Rivers Flashcards
Course of a river
the path of a river as it flows down hill
What do rivers form as they flow downhill?
Channels and valleys
Describe the upper course of a river
gradient - steep
valley and channel shape - v-shaped valley, steep sides narrow, shallow channel
Describe the middle course of a river
gradient - Medium
valley and channel shape - gently sloping valley sides wider, deeper channel
Describe the lower course of a river
gradient - gentle
valley and channel shape - very wide, almost flat valley, deep channel
Vertical erosion
deepens the river valley (and channel) making it V-shaped
dominant in the upper course of the river
high turbulence caused the rough angular particles to be scraped along the river bed - causing intense downwards erosion
Lateral erosion
widens the river valley (and channel) during the formation of meanders
dominant in the middle course and lower course
What determines the change in the shape of the valley and channel?
it depends on wether erosion or deposition is having the most impact (is the dominant process)
Freeze-thaw weathering
happens when temperature alternates above and below 0°c (freezing point of water)
water gets into rock that has cracks - when water freezes it expands which puts pressure on the rock
when water thaws it contracts which releases pressure on the rock
repeat freezing and thawing - widens cracks and causes rock to break up
Four processes of erosion
Hydraulic action
Abrasion
Attrition
Solution
Hydraulic action
force of the water breaks rock particles away from the river channel
Abrasion
eroded rocks picked up by the river scrape and rub against the channel - wearing it away
most erosion happens by abrasion
Attrition
eroded rocks picked up by the river smash into each other and break into smaller fragments
edges get rounded off as they rub together
further material travels more eroded it gets - attrition causes particle size to decrease between river source and its mouth
Solution
river water dissolves some types of rock such as chalk and limestone
Four processes of transportation
Traction
Suspension
Saltation
Solution
Traction
large particles like boulders are pushed along the river bed by the force of the water
Suspension
Small particles like silt and clay are carried along by the water
Saltation
pebbled-sized particles are bounced along the river bed by the force of the water
Solution
Soluble material are dissolved in the water and carried along
Deposition
when a river drops eroded material its transporting
When does deposition occur?
when a river slows down (loses velocity)
Why does a river slow down and deposit material?
1) volume of water in river falls
2) amount of eroded material in the water increases
3) water is shallower (inside of a bend)
4) river reached its mouth
Where are waterfalls and gorges found?
upper course of a river
How does a waterfall form?
1) when a river flows over an area of hard rock followed by an area of softer rock
2) softer rock is eroded (by hydraulic action and abrasion) more than the hard rock - creates a ‘step’ in the river
3) as water goes over the step it erodes more and more of the softer rock
4) steep drop is eventually created - this is called a waterfall
How does a gorge form?
1) once a waterfall forms the hard rock is eventually undercut by erosion - becomes unsupported and collapses
2) collapsed rocks are swirled around at the foot of the waterfall where they are erode the softer rock by abrasion - this creates a plunge pool
3) over time more undercutting causes more collapses - the waterfall will retreat leaving behind a steep-sided gorge
How are interlocking spurs formed?
1) in the upper course of a river - most erosion is vertically downwards
this creates steep-sided V-shaped valleys
2) the rivers aren’t powerful enough to erode laterally - have to move around the high hillside that are in their paths on either side
3) hillsides that interlock with each other as the river moves around them are called interlocking spurs
Where do rivers develop meanders (large bends)?
Middle and lower courses
in areas where there area both shallow and deep section in the channel
Where is the current faster in a meander?
Outside of the bend - the river channel is deeper (there’s less friction to slow down the water)
so more erosion (abrasion and hydraulic action) take place on the outside of the bend forming river cliffs
Where is the current slower in a meander?
Inside of the bend - river channel is shallower (there’s more friction to slow down the water)
so eroded material is deposited on the inside of the bend forming slip-off slopes
How does a meander form into an ox-bow lake?
erosion causes the outside bend to get closer until there’s only a small bit of land left between the bends - called neck
river breaks through this land - usually during a flood
the river flows along the shortest course
deposition eventually cuts off the meander - forming an ox-bow lake
Flood plain
the wide valley floor on either side of a river which occasionally gets flooded
What happens when a river floods onto a flood plain?
water slows down and deposits the eroded material that it is transporting
this builds up the flood plain (makes it higher)
How do meanders effect flood plains?
meanders migrate across the flood plain - makes it wider
meanders migrate also down stream - flattens out the valley floor
deposition that happens on the slip-off slopes of meanders -builds up the flood plain
Where are flood plains, levees and deltas found?
Lower course of a river
Levees
natural embankments along the edges of a river channel
How are levees formed?
during a flood - eroded material is deposited over the whole flood plain
heaviest material is deposited closest to river channel - as it gets dropped first when the river slows down
over time the deposited material builds up - creating levees along the edges of the channel
Deltas
low lying areas where a river meets the sea or lake
How are deltas formed?
rivers are forced to slow down when they meet the sea or a lake - causes them to deposit the material they are carrying
if the sea doesn’t wash away the material - it builds up and the channel gets blocked
this forces the channel to split up into lots of smaller rivers called distributaries
eventually material builds up so much that low-lying areas of land called deltas are formed
Sediment load
material carried by a river
What 3 factors help shape river landscapes and affect sediment load?
Climate
Geology
slope processes
Rivers in wetter climates have…
higher discharge as there is more water entering the river channel
How does higher river discharge affect erosion?
increases rate of erosion
if a river has higher volume of water it has more power to erode river banks and bed
this adds material to rivers load
How does higher river discharge affect the landscape?
shapes the landscape
forms:
V-shaped valleys in the rivers upper course (through vertical erosion)
wide flat flood plain in the lower course (through lateral erosion)
How does higher river discharge affect transportation?
transportation increases
because the river has more energy to carry material
How does higher river discharge affect weathering?
weathering increases the river sediment load and can affect the shape of the landscape
for example freeze thaw weathering makes rockfall more likely
Rate of erosion of rivers flowing through hard rock
Slower rate - hard rock is more resistant
this means river will have low sediment load
Rate of erosion of rivers flowing through softer rock
Higher rate
adds more material to rivers sediment load
valley sides for landscapes with more resistant rock
steeper valley sides
valley sides for landscapes with less resistant rock
gentle sloping valley sides
How does vertical erosion effect rivers?
makes valley sides steeper - increases movement of material down slopes
How does mass movement affect rivers?
can add large amounts of material to rivers load
more likely during cold weather - freeze thaw weathering loosens material
and during periods of intense rainfall - saturated ground becomes heavier and less stable
Soil creep
when soil particles move down slope due to gravity
caused by the expansion and contraction of soil
water adds weight to soil and makes it expand - causes it to move down the slope
when soil dries out it contracts
adds lots of fine material to rivers load
River discharge
the volume of water that flows in a river per second
measured in cumecs - cubic metres per second
Storm hydrographs
show the changes in river discharge around the time of a storm
Peak discharge
highest discharge in the period of time you’re looking at
Lag time
delay between peak rainfall and peak discharge
happens because most rainwater doesn’t land directly in the river channel - there’s a delay as rainwater gets to the channel
gets there by:
flowing quickly over land - surface runoff
soaking into the ground (infiltration) flowing slowly underground
Rising limb
increase in river discharge as rainwater flows into the river
Falling limb
decrease in river discharge as the river returns to its normal level
How does increases surface runoff affect lag time and a Storm Hydrograph?
Lag time - reduced
discharge increases and the hydrograph will be steeper - more water gets into the river in a shorter period of time
How does geology affect runoff?
water cant infiltrate impermeable rock - more runoff
How does rock type affect runoff?
more impermeable soils cant absorb as much water as sandy soils - increases surface runoff
shallower soils become saturated more quickly than deeper soils
how does slope affect runoff?
steeper the slope, the less infiltration - higher the surface runoff
How does drainage basin affect lag time and discharge?
Circular drainage basins - shorter lag time and higher discharge than narrow basins
this is because water reaches the main river channel at the same time
Narrower basin - water from the far end of the basin takes long time to reach the main channel
How do antecedent condition affect runoff?
previously wet or very cold weather can increase runoff
because water cant infiltrate saturated or frozen soil
How does urbanisation affect runoff and discharge?
water cant infiltrate impermeable surface (concrete/tarmac)- more runoff
gutters and drains quickly take runoff to rivers - rapidly increases discharge
How does deforestation affect runoff and discharge?
trees take up water from ground and store it which reduces runoff
cutting them down - increases runoff and causes more water to enter river channel - increases discharge
Two main factors in the UK increasing risk of flood
Increased frequency of storms -
consequence of climate change
more periods of wet weather means ground is more saturated - makes floods more likely
Land use change -
as population grows - more pressure to expand urban areas which leads to increase in impermeable surfaces
removing vegetation and permeable surfaces - eater than would have been stored in the soil/plant/tree now flow downstream quickly
development (houses) on floodplains - areas are more prone to flooding
Threats of flooding to people (5)
1) killed or injured by flood water
2) infrastructure (roads/bridges/rail lines) damaged or destroyed
3) flood water contaminated with sewage - lack of clean drinking water
4) homelessness
5) businesses forced to shut due to flood damage and disrupted power supplies - leads to loss of livelihoods
Threats to the Environment (4)
1) flood water contaminated with sewage and rubbish can pollute rivers - damage wildlife habitats
2) farmland ruined due to silt and sediment deposited after flood
3) river banks eroded causing huge changes to river landscape
4) force of flood water can uproot plants/trees and standing flood water causes those that survive to die
Hard engineering examples
Flood walls
embankments
flood barriers (flood gates)
flood barriers (demountable)
Soft engineering examples
Flood plain retention
River restoration
Flood walls
artifice barriers built along river banks
designed to increase the height of river banks - allowing the river channel too hold more water
very expensive, not asthetically pleasing
Enbankments
high banks that area built along or near the river banks
stop river flooding into built-up areas during a flood
protects building and infrastructure on the flood plain
expensive
risk of severe fooding in water rises above level of enbankment
Flood gates
such as Thames Barrier
built on river estuaries to stop flooding from storm surges/high tides
can be shut when there is a surge forecast to prevent flooding and protect large areas if land - Central London
very expensive to build
need to maintained regularly
Flood plain retention
strategy involves maintaining rivers flood plain - by building on it
helps slow flood water down and maintain flood plains ability to store water
no money has to be spent
restrict development and cant be used in urban areas
River restoration
involves making the river more natural - by removing man-made levees so that the flood plain can flood naturally
less risk of flooding downstream as discharge is reduced
river is left in natural state - little maintenance required
