River Processes and Pressures - GCSE Edexcel B

Erosion
1. Hydraulic action - Force of the river against the banks can cause air to get trapped in cracks and crevices.The pressure weakens the banks and gradually wears it away.
2. Attrition - rocks being carried by the river smash together and break into smaller, smoother and rounder particles.
3. Corrosion - acids in the water eat away at certain types of rock (soft less resistant) e.g limestone
4. Abrasion - rocks carried along by the river can wear down the river bed and banks
Transportation
1. Solution - minerals are dissolved in the water and carried along in solution.
2. Suspension - fine light material is carried along in the water.
3. Saltation - small pebbles and stones are bounced along the river bed.
4. Traction - large boulders and rocks are rolled along the river bed.
River Profiles
1. Upper Course
  1. Long Profile - A river is steeper towards the source of the river as it is often found in mountainous areas.
  2. Cross profile - in the upper course a river tends to be quite narrow and shallow. The bottom and sides tend to be littered with many large angular rocks, causing a lot of friction for the water flowing past them,so creates a lot of turbulence and friction.This slows the water down as it uses its energy to overcome friction - in the upper course a river is flowing at its slowest velocity. valley sides are steep and v-shaped as there is lots of vertical erosion. There is lots of vertical erosion because the large angular rocks and boulders are transported down the river by traction, this grinds away at the river bed through abrasion.The transportation of this material is aided by the force of gravity and the amount of energy the water has because of gravity as the gradient of the river is steep.
2. Middle Course
  1. Long Profile - the gradient of a river becomes less steep further towards the mouth of a river as the land begins to level off and the river erodes towards its base level.
  2. Cross Profile - a river increases in width and becomes more u-shaped because there is more lateral erosion,it also increases in depth as some vertical erosion still takes place.
3. Lower Course
  1. Long Profile - lower part of a river has the greatest cross section, highest hydraulic radius, greatest velocity (because of decrease in chanel bed roughness) and discharge.The gradient of the river is smaller as the river approaches the sea or lake where the land is much flatter.
  2. Cross profile - velocity and energy increasses due to increased discharge.The river performes more lateral erosion making the chanel wider and smoother. As a result there is less turbulence and friction, making the flow more efficient. At this stage the water is flowing at its fastest and the river is deep in places where the water is flowing the quickest.
4. Long profile - on a graph of the long profile of a river there will be 'notches'/irregularities in the line. These could be because of changes in rock type, sea level or natural features like waterfalls in the upper course of a river.
The Bradshaw Model
1. Average Velocity - Chanel bed roughness decreases further downstream so the river does not require to use its energy to overcome friction like it does in the upper course (its more efficient). For this reason the velocity increases as the river maintains its energy.
2. Channel Depth - This increases as some vertical erosion still continues in the middle course and hydraulic action and abrasion on the outside bend of meanders causes parts of a river to become deeper. The channel depth is most affected because increased erosion of particles means tht there is a lot more river bed exposed so erosion of the river bed through hydraulic action occurs more easily.
3. Load Particle Size - Most rocks and pebbles come from the source of the river when they fall.As they move downstream they become more and more eroded by attrition and corrosion so their size is reduced.When there is low flow particles are stored on banks so they can be broken down by weathering (particularly freeze-thaw) but this depends on how long they are stored.
4. Discharge - discharge increases largely further downstream as more streams and tributaries join the river and if there is a confluence of two rivers.
5. Occupied Channel Width - channel width increases as more lateral erosion takes place downstream unlike upstream where erosion is predominantly vertical.
6. Gradient - This decreases downstream because the land levels out as the river travels further towards the sea from the source which is in mountainous areas; the lower course is in flood plains and the mouth may be in deltas.
7. Load Quantity - A river gets wider and deeper downstream, so it has a larger capacity to hold more material.Through transportation large rocks tend to break apart into smaller rocks through attrition so the overall amount of particles in the river increases.
8. Channel Bed Roughness - This decreases as you go downstream because rocks have been eroded as they travelled, so they are smaller and smoother.
River Course Features
1. Lower Course
  1. Levees-Levees are natural embankments along the banks of a river.Rivers can carry a lot of material so when a river floods it carries the material with it.The water immediately loses velocity so heavy material is dropped first on the edge of a river as it does not have enough energy to carry it. Lighter material is transported further as less energy is required to carry it.When flooding occurs again the banks get higher and higher;people sometimes build these even higher to reduce the risk of flooding.
  2. Floodplains - A floodplain is a wide, flat area of land either side of a river in its lowercourse.It is formed by both erosion and deposition.Lateral erosion is caused by meanders eroding the outside of their bends-this makes the valley floor wide and flat. When the river floods,the flood water spreads out on the valley floor,slows down and deposits sediments it was carrying;This can be beneficial for farmers because it can help fertilise the soil.
2. Middle course
  1. Meanders - A meander is a bend in a riveri n the middle course.In this section the river flows faster as the bed is smoother so there is less friction.Water flows faster on the outside of the river bend.Hydraulic action and abrasion undercuts the river bank and creates a river cliff.The water flows slowly aroun the inside of the river bend so it cant carry a lot of material so deposits it.This is called a slip off slope.
  2. Oxbow Lakes - An oxbow lake occurs on the bend of a river.Water erodes the the outside bend of a river through hydraulic action an abrasion and deposits sand/silt on the inside.The neck of the meander gets closer together and eventually (normally when a river floods) it pushes through the neck to follow the shortest course.The old bend is abandoned by the river and gradually dries up.
3. Upper Course
  1. Waterfall - When a river meets a band of less resistant softer rock,hydraulic action and abrasion causes the rock to erode, causing a undercutting.Because the underlying softer rock erodes more quickly than the more resistant rock.The unsupported more resistant rock overhangs,becomes unstable through the force of gravity and eventually collapses onto the river bed.This and other rocks causes abrasion of the river bed.Hydraulic action also helps to create a deep plunge pool.This process is repeated so the waterfall retreats upstream and overtime a steep sided river valley (gorge) is formed.
  2. Interlocking Spurs - Streams that flow down steep valley sides into the river perform vertical erosion so the sides of the tributaries and streams protrude outwards.The bed is littered with rocks and debris which are occasionally after storms.This material wears down the bed by abrasion as it is transported by traction and saltation.Areas of the landscape that the river flows through contains hard rock , this means that the river erodes these areas much more slowly than the softer rock.this means that the river winds and bends as interlocking spurs are created. interlocking spurs are kind of like the interlocking parts of a zip. when a river runs over alternating layers of hard and soft rock,rapids and waterfalls may form.
Weathering - The breakdown of rocks and minerals by physical and chemical processes.This involves little/no movement of material unlike erosion.
1. Physical Weathering (Freeze - thaw) - This happenes when rainwater enters a crack or gap in a rock and freezes when temperatures fall below 0 degrees.The water expands as it turns into ice and experts pressure on the rock.This is repeated overtime so eventually the rock cracks into smaller pieces.
2. Chemical Weathering - When rain water is slightly acidic (acid rain) it can react with weak minerals within the rock causing them to dissolve and the rock to decay and wear away.This can happen if the air polluted by factory /vehicle emissions,acidic rain usually occurs close to areas where there is lots of these emissions.
3. Biological Weathering - The roots of plants can grow into cracks in a rock and split the rock appart.
River Valleys
1. Weathering and Mass Movement - Weathering causes the valley sides to break down which allows mass movement - The weathered material is transported down the valley sides to the river, via rockfall or sliding where it becomes part of the load
2. Gorges- Gorges have near vertical sides because there is little weathering or mass movement,the river cuts downwards rapidly;perhaps the land is being lifted up too.in different conditions the valley might be wider because weathering and mass movement operate fast,as in a tropical climate,rivers erode slowly.
3. Geology - where a river flows over hard resistant rock the valley sides tend to be steep and the sediment load is small because erosion is slow.When a river flows over bands of softer less resistant rock the valley sides tend to be gentile and the sediment load is too large because erosion is rapid.
4. Factors That Affect the Shape of River Valleys
  1. The speed of weathering;if scree piles up, weathering rapidly takes place.
  2. The speed of mass movement.
  3. How quickly the river can remove the material brought by mass movement.
  4. If the river has plenty of energy, it takes the material away and uses it to help erode the valley making it steeper (abrasion)
Key Terms
1. Rock Outcrops - exposures of bedrock above the surface of the ground.
2. Scree - accumulated weathering debris below a crag or other exposed rock face.Larger boulders will accumulate at the base of the scree, carried by greater energy/flow.
3. Mass Movement - the movement of broken (weathered) rock,minerals,soil and vegetation from steep valley sides to a river as a result of the force of gravity. e.g. Weathering takes place on valley sides, usually making it less steep overtime as material is moved from the top of a slope to the bottom. However the river itself erodes its own channel by wearing away the bed and banks as well as eroding the base of the valley side, making it steeper and leading to mass movement.
4. Landslides - Landslids occur when a hill or mountainside becomes unstable because of factors such as erosion;this can overly steepen slopes.Earthquakes can cause landslides because they cause soil to move - on steep slopes these soil slips cause landslides.When steep slopes become saturated with repeated heavy rainfall landslides can occur, especially if there is no root support.Human actions can enhance the lack of root support through deforestation.This stops water from being able to be absorbed by the roots so hillsides become heavily saturated.
5. Soil Creep - Individual particles of soil slowly move down valley sides because of gravity and collect at the bottom of the valley sides.
6. Slumping - This happens when the bottom of a valley side is eroded by the river.This makes the slope steeper and the valley side material can slide downwards in a rotational manner, often triggered by saturation due to rainfall, which both 'lubricates' and makes it much heavier.
Flooding
1. Factors That Encourage Flooding
  1. A Steep -Sided Channel - a river channel surrounded by steep slopes causes fast surface run-off.
  2. Lack of Vegetation or woodland - trees and plants intercept precipitation (they can catch or absorb water).If there is little vegetation in the drainage basin then surface run-off will be high.
  3. Drainage basin, consisting mainly of impermeable rock - this will mean that water cannot percolate (pass through) through the rock layer, and so run faster over the surface.
  4. A Drainage Basin in an Urban Area - these consist largely of impermeably concrete, which encourages overland flow.Drains and sewers takes water quickly and directly to the river channel.Houses with sloping roofs further increase the amount of run-off.
2. A flood ocurs when a river breaks its banks and water spills onto the flood plain.Tthe faster the water from hesvy rsinfsll reaches the river chsnnel the more likely the river is to flood.
3. Flood Management - flood management techniques often involve lengthening the amount of time it takes for water to reach the river channel, so increasing the lag time.
  1. Hard-Engineering Options V.s Soft-Engineering options - hard options tend to be more expensive and have a greater effect on the river and surrounding landscape.soft - engineering options are more ecologically sensitive. Effective flood management should be economically,environmentally and socially sustainable.Sustainable strategies allow management without compromising the needs of future generations
    1. Hard-Engineering Options
      1. Dam Construction -Dams are often built along the course of a river in order to control the amount of discharge.Water is held back by the dam and released in a controlled way.This controls flooding.Water is usually stored in a reservoir behind he dam.This water can then be used to generate hydroelectric power or for regeneration purposes. Building a dam can be very expensive .Sediment is oftentrapped behind the wall of the dam,leading to erosion further downstream.Settlements and agricultural land may be lost when a river valley is flooded to form a reservoir.
      2. River Engineering - The river channel may be widened or deepened allowing it to carry more water.A river channel may be straightened so that water can travel faster along the course.The channel course of a river can also be altered diverting flood water away from settlements.Altering the river channel may lead to a grater risk of flooding downstream as the water is carried there faster.
    2. Soft-Engineering Options
      1. Afforestation - trees are planted near to the river so there is a greater interception of rainwater and lower river discharge.This is a relatively low cost option which enhances the environmental quality of the drainage basin.
      2. Managed flooding - the river is allowed to flood naturally in places, to prevent flooding in other areas i.e. near settlements.
      3. Planning - local authorities andnational governments intraduce policies to control urban development close to or on the flood plain.This reduces the chance of flooding and the damage to property. There can be resistance to development restrictions in areas where there is a shortage of housing.Enforcing planning regulations and controls may be harder in LEDCs.
  2. Different interest groups have different views about flood management.
    1. Governments and developers prefer large, hard engineering options because by building a dam you can generate income as profits can be made.
    2. Environmental groups and local residents often prefer softer options as they cause little damage to the environment and do not involve resettlement of communities.
4. The Impact of Flooding
  1. Floods can cause damage to homes and possessions as well as disruption to communications. However flooding can have positive impacts on an area - flooding deposits fine silt (alluvium) on to the floodplain making it very fertile and excellent for agriculture.People living on or near to flood[plains may rely upon regular flooding to help support their farming and therefore provide food.
  2. LEDC's tend to be more affected by the effects of flooding more than MEDC's.This is partly because LEDC's have more farms, and farming communities are attracted to fertile floodplains LEDC's often do not have the resources to prevent flooding or deal with the aftermath of flooding.
River Background
1. The Hydrological Cycle - seas and oceans contain 97% of the world's water, and ice holds 2%.That leavs just 1% of the world's water as fresh water on land or in the air .This water is ecycled again and again through the process of evaporation ,condensation and water transfers such as surface run-off.
  2. Key Definitions
    1. Precipitation - Rain,snow, sleet or hail that falls on the ground.
      1. Interception - when tree leaves and branches 'catch' precipitation and stop it from reaching the ground.
        Evaporation - when water is turned from a liquid to a gas.
        Transpiration - when water is lost from the leaves through water vapour.
        Infiltration - when surface water soaks into the soil.
        Soil moisture - when water is stored in the soil.
        Surface storage - when water is stored on the Earth's surface (puddles).
        Percolation - when water moves through permeable rocks .
        Groundwater - when water is stored in rocks underground.
        Through-flow - when water flows through the soil.
        Groundwater flow - when water flows in rivers in rock underground.
        Surface run-off - when water flows over the surface of the Earth.
        River channel - when water flows in the river channel.
2. Terminology For The Study of Rivers
  2. Drainage basin - the area of land drained by a river.
    1. Catchment area - the area within a drainage basin.
      1. Watershed - the edge of highland surrounding a drainage basin.It marks the boundary between two drainage basins.
        Source - the beginning or start of a river.
        Confluence - the point at which two rivers or streams join.
  3. Tributary - a stream or smaller rive which joins a larger stream or river
    1. Mouth - the point where the river comes to the end, usually when entering a sea.

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