Rivers - Physical Geography

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Soli Creep
Slow movement. Induvidual soil particles are pushed to the surface by wetting, heating or freezing of water. They fall under the influence of gravity and net movement is downslope. Rates of movement are slow, (1mm a year in the UK).
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Rain-Splash Erosion
Slow movement. On flat surfaces, raindrops compact the soil and dislodge particles equally in all directions. On steep slopes this causes erosion downslope.
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Terracettes
Small terraces from a few centimeters to 0.5m across the face of a slope caused by soil creep of solifluction.
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Surface Wash
Flow Movement. This occurs when the soil's infiltration capacity is exceeded and can lead to the formation of gullies. In Britain this mainly occurs in winter as waters drains across saturated or frozen ground.
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Sheetwash
Flow movement. This is the unchanelled flow of water over a soil surface. Often transports material dislodged by rain splash. During the Boscastle floods, sheetwash from the shallow moorland peat caused steep, narrow gulleys to form.
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Throughflow
Flow movement. Water moving down through the soil.
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Landslides
Fast mass movement. Sliding material causes large slumped terraces to form at the bottom of slopes. Eg In January 2006 prolonged heavy rain led to landslides in Indonesia killing 120 people.
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Rockfalls
Fast mass movement. Rock falls occur on steep slopes (>70°). Erosion and weathering open lines of weakness, causing the rocks to fall under the force of gravity. Falls lead to scree slopes and large slumped terraces.
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Slumps
Fast mass movement. Slumps occur on weaker rocks, especially clay and have a rotational movement along a curved slip plane. Frequently, the base of a cliff has been undercut and weakened by erosion therefore reducing its strength.
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Factors affecting slopes: Climate
In general, slopes in temperate environments are rounder, due to chemical weathering, whereas slopes in arid environments are jagged or straight because of mechanical weathering.
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Factors affecting slopes: Rock type and structure
Hard resistant rocks, such as granite, basalt and carboniferous limestone form upland rugged areas. Weaker rocks such as chalk and clay from low-lying landscapes.
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Factors affecting slopes: Aspect
In the UK, North facing slopes remain shaded. Whereas south facing slopes experienced many cycles of freeze - thaw. Solifluction and and over-land runoff lower the level of the slope. This results in an asymmetric slope.
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Mechanical weathering: Freeze-thaw
This occurs when water in joints and cracks freezes at 0℃, expands by 10% and then exerts pressue up to 2100kg/cm2. Most common in areas where moisture is plentiful and there are frequent fluctuations above and below freezing.
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Mechanical weathering: Salt crystal growth
1) In areas of approximately 26-28℃, sodium sulphate and sodium carbonate expand by 300%. 2) When water evaporates, salt crystals may be left behind to attack the structure.
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Mechanical weathering: Pressure release
This is the process where overlying rocks removed by erosion cause underlying ones to expand and fracture parallel to the surface.
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Chemical weathering: Carbonation solution
Rocks containing calcium carbonate (chalk and limestone). Rainfall and dissolved carbon dioxide forms a weak carbonic acid. The calcium carbonate reacts with this and becomes soluble, it is then removed by percolating water.
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Chemical weathering: Hydrolysis
This occurs on rocks with orthoclase feldspar (eg Granite). Orthoclase racts with water leaving china clay as the end product.
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Chemical weathering: Hydration
This is the process where certain minerals absorb water, expand and change.
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Chemical weathering: Oxidation
This occurs when compounds of Iron react with oxygen to produce a reddish-brown coating.
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Biological weathering
Biological weathering involves both mechanical impacts such as the growth of roots, and chemical impacts such as the release of organic acids.
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Factors affecting weathering: Climate
Weathering is related to moisture availabilty and average annual temperature. Frost shattering increases as the number of freeze-thaw cycles increases. Chemical weathering increases with moisture and heat.
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Factors affecting weathering: Geology
Rock type influences the rate and type of weathering in many ways due to: Chemical composition, The nature of cements in sedimentary rock, and Joints and bedding planes.
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Factors affecting river flow and velocity: Turbulent flow
Turbulence provides the upward motion to the flow that allows the lifting and support of fine particles. Turbulent flows occur in meandering channels, with high velocites, and cavitation in which pockets of air explode under high pressure.
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Factors affecting river flow and velocity: Laminar flow
The movement of water in a series of sheets (or laminae). It is common in groundwater and in glaciers, only occurs in the bed and lower course of a river. Conditions: shallow channels, smooth & straight channels, low velocities.
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Factors affecting river flow and velocity: Channel Shape
The efficiency of a stream's shape is measured by its hydraulic radius, which is the hydraulic radius divided by the wetted perimeter. The higher the ratio, the more efficient the stream and the smaller the frictional loss.
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Factors affecting river flow and velocity: Channel Roughness
Channel roughness causes friction that slows down the velocity of the water. Friction is caused by: boulders, trees, vegetation, irregularities in the riverbed and contact between the water and the bed and bank.
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Transport: Suspension
The smallest particles (silts and clays) are carried in suspension as the suspended load. This is especially important during times of flood.
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Transport: Saltation
Larger particles (sands, gravels, very small stones) are transported in a series of 'hops' as the saltated load.
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Transport: Traction
Pebbles are shunted along the bed as the tracted (bed) load.
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Transport: Solution
In areas of calcareous rock, material is carried in solution as the dissolved load.
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Capacity
The total load that a stream can carry.
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Competance
The size of the largest particle that a stream can carry.
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Sediment Yield
The amount of material (debris) carried away by water. Influenced by relief, tectonic activity, rainfall, erodable sediments, intense human activity, and limited vegetation cover.
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Hjulstrom curves
The smallest and largest particles require high velocites to lift them. Higher velocites are required for entrainment than for transport. When velocity falls below a certain level those particles are deposited.
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Entrainment
Picking up particles from the ground/bed of a river.
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Unconsolidated rocks
Loose, broken material that is often easily eroded.
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Consolidated rocks
solid rocks or sediments that have been bonded together by a sedimentary cement.
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Erosion: Abrasion
The wearing away of the bed and bank by the load carried by a river.
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Erosion: Attrition
The wearing away of the load carried by a river. It creates smaller, rounder particles.
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Erosion: Hydraulic Action
The force of air and water on the sides of rivers and in cracks. Hydraulic action occurs as a river tumbles over a waterfall and erodes the rocks below.
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Erosion: Solution
The removal of chemical ions, especially calcium. It is a form of erosion especially effective on chalk and limestone.
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Factors affecting the rates of erosion: Load
The heavier and sharper the load the greater the potential for erosion.
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Factors affecting the rates of erosion: Velocity
The greater the velocity the greater the potential for erosion.
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Factors affecting the rates of erosion: Gradient
Increased gradient increase the rate of erosion.
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Factors affecting the rates of erosion: Geology
Weak unconsolidated rocks such as sand and gravel are more easily eroded than consolidated rocks.
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Factors affecting the rates of erosion: pH
Rates of solution are increased when the water is more acidic.
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Factors affecting the rates of erosion: Human Impact
Deforestation, dams and bridges interfere with the natural flow of a river and frequently end up increasing the rate of erosion.
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Causes of Deposition:
Shallowing gradient which decreases velocity and energy, A decrease in the volume of water in the river, an increase in the friction between the water and the channel, human obstructions such as dams.
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Landforms produced by erosion: V-Shaped Valleys
Weathering and mass movements occur on the valley sides while the river erodes the base of the slopes. Influenced by: Resistance of the rocks, climate, location along the course of the river, rate of downward erosion by the river.
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Landforms produced by erosion: Waterfalls and Gorges
Undercutting, weight of water causes pressure, Hydraulic action, Abrasion, Collapse, Retreat. Some waterfalls may be initiated by faulting Eg Victoria Falls Zimbabwe. While others are caused by a drop in sea level.
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Waterfalls, gorges and human activity: Advantages
Sudden change in gradient is ideal for the generation of hydroelectricity, scenic/recreational attractions, the gorges of incised meanders provided safe defensive locations for settlement, The narrow valleys are relatively easily dammed.
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Waterfalls, gorges and human activity: Disadvantages
Transport is severely affected - the valleys are often too narrow to accomodate roads of rail and crossing gorges makes bridging necessary, Retreat of waterfalls limits the life of constructions.
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Landforms produced by erosion: Potholes
A pothole is a hole in the base of a stream or river. It is formed through abrasion by pebbles held up by turbulent flow in the eddies of the stream.
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Landforms produced by erosion and deposition: Meanders
When stream velocity has increased and the load has become finer, in relatively straight channels, meanders develop as this is the only way the stream can use up the energy it now possesses.
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Cross-section of a meander:
Meanders have an asymmetric cross section. They are deeper on the outside bank and shallower on the inside bank.
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Formation of a meander:
The development of POOLS and RIFFLES in a straight channel and the THALWEG begins to flow from side to side. This movement of energy causes variations in erosion and deposition. Thes variations cause river cliffs and slip off slopes.
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Thalweg:
The line of maximum velocity of a river.
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Helicoidal flow:
The spiraling flow motion that occurs in a meander.
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River Cliff:
The steep-sided slope on the outer bend of a meander.
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Slip-off slope:
The gentle slope deposited on the inner bend of a meander.
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Channel Characteristics and meanders:
Meanering is more pronounced when the bed load is varied, Meander wavelength increases in streams that carry coarse debris, Meandering best develops at or near the bankful stage, Wavelength is 6-10 times the width of the channel.
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Bankfull Stage:
Where the channel variables (depth, width, and velocity) of a river are unable to cope with additional water.
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Formation of Oxbow Lakes:
Lateral erosion, caused by centrifugal forces, is concentrated on the outer bank of a meander. During times of flooding, erosion increases causing the river to break through the neck. The old meander is closed off by deposition.
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Landforms produced by deposition: Braided channels
A river becomes braided when the main channel separates into a number of smaller interlocking channels. River does not have the capacity to transport its load.
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Causes of channel braiding:
A channel gradient that is slightly steeper than that of a meandering stream, A load that contains a large proportion of coarse material, A highly variable discharge.
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Landforms produced by deposition: Floodplains
Repeated flooding and deposition lead to the build up of alluvium and forms a river floodplain. This is a low relief feature filling the river valley.
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Alluvium:
Fine sediments of silt and clay transported in suspension.
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Landforms produced by deposition: Levees
When the river floods, its velocity drops as the floodwaters spill out of the channel, therefore the heaviest and coarsest sediments are the first to be deposited and overtime they will build up into levees.Landforms produced by deposition:
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Landforms produced by deposition: Deltas
Deltas form when a river is carrying a large volume of sediment and it enters a still body of water such as a sea or lake. Deposition is increased if the water is salty as flocculation occurs. Vegetation also increases the rate of depostion.
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Features associated with negative changes in base level: Intrenched/Incised meanders
Meanders that develop on a floodplain will maintain their form as the river cuts down towards its new base level. Where this process has continued for some time, the river will flow in a meander gorge.
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Features associated with negative changes in base level: River terraces
When a river cuts down into its floodplain, the remnants of the floodplain form a terrace with a steep slope. When this process repeats a sequence of terrace 'steps' will form.
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Features associated with negative changes in base level: Knickpoints
Knickpoints are sudden changes in gradient along a river where the river changes from deposition to erosion, caused by negative changes in base levels due to lithology or human actions such as damming or deforestation.
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Lithology:
The properties of rocks - mineral composition, cracks, joints.
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Uses of River Basins as a resource:
Water source, opportunities for industrial/residential development, fertile soil for agriculture, a means for transportation, a source of power, a means of flood defence, a conservation value.
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Other cards in this set

Card 2

Front

Slow movement. On flat surfaces, raindrops compact the soil and dislodge particles equally in all directions. On steep slopes this causes erosion downslope.

Back

Rain-Splash Erosion

Card 3

Front

Small terraces from a few centimeters to 0.5m across the face of a slope caused by soil creep of solifluction.

Back

Preview of the back of card 3

Card 4

Front

Flow Movement. This occurs when the soil's infiltration capacity is exceeded and can lead to the formation of gullies. In Britain this mainly occurs in winter as waters drains across saturated or frozen ground.

Back

Preview of the back of card 4

Card 5

Front

Flow movement. This is the unchanelled flow of water over a soil surface. Often transports material dislodged by rain splash. During the Boscastle floods, sheetwash from the shallow moorland peat caused steep, narrow gulleys to form.

Back

Preview of the back of card 5
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