drainage basin
source
tributary
confluence
watershed
mouth
- key terms
drainage basin - an area of land drained by a river and its tributaries
source - the start of a river
tributary - a small stream that joins a larger river
confluence - where a tributary joins a larger river
watershed - the edge of a river basin
mouth - the end of a river, usually where a river joins the sea
How the long and cross profile of a river changes downstream
upper
v shaped valley, steep sided, narrow, shallow
steep gradient
at the source in the mountains, water velocity varies a lot
where the water is shallow and turbulent, there is friction with the banks, slowing the rate of flow
where there are rapids, it means the channel narrows and the river becomes deeper - faster
middle
wider, deeper
gently sloping
further downstream, the channel is much deeper due to tributaries bringing in additional water
less water in contact with bed and banks - velocity increases
lower
very wide, almost flat
very gentle gradient
vertical and lateral erosion
one is usually dominant at different points along the river
vertical - deepens valley and channel, upper course
high turbulence causes the rough, angular particles to be scraped along the river bed - intense downwards erosion
lateral - widens valley and channel during the formation of meanders
middle and lower course
processes of erosion
hydraulic action - the force of the water hitting the river bed and banks causes rock particles to break away
abrasion - eroded rocks carried by the river repeatedly scrape against the channel, wearing it away
attrition - erroded rocks carried by the river knock against each other and break into smaller fragments. edges get rocnded off as they rub together.
solution - river flowing over limstone or chalk will dissolve it as the water is slightly acidic
transportation processes
traction - large particle like boulders are pushed along the river bed by the force of water
saltation - pebbles are boundes along the river bed by the force of the water
suspension - small particles like silt and clay are carried along inn the river
solution - soluble materials like limestone dissolve in the water and are carried along
deposition
occurs when river velocity decreases - less energy to transport sediment, when the amount of eroded material increases, when the water is shallower or when the river reaches its mouth
larger rocks tend to be deposited in the upper course of a river. transported short distanced by traction during very high flow
finer sediment is carried further downsteam by suspension and deposited on the river bed or banks when velocity is slowed by friction
deposion occurs at the river mouth due to the interaction with tides and the very gentle gradient reducing the river’s velocity
waterfalls and gorges
found in the upper course of a river
form where a river flows over an area of hard rock which sits above an area of softer rock
the softer rock is eroded by hydaulic action and abrasion more than the hard rock, creating a “step” in the river
as more water flows over the step, it erodes more of the softer rock
a steep drop is eventually created, a water fall
hard rock is eventually undercut by erosion - unsupported and collapses
collapsed rocks are swired around at the foot of the waterfall where they erode the softer rock by abrasion - deep plunge pool
overtime, more undercutting causes more collapses. waterfall retreats, leaving behind a steep sided gorge
interlocking spurs
in the upper course, most erosion is vertical - steep sided v shaped valleys
rivers lack the power to erode laterally - have to wind around the high hillsides that stick out into their paths on either side
meanders
rivers develop large bends in their middle and lower courses, in areas where the channel has both deep and shallow sections
the current is faster on the outside of the bend because the river chanel is deeper - less friction
more erosion takes place here, forming river cliffs
current is slower on the inside of the bend because the river channel is shallower
eroded material is deposited on the inside forming slip off slopes
oxbow lakes
erosion causes the outside bends of meanders to get closer as they start to erode towards each other
there is only a small bit of land left between the bends (called the neck)
the neck narrows until it is completely broken through by the river (usually during a flood) to form a new straighter channel and this becomes the shortest course for the river to take
deposition eventually cuts off the meander, forming an ox bow lake
pools and riffles
meandering streams carrying coarse sediment may develop alternating deep sections (pools) and shallow sections (riffles)
pools are usually found on the outside bends of meanders where, during periods of high flow, the faster flow erodes a deep channel
riffles result from the deposition of coarse sediment, also at times of high flow and are characterised by more turbulent slower flowing water
during low flow, the water tends to flow more slowly through a pool section, depositing a fine mudd sediment
during low flow, water may flow slightly faster in a riffle section, accounting for the lack of fine sediment here.
flood plains
the flood plain is the wide, flat valley floor on either side of a river in the middle and lower courses
occasionally floods
when rivers flood, the water slows down, loses energy and deposits silt here which builds up the flood plain
meanders widen as they migrate across flood plains laterally and when they reach the edge of a floodplain, they erode the valley side
over time, meanders also migrate downstream and the deposition that happens on the slip off slopes of meanders also builds up the flood plain
used for farming as the deposited soils are very fertile
levees
levees are natural embankments (raised banks) along the edges of a river channel in its slower course
during a flood, water flows over the sides of the channel at a low velocity
eroded material is deposited over the whole flood plain
the heaviest material is deposited closest to the river channel when the river loses energy during low flow
over time, the deposited material bulds up, creating levees along the edges of the channel
estuaries
found at river mouths
land is close to sea level and and the river valley is at its widest
water here is tidal - river level rises and falls
when the water floods over the banks of the river, it carries silt and sand onto the valley floor.
at high tide, river water is unable to be discharged into the sea
the rivers velocity falls and sediment is deposited
at low tide, these fine deposits form extensive mudflats which develop into saltmashes - important natural habitats
the wide muddy banks are exposed at low tide
where is the river tees?
north east england
source is high in the pennine hills near cross fell (height 893m)
from there it flows roughly east for around 128km to reach the North Sea at middlesbrough
high force waterfall and gorge
located close to forest in teesdale in the river’s upper course
river drops 20m as a single sheet of water into the foaming and turbulent plunge pool below
it then continues its course through a gorge
the waterfall was formed due to a band of igneous rock which cuts across the river valley
as the river is unable to erode this tougher rock, the river has formed a step in its long profile
the underlying darker rock is limestone, a less resistant rock
as the river plunges over the waterfall, it undercuts the weaker limestone, forming and overhang
this eventually collapses and the waterfall gradually retreats upstream to form a gorge
meanders, levees and floodplains near darlington
flooding
land that is not normally underwater becomes inundated
a river flood occurs when a river channel can no longer hold the amount of water flowing in it
water overtopss the banks and floods the adjacent land - the floodplain
physical factors affecting flood risk
precipitation - torrential rainstorms can lead to sudden flash floods as river channels cannot contain the volume of water flowing into them. steady rainfall over several days can also saturate the soil, leading to flooding in lowland river basins
geology - impermeable rocks such as shales and clays encourage water to flow overland and into river channels , speeding up water flow and making flooding more likely
steep slopes - encourage rapid transfer of water towards river channels, increasing the risk of flooding.
human factors affecting flood risk
urbanisation - building on a floodplain creates impermeable surfaces. water is transferred quickly to drains and sewers and then into urban river channels. this rapid movement makes flooding more likely. man made drains quickly transport water to rivers, increasing discharge
deforestation - trees intercept rainwaiter on their leaves, which then evaporates. trees also take up and store water from the ground so cutting down trees increases the volume of water that enters the river channel
agriculture - in arable farming, soil is left unused and exposed for some time, leading to more surface run off. this is incrased if the land is ploughed up and down steep slopes as water can flow quickly along the furrows
hydrograph
discharge is the volume of river water flowing per second, measured in cumecs - cubic metres per second
hydrographs show how the discharge at a certain point in a river changes over time in relationn to rainfall
peak discharge - the highest discharge in the period of time you’re looking at
lag time - the delay between peak rainfall and peak discharge
rising limb - the increase in river discharge as rainwater flows into the river
falling limb - the decrease in river discharge as the river returns to its normal level
lag time occurs as most rain water doesn’t land directly in the river channel. it flows quickly over land or infiltrates into the ground and flows slowly underground to the channel
shorter the lag time, the higher the risk of flooding
hard engineering
man made structures to prevent or control natural processes
usually very expensive
preferred option for protecting expensive property or land
dams and reservoirs
barriers built across the rivers, usually in the upper course
a reservoir is formed behind it
benefits
store water - irrigation and water supply
prevents floods downstream
recreation
can be used to generate hydro-electric power
drawbacks
expensive to build
the reservoir can flood existing settlements - people moved from their homes
material is deposited in the reservoir so farmland downstream can become less fertile
channel straightening
meanders are removed by building straighter, artificial channels
benefits
water leaves the area more quickly rather than building up - reduces flood risk if built in a vulnerable area
drawbacks
may shift the flooding problem downstream
faster moving water may cause more erosion downstream
in some places, straightened sections of the river are lined with concrete to speed up flow and prevent the banks collapsing. unattractive and unnatural. damage wildlife habitats
embankments
raised walls built along river banks
benefits
allows river to hold more water before flooding occurs
sometimes mud dredged from the river may be used - cheaper, sustainable and more natural looking
drawbacks
expensive, especially if concrete or stone is used
risk of severe flooding if the water rises above the level of the embankments or if they break
the concrete is less natural looking
flood relief channels
build to divert water around urban areas or to divert excess water if the river level gets too high
benefits
during high flow, sluice gates can be opened to allow excess water to flow away into the flood relief channel , reducing the threat of flooding
drawbacks
there will be increased discharge where the relief channel rejoins the river or joins another river, causing flooding in that area
soft engineering
working with natural river processes to manage the flood risk
no artificial structures
floodplain zoning
restricts different land uses to certain locations on the flood plain that are likely to be affected by a flood
benefits
areas close to the river and at risk from flooding can be kept clear of high value land uses such as housing and industry
instead these areas can be used for pasture, parkland or playing fields
can reduce overalll losses caused by flood damage
fewer impermeable surfaces - flood risk reduced
drawbacks
the expansion of an urban area is limited if there are no other suitable building sites
can’t help in areas with existing buildings
difficult to implement on floodplains that have already been developed as it can cause land prices to fall
river restoration
where the course of a river has been artificially altered, this can return it to its original course
uses the natural processes and features of a river to slow down the likelihood of a major flood downstream
benefits
discharge is reduced - less risk of flooding downstream
little maintenance is needed
better habitats for wildlife
drawbacks
local flood risk can increase, especially if nothing is done to prevent major flooding
flood warnings and preparation
rivers and river basins are monitored remotely using satellites and remote technology. instruments are used to measure rainfall and river level
the environment agency issues flood warnings through various media
buildings are modified to minimise flood damage
residents can prepare sandbags and flood boards prior to the flood
benefits
warnings give people time to move their possesions upstairs and put in sandbags
reduces the impact of flooding
drawbacks
warnings don’t prevent floods
people may not have access to warnings
modifying buildings is expensive
preparation doesn’t guarantee safety from a flood and it could give people a false sense of security
planting trees
afforestation
trees intercept rain water, slowing transfer
soaked up
benefits
discharge and flood risk decrease
floodimpact reduced - less buildings here to damage
relatively cheap
wildlife habitats
drawbacks
less land for farming and building