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.
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.
12.
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.
16.
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.
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)
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.
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.
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
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
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