River Landform s

7/29/2019 River Landform s

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Rivers: Profiles &

Landforms

Higher Geography

The Hydrosphere


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River ProfileSource

of river

UpperCourse

MiddleCourse

LowerCourse


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River speed

Fastest??

Moderate??

Slow??


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Upper Course

Lower Course


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Contrasting river landformsfrom source to mouth

Channel features

Valley features

Long profile


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Upper Course - Channel features

River channel is rocky. Covered with various shapes and sizes

of boulder.

Discharge is low.

Under flood conditions rivers energy is

expended on vertical erosion with

hydraulic action and corrosion processesat work.

Potholes may form.


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Vertical erosion


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Pothole formation


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Upper Course - Valley features

Valley sides are steep and form a V shaped

cross section.

Interlocking spurs.

V shaped valley

Steep sidesZig-zag bends(interlocking

spurs)


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V shaped valley

Form due to a combination ofthe following processes:

Vertical erosion by the river

itself.

Physical weathering (eg:frost action) which provides

debris to move down slope.

Mass movement (inc: soilcreep & landslides) to movedebris down slope.


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Interlocking

spurs

River flows aroundinterlocking spurs


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Upper CourseLong Profile

Generally the gradient is steep and the

profile is uneven, particularly where

waterfalls and rapids form.


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Waterfall formation

Hard Rock

LavaSoft Rock Sandstone or Conglomerates

Soft rock is easy toerode, but the hard rockis resistant.

So over time a ledgedevelops.


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Waterfall formation

The water rushes over theledge and erodes a plunge

pool by abrasion andhydraulic action.

Hard Rock



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Waterfall formation

The ledge collapses intothe plunge pool, wherethe debris helps tospeed up the erosion.

Hard Rock



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Waterfall formation

The process is repeatedand the waterfall

gradually retreatsupstream, carving out agorge.

Hard Rock



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WATERFALL


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PLUNGE

POOLUNDERCUTTINGOF SOFT ROCK

OVERHANG

WATERFALLRETREATS . .UPSTREAM . .


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Formation of rapids

Resistant rock Less Resistant Rock


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Features in the Upper Course

(closer to the source)

1. Steep V-shapedvalleys

As a river cuts down

vertically into its bed,the valley side walls

become degraded by

weathering and mass

movement, so thatthe valley itself adopts

a V-shaped profile.

2. Inter-lockingspurs

Water will find the

course of leastresistance, it

therefore moves

around hard rock. The

hard rock sticks outas spurs which inter

lock.


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Upper course

3. Potholes

A circular depressionon a river bed carved

out of solid rock Pebbles caught ineddy currents andwhisked around within

a small natural crackor hollow

Commonly found

under waterfalls or

rapids where

hydraulic action is asignificant process.

Abrasion is also at

work here.


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Upper course

4. Waterfalls

A sudden step in a rivers

long profile.

The result of a toughermore resistant band of

rock cutting across the

valley.

Waterfalls can migrate

backwards to produce a

steep sided valley or

gorge.


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Waterfalls

See diagrams


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Niagara Falls

At Niagara a hard

band of limestone

overlies softer shales

and sandstones. The Niagara River

plunges 50 m causing

the falls to retreat by

1m a year and socreating the Niagara

Gorge.


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Upper course 5. RAPIDS

Cedar Rapids, USA

Rapids can also be known ascataracts.

They develop where thegradient of the river bed

increases without a suddenbreak like a waterfall.

Or where the river flow over aseries of gently dipping bandsof harder rock.

Rapids increase the turbulenceof a river and hence its erosivepower.

See diagram


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RAPIDS
http://www.geekphilosopher.com/bkg/waterRapids.htm


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Middle Course - Channel features

Channel is now wider and has smoother

banks and bed compared to the upper

course.


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Middle Course - Valley features

River erosional energy is now increasingly

expended horizontally rather than vertically.

Lateral erosion by the rivers meanders

broadens the valley floor into a narrow flood

plain.

Meanders gradually shift their course

downstream.


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Middle Course 1


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Middle Course 2


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MeandersA meander starts as a slight bend:

Water flows faster on the outer curve ofthe bend (more energy), and slowest on theinner curve (less energy).

So the outer bank gets eroded whilematerial is deposited at the inner bank.

Over time the outer bank gets worn away(river cliff) and the inner one builds up(river beach). The bend grows into ameander.


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Meanders(refer to previous notes and diagrams)

Alternating series of irregularities develop

Poolsdeeperstretches ofslow moving water

Rifflesshallowersection offasterflow,

flowing above coarser material River develops a winding or sinuous course

Fasterflow on outer bend results in erosion

and formation of River Cliff

Slowerflow on inside of bend results in

deposition and formation of Slip-off Slope


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Meanders

Meanders develop and

migrate laterally and

downstream

Helicoidal flow further

assists meander

formation and transports

sediment from river cliff to

the slip-off slope on the

inside of the next bend.

MEANDERS


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MEANDERS

Flood plain

MeandersMost erosionon the outsideof the bend .. Fastest flow

Possible breakthrough point

Possibleox-bowlake


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Meanders


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Middle Course - Profile


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Middle Course - Profile

Gradient is now less steep.

Overall the profile is smooth but anoutcrop of resistant rock could still cause a

waterfall.


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Middle Course 1.MEANDERS

In the middle course of

the river the discharge

has increased as more

tributaries feed into the

river. Gradient has decreased

so instead of vertical

erosion, lateral erosion

takes over. See overhead for 3 types of flow

in the river channel


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Meanders

Rivers follow the path

of least resistance so

it is often forced to

follow a windingroute.

Fastest flow is on the

outside of the bend


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What causes meandering?

A) An obstruction inthe channel

- This causes thewater to be thrownto the outside of thechannel as the waterfinds the path ofleast resistance

- See diagram on overhead

B) Helicoidal flowSurface water flows towardsthe outer (concave) banks

the water descendseroding & undercutting the

bankThe bottom flow is towards

the inner (convex) bank

- eroded material is carriedalong the bed & deposited

on the slip of slope area

See diagram on overhead


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Fluvial LandformsMiddle course of

river


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What causes meanders?

C) In straight

channels there is a

regular pattern of

shallow areas (calledriffles) and deep

areas (called pools).

Spacing is usually 5

to 6 times bed width


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river

There is less friction in pools.

Thus, river flows faster and has more erosive

power.

Increased friction in the riffles slows water

down and more deposition takes place.

Continuous erosion in the pools and deposition

in the river accentuates the slight bends of a

river. See diagram on overhead


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river


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river


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Middle Course

Meanders will migrateacross the plain anddownstream

See diagram on overhead

2. Ox-bow Lakes

Handout provided

- also known as a cutoffor a mortlake

- it is a crescent shapedlake originating in ameander


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Lower Course - Channel features

The channel is now at its broadest and deepest.

Bedload is carried entirely in suspension and is

solution.

Deposition now dominates particularly duringfloods.

Erosion also occurs in the formation ofmeanders


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Lower CourseValley features

Thanks to lateral erosion the valley sides

may now be several kilometres away.

Typically it may also contain the following

features:

Floodplain & natural leves

Braided channels

Meanders

Oxbow lakes

Estuaries and deltas


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Lower Course


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Natural leves

As the river floods, sediment is dropped over allthe flooded areas but most falls along the riverchannel itself.

This sediment raises the height of the banks isflooding occurs regularly

Levees themselves do not prevent floodingbecause as the banks are raised, moresediment is dropped on the river bed, raising thewater level.


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Gentle valley sides

Flat floodplain

Layers of siltDeposited during floods

Coarse materialForms naturallevees

River is actually flowing above

The floodplain !!

Levees is oftenartificiallystrengthened


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Braided channels

Formed by the choking of the main

channel by the deposition of a

considerable amounts of the river load.

The channel splits into several smaller

channels which flow around fresh islandsof deposited material before rejoining.


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Braided channels


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Braided channels

O


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Ox-bow lakesDownstream migration of meanders produce pronounced meander loops which

may form ox-bow lakes during flood conditions

1 Lateral erosion on outside of bends of pronounced meander

2 Narrow neck of meander gradually becomes narrower

3 Neck is cut through by river during floods and river forms new straighter

channel

4 Cut-off is sealed by deposition

5 Ox-bow lake begins to silt up(Core Higher, P 65, Fig 3.36)


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Oxbow lakes

An Oxbow lake starts as a meander.During a flood the river cuts across themeander, forming a new channel.

Sediment is deposited along the sides ofthe new channel. The loops gets sealedoff and an oxbow lake forms.

The water in it becomes stagnant. Thelake will remain sealed of until either theriver floods into it or it dries out.


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Oxbow lakes


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NARROW MEANDER NECK

FUTUREOX-BOW LAKE


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Estuaries and deltas

An estuary is where a river meets the sea

(it is tidal).

Deltas are essentially the seaward

extension of the floodplain and form where

tides are too weak to remove depositsediment.


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Lower Course


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Lower course -1. Floodplains

AGGRADATION occurs as the river

bed is silted up due to deposition.

There is still some lateral erosion to

wear away the banks and widen the

valley floor, but this is weak.

A FLOODPLAIN is a mostly flat

area of land bordering a river that

is subject to periodic flooding.

It is made up of silts and sands

which have been deposited over

many years by the river


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Floodplain Formation

Formed in 2 ways:1. When river in flood

- Wetted perimeter increases,so more friction

- Deposition of suspendedload

- Alluvium deposited, andfloodplain builds up in heightover time

- E.g. it has proved impossibleto drill down to bedrock inlower Nile valley due to depth

of alluvium- Alluvium provides a very

fertile soil

2. By migration of meanders

- Deposition occurs on insidebend of meander

- Point bar deposits build up toform a slip off slope

- Ultimately the whole valleyfloor is affected as meanderswander across it

The floodplain can be widenedby lateral erosion of themeanders.

The edge of the f.plain is oftenmarked by a prominent slopeknown as the BLUFF LINE.


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Lower Course Features

2. LeveesHandout provided

Natural levees producedduring floods

Artificial levees built toprotect from flooding e.g.levees on Mississippistretch for 3,500 mileswith average height of 7mand reaching 15m inplaces.


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Lower course features

3. Terraces- These form when sea level

drops

- This could be due to tectonicuplift of plates e.g. Turkey

- Or isostatic uplift due todeglaciation (ice melts so lessweight and land rises e.g. inBritain)

- Due to the drop in sea level theriver has much more erosivepower in its lower reaches

- So the river cuts down into itsbed and the old floodplain maybe left perched above thechannel to form a TERRACE


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4. BraidingBRAIDING is the subdivision of a

river due to the build up of

deposited material in the channel.

Why does this happen?

A) Easily eroded river banks e.g. if

made of loose sands and gravels.Braiding occurs in rivers which for

short periods of the year carry a

very heavy load in relation to their

velocity e.g.

B) A glacial out-water stream ladenwith sediment eroded by glacier.

C) A stream during snowmelt in

Alpine or Arctic areas.

D) Rapid and frequent variation in

stream discharge


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Braiding continued

Why does the river divide?- When discharge falls

competence and capacity arereduced.

- The channel becomes chokedwith material which it cannot carry.

- Load is deposited.- Dumped material forms islands in

the channel called EYOTS (orBRAID BARS).

- River is forced to divide aroundEYOTS.

- The location and number of

channels and islands is notpermanent, these move aboutmaking navigation difficult.

- The smaller subdivided channelsare faster and can cut into thebed, steepening the gradient andincreasing efficiency.

Generally braided channels occur

in summer when discharge is low.In winter greater discharge

enables material to be transported

again and the river can fill the

whole width of its channel again.


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Lower course 5. Deltas

A DELTA is the flat alluvial area atthe mouth of some rivers where

the mainstream splits up into

several DITRIBUTARIES.

Flooding from distributaries builds

up the delta. The largest deltasin the world are the Ganges, Nile

and Mississippi.

Conditions for formation:

A) Huge amount of sediment in the

river

B) River gradient is low

C) Little current or tidal action as

this would sweep sediment away

Most of the worlds deltas are

decaying because people builddams which hold back sediment.

Tides are also changing and getting

higher, so eroding deltas.


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Mississippi Delta

BIRDS FOOT DELTA

River has many distributaries

which extend out to sea like

the claws of a birds foot


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Nile Delta

ARCUATE DELTA has a

rounded, convex outer

margin.

When Mississippi and Nile

reach the sea, the meeting

of fresh and salt water

produce an electric charge

which causes clay

particles to coagulate andto settle on the seabed,

this process is called

FLOCCULATION.


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Tiber Delta

CUSPATE DELTA

Material brought down by

the river is spread out

evenly on either side of

its channel.Deltas are:

- Fertile

- High flood risk areas

- Difficult to navigate as

shallow and change

shape

Ri


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River

Running water is the most important agent oferosion on the continents and the streamvalleys are the most common landforms.

Rivers flowing to the oceans drain about 68 %of the Earth's land surface. The remainder ofthe land either is covered by ice or drains toclosed basins.

River gradually mould the land by erodingaway the material in some place anddepositing it in other


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A river system consists of a main channel (trunk stream) and all of the

tributaries that flow into it or joining the trunk stream.

A RIVER SYSTEM CAN BE DIVIDED INTO THREE SUBSYSTEMS:

collecting system (branches) -- consisting of a network of tributariesin the headwater region, collects and funnels water and sediment to

the main stream transporting system (trunk) -- the main trunk stream, which functions

as a channelway through which water and sediment move from the

collecting area toward the ocean. (Erosion and deposition also occur

in a river's transporting system)

dispersing system (roots) -- consists of a network of distributaries atthe mouth of a river (delta), where sediment and water are dispersed

into an ocean, a lake, or a dry basin

Partsof River


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tributary : a stream flowing into or joining a larger

stream distributary : numerous stream branches into which a

river divides where it reaches its delta

upstream : moves toward headwater (up the regional

slope of erosion) downstream : moves toward mouth of river (delta)

Delta : a large, roughly triangular body of sediment

deposited at the mouth of a river

Meander : a broad, looping bend in a river

Braided : river is divided into multiple channels by

alluvial islands. Braided rivers tend to have steeper

gradients

Partsof River

Drainage Pattern


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Irregular branching pattern (tree

like) in many direction.

It is common in massive rocks and

in flat lying strata

Due to strong resistance of rocks

headward development of valley is

negligible.

Drainage Pattern

Dendritic

Parallel Parallel or sub-parallel drainage

formed on sloping surface.

Common in terrain with

homogeneous rocks. Development of parallel rills, gullies

or narrow channels are commonlyseen on steeper and gently slopingsurface


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Streams radiates out from the center of the

topograhic high

common in Volcanic terrain

Channels marked by right-angle bends

Commonly due to presence of joints

and fractures in the massive rocks or

foliation in metamorphic rocks


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Consequent Stream:


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Consequent Stream:

A stream following the slope.

Development of slope may

be due to sudden tectonic

uplift

Subsequent Stream:

A stream that develops later on, carving the

softer rocks and flow at almost right angle to

the original slope of the land

FLUVIAL LANDFORMS


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Alluvial Fan


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Alluvial fans are fan-shaped deposits ofwater-transportedmaterial (alluvium).

They typically form atthe base oftopographic features

where there is amarked break in slope.

Consequently, alluvialfans tend to be coarse-grained, especially attheir mouths. At their

edges, however, theycan be relatively fine-grained.


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Braided Channel

Levee


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s

Oxbow


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lakes

Riversystem


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system

Headwat

er

TributariesTrunk

stream

Distributaries


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River Landform s

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