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Theory:
Geomorphology
      or
  Landforms
Geography fluvial landforms
Geomorphology
 (from Greek: γῆ, ge, "earth"; μορφή, morfé, "form"; and
  λόγος, logos, "study") is the scientific study of landforms
  and the processes that shape them. Geomorphologists seek
  to understand why landscapes look the way they do, to
  understand landform history and dynamics, and to predict
  future changes through a combination of field
  observations, physical experiments, and numerical
  modeling. Geomorphology is practiced within physical
  geography, geology, geodesy, engineering geology,
  archaeology, and geotechnical engineering, and this broad
  base of interest contributes to a wide variety of research
  styles and interests within the field
Fluvial
 refers to the processes associated with rivers and streams
  and the deposits and landforms created by them. When the
  stream or rivers are associated with glaciers, ice sheets, or
  ice caps, the term glaciofluvial or fluvioglacial is used.
 The Bradshaw Model is a geographical model which
  describes how a river's characteristics vary between the
  upper course and lower course of a river. It shows that
  discharge, occupied channel width, channel depth and
  average load quantity increases downstream. Load particle
  size, channel bed roughness and gradient are all
  characteristics which decrease downstream.
Drainage Basins features
The land based part of the hydrological
cycle is called the Drainage Basin System
Drainage Basin
Geography fluvial landforms
Drainage basin features
 A drainage basin is the area of land which is drained by a river.
 When water reaches the surface there are a number of routes
    which it may take in its journey to reach the river.
   The edge of a drainage basin is characterised by the highest
    points of land around the river, this is known as the watershed.
   The point at which a river starts is called its source.
   As the river continues to flow down stream it may be joined by
    smaller rivers called tributaries.
   The point at which these smaller rivers join the main river is
    known as a confluence.
   As the river continues its journey, eventually reaches the sea - the
    point where the river flows into the sea is known as the river
    mouth.
Longitudinal profile
Fluvial/River- Areas
 Rivers - Source to Mouth Having understood the basics
  of a Drainage Basin we now need to consider the journey
  that a river within a Drainage Basin takes from its
  beginning to its end.
 The path the river follows from its source to mouth is
  known as the river's course.
 When studying rivers we often divide it into 3 main
  sections, the upper course; middle course and lower
  course.
 Each part of the river has distinctive features which form
  and the characteristics of the river and its surrounding
  valley change downstream.
Geography fluvial landforms
River Processes
 As a river flows along its course it undertakes 3 main
  processes which together help to shape the river
  channel and the surrounding valley.
 These processes are erosion, transport and
  deposition.
RIVER EROSION
 River erosion is the wearing away of the land as the water flows
    past the bed and banks. There are four main types of river
    erosion. These are:
   Attrition - occurs as rocks bang against each other gradually
    breaking each other down (rocks become smaller and less
    angular as attrition occurs)
   Abrasion - this is the scraping away of the bed and banks by
    material transported by the river
   Solution - chemicals in the river dissolve minerals in the rocks
    in the bed and bank, carrying them away in solution.
   Hydraulic Action - this is where the water in the river
    compresses air in cracks in the bed and banks. This results in
    increased pressure caused by the compression of air, mini
    'explosions' are caused as the pressure is then released gradually
    forcing apart parts of the bed and banks.
RIVER TRANSPORT
 Material may be transported by a river in five main ways:
  floatation; solution; suspension; saltation and traction.
 The type of transport taking place depends on...
    (i) the size of the sediment and
    (ii) the amount of energy that is available to undertake the
     transport.
    The chemical composition of the parent rock from which sediments
     originate.
 In the upper course of the river there is more traction and
  saltation going on due to the large size of the bed-load, as a river
  enters its middle and lower course there is a lot of finer material
  eroded from further upstream which will be carried in
  suspension.
Geography fluvial landforms
DEPOSITION
 is where material carried by the river is dropped.
 occur when there is no longer sufficient energy to
  transport material.
 May result in the formation of features such as slip off
  slopes (on the inner bends of meanders); levees
  (raised banks) alluvial fans; meanders; braided
  streams and the floodplain.
 Remember - it is the largest material that will be
  dropped first as it requires the most energy to be
  transported. Eroded material carried in suspension
  and solution will be dropped last.
Stream flow-Upper course
Cross Profile-Upper course
Upper course
Key Term Check
 V-shaped Valley - a valley which resembles a 'v' in
  cross section. These valleys have steep sloping sides
  and narrow bottoms.
 Interlocking Spur - spurs are ridges of more resistant
  rock around which a river is forced to wind as it passes
  downstream in the upper course.
   Interlocking spurs form where the river is forced to
    swing from side to side around more resistant ridges.
 Load - collective term for the material carried by a
 river
How does a v-shaped valley form?
 1. Vertical erosion (in the form of abrasion, hydraulic action and
    solution) in the river channel results in the formation of a steep
    sided valley
   2. Over time the sides of this valley are weakened by weathering
    processes and continued vertical erosion at the base of the valley
   3. Gradually mass movement of materials occurs down the valley
    sides, gradually creating the distinctive v-shape.
   4. The material is gradually transported away by the river when
    there is enough energy to do so.
   As the river flows through the valley it is forced to swing from
    side to side around more resistant rock outcrops (spurs). As
    there is little energy for lateral erosion, the river continues to cut
    down vertically flowing between spurs of creating interlocking
    spurs.
Upper Course of the River: Waterfalls
                           Another feature found in
                           the upper course of a
                           river, where vertical
                           erosion is dominant, is a
                           waterfall. The highest
                           waterfall in the world is
                           the Angel Falls in
                           Venezuela (see picture
                           right) which have a drop
                           of 979m. Other
                           particularly famous
                           examples include Niagara
                           Falls (North America),
                           the Victoria Falls (on the
                           Zambia / Zimbabwe
                           border) and the Iguazu
                           Falls (South America).
Waterfall Formation
The formation of Waterfalls
 Waterfalls are found in the upper course of a river. They usually occur where a
    layer/band of hard rock lies next to soft rock. They may start as rapids.
   As the river passes over the hard rock, the soft rock below is eroded (worn
    away) more quickly than the hard rock leaving the hard rock elevated above the
    stream bed below.
   The 'step' in the river bed continues to develop as the river flows over the hard
    rock step (Cap Rock) as a vertical drop.
   The drop gets steeper as the river erodes the soft rock beneath by processes
    such as abrasion and hydraulic action.
   A plunge pool forms at the base of the waterfall.
      This erosion gradually undercuts the hard rock and the plunge pool gets
       bigger due to further hydraulic action and abrasion.
      Eventually the hard cap rock is unsupported and collapses.
      The rocks that fall into the plunge pool will continue to enlarge it by abrasion as
       they are swirled around.
      A steep sided valley known as a gorge is left behind and as the process
       continues the waterfall gradually retreats upstream.
Cascades and rapids
Key Term Check
 Cap Rock - layer of hard resistant rock forming the 'step'
    over which the 'falls' occur in a waterfall.
   Waterfall - a cascade of water over a hard rock step in the
    upper course of a river
   Plunge Pool - a deep pool beneath
   Gorge - a steep sided valley left behind as a waterfall
    retreats upstream
   Abrasion - where rocks and boulders scrape away at the
    river bed and banks
   Hydraulic Action - where the force of water compresses
    air in cracks resulting in mini-explosions as the increased
    pressure in the cracks is released.
Upper Course of the River:
V-Shaped Valleys
 In the upper course of a river, water flows quickly
  through a narrow channel with a steep gradient; as it
  does so it cuts downwards. This in known as vertical
  erosion.
 This vertical erosion results in a number of
  distinctive landforms including the steep sloping v-
  shaped valley through which the river flows in its
  upper course.
V-Shaped Valley
Cross Profile Middle course
Cross Profile Lower course
Drainage patterns
Formation of Drainage Patterns
Drainage Pattern        Reasons for formation
Dendritic               Associated with uniform sedimentary or igneous
                        rock
Parallel                Associated with fold mountains
Trellis                 The river is rock controlled associated with
                        alternating layers of variable resistance (hard and
                        soft) igneous and sedimentary rocks
Rectangular (angular)   The river is rock controlled and is associated with
                        igneous rock.
Radial
                        The is a valley/depression/low lying area
Radial Centripetal
Redial Centrifugal      There is a Mountain/high lying area
Deranged/contorted      Associated with glacial erosion /glaciations
Drainage patterns
Drainage Patterns-In 3D
Assessment
Solutions
Hydrographs and River Discharge
Stream order
What are Hydrographs?
 The amount of water in a river at any given point and time is known as the discharge
  which is measured in cumecs (cubic metres per second). This can be calculated by
  multiplying river velocity by channel volume at a given point and time.
 Hydrographs are graphs which show river discharge over a given period of time and
  show the response of a drainage basin and its river to a period of rainfall.
 A storm hydrograph shows how a river's discharge responds following a period of heavy
  rainfall. On a hydrograph, the flood is shown as a peak above the base (normal) flow of
  the river. Analysis of hydrographs can help hydrologists to predict the likelihood of
  flooding in a drainage basin. The response of a river to a rainfall event can be measured in
  terms of the lag time - the time between peak rainfall and peak discharge. Rivers with a
  short lag time respond rapidly to rainfall events and are therefore more prone to flooding
  than rivers with a longer lag time
 River discharge does not respond immediately to rainfall inputs as only a little of the
  rainfall will fall directly into the channel. The river will start to respond initially through
  inputs from surface runoff (the fastest flow of water) and its discharge will later be
  supplemented through inputs from throughflow and groundwater flow.
Variations in the shape of a Hydrograph
 The shape of a hydrograph is determined by the speed in which flood
  waters are able to reach the river. The nature of the drainage basin
  therefore has a great influence on the way a river responds to a
 river as it will determine the types and speeds of the flow of water to
  the river.
 The fastest route to the river is via overland flow. If most of the water
  in a drainage basin travels in this way, a river will respond quickly to
  heavy rainfall and the hydrograph shape will be 'peaky' (graph A) with
  steep rising and recessional limbs. The lag time will be short and there
  will be a greater risk of flooding.
 Where more water is able to pass into the soil and travel to the river via
  throughflow / groundwater flow, there will be a slower rise in
  discharge and the river will respond slower (graph B). The lag time will
  be longer and the risk of flooding will be much lower.
Drainage Basin Shapes
Variations in drainage basins
Factors affecting a flood hydrograph
 Characteristics of the Drainage Basin
Permeability
 Impermeable rocks (e.g. granite) and soil (e.g. clay)
  will not allow water to pass through, resulting in large
  amounts of surface runoff and a greater flood risk as
  rivers respond quickly - results in a short lag time.
 Permeable rocks and soil have a high infiltration
  capacity and will absorb water quickly, reducing
  overland flow - results in a longer lag time
 A drainage basin with a steep gradient will result in
  greater overland flow and a shorter lag time than
  where the gradient is less steep allowing more time for
  infiltration to occur.
Type and amount of Precipitation
Type and amount of rain
 heavy rain results in rapid saturation of the upper soil
  layers and the excess water therefore reaches streams
  quickly as surface runoff (short lag time)
 - slow light rain can be absorbed by infiltration and
  the river takes longer to respond to rainfall as water
  takes longer to pass through the drainage basin via
  throughflow and groundwater flow (longer lag time)
Land Use and Human Impact
Human Impact
 Man made surfaces such as concrete and tarmac are
  impermeable therefore rivers in urban drainage basins
  tend to have short lag times due to higher amounts of
  surface runoff and drainage systems taking water to rivers
  quickly.
 Vegetated areas help to reduce flood risk by increasing
  the time it takes for water to reach a river (longer lag
  time) by encouraging infiltration (roots opening up the
  soil), intercepting water by their leaves and taking up water
  in their roots.
 areas cleared by deforestation will respond quickly to
  rainfall due to the reduced interception
Size of the Drainage Basin
 Large Drainage Basin - water will take longer to reach
  the river (long lag time)
 Small Drainage Basin - water will enter the river
  quicker (short lag time)
Present conditions of the Drainage Basin
 If the soil has already been saturated by heavy rain its
  infiltration capacity will be reduced and further rain
  will go as surface runoff.
 If the soil is dry it will be able to absorb more water
  during infiltration and therefore the lag time will be
  longer.
 If the ground surface is frozen lag time is short as
  water cannot infiltrates and passes quickly to the river
  as runoff.
River flow Management




The presence of a dam will allow flow to be controlled, reducing flood risk
and allowing rivers to gradually respond to heavy rainfall in a controlled
way
Exam Tip
 Make sure you are able to calculate lag time - you may
  be given a hydrograph in an exam and be expected to
  give the lag time
 When quoting lag time, discharge, rainfall etc.. from a
  hydrograph make sure you include the relevant units
  in your answer! (i.e. hours, cumecs, mm, respectively.)
 Make sure you are able to discuss the factors that result
  in long or short lag times and thus affect the likelihood
  of a drainage basin flooding.
Key Terms Check:
 Discharge - this is the amount of water in a river at any given point and time. Discharge
  is measured in cumecs (cubic metres per second)

 Velocity - speed of a river (measured in metres per second)

 Hydrograph - a graph showing changes in river discharge over time in response to a
  rainfall event.

 Lag time - the time taken between peak rainfall and peak discharge

 Rising Limb - shows the increase in discharge on a hydrograph

 Falling Limb - shows the return of discharge to normal / base flow on a hydrograph

 Peak Rainfall - maximum rainfall (mm)

 Peak Discharge - maximum discharge (cumecs)
Stream capture / Stream capture or
River capture or Stream piracy
Stream capture / Stream capture or River capture or
Stream piracy
Headwords Erosion
Abstraction
Mechanisms of river capture
 Erosion, either
    Headward erosion of one stream valley upwards into
     another, or
    Lateral erosion of a meander through the higher ground
     dividing the adjacent streams.
 Natural damming, such as by a landslide or ice sheet.
    Within an area of karst topography, where streams may
     sink, or flow underground (a sinking or losing stream)
     and then reappear in a nearby stream valley.
Features of stream piracy
Assessment
Lower Course of the River -
Floodplains and Levées
Stream Piracy
Moving between the Middle and
Lower Course of the River
 As a river continues its journey towards the sea, the valley
  cross section continues to become wider and flatter with an
  extensive floodplain either side of the channel. The river
  erodes laterally and deposition also becomes important. By
  the time it reaches the lower course the river is wider and
  deeper and may contain a large amount of suspended
  sediment.
 When the river floods over the surrounding land it loses
  energy and deposition of its suspended load occurs.
  Regular flooding results in the building up of layers of
  nutrient rich alluvium which forms a flat and fertile
  floodplain
When the river water bursts its bank, the shallower depth of water flowing over
the surface results in frictional drag and a consequent reduction in velocity
(speed) of flow. This results in the loss of energy and therefore deposition
occurs. The heaviest materials are deposited first as these require the most
energy to be transported and therefore build up around the sides of the river
forming raised banks known as Levées. Finer material such as silt and fine clays
continuing to flow further over the floodplain before they are deposited.
Floodplain & Levees
 Floodplain - the area of land around a river channel
  which is formed during times of flood when the
  amount of water in a river exceeds its channel capacity
  and deposition of rich silt occurs.
 Levées - a raised river bank (can be natural features
  formed by deposition or artificial structures built to
  increase channel capacity and reduce flood risk)
Floodplain & levees
Having studied the characteristics of a river
in its upper reaches we now need to follow
the river as it enters its middle course.
 Here the river channel has become much wider and
  deeper as the channel has been eroded and the river has
  been fed by many tributaries upstream. Consequently,
  despite the more gentle gradient the velocity of flow may
  be as fast as in the uplands. As well as changes in the river
  channel, its surrounding valley has also become wider
  and flatter in cross-section with a more extensive
  floodplain.
 One of the most distinctive features of the river in the
  middle course is its increased sinuosity (a winding bend
  or curving movement). Unlike the relatively straight
  channel of the upper course, in the middle course there are
  many meanders (bends) in the river.
Middle Course of the River
- Meanders & Ox-bow Lakes
Meander Formation
Meander
Meander-Formation
 Meanders form due to the greater volume of water
 carried by the river in lowland areas which results in
 lateral (sideways) erosion being more dominant than
 vertical erosion, causing the channel to cut into its
 banks forming meanders.
Meander-Formation
 1. Water flows fastest on the outer bend of the river where the
  channel is deeper and there is less friction. This is due to
  water being flung towards the outer bend as it flows around the
  meander, this causes greater erosion which deepens the
  channel, in turn the reduction in friction and increase in energy
  results in greater erosion. This lateral erosion results in
  undercutting of the river bank and the formation of a steep
  sided river cliff.
 2. In contrast, on the inner bend water is slow flowing, due to
  it being a low energy zone, deposition occurs resulting in a
  shallower channel. This increased friction further reduces the
  velocity (thus further reducing energy), encouraging further
  deposition. Over time a small beach of material builds up on the
  inner bend; this is called a slip-off slope.
Meander-Landforms
Remember
 A meander is asymmetrical in cross-section (see
  diagram on previous slide).
 It is deeper on the outer bend (due to greater
  erosion)
 and shallower on the inside bend (an area of
  deposition).
 Over time meanders gradually change shape and
  migrate across the floodplain. As they do so meander
  bends becomes pronounced due to further lateral
  erosion and eventually an ox-bow lake may form.
Meandering and oxbow lake
Ox-Bow Lake formation
 As the outer banks of a meander continue to be eroded through
    processes such as hydraulic action the neck of the meander
    becomes narrow and narrower.
   Eventually due to the narrowing of the neck, the two outer bends
    meet and the river cuts through the neck of the meander. The
    water now takes its shortest route rather than flowing around
    the bend.
   Deposition gradually seals off the old meander bend forming a
    new straighter river channel.
   Due to deposition the old meander bend is left isolated from the
    main channel as an ox-bow lake.
   Over time this feature may fill up with sediment and may
    gradually dry up (except for periods of heavy rain). When the
    water dries up, the feature left behind is known as a meander
    scar
Key Terms Check
 Meander - a bend in a river
 River Cliff - a small cliff formed on the outside of a
  meander bend due to erosion in this high energy zone.
 Slip off Slope - a small beach found on the inside of a
  meander bend where deposition has occurred in the low
  energy zone.
 Ox-bow lake - a lake formed when the continued
  narrowing of a meander neck results in the eventual cut
  through of the neck as two outer bends join. This result in
  the straightening of the river channel and the old meander
  bend becomes cut off forming an ox-bow lake.
 Meander scar - feature left behind when the water in an
  ox-bow lake dries up.
Mass Movement/Wasting
 Mass wasting is the down-slope movement of
  rock and sediments due to the force of gravity.
 Types of mass movements
  1.   Soil creep
  2.   Mud flow
  3.   Earth flow
  4.   Solifluction
  5.   Landslide
  6.   Land slumps/slip
  7.   Rockfalls
Mass Wasting/Movements
Slope Elements
Assessment
Assessment
Rock structure
Horizontal strata
Inclined/Tilted strata
Massive
Rock structure
Mesa
Butte
Horizontal-Conical Hill
Horizontal Strata Features
Inclined/Tilted-Rock Strata
 Cuesta -a ridge with a steep face on one side (scarp
  slope) and a gentle slope (Dip slope) on the other
 Homoclinal Ridge
 Hogsbacks
Cuesta
Homoclinal Ridge
Hogsbacks
Massive Rock
 Dome
 Tors
Dome
Dome
Tor
Tor-Formation
Karst Topography
-a limestone landscape, characterized by caves, fissures, and
underground streams
Geography fluvial landforms
Geological Terms
 Aquifer (water-bearing rock)- is a layer of permeable
  rock, sand, or gravel through which ground water
  flows, containing enough water to supply wells and
  springs.
 Aquiclude (impermeable rock) is a layer of rock,
  sediment, or soil through which ground water cannot
  flow.
 Aqueduct-is a structure in the form of a bridge that
  carries a canal across a valley or river
Fluvial Related Terms
 Lacustrine - of or relating to a lake
 Maritime - of or relating the sea
 Oceanic - of or relating to an ocean
 Palustrine - of or relating to a marsh
The End

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Geography fluvial landforms

  • 1. Theory: Geomorphology or Landforms
  • 3. Geomorphology  (from Greek: γῆ, ge, "earth"; μορφή, morfé, "form"; and λόγος, logos, "study") is the scientific study of landforms and the processes that shape them. Geomorphologists seek to understand why landscapes look the way they do, to understand landform history and dynamics, and to predict future changes through a combination of field observations, physical experiments, and numerical modeling. Geomorphology is practiced within physical geography, geology, geodesy, engineering geology, archaeology, and geotechnical engineering, and this broad base of interest contributes to a wide variety of research styles and interests within the field
  • 4. Fluvial  refers to the processes associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvioglacial is used.  The Bradshaw Model is a geographical model which describes how a river's characteristics vary between the upper course and lower course of a river. It shows that discharge, occupied channel width, channel depth and average load quantity increases downstream. Load particle size, channel bed roughness and gradient are all characteristics which decrease downstream.
  • 6. The land based part of the hydrological cycle is called the Drainage Basin System
  • 9. Drainage basin features  A drainage basin is the area of land which is drained by a river.  When water reaches the surface there are a number of routes which it may take in its journey to reach the river.  The edge of a drainage basin is characterised by the highest points of land around the river, this is known as the watershed.  The point at which a river starts is called its source.  As the river continues to flow down stream it may be joined by smaller rivers called tributaries.  The point at which these smaller rivers join the main river is known as a confluence.  As the river continues its journey, eventually reaches the sea - the point where the river flows into the sea is known as the river mouth.
  • 11. Fluvial/River- Areas  Rivers - Source to Mouth Having understood the basics of a Drainage Basin we now need to consider the journey that a river within a Drainage Basin takes from its beginning to its end.  The path the river follows from its source to mouth is known as the river's course.  When studying rivers we often divide it into 3 main sections, the upper course; middle course and lower course.  Each part of the river has distinctive features which form and the characteristics of the river and its surrounding valley change downstream.
  • 13. River Processes  As a river flows along its course it undertakes 3 main processes which together help to shape the river channel and the surrounding valley.  These processes are erosion, transport and deposition.
  • 14. RIVER EROSION  River erosion is the wearing away of the land as the water flows past the bed and banks. There are four main types of river erosion. These are:  Attrition - occurs as rocks bang against each other gradually breaking each other down (rocks become smaller and less angular as attrition occurs)  Abrasion - this is the scraping away of the bed and banks by material transported by the river  Solution - chemicals in the river dissolve minerals in the rocks in the bed and bank, carrying them away in solution.  Hydraulic Action - this is where the water in the river compresses air in cracks in the bed and banks. This results in increased pressure caused by the compression of air, mini 'explosions' are caused as the pressure is then released gradually forcing apart parts of the bed and banks.
  • 15. RIVER TRANSPORT  Material may be transported by a river in five main ways: floatation; solution; suspension; saltation and traction.  The type of transport taking place depends on...  (i) the size of the sediment and  (ii) the amount of energy that is available to undertake the transport.  The chemical composition of the parent rock from which sediments originate.  In the upper course of the river there is more traction and saltation going on due to the large size of the bed-load, as a river enters its middle and lower course there is a lot of finer material eroded from further upstream which will be carried in suspension.
  • 17. DEPOSITION  is where material carried by the river is dropped.  occur when there is no longer sufficient energy to transport material.  May result in the formation of features such as slip off slopes (on the inner bends of meanders); levees (raised banks) alluvial fans; meanders; braided streams and the floodplain.  Remember - it is the largest material that will be dropped first as it requires the most energy to be transported. Eroded material carried in suspension and solution will be dropped last.
  • 21. Key Term Check  V-shaped Valley - a valley which resembles a 'v' in cross section. These valleys have steep sloping sides and narrow bottoms.  Interlocking Spur - spurs are ridges of more resistant rock around which a river is forced to wind as it passes downstream in the upper course.  Interlocking spurs form where the river is forced to swing from side to side around more resistant ridges.  Load - collective term for the material carried by a river
  • 22. How does a v-shaped valley form?  1. Vertical erosion (in the form of abrasion, hydraulic action and solution) in the river channel results in the formation of a steep sided valley  2. Over time the sides of this valley are weakened by weathering processes and continued vertical erosion at the base of the valley  3. Gradually mass movement of materials occurs down the valley sides, gradually creating the distinctive v-shape.  4. The material is gradually transported away by the river when there is enough energy to do so.  As the river flows through the valley it is forced to swing from side to side around more resistant rock outcrops (spurs). As there is little energy for lateral erosion, the river continues to cut down vertically flowing between spurs of creating interlocking spurs.
  • 23. Upper Course of the River: Waterfalls Another feature found in the upper course of a river, where vertical erosion is dominant, is a waterfall. The highest waterfall in the world is the Angel Falls in Venezuela (see picture right) which have a drop of 979m. Other particularly famous examples include Niagara Falls (North America), the Victoria Falls (on the Zambia / Zimbabwe border) and the Iguazu Falls (South America).
  • 25. The formation of Waterfalls  Waterfalls are found in the upper course of a river. They usually occur where a layer/band of hard rock lies next to soft rock. They may start as rapids.  As the river passes over the hard rock, the soft rock below is eroded (worn away) more quickly than the hard rock leaving the hard rock elevated above the stream bed below.  The 'step' in the river bed continues to develop as the river flows over the hard rock step (Cap Rock) as a vertical drop.  The drop gets steeper as the river erodes the soft rock beneath by processes such as abrasion and hydraulic action.  A plunge pool forms at the base of the waterfall.  This erosion gradually undercuts the hard rock and the plunge pool gets bigger due to further hydraulic action and abrasion.  Eventually the hard cap rock is unsupported and collapses.  The rocks that fall into the plunge pool will continue to enlarge it by abrasion as they are swirled around.  A steep sided valley known as a gorge is left behind and as the process continues the waterfall gradually retreats upstream.
  • 27. Key Term Check  Cap Rock - layer of hard resistant rock forming the 'step' over which the 'falls' occur in a waterfall.  Waterfall - a cascade of water over a hard rock step in the upper course of a river  Plunge Pool - a deep pool beneath  Gorge - a steep sided valley left behind as a waterfall retreats upstream  Abrasion - where rocks and boulders scrape away at the river bed and banks  Hydraulic Action - where the force of water compresses air in cracks resulting in mini-explosions as the increased pressure in the cracks is released.
  • 28. Upper Course of the River: V-Shaped Valleys  In the upper course of a river, water flows quickly through a narrow channel with a steep gradient; as it does so it cuts downwards. This in known as vertical erosion.  This vertical erosion results in a number of distinctive landforms including the steep sloping v- shaped valley through which the river flows in its upper course.
  • 33. Formation of Drainage Patterns Drainage Pattern Reasons for formation Dendritic Associated with uniform sedimentary or igneous rock Parallel Associated with fold mountains Trellis The river is rock controlled associated with alternating layers of variable resistance (hard and soft) igneous and sedimentary rocks Rectangular (angular) The river is rock controlled and is associated with igneous rock. Radial The is a valley/depression/low lying area Radial Centripetal Redial Centrifugal There is a Mountain/high lying area Deranged/contorted Associated with glacial erosion /glaciations
  • 40. What are Hydrographs?  The amount of water in a river at any given point and time is known as the discharge which is measured in cumecs (cubic metres per second). This can be calculated by multiplying river velocity by channel volume at a given point and time.  Hydrographs are graphs which show river discharge over a given period of time and show the response of a drainage basin and its river to a period of rainfall.  A storm hydrograph shows how a river's discharge responds following a period of heavy rainfall. On a hydrograph, the flood is shown as a peak above the base (normal) flow of the river. Analysis of hydrographs can help hydrologists to predict the likelihood of flooding in a drainage basin. The response of a river to a rainfall event can be measured in terms of the lag time - the time between peak rainfall and peak discharge. Rivers with a short lag time respond rapidly to rainfall events and are therefore more prone to flooding than rivers with a longer lag time  River discharge does not respond immediately to rainfall inputs as only a little of the rainfall will fall directly into the channel. The river will start to respond initially through inputs from surface runoff (the fastest flow of water) and its discharge will later be supplemented through inputs from throughflow and groundwater flow.
  • 41. Variations in the shape of a Hydrograph  The shape of a hydrograph is determined by the speed in which flood waters are able to reach the river. The nature of the drainage basin therefore has a great influence on the way a river responds to a  river as it will determine the types and speeds of the flow of water to the river.  The fastest route to the river is via overland flow. If most of the water in a drainage basin travels in this way, a river will respond quickly to heavy rainfall and the hydrograph shape will be 'peaky' (graph A) with steep rising and recessional limbs. The lag time will be short and there will be a greater risk of flooding.  Where more water is able to pass into the soil and travel to the river via throughflow / groundwater flow, there will be a slower rise in discharge and the river will respond slower (graph B). The lag time will be longer and the risk of flooding will be much lower.
  • 44. Factors affecting a flood hydrograph  Characteristics of the Drainage Basin
  • 45. Permeability  Impermeable rocks (e.g. granite) and soil (e.g. clay) will not allow water to pass through, resulting in large amounts of surface runoff and a greater flood risk as rivers respond quickly - results in a short lag time.  Permeable rocks and soil have a high infiltration capacity and will absorb water quickly, reducing overland flow - results in a longer lag time  A drainage basin with a steep gradient will result in greater overland flow and a shorter lag time than where the gradient is less steep allowing more time for infiltration to occur.
  • 46. Type and amount of Precipitation
  • 47. Type and amount of rain  heavy rain results in rapid saturation of the upper soil layers and the excess water therefore reaches streams quickly as surface runoff (short lag time)  - slow light rain can be absorbed by infiltration and the river takes longer to respond to rainfall as water takes longer to pass through the drainage basin via throughflow and groundwater flow (longer lag time)
  • 48. Land Use and Human Impact
  • 49. Human Impact  Man made surfaces such as concrete and tarmac are impermeable therefore rivers in urban drainage basins tend to have short lag times due to higher amounts of surface runoff and drainage systems taking water to rivers quickly.  Vegetated areas help to reduce flood risk by increasing the time it takes for water to reach a river (longer lag time) by encouraging infiltration (roots opening up the soil), intercepting water by their leaves and taking up water in their roots.  areas cleared by deforestation will respond quickly to rainfall due to the reduced interception
  • 50. Size of the Drainage Basin  Large Drainage Basin - water will take longer to reach the river (long lag time)  Small Drainage Basin - water will enter the river quicker (short lag time)
  • 51. Present conditions of the Drainage Basin  If the soil has already been saturated by heavy rain its infiltration capacity will be reduced and further rain will go as surface runoff.  If the soil is dry it will be able to absorb more water during infiltration and therefore the lag time will be longer.  If the ground surface is frozen lag time is short as water cannot infiltrates and passes quickly to the river as runoff.
  • 52. River flow Management The presence of a dam will allow flow to be controlled, reducing flood risk and allowing rivers to gradually respond to heavy rainfall in a controlled way
  • 53. Exam Tip  Make sure you are able to calculate lag time - you may be given a hydrograph in an exam and be expected to give the lag time  When quoting lag time, discharge, rainfall etc.. from a hydrograph make sure you include the relevant units in your answer! (i.e. hours, cumecs, mm, respectively.)  Make sure you are able to discuss the factors that result in long or short lag times and thus affect the likelihood of a drainage basin flooding.
  • 54. Key Terms Check:  Discharge - this is the amount of water in a river at any given point and time. Discharge is measured in cumecs (cubic metres per second)  Velocity - speed of a river (measured in metres per second)  Hydrograph - a graph showing changes in river discharge over time in response to a rainfall event.  Lag time - the time taken between peak rainfall and peak discharge  Rising Limb - shows the increase in discharge on a hydrograph  Falling Limb - shows the return of discharge to normal / base flow on a hydrograph  Peak Rainfall - maximum rainfall (mm)  Peak Discharge - maximum discharge (cumecs)
  • 55. Stream capture / Stream capture or River capture or Stream piracy
  • 56. Stream capture / Stream capture or River capture or Stream piracy
  • 59. Mechanisms of river capture  Erosion, either  Headward erosion of one stream valley upwards into another, or  Lateral erosion of a meander through the higher ground dividing the adjacent streams.  Natural damming, such as by a landslide or ice sheet.  Within an area of karst topography, where streams may sink, or flow underground (a sinking or losing stream) and then reappear in a nearby stream valley.
  • 62. Lower Course of the River - Floodplains and Levées
  • 64. Moving between the Middle and Lower Course of the River  As a river continues its journey towards the sea, the valley cross section continues to become wider and flatter with an extensive floodplain either side of the channel. The river erodes laterally and deposition also becomes important. By the time it reaches the lower course the river is wider and deeper and may contain a large amount of suspended sediment.  When the river floods over the surrounding land it loses energy and deposition of its suspended load occurs. Regular flooding results in the building up of layers of nutrient rich alluvium which forms a flat and fertile floodplain
  • 65. When the river water bursts its bank, the shallower depth of water flowing over the surface results in frictional drag and a consequent reduction in velocity (speed) of flow. This results in the loss of energy and therefore deposition occurs. The heaviest materials are deposited first as these require the most energy to be transported and therefore build up around the sides of the river forming raised banks known as Levées. Finer material such as silt and fine clays continuing to flow further over the floodplain before they are deposited.
  • 66. Floodplain & Levees  Floodplain - the area of land around a river channel which is formed during times of flood when the amount of water in a river exceeds its channel capacity and deposition of rich silt occurs.  Levées - a raised river bank (can be natural features formed by deposition or artificial structures built to increase channel capacity and reduce flood risk)
  • 68. Having studied the characteristics of a river in its upper reaches we now need to follow the river as it enters its middle course.  Here the river channel has become much wider and deeper as the channel has been eroded and the river has been fed by many tributaries upstream. Consequently, despite the more gentle gradient the velocity of flow may be as fast as in the uplands. As well as changes in the river channel, its surrounding valley has also become wider and flatter in cross-section with a more extensive floodplain.  One of the most distinctive features of the river in the middle course is its increased sinuosity (a winding bend or curving movement). Unlike the relatively straight channel of the upper course, in the middle course there are many meanders (bends) in the river.
  • 69. Middle Course of the River - Meanders & Ox-bow Lakes
  • 72. Meander-Formation  Meanders form due to the greater volume of water carried by the river in lowland areas which results in lateral (sideways) erosion being more dominant than vertical erosion, causing the channel to cut into its banks forming meanders.
  • 73. Meander-Formation  1. Water flows fastest on the outer bend of the river where the channel is deeper and there is less friction. This is due to water being flung towards the outer bend as it flows around the meander, this causes greater erosion which deepens the channel, in turn the reduction in friction and increase in energy results in greater erosion. This lateral erosion results in undercutting of the river bank and the formation of a steep sided river cliff.  2. In contrast, on the inner bend water is slow flowing, due to it being a low energy zone, deposition occurs resulting in a shallower channel. This increased friction further reduces the velocity (thus further reducing energy), encouraging further deposition. Over time a small beach of material builds up on the inner bend; this is called a slip-off slope.
  • 75. Remember  A meander is asymmetrical in cross-section (see diagram on previous slide).  It is deeper on the outer bend (due to greater erosion)  and shallower on the inside bend (an area of deposition).  Over time meanders gradually change shape and migrate across the floodplain. As they do so meander bends becomes pronounced due to further lateral erosion and eventually an ox-bow lake may form.
  • 77. Ox-Bow Lake formation  As the outer banks of a meander continue to be eroded through processes such as hydraulic action the neck of the meander becomes narrow and narrower.  Eventually due to the narrowing of the neck, the two outer bends meet and the river cuts through the neck of the meander. The water now takes its shortest route rather than flowing around the bend.  Deposition gradually seals off the old meander bend forming a new straighter river channel.  Due to deposition the old meander bend is left isolated from the main channel as an ox-bow lake.  Over time this feature may fill up with sediment and may gradually dry up (except for periods of heavy rain). When the water dries up, the feature left behind is known as a meander scar
  • 78. Key Terms Check  Meander - a bend in a river  River Cliff - a small cliff formed on the outside of a meander bend due to erosion in this high energy zone.  Slip off Slope - a small beach found on the inside of a meander bend where deposition has occurred in the low energy zone.  Ox-bow lake - a lake formed when the continued narrowing of a meander neck results in the eventual cut through of the neck as two outer bends join. This result in the straightening of the river channel and the old meander bend becomes cut off forming an ox-bow lake.  Meander scar - feature left behind when the water in an ox-bow lake dries up.
  • 79. Mass Movement/Wasting  Mass wasting is the down-slope movement of rock and sediments due to the force of gravity.  Types of mass movements 1. Soil creep 2. Mud flow 3. Earth flow 4. Solifluction 5. Landslide 6. Land slumps/slip 7. Rockfalls
  • 86. Mesa
  • 87. Butte
  • 90. Inclined/Tilted-Rock Strata  Cuesta -a ridge with a steep face on one side (scarp slope) and a gentle slope (Dip slope) on the other  Homoclinal Ridge  Hogsbacks
  • 95. Dome
  • 96. Dome
  • 97. Tor
  • 99. Karst Topography -a limestone landscape, characterized by caves, fissures, and underground streams
  • 101. Geological Terms  Aquifer (water-bearing rock)- is a layer of permeable rock, sand, or gravel through which ground water flows, containing enough water to supply wells and springs.  Aquiclude (impermeable rock) is a layer of rock, sediment, or soil through which ground water cannot flow.  Aqueduct-is a structure in the form of a bridge that carries a canal across a valley or river
  • 102. Fluvial Related Terms  Lacustrine - of or relating to a lake  Maritime - of or relating the sea  Oceanic - of or relating to an ocean  Palustrine - of or relating to a marsh

Editor's Notes

  1. Mesa