geography unit 2: rivers

upland area

pennines

lowland area

anglesey

river in uplands

river severn

river in lowlands

river thames

fluvial erosion

erosion in rivers

erosion

the wearing away of rocks by a moving agent with the materials transported away

hydraulic action

the force of the water erodes the river bed as air is forced into cracks causing mini explosions, most effective in fast flowing rivers.

abrasion

stones are thrown at the river bed and the river banks causing material to break off (sandpaper)

attrition

stones knock against each other causing pieces to break off, becoming smoother and rounder

solution

rocks like chalk or limestone are eroded by slightly acidic river water which dissolves it

vertical erosion

downward erosion which deepens the river

\

lateral erosion

sideward erosion which widens the river channel

fluvial transport

process which the river carries its load

transport

sediment carried within a river

Load

load varies in size, from large boulders in the upper course to small sediment in the lower course

\
load mainly comes from weathered material that has rolled down hillside into the river and from the eroded bed and banks of the river

solution

minerals are dissolved in the water and carried along in the water

suspension

fine, light materials are carried along on the surface of the water

saltation

small pebbles are bounced along the river bed

traction

large boulders and rocks are rolled along the river bed (bed load)

deposition

materials in a river are dropped due to a decrease in velocity

why do rivers deposit sediment

due to a decrease in velocity

where does deposition occur

river enters a shallow area (when it floods and touches the floodplain)

at the base of a waterfall

on the inside bend of a meander

towards the mouth of a river where it meets the sea

source

the area in which a river begins

mouth

where a river ends its journey flowing into a lake or sea

tributary

a smaller river that joins a larger river

confluence

the point at which two rivers join

watershed

the boundary between two drainage basins marked by a ridge of high land

drainage basin

the area which is drained by a river and its tributaries

drainage density

the total length of all the streams in the basin divided by the total area of the basin

long profile

shows the gradient of the river from the source to where it enters the sea

upper course

vertical erosion

weathering

steep gradient

v shaped valley

small discharge

middle course

lateral erosion

transportation

discharge is higher due to tributaries

meanders

river cliffs

lower course

discharge at its highest

transportation

deposition

levees ox bow lakes

cross profile

shows the shape of the valley not the river

upper valley

v shaped valley

erosion

load size is large

vertical erosion

middle valley

deeper and wider

lateral erosion

load size is smaller

bradshaw model

model showing the changes of a river from the upper course to the lower course

load quantity

how much material being carried

load particle size

material size

formation of a v shaped valley

vertical erosion lowers the river bed

\
sides of the valley are subjected to weathering which weakens the sides of the valley, moves down slope (mass movement)

\
the gradient decreases of the valley sides

\
interlocking spurs

formation of interlocking spurs

river doesnt have a lot of energy and the rock is hard to weather so the valley is v shaped

\
freeze thaw weathering widens the valley leaving more material to erode with

\
the river cant erode harder rock so it winds and bends around the rock forming interlocking spurs

formation of a waterfall

a river meets a band of softer rock

\
the underlying, softer rock is eroded away more quickly

\
processes of erosion such as abrasion cause undercutting

\
the more resistant rock is left overhanging and unsupported

\
eventually the harder rock collapses onto the riverbed

\
the rock causes abrasion of the river bed

\
hydraulic action also helps to form a deep plunge pool with eddy currents

\
this process is repeated as the waterfall retreats

\
a gorge is created

gorge

a steep sided river valley

formation of a meander

in the middle course the river flows over flatter land, lateral erosion dominates as the river swings in large bends called meanders which are constantly changing shape

formation of an ox bow lake

during high flow conditions the meander neck is broken through the river now flows by the old meanders as it is shortest

\
the meander neck narrows due to lateral erosion on the opposite sites of meander bend

\
deposition occurs at the edge of the new straight section cutting off the meander leaving an ox bow lake which will silt up over time and form marshland

what is a levee

natural embankments deposited on either sides of the river channel

formation of levees

during a flood the water flows over into the floodplain and an increase in friction causes large sediment to be deposited instantly on each side, deposition makes the levees higher and higher

what is a floodplain

the area either side of a river in its lower course which is prone to flooding

formation of a floodplain

when a river floods, material is carried from the river to the floodplain, the finer silt are deposited further than the larger material, layers of material will build up over time continuing to build up the floodplain

bluff

area of higher ground which mark a floodplain

alluvium

fine silt

what is an estuary

a wide sheltered body of water found at a river’s mouth

formation of an estuary

a large river entered the sea at a narrow mouth and after the ice age melting ice caused a rise in sea level ,this caused the sides of river to become flooded. The original channel of the river is now on the estuary floor where it provides a deep channel for shipping

characteristics of an estuary

high tidal range

may be very wide

mudflats visible at low tide

how are estuary mudflats formed

they are formed in sheltered areas, as the water transports material down the sea an incoming tide transports the material up the estuary, deposition occurs downstream as fresh and saltwater mix (velocity drop). This builds up layer of mud called mudflats

how are salt marshes formed

mudflats will become colonised by vegetation forming slat marshes

the river severn

* longest river in britain
* source in the cambrian mountains
* 600m area
* 2650 mm of rainfall

lag time

the difference between peak rainfall and peak discharge

discharge

the amount of water flowing in a river

hydrograph

a graph to show river discharge and rainfall over time

what is discharge measured in

cumecs

cubic meters per second

how can hydrographs be used

to predict flooding

physical factors affecting storm hydrographs

* size of drainage basins
* steep sided basins
* high drainage density
* saturation
* rock type
* vegetation
* precipitation

size of drainage basin

large catch more precipitation so have a higher peak discharge

small have a shorter lag time as rain doesnt have to travel as far

steep sides

shorter lag times as water flows more quickly on the steep slopes

high drainage density

basin that have more streams drain quicker so they have a shorter lag time

saturation

if the basin is already saturated this increases surface runoff , shortening the lag time

rock type

if rock is an impermeable surface, surface runoff will increase and lag time will be shortened

vegetation

the more vegetation the more interception and slow the rain will enter the river this increases lag time

precipitation

heavy storms result in more water entering the drainage basin which results is a higher discharge

if there is snow the lag time will increase but the peak discharge will increase when there is rapid melting of snow

human factors affecting storm hydrographs

deforestation

draining wetlands

digging artificial drains

downhill ploughing

compaction by animals or machinary

deforestation

less trees means less interception increasing SRO and discharge and a shorter lag time

draining wetlands

lag time will shorten, discharge increases as more water enter the river due to the wetland being drained as before it acted like a sink ,this means that water will flow directly into the river

digging artificial drains

peak discharge will increase and lag time decrease as artificial drains carry water straight to the river

downhill ploughing

peak discharge will increase and lag time decrease as the land is ploughed towards the river causing the water to flow easily into the river carrying sediment decreasing the river capacity

compaction by animals or machinary

soil is compacted by animal hooves or machinery which means there is more SRO, higher discharge and shorter lag time

what is hard engineering river management

involves the construction of structures built to control the flow of water

what is channel straightening

removing meanders to straighten a river channel

benefits and costs of channel straightening

benefits

* water moves quickly away from urban areas
* navigation improved
* reduces flood risk in prone areas
* reduces insurance costs

\
costs

* expensive
* ugly
* increases flood risk downstream
* aquatic habitats affected

what is a dam and reservoir

a dam is a structure across a river to control the flow of water and a reservoir is where water is stored

benefits and costs of

benefits

* large storage capacity
* generate electricity
* controlled release of water
* source of drinking water

\

costs

* expensive
* people displaced
* large area of land flooded
* sediment is trapped behind the dam

what is an embankment

building up the banks of a river creating levees or creating walls

benefits and costs of embankments

benefits

* increases river capacity
* new habitats created
* provides walkways
* reduces flood risk

\

cost

* ugly
* expensive
* ongoing maintenance
* if embankments fails flooding is more serious

\

what is a flood relief channel

an artificial channel to divert water

benefits and costs of flood relief channel

benefits

* flood risk reduced near urban areas
* new habitats created
* recreational activities (fishing etc)
* reduces insurance cost

\
costs

* expensive
* habitats disturbed
* ongoing maintenance
* ugly is concrete is used

\

what is soft engineering river management

adapting to a river and learning to live with it

what is afforestation

trees being planted to increase interception

benefits and costs of afforestation

benefits

* interception reduces surface run-off
* increases carbon storage
* creates new habitats and increases biodiversity
* relatively inexpensive

\
costs

* loss of potential grazing land
* changing the appearance of the countryside
* can increase soil acidity

\

what is flood warning and prep

an alert system to the risk of flooding

benefits and costs of flood warning and prep

benefits

* cheap and dependent on communication
* if warned in advance people can protect valuables
* ensures safety without the cost of hard engineering

\
costs

* only effective if people listen and take action
* not everyone has access to digital communications
* floods continue to occue

\

what is flood plain zoning

land in a river valley is used in a way to minimise flooding

benefits and costs of flood plain zoning

benefits

* impermeable surfaces are not increased
* low cost as it only involves administration
* traditional water meadows protected
* creates welcome green space

\
costs

* limited impact as most floodplains are developed
* house prices inflated due to lack of housing stock
* other greenfield sites affected

\

what is river restoration

returning an engineered river to its natural state

benefits and costs of river restoration

benefits

* creates new wetland habitats and increases biodiversity
* increased water storage in areas affected by flooding
* reduces risk of flooding downstream

\
costs

* possible loss of agricultural land
* can be very expensive

\

warm, moist air blowing from the sw meets the cumbrian mountains

it cools, condenses, producing cloud and relief rain (314mm in 24hrs)

the ground was saturated so

water could not infiltrate the ground, increasing surface runoff

the drainage density in the cumbrian mountains around the river cocker and derwent is high so

more water is entering the river from the drainage basin causing flooding

deforestation has occurred in the cumbrian mountains over the past few decades resulting in

less interception, increasing surface runoff and a higher discharge

cockermouth is at the confluence of the rivers derwent and cocker resulting in

more water joining together increasing the discharge which causes flooding

the derwent and cocker river were already swollen with previous rainfall so

meaning you didnt need much more water to fill the river which causes flooding