Chapter 11

Rivers: Shaping our landscape

Learning outcomes

In this presentation you will learn:

• Common terms associated with rivers

• About the three stages of a river

• About the processes of river erosion

• About the processes of river transportation

• About features formed by river erosion and deposition

• How human activity affects rivers

Common terminology

Source The start of a river

Course The route a river takes

Confluence The point at which two rivers join together

Tributary A smaller river that flows into a larger river

Mouth The end of a river – enters the sea

Drainage

basin

Area of land drained by a river

Watershed Highland surrounding drainage basin

Estuary Part of river mouth that is tidal

Exam hint: You need to understand common

terminology associated with rivers.

Common terminology

Describe each term

above.

Stages of a river

Three stages:

Upper or

youthful stage

Middle or

mature stage

Lower or old

stage

Exam hint: You must know the three stages of a river.

Stages of a river Upper or youthful

stage

Middle or mature

stage

Lower or old stage

Steep, fast-flowing

Little water

Steep, V-shaped

valley

Narrow, shallow

channel

High bed load (larger

rocks)

Starts to slow down

More water

Still eroding

Open, gently sloping

valley with flood plain

Wider, deeper

channel

More suspended

sediment

Slow-moving river

Much more water

Depositing, not

eroding

Open, gently slowing

valley with flat and wide

flood plain

Very wide, deep

channel

Suspended sediment

Features:

V-shaped valleys,

interlocking spurs,

waterfalls

Features:

Meanders, oxbow lakes

Features:

Flood plains, levees

Exam hint: You need to be able to describe a river at each stage.

Stages of a river

Name two features

associated with each

stage.

The work of rivers

Shapes the

landscape:

Erosion

River wears the

landscape away

Transportation

River carries the

material it has

eroded

Deposition

River drops off

material it was

carrying

Exam hint: You need to understand how a river shapes the landscape.

River erosion – processes

Hydraulic

action

Force of moving water wears away

river bed

Abrasion Stones carried by water wear away

banks

Attrition Stones are worn down as they hit

each other

Solution Rocks (e.g. limestone) are

dissolved by acids in water

Exam hint: You need to be able to describe how a river erodes.

River transportation

• Transportation occurs

in a number of ways

• Method depends on:

size of material

size and speed of river

• Material transported

by river is its load

Exam hint: You need to know how a river transports its load.

River transportation – methods

Rolling Large stones are rolled along river

bed – known as traction

Bouncing Smaller pebbles are bounced along

river bed – known as saltation

Suspension Light materials float along in water

Solution Materials dissolve and are carried

along

River deposition

• Shapes land by leaving sediment in new places

• Makes new land or adds to existing

• Causes of deposition:

Reduction in river’s speed – where a river enters a

sea or lake or during dry spell

Increase in load size – when tributary joins or after

heavy rain

Reduction in river’s volume – during dry spell

When gradient levels off – when river reaches flatter

land

The youthful stage (upper course)

• River cuts downwards

• Gradient is steep and river channel narrow

• Vertical erosion

V-shaped valleys

Interlocking spurs

Waterfalls

V-shaped valleys • Steep-sided valley

• Shape of a ‘V’

• Shaped by vertical erosion

• River bed made deeper by hydraulic action

• Mechanical weathering and mass movement weather sides of valley

• Examples: upper course of Rivers Liffey, Lee and Moy

Exam hint: You must be able to describe how V-

shaped valleys are formed.

Interlocking spurs

• Areas of high ground

• Jut out at both sides of V-shaped valley

• River winds and bends around hard rock as it erodes

• Still erodes vertically

• Creates interlocking spurs

• Example: Upper course of Rivers Barrow and Slaney

Exam hint: You must be able to

describe how interlocking spurs are

formed.

Waterfalls

• Vertical drop in course of river – youthful stage

• Band of hard rock across band of soft rock

• Soft rock eroded quicker – drop is created

• Material creates a plunge pool at base

• Enlarges over time

• Hydraulic action and abrasion cause undercutting

• Creates overhang – eventually collapses

• This is repeated and waterfall retreats over time

Exam hint: You need to know how waves are formed.

Waterfalls

Name another example of a

waterfall.

The mature stage (middle course)

• River has more energy

• Higher volume of water

• Tributaries or other rivers have joined

• Gradient is gentle

• Lateral erosion has widened river channel

• River channel has deepened

• River carrying more material

Meanders

• Curves or bends

• River erodes laterally

• Right then left

• Forms large bends

• Horseshoe-like loops

• Meanders formed by deposition and erosion

• Hydraulic action erodes outside of bend

• Material deposited on inside of bend

• Example: Middle course of River Shannon

Exam hint: You must be able to describe how

meanders are formed.

Oxbow lakes • Horseshoe-shaped lake

• Meander cut off from river

• Neck of meander becomes tight

• During flood – more discharge

• River breaks through

• Loop cut off – sediment dropped at entrance

• Forms oxbow lake

• Dries up – oxbow scar

• Examples: River Liffey and River Moy

Exam hint: You must be able to describe how

oxbow lakes are formed.

Oxbow lakes [Insert

11.21.01

11.21.02]

The old stage (lower course)

• Slow-moving river

• Much more water

• Deposition, not erosion

• Gently sloping valley

• Very wide and very deep channel

Flood plains

• Wide, flat area of land either side of river

• River nearing sea, deposition main process

• Large amount of sediment – alluvium

• River spread out during flood and deposits load

• Makes land fertile

• Highest point reached – bluff line

Exam hint: You must be able to describe flood

plains and their features.

Flood plain

• Examples: Lower course of River Moy and River Liffey

Levees

• Build up of alluvium on banks of river

• Caused by floods

• Low flow – materials deposited on river bed

• Flood – water leaves channel and deposits heavier material on banks

• After many floods – river bank builds on both sides

• Examples: Mature stage of Rivers Liffey and Moy

What are artificial levees and

what are they used for?

Exam hint: You need to know

how waves are formed.

Deltas

• Triangular/ fan-shaped – at mouth of river

• Features of deposition – old stage

• River mouth choked with sediment

• River channel splits into smaller channels

• Form under certain conditions:

Transporting large amount of sediment

Sea – small tidal range, weak currents

Shallow at mouth of river

Examples: Mississippi River in USA and the Nile delta in

Africa

Exam hint: You need to know how waves are formed.

Human activities and rivers

• Humans use rivers in variety of ways

• Interactions can be beneficial or harmful

• Interactions include:

Transport

Tourism

Fishing

Settlement

Pollution

Flooding

Hydroelectric power (HEP)

Human activities and rivers

Transport Used to transport people and goods, e.g. Limerick has port

function, River Rhine is important transport route in Europe

Tourism Use rivers for cruising and fishing, e.g. Carrick-on-Shannon

and Athlone on River Shannon are tourist destinations

Fishing Popular with anglers, many popular for salmon fishing, e.g.

River Moy

Settlement Settlement along river banks – provides food, water and

transport

Pollution Farming – spread slurry; domestic – waste material from septic

tanks; industrial – factories dump waste

Flooding Cutting down trees – surface run-off and erosion, can lead to

deposition, river rises and bursts banks

HEP HEP stations built using dams, large concrete wall to control

flow of river, can have both positive and negative impact

Exam hint: You need to be able to describe how human

interactions impact on rivers.

HEP: positive and negative aspects

Positive aspects Negative aspects

Generate cheap, clean,

renewable electricity

Water builds up in

reservoir – water supply

Reservoir can be used

for leisure activities –

fishing and water sports

Creation of reservoir –

can lose agricultural land

Families need to be

relocated

New roads and bridges

have to be built

Fish stocks can be

affected

Exam hint: You may be asked to discuss the positive and

negative aspects of HEP.

Inniscarra, Co. Cork

• Location: Inniscarra and

Carrigadrohid

• Technology: Hydro

• Capacity: 27 MW

• Commissioned: 1957

• Part of Lee Hydroelectric

Scheme

• Located west of Ballincollig

• Buttress dam, 44 m

• Parts of Lee valley flooded to

create reservoirs

• Inniscarra – 530 h (5 km²)

• 40 k of bank can be fished

Exam hint: You need to be able to describe a HEP scheme you have studied.

Sample answer

i) Rivers are often used to generate hydroelectric power. Describe one positive and one negative effect of a HEP scheme you have studied.

Answer: Positive – Inniscarra reservoir is used for angling.

Negative – Agricultural land was lost when the Lee valley was flooded to create reservoirs for generating electricity.