Test
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Name and explain the 4 ways that a river
transports its load?
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Name and explain 5 ways that the river erodes?
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Give three reasons why a river deposits?
Features
source drainage basin –
watershed –
tributary -.
confluence –
mouth
Definition
Features
Definition
source -
point at which a river starts.
drainage basin –
the total area of land drained by a
river and its tributaries.
watershed –
the area, normally a ridge of high
ground that separates one drainage
basin from another.
tributary -.
a small river / stream which joins the
main river
confluence –
the point at which a tributary joins
the main river.
mouth
the point at which a river enters a
lake or the sea.
Drainage Patterns
Dendritic: Tree like branch pattern where tributaries converge into a main river.
Develops on gently sloping land of uniform rock type. E.g. Shannon
Radial: Streams move out from a central high point - a hill or mt. Wicklow mts.
Trellised: Tributaries join at right angles. Occurs where land has been folded or varied
rock bands. E.g.. Lee
Deranged: Random and chaotic pattern. Found in marshy lowlands & clay soils.
E.g.. Cavan area.
Drainage Patterns
River Discharge: Volume of water passing a certain point at a given time in a river
channel. Greater the discharge = great capacity to erode & transport.
Discharge greater in winter.
Shown on a hydrograph…. Shows suitability for HEP, water supply & irrigation.
Also used to predict flood risk & flood control measures.
River Velocity: (speed) Effected by 1. Gradient… steeper = faster
2. Discharge… larger = more energy
3. Channel shape… more contact with bed &
banks (wetted perimeter) - more friction - reduce energy & speed - 95% of
energy used for moving
4. Channel roughness… uneven bed & banks,
potholes & boulders - friction & turbulent water - reduce energy & speed
River Flow: Mainly turbulent flow, this increases with velocity-- movement in
all directions – necessary for transportation and erosion
Processes of Erosion
Hydraulic action: The force of the water….. Stronger current = greater erosion.
turbulent and eddying water can undermine banks causing bank caving.
Cavitation (air bubbles collapsing forming tiny shock waves also loosen soil.
Solution: Chemical erosion of rocks, dissolving minerals which are carried in
solution, particularly in areas of chalk or limestone
Abrasion: River armed with material (pebbles, sand, gravel) which it uses to
erode. Uses it load to erode.Greatest erosive power when in flood. Pebbles
whirled by eddies form potholes.
Attrition: River load is worn down as it hits off itself, pebbles are rounded and
smooth
Vertical erosion: Downward erosion of the bed
Lateral erosion: Side wards, erodes the bands
Headward erosion: River erodes in the opposite direction to which it flows.
Processes of Transportation
Solution: The dissolved load.
Suspension: Visible cloud of sediment (brown colour) -- clay & silt carried within
the flow.
Saltation: Smaller particles are bounced along the river bed subject to the force of the
water.
Traction: Heavy load is rolled along the river bed.
Deposition
Material sorted by deposition… heaviest dropped first then lightest (clay & silt) last =
alluvium.
Depends occurs when:
1. Velocity… reduced as it enters sea or lesser gradient.
2. Discharge…. Less water due to less precipitation
3. Increased load…. Increased soil erosion after a flood or if new tributary joins
River profile: slope from source to base.
Always trying to reach its perfect graded profile but it never
happens due to differences in rock type, presence of lakes
etc.
1
•
2
3
Small stream… vertical erosion…. deep valley
2. Tributaries join… lateral erosion…wider valley
3.
Full size river… deposition…. valley filled in
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Youthful stage: Dominant process is erosion
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Features:
V shaped valley: Narrow, steep sided and deep valley….
Due to vertical erosion. River is weak but fast flowing &
volume is small. Hydraulic action attacks the bed, cutting
downwards. Corrosion cause rocks to break down &
increases vertical erosion --- steep slopes & narrow
floor develop as v shape.
River follows a zig-zag course as it finds the path of least
resistance which leads to interlocking spurs
Weathering and mass movement also aid in the
formation of the V-shaped valley
E.g.. Barrow, Co Laois.
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Waterfall: A layer of resistant rock lies beside or across soft rock. Soft
rock erodes faster --differential erosion - a fall develops as the river
bed is steepened. Water drops on the underlying soft rock and wears it
away.
Water backlashes and mist rots rock to form a cave. The overhang
collapses since the rock is undercut. This collapsed rock swirls & carves
out a plunge pool.
Hydraulic action: Speed is increased as it approached the fall -
increase hydraulic action
Attrition: Eroded the soft rock and deepened the plunge pool. It
dissolves, weakens & breaks up rocks.
Abrasion: Eroded material used to gouge out a plunge pool.
Waterfall retreats upstream -- headward erosion creating a gorge
Niagra falls has created a 12km gorge, retreating upstream by 1m a year. Has
been reduced as some water now diverted for HEP
E.g. Powerscourt Co Wicklow. Dargle.
Torc, Killarney Co Kerry. Flesk
Mature stage
Meander & ox-bow lake
Meanders are curves or bends in a river. Formed by erosion &
deposition.
Water flows faster on the concave (outside) & slower on the convex
(inside). The outside bend increases in size due to hydraulic action
& abrasion. This lateral erosion increases the curve size.
Secondary flows (corkscrew in shape) move material from the
convex & deposit it on the concave. This deposition of sand &
shingle forms a point bar.
The meander becomes pronounced & a neck develops. During a
flood the rivers cuts through he neck & forms a new course.
In time the river deposits along its banks & cuts off the bend
forming an ox-bow lake and eventually a meander scar.
E.g.. Nore Co Kilkenny, Suck, Co Galway.
Flood Plain
A flat low-lying area flooded by a river.
Processes: Corrasion, attrition, solution.
Lateral erosion predominates. River floods, overflows its
banks & deposits silt. Coarser material deposited close to
banks & alluvium (finer) deposited on outer flood plain.
Edge of flood plain marked by a bluff line (slope). Material
accumulates over time. Forms levees on river banks.
Drained flood plains are very fertile…. good agric. land.
E.g.. Shannon, south of Athlone.
Lower Course
Delta: A flat area of alluvium deposited by a river when it enters
a lake or the sea. Velocity of river decreases and it deposits its
load. Flocculation is where sediments clot on contact with sea
water.
Heavier material is laid down first & the lighter material carried
further. The tides are unable to remove the sediments and the
river channel becomes choked.
Bottomset Beds are layers of fine sediment carried out furthest
Foreset Beds are thicker coarser layers that are laid closer to the
river
Topset beds are a mixture of material deposited nearest to the
land
These form islands or braids. This causes the mouth of the river to
divide into channels called distributaries.
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Estuarine Delta: Forms at mouth of river as material is
deposited along the banks of the river. An island may
form in the centre of the river. E.g.. Rhine Delta or
Shannon
Lacustrine Delta: Formed as a river flows into a lake.
Alternate layers of fine (summer deposits) and coarse
material (winter deposits). EG Lough Tay co Wicklow.
Arcuate Delta: triangular Shape, sea currents strong
keep the outer edge straight e.g. Nile Delta
Bird’s Foot Delta: Relatively small number of
distributaries extend into sea like a birds foot e.g.
Mississippi Delta
Change in base level: River rejuvenation
1.
Land level rises relative to the sea (isostatic) Uplift after a
glacial period ends.
2.
Sea level falls relative to the land.( eustatic) Onset of ice age
3.
Tectonic movement causes folding and uplifting.
4.
River gets new energy as its gradient increases & the land
emerges from the sea due to any of the above.
It has an
increased ability to erode.  rejuvenation
Knickpoint: The intersection between the old and new profile.
Often marked by a waterfall or weir. Due to headward erosion.
EG Barrow, Bagenalstown Co Carlow.
Paired Terraces: Vertical erosion abrasion & hydraulic action
erodes the clay silt and bedrock and forms a new narrower
floodplain at a lower level. Lateral erosion creates a pair of
terraces as margins of the floodplain. A series of these occurs
with rejuvenation & are left high above the new floodplain.
EG Dodder, Terenure, Dublin
Incised meander: If downcutting is severe the river cuts a
deep meander into the valley floor.
EG Nore, Thomastown Co Kilkenny,
Drainage pattern of south of Ireland
A chalk surface was uplifted & faced south.
Consequent streams…. Follow direction of slope, flowed south
Eroded and exposed sandstone anticlines. Rivers cut into
landscape and superimposed themselves.
Subsequent streams… tributaries flowed at right angles to the
consequents. Developed as they flowed on the chalk. These
streams, by headward erosion, cut through to new catchment
areas. They intercepted the consequent & changed its direction
of flow. This is called river capture.
C2
C1
S1
S2
Wind gap
A beheaded stream remains & the bend where the river was
captured is called the elbow of capture. Below this a wind
gap marks the now dry former course of the beheaded
stream. The beheaded stream is a misfit stream as it is very
small in relation to the valley through which it flows.
The chalk was eroded & tributaries flowed down the
anticline, joining at right angles. This gives us a trellised
pattern of drainage along the Suir & Blackwater rivers.