (eg. Aswan Dam)
50
What are the types of sea level change?
Eustatic and isostatic
51
What is eustatic sea level change?
- A worldwide rise or fall in sea level due to an increase or decrease in the volume of water in the oceans
- Caused by climate change (glacial and interglacial periods)
52
What is isostatic sea level change?
- Local sea level change due to rise and fall of the land due to loading and unloading
- Caused by tectonic uplift or ice removal leading to isostatic rebound
53
What are the landforms produced by sea level rise?
SUBMERGENCE
- Rias
EXAMPLE = Bristol Channel and Lower Severn Estuary
54
What are landforms produced by sea level fall?
EMERGENCE
- River terraces
- Floodplains
_ Incised Meanders
