1C - rivers

drainage basin

source

tributary

confluence

watershed

mouth

- key terms

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

• medium 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