Geomorphology Flashcards
Define a drainage basin
The bowl-shaped depression from which water drains off the land.
Area occupied by a river system
What flows in a drainage basin?
A river system or stream network
What is a river system/stream network made up of?
A number of individual stream channels that join together
What are tributaries?
Rivers that join the main stream
What is the confluence?
The point where one river flows into another river in the drainage basin
What is a watershed?
The high ground that separates one drainage basin from the drainage basin that lies alongside it
The size and shape of a drainage basin is dependent on…
The position of the watershed
What is an area of high ground that separates tributaries?
A spur or interfluve
Define the mouth of a river
Where the river ceases to flow as it flows into a lake or the sea
Define the catchment area
The area of land that “catches” the rain that falls and that supplies water to a river system
What is the main source of water that reaches a drainage basin?
Precipitation - mainly rainfall
What is interception
When water is intercepted by the vegetation
what is retention?
when water is retained on teh surface
What is evaporation?
when water evaporates back into the atmosphere
what is evapotranspiration
when water is given off by plants and then evaporates
What is infiltration
when water seeps into the ground
What 5 things can occur when precipitation doesn’t become runoff?
interception
retention
evaporation
evapotranspiration
infiltration
What is surface runoff?
precipitation that is not intercepted, retained, evaporated or infiltrated which flows on the surface of Earth
What two kinds of surface runoff are there?
Overland flow (sheetflow) - water that flows across the land after rain
Channel flow (streamflow) - water that flows in distinct channels called streams or rivers
What is groundwater?
Water that seeps underground
what is groundwater that seeps through the soil?
throughflow
what is groundwater that seeps into river
baseflow
What will happen (to any precipitation) if there is a steep gradient on the land?
more runoff and less infiltration
Define permeable rock
rock that has pores or spaces through which water can pass
In what kind of soil will there be less runoff and more infiltration?
In porous soil
In unsaturated soil
What will happen (in terms of runoff and infiltration) if evaporation rates are high?
There will be more runoff and infiltration
What will happen (in terms of runoff and infiltration) if rainfall is high?
There will be more runoff and infiltration
What type of rainfall causes a lot of runoff?
Heavy thunderstorms
What type of rainfall results in a lot of infiltration?
Soft gentle rainfall
What is the difference between permeability of rocks and the resistance of rocks?
Permeability: Degree to which water moves through rocks
Resistance: relates to how easily the rock can be weathered and eroded
What are porous rocks?
Rocks that have many linked pores so that the water can move through the rocks
Describe the water-holding characteristics of sandstone
Porous rock
Therefore, permeable
Describe the water-holding characteristics of shale and clay
Not porous
Therefore, impermeable
Describe the water-holding characteristics of granite
impermeable - unless it has cracks and joints in it
Describe the water-holding characteristics of dolerite
Dolerite dykes are impermeable
What happens once groundwater infiltrates through the soil?
It percolates through permeable rocks until it comes to a layer of impermeable rock
What happens in the pores and cracks of the permeable rock?
Water collects here
What is the water table?
The upper level of the saturated rock beneath earth’s surface
It can be interpreted as the dividing line between the permanently wet layer of rock and the dry area above it
define an aquifer
rocks that hold water
what are aquicludes?
rocks that do not allow water to enter them
The water table is closer to the surface of the land in….
the wet season
Water percolates through permeable rock but ___ through soil
infiltrates
What shape does the water table take on?
The shape of the land.
What happens when the water table intercepts the surface at rivers and springs?
groundwater reaches the surface
How do people access groundwater?
By sinking boreholes
what is the type of river dependent on?
the height of the water table beneath the earth’s surface
What determines whether a river recieves baseflow or not?
The level of the water table in relation to the bed of the river
What is the cheif factor that determines the level of the water table?
the annual rainfall
What is baseflow?
The groundwater that seeps into the river
Where are permanent rivers found?
On the S and E coasts of SA
What are Two characteristics of a permanent river?
- the river flows all year
- water table always lies above the bed of the river so groundwater always seeps into the river
Where are periodic rivers found in SA?
In the interior
What are TWO characteristics of periodic rivers
the river flows only in the rainy season
the water table lies above the bed in rainy season but below the bed in the dry season
Consequently, the groundwater only seeps into the river in the rainy season
Where are episodic rivers found in SA?
In the western parts of SA
Name TWO characteristics of an episodic river
- flows only after heavy rain
- water table always lies below the riverbed
Which is the only exotic river in SA?
The Orange River
Name two characteristics of an exotic river
source of river in area with great deal of rain
river is supplied with groundwater only where it rises
Where will water erode a river channel?
Where the gradient is steeper and the rock is more easily eroded
What does the drainage pattern of a river system in a drainage basin show?
The arrangement of channels in relation to one another
What two things is the drainage pattern usually related to?
topography (shape of land)
geology (rock structure) of the area
Describe the dendritic drainage pattern
tree-shaped
tributaries join at acute angles
distinct interfluves are present
What conditions are required to attain the dendritic drainage pattern?
- Rock uniformally resistant to erosion
- Horizontal sedimentary / massive igneous rock
- uniform slope
This pattern is not dependent on either the topography or the geology of the area
Describe the trellis drainage pattern
- tributaries are short
- tributaries meet the main stream at right angles
- it is a longitudinal pattern
- rivers cut gaps or poorts through the ridges formed by the hard rock
What conditions are required for the trellis drainage pattern?
- alternate layers of hard and soft rock
- parallel ranges of fold mountains
- strong main stream
This pattern is dependent on both the topography and geology of the area
What are some of the charateristics of the rectangular drainage pattern?
- the surface water flows in the exposed joints
- tributaries join one another at right angles
- there are right-angled bends in the individual streams
What are the required conditions to form the rectangular drainage pattern?
- It occurs if there is well-jointed igneous rock such as granite in the area.
This pattern is determined by the geology of the area (not the topography)
What is another word for a radial drainage pattern?
centrifugal
What are is one characteristic of the radial/centrifugal drainage pattern?
Streams radiate outwards from a central high-lying area
What are the conditions required to develop a radial/centrifugal drainage pattern?
- Develops on dome-shaped features
This pattern is dependent on the topography but not the geology of the area
What is one characteristic associated with the centripetal drainage pattern?
Streams flow inwards towards the central depression
What are the conditions required for the centripetal drainage pattern?
- It develops around a low-lying area such as a depression or swamp
This pattern is dependent on the topography but not the geology of the area
What are some characteristics of teh deranged drainage pattern?
It is a haphazard pattern
There are many lakes and swamps
There are a few short streams
What are some of the conditions required for a deranged drainage pattern?
- It is found in either a geologically young area or in an area that has experienced glaciation.
Both topography and geology have no influence on this pattern
What are some of the characteristics of a parallel drainage pattern?
Rivers flow parallel to each other
What are some of the conditions required for a parallel drainage pattern?
- Where rivers form in areas of uniform gradient
This pattern is governed by only the topography of the area
What does drainage density indicate?
The amount of runoff in the drainage basin system
What can the density of the stream network be described as?
LOW
MEDIUM
HIGH
VERY HIGH
What doers it mean if the drainage density of a river is low?
There are very few streams forming the network of one drainage basin
How can you calculate the drainage density of an area?
Total length of the streams in the drainage basin (KM) / Total area of the river basin (KM2)
What else can be used to describe drainage density?
Texture
What are four ways that the texture of the drainage basin can be described?
coarse
medium
fine
superfine
Does a low-density network have a fine texture?
No, it has a coarse texture
Why does drainage density vary?
When there is a high runoff (same factors that determine amount of runoff and infiltration)
Rock resistance (stream channels form more easily if the rock is less resistant)
What does stream order indicate?
The degree to which the channels are linked in the stream network.
Will the main stream flowing out of a drainage basin with a high drainage density have a high or low stream order value?
High stream order value
Define the discharge of a river
The amount of water passing a point in a certain amount of time
What is the discharge of a river measured in?
Cumecs
This means Cubic metres per second
Can also be measured in centimetres per second
Name some characteristics of laminar flow
- Riverbed is level and even
- water flows in layers
- greatest speeds occur just below the surface
- speeds reached are greater
- less erosion (lower layers of water protect the river channel)
Name some characteristics of turbulent flow in a river
- Uneven and steep
- water continually changes levels
- flows with a bubbling action
- much erosion takes place (sand particles are lifted by the water)
Where does a river rise?
At its source
Give four processes that a river performs as it flows towards the sea
- erodes land
- transports eroded material (called load)
- water must move in the channel
- river may deposit its load
What are the processes performed by the river dependent on?
The energy of the river
When does a river has lots of energy and when does a river have little energy?
Lots of energy - when river is flowing fast
Little energy - when river is flowing slowly
Why does the river flow fastest just below the surface (described as “in the centre”)?
This is where there is the least friction
What is the rate (speed) at which the water flows dependent on?
Gradient
Volume
Channel shape
Channel roughness
Why do rivers with a large volume flow faster?
As less water is slowed down by friction
Why can deeper channels achieve greater speeds of river flow?
There is less friction
Why will a rough river channel flow slower?
There will be more turbulence and therefore less speed
What is the stream capacity?
The maximum load that the stream can transport in relation to:
- speed
- volume
- size of particles
A river that flows quickly and has great volume is able to ___ as it has a large stream capacity.
carry a greater load
What are the FIVE ways in which a river’s load can be transported?
flotation
solution
suspension
saltation
traction
What is floation of a river’s load?
When very fine particles float on the surface of the water and are moved downstream
What is the flotation load made up mostly of?
Leaves and twigs
Describe what it is when particles making up a river’s load are transported in solution
Minerals are dissolved in the water and carried in this way.
A solution load is invisible
What is it when particles making up a river’s load are transported in suspension?
When smaller particles of sand and silt are suspended in the water and moved by the water
Suspension load causes a river to be…
muddy
What is saltation?
When pebbles bounce along the riverbed
What is traction?
When heavy, large boulders are rolled, pushed or bumped along the riverbed
What makes up the bedload of a river?
The saltation load
AND
The traction load
Why does a river need energy?
To carry the load
To overcome friction
To flow
What will happen if a river has excess energy?
The river will erode the channel
What does the rate at which erosion in a river occuring depend on?
How much excess energy the river has
The resistance of the rocks that make up the channel
What is erosion by hydraulic action?
When the channel is eroded by the force of the water pounding on th erocks that form the sides and the bed of the channel
What is river erosion by abrasion?
When eroded material carried by the water scrapes against the sides of the channel, causing the channel to be worn away
What is pothole scouring?
When larger particles swirl in hollows on the river bed and make them larger
What is attrition (in terms of river erosion)?
When small particles bump against one another, becoming smoother and rounder
What is corrosion (in terms of river erosion)?
When acids in the water cause the rock that makes up the channel to dissolve
What is river erosion by undermining?
when the banks are undercut by the force of the water
What is headward erosion?
When the channel becomes longer as erosion towards the source of the river occurs
What is vertical erosion?
When the river channel becomes deeper as erosion of the bed occurs
What is lateral erosion?
When the channel becomes wider as the sides of the channel are eroded
Is there lots of energy for erosion down the steep slopes of a mountain? Why / why not?
There is little excess energy for erosion because there is lots of friction:
- Small volume
- Small narrow channel
What is responsible for the erosion of the sides of a valley?
mass wasting and sheetflow
NOT erosion by the river
What is a longitudinal profile?
a diagram that shows the length of a river’s course from its source in the mountains to its mouth at sea level
what type of river profile indicates the gradient down which the river flows?
longitudinal profile
What shows the shape of a river valley at an imaginary line drawn across the river valley?
The transverse profile
OR
The cross profile
Where the gradient is steep as indicated by the longitudinal profile, the valley shape is…
a narrow V-shape
What 3 stages can the COURSE of the river be divided into?
Upper course
Middle course
Lower course
What type of cross profile will form if the river has a lot of energy to erode downwards?
A deep, narrow steep-sided profile
A river can have different cross profiles along its course, depending on….
the climatic and geological conditions of the area through which it is flowing
Hydraulic action and abrasion result in ___ and ___ in the middle course of a river (where it has a more open V-shaped profile)
lateral erosion and undermining
What two processes can lower the valley sides?
sheetflow and mass wasting
What can the very flat, open V-shaped valley in the lower course of a river also be called?
the flood plain
A river that has a straight course will have a ___ cross profile
symmetrical
A meandering river will develop a ___ cross profile. This is because the erosive force of the water tends to be stronger on the ourside of the meander.
asymmetrical
Is the cross profile of a river always similar to that of the valley in which it is found?
No
What shape are most channel cross profiles?
Rectangular in shape
What is the steep bank of a river called (when looking at the cross profile)
river cliff
OR
undercut bank
What is the gently-sloped bank of a river called (when looking at the cross profile)
slip-off slope
The longitudinal profile is the visual representation of the ___ and ___ of the river
height and length
The sea is the ___ of the river as this is the lowest level to which a river can erode the landmass
permanent base level
What kind of base levels are found along a river’s course?
Tempory/Local base levels
What is a temporty base level?
Any factor present that interrupts the vertical erosion of the river
Why is a temporary base level regarded as temporary?
Eventually the river erodes through the resistant rock
What does “fluvial” mean?
relating to a river
What type of erosion deepens the valley?
Vertical erosion
What forms as the river twists and turns round obstacles of hard rock?
Interlocking spurs form
What type of erosion gradually widens the valley floor?
lateral erosion
Does lateral erosion begin on the concave or convex banks of river bends?
Concave banks of river bends
What happens once the end of a spur is cut off by lateral erosion?
a truncated spur forms
What is a gorge?
A very deep, steep-sided valley formed by an energetic rover in a humid environment
What is a canyon?
A very deep, steep-sided valley formed by an exotic river in an arid environment.
Found in an area of horizontal rock strata of varied resistance to erosion
What is the difference between a canyon and a gorge?
A gorge is found in a humid environment
A canyon is found in an arid environment
How do waterfalls and rapids form?
By a resistant layer of rock across the course of the river, leading to a sharp change in the gradient
What determines whether a waterfall or rapid will form?
The angle at which the resistant rock layer dips
What is the resistant layer of rock which forms a waterfall sometimes referred to?
fall-maker
OR
lip of the waterfall
What causes the position of a waterfall to move upstream?
- The undercutting of the softer rock beneath the lip
- the lip collapses
What often occurs downstream of a waterfall? Why?
A deep gorge often forms downstream of a waterfall.
This is because the headward erosion results in the formation of a deep gorge downstream of the waterfall
What is the name for deep, circular depressions or hollows on the bed of a river?
potholes
How are potholes formed?
When pebbles collect in depression on the bed of the river.
Through abrasion and pothole scouring, the depressions become deeper
What can cause the flow of a river to slow down?
a reduction in the gradient
Eg) when the river flows off the mountatin onto the plain downstream
What can cause the load of a river to increase?
Deforestation causes increased erosion of topsoil in the drainage basin.
What is the name of the fertile material deposited by a river?
Alluvium
What 4 situations can cause a river to deposit its load?
- Speed of river decreases (reduction in gradient of river)
- Volume of water decreases (usually after a flood)
- Channel of river becomes wider (flowing on floodplain)
- When load of river increases (deforestation)
What can form at the base of mountains when a river that is carrying a lot of sediment flows onto the flatter plain?
alluvial fans
Sediment of an alluvial fan is deposited in what shape?
In the shape of a fan
The river splits into ___ as a result of an alluvial fan
distributaries
What are distributaries
smaller streams leading from the main stream to flow round the deposited material of an alluvial fan
What are raised banks of a river in its lower course called? Why do they form?
levees
They form from the deposition of coarse material as the river flows over the sides of the channel when it floods
What can form from the water that lies on the floodplain?
back swamps
What forms when a river breaks through the neck of a meander when it is in flood, causing the meander loop to be cut off from the main stream?
An ox-bow lake
What forms once an ox-bow lake dries up?
A meander scar
What is it called when a river bends and winds across the floodplain?
a meandering channel pattern
What lies just above the bedrock in the lower course of a river valley?
alluvium (deposited fertile material)
What are the raised areas at the edge of the flooodplain that are the remains of the old valley sides called?
bluff
What is a tributary that flows alongside the river until it is able to flow into the main stream where there is a break in the levee called?
a yazoo stream
Where does depostion occur and where does erosion occur in the cross section of a river?
Erosion - outer bank
Deposition - inner bank
What is a channel pattern?
The pattern made by a single channel
What is the difference between a channel pattern and a drainage pattern?
A channel pattern is the pattern made by a single channel
A drainage pattern is the pattern made by the way in which individual channels join one another
How does a delta form?
- River slows down as it enters the sea
- deposits its load at the mouth of the river
- deposited sediment at the mouth of the river forms a delta
When a delta forms, which particles are deposited first and which particles settle further out to sea?
Heaviest particles are deposited first and the finer particles settle further out to sea
Describe the land which makes up a delta
Flat, fertile land
What does the river split up into in order to flow through the delta?
Distributaries
What shape is a arcuate delta?
Shaped like a fan
What shape is a digitate delta?
Look like birds’ feet
What shape is a cuspate delta?
Shaped like the cusps of teeth
What shape is a estuarine delta?
It is formed in the funnel-shaped mouth of a river
Define grade
When the gradient is just suffficient for the transportation of the load under existing volume and channel shape conditions.
This is when erosion rates are in equilibrium with deposition rates
What shape of profile does a graded river have?
A smooth, concave-shaped profile
What is a multi-concave profile?
When streams have graded and ungraded segments separated from each other by sharp gradient changes at tempory base levels
What does grade depend on?
Stream load capacity
This, in turn, depends on:
- velocity of river
- volume of river
- shape of river
- nature of river
What is an overgraded river?
When the load is less than the capacity
This means the stream can collect more material through erosion
This decreases the gradient and the speed becomes slower (only until stream capacity is reached and the river has achieved grade)
What is an undergraded river
When the load is greater than the stream capacity
This makes the river deposit material
The gradeint becomes steeper and the river can transport a greater load
In this way, the river has graded itself
What is a rejuvinated river?
Simply an overgraded river
Define rejuventation
When a river gains renewed energy and erodes downwards again
When can rejuvenation occur?
- If the gradient of the river increases
- drop in sea level
- uplift of the landmass
- If the volume of the river increases
- increased rainfall
- capture of another river
- incresed runoff due to the clearing of the vegetation
Which FOUR landforms can form as a result of rejuvenation?
Incised (entrenched) meanders
waterfalls (at the knickpoint)
valleys within valleys
terraces
What is it called when the position and gradient of a watershed is altered so that the gradient is the same on each side of the watershed?
Abstraction
Where in SA is abstraction frequently occuring?
On the eastern side of the Drakensberg mountain range
When does abstraction occur?
When the stream on one side of a watershed is more energetic than the stream on the other side of the watershed
Describe the process of abstraction
- a more energetic stream erodes at a faster rate
- this lowers the watershed and moves it in the direction of the less energetic stream
- abstration stops when the gradients of both streams are the same
Give 3 reasons why a river that erodes a watershed could be more energetic
- It might have been flowing down a steeper gradient
- It might have has a great volume (due to it being in an area with higher rainfall)
- It might have been flowing over less resistant rock
What causes a more energetic river to increase the size of its drainage basin?
abstraction
when does river capture occur?
When a river with more energy intercepts another river and “captures” the water of the other river
What is the captor stream?
The river with more energy that captured another river
What is a misfit stream?
the river that lost its headwaters
A misfit stream can also be called a beheaded stream
What is the captive?
The river that was captured
What is a wind gap?
The part of the old valley of the captured river that is now dry and filled with sand and gravel.
Where does a knickpoint waterfall occur in terms of river capture?
At the point where the captured (captive) stream flows into the captor stream
What can cause river capture?
- Headward erosion of the more energetic river (this can lengthen the channel upstream)
- Lateral erosion on the outer bend of a meandering river
- Abstraction of a watershed (the abstracting river can intercept the course of the river flowing down the other side of the watershed)
What are the results of river capture?
- Captor gains water
- more energy
- erosion take place at a faster rate
- Misfit loses water
- less energy
- has to deposit much of its load
Name some things that you can identify to help you see the point of river capture
- identify a sharp bend in one of the streams (this shows the elbow of capture)
- Look for a knickpoint waterfall close to the elbow
- Find the wind gap close to the elbow
- Identify the stream that flows down a steeper gradient
- try to recognise the misfit stream
Rivers with superimposed and antecendent drainage patterns have river coursers that do / do not relate to the rock structures or topography of the area in which the river is flowing
do not
In a superimposed drainage pattern, does the river avoid the resistant rock?
No, it has eroded a course through the mountains
TRUE OR FALSE:
The course of a river (when associated with a superimposed drainage pattern) has a relation to the present geology and topography of the area
False,
The course of the river in a superimposed drainage pattern has no relation to the present geology and topography of the area.
What rocks does a superimposed drainage pattern develop on?
On rocks that have since been removed by erosion
Is a superimposed drainage pattern flowing on older or younger rocks that have been uncovered?
Older rocks that have been uncovered
Are superimposed rivers younger than the structures over which they flow?
Yes
What is a good example of a superimposed drainage pattern in SA?
The Vaal River near Parys
Does a antecendent river develop its course on a former landscape?
Yes
What has taken place in order for an antecendent drainage pattern to form?
Folding or faulting
Does the stream of an antecendent river (where the landscape undergoes folding and faulting) maintain its course?
Yes
How does the antecendent stream maintain its course along the river?
It erodes through the anticline at the same rate at which it is forming
Are antecendent rivers younger than the structures through which they flow?
No, they are older
Do antecendent rivers flow around the high-lying ground?
No, they cut poorts through the anticlines
What is a good example of an antecendent river in South Africa?
The Orange River
Define catchment area
The area that receives rainfall and supplies water to the river
When will SA experience severe water shortages? What should we do about it?
2025
The management of catchment areas, drainage basins and rivers is therefore vital for people and the sustainable development of the country
What is another reason for managing catchment areas, besides due to predicted water shortages?
To prevent floods that can cause major damage
What are natural storage areas of water?
Wetland areas within the drainage basin
Why do wetlands need to be conserved (in terms of management of catchment areas and drainage basins)?
They need to be able to release water slowly to supply rivers with water, also to filter the water, catching sediments and pollution
How can people alter the flow characteristics and quality of rivers?
- building dams, canalising rivers, cutting through meander loops and diverting river courses
- implementing inter-basin transfer schemes
- making use of wetlands for farming and draining wetlands for settlement
- clearing the vegetation, which increases runoff and adds sediment to the river
- allowing the spread of alien vegetration alongside rivers
- polluting the river through refuse deposits, sewerage, setergents, chemicals
- construction of impermeable tar and concrete surfaces
- building on a river’s floodplain
What is the main negative impact of alien vegetation?
It uses a lot more water
This makes it deplete the underground water supply
How can people pollute a river?
refuse deposits
sewerage
detergents
chemicals
How can people prevent or minimise flooding by rivers?
- Entire drainage basin must be studied
- Wetlands must be preserved
- dams must be correctly positioned
- meander loops can be cut through (allows the water to flow downstream quicker)
- conservation methods to reduce removal of vegetation
What does RHP stand for? What does it aim to do?
River Health Programme
- It assesses the biological habitats of fish, aquatic organisms and river vegetation and the general health of rivers
- It then makes recommendations so that authorities can work towards managing rivers in order to achieve the desired health of the river
Who controlls the implementation of the RHP?
The Department of Water Affairs and Forestry (DWAF)
Each province has a network of people and institutions involved in monitoring river health
What does the term “riparian zone” refer to?
River banks
