Geomorphology (from Greek: , ge, "earth"; , morf, "form"; and ,
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 rivers
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 hydrologicalcycle is called the
Drainage Basin System
Drainage Basin
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 rivers 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.
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.
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 PatternsDrainage Pattern Reasons for
formationDendritic Associated with uniform sedimentary or igneous
rockParallel Associated with fold mountainsTrellis The river is
rock controlled associated with alternating layers of variable
resistance (hard and soft) igneous and sedimentary rocksRectangular
(angular) The river is rock controlled and is associated with
igneous rock.Radial The is a valley/depression/low lying areaRadial
CentripetalRedial Centrifugal There is a Mountain/high lying
areaDeranged/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
rivers 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 ManagementThe presence of a dam will allow flow to
be controlled, reducing flood riskand allowing rivers to gradually
respond to heavy rainfall in a controlledway
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 orRiver capture or Stream
piracy
Stream capture / Stream capture or River capture orStream
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 Leves
Stream Piracy
Moving between the Middle andLower 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 overthe surface results in frictional drag and a
consequent reduction in velocity(speed) of flow. This results in
the loss of energy and therefore depositionoccurs. The heaviest
materials are deposited first as these require the mostenergy to be
transported and therefore build up around the sides of the
riverforming raised banks known as Leves. Finer material such as
silt and fine clayscontinuing 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. Leves - 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 riverin its upper
reaches we now need to followthe 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 structureHorizontal strataInclined/Tilted
strataMassive
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, andunderground streams
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