what you need to know about rivers david redfern philip allan publishers © 2015

Philip Allan Publishers © 2015

What you need to know about rivers

David Redfern

The River Helmsdale at Kildonan

Photo: Michael Redfern


Essential definitions (1)Drainage basin Area of land drained by a river and its tributaries.

Watershed Boundary between two drainage basins.

Source Where a river starts.

Mouth Where a river ends (usually at the sea or a lake).

Tributary A smaller stream/river which flows into a larger one.

Confluence The point where two rivers meet.

Drainage density The length of all the rivers in a drainage basin, divided by the area of the drainage basin. The higher the drainage density, the greater the risk of flooding.


Essential definitions (2)

Inputs Ways in which water enters the system through precipitation (rain, snow etc.)

Outputs Ways in which water is lost to the system, either when the rivers carry it to the sea or through evapotranspiration.

Evapotranspiration The loss of moisture directly from water surfaces such as rivers and lakes (evaporation) or from vegetation (transpiration).

Interception When trees, other plants etc. ‘interrupt’ the flow of water to the ground.


Essential definitions (3)

Surface runoff Water flowing directly overland to the river (sometimes called overland flow).

Infiltration Water passing through the earth surface inthe drainage basin into the soil layer.

Throughflow The movement of water through the soil towards the river channel.

Percolation The movement of water from the soil layer to the rock layer.

Groundwater flow (sometimes called base flow) The movement of water through the rock layer towards the river channel.


The drainage basin hydrological cycle


Flood hydrographs (1)

A flood or storm hydrograph shows how a river responds to one particular period of heavy

rainfall.

‘Lag time’ is the time between the peak rainfall and the peak discharge of the river.

Lag times can vary depending on the relief of the drainage basin, the underlying rock type, the

vegetation, the land use and the drainage density.

A river regime shows how the discharge of a river varies over a longer period of time —

usually a year.

Be aware of the factors that change the speed, and amount, of precipitation that reaches a

river. Think of a river as system with:

•Inputs

•Stores

•Transfers

•Outputs


Flood hydrographs (2)

This is a ‘flashy’ hydrograph with a

short lag time showing that the river

has risen quickly in response to heavy

rainfall. On other occasions, the lag

time may be longer, resulting in a

‘subdued hydrograph’


Methods of erosion

Hydraulic action Occurs when the sheer force of the water dislodges particles from the river beds and banks.

Abrasion (also known as corrasion) Occurs when smaller material, carried in suspension, rubs against the banks of the river, wearing them away with a sand-papering action.

Attrition Occurs when boulders and other materials being transported by the river collide and break up into smoother, smaller pieces.

Corrosion (also known as solution) Occurs when acids in the water dissolve rocks such as limestone, which form the banks and bed of a river.


Methods of transportation

Rivers pick up and carry material as they flow downstream. A river may transport material infour different ways:

Traction Large boulders and rocks are rolled along the river bed.

Saltation Small pebbles and stones are bounced along the river bed.

Suspension Fine light material is carried along in the water.

Solution Minerals are dissolved in the water.

Deposition occurs when a river lacks the energy to carry its load — perhaps after a dry spell or on the inside of a meander where velocity is lower or where the river enters the sea.


The Hjulström curveLogarithmic scale (each

cycle is a ten-fold increase)

Logarithmic scale showing wide range of particle size

Critical erosion velocity (CEV). At this

speed river will entrain particles

Sand is the first size to be entrained

Critical settling velocity. At this speed the river begins to deposit grains of differing sizes, coarsest

first.

Once entrained particles can be transported at lower

velocities than CEV

Fine particles are cohesive and difficult

to entrain

Very fine-grained clays and muds are

suspended in virtually still water


As you go downstream...

There is an increase in:

• Velocity

• Discharge

• Load amount

• Cross-sectional area

• Efficiency.

There is a decrease in:

• Gradient

• Roughness

• Friction

• Turbulence

• Load size


The upper course (1)


The upper course (2)

In this stage:

• The river is high above sea level and has lots of potential energy which it uses largely in

vertical erosion.

• The river valley is often V-shaped with interlocking spurs.

• The channel is narrow and shallow with a large, angular bedload.

• The channel has a steep gradient, especially at rapids and waterfalls, where the velocity of

the water is relatively high.

• However, the overall velocity is low as so much energy (up to 95%) is lost due to friction

with the banks and beds.

• The water is often very clear as there has been little abrasion and attrition — so the

suspended load is very small.


The middle course (1)


The middle course (2)

As a river flows downstream, the gradient becomes less steep and lateral (sideways) erosion

becomes more important. The river then starts to meander.

The flow is always faster on the outside bend of a meander. This means that the water has

more power to erode its bed and so it is also deeper here.

Meander migration starts.

The water will also erode/undercut the river banks to form a steep-sided river cliff.

On the inside bend, the water flows more slowly; the water is shallower and deposition of

material will lead to the build up of a river beach (sometimes called a slip-off slope or point

bar).


Photo: Kevin Eaves/Fotolia

A meander on the River Lune in Cumbria


The lower course


The lower course

In the lower course, the river becomes wider and deeper.

The velocity also increases because there is less friction with the banks and bed.

The bedload is smaller and more rounded as a result of the process of attrition.

At this stage, the river will be carrying a large load of suspended material (brought from

further upstream) and so deposition becomes the most important process.

Causes of flooding

Causes of flooding

Human

Physical

Urban growth

Deforestation

Poor agricultural

practices

Populationgrowth

Saturated ground

Impermeable rock

Snowmelt

Prolonged rainfall

Intense rainfallAll linked to case studies

Impacts of flooding

Impacts of flooding

Human, social and economic

Physical/environmental

Disease

Damage to property/

dispossession

Casualties

Recharge groundwater

stores

Scale of flood

Meander cutoff/ levee

breach

Deposition of silt

All linked to case studies

Loss of crops/farm

animals, food shortages

Overall cost/Insurance Infrastructure/

business damage

Social

Economic


This resource is part of GEOGRAPHY REVIEW, a magazine written for A-level students by subject experts. To subscribe to the full magazine go to: http://www.hoddereducation.co.uk/geographyreview

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what you need to know about rivers david redfern philip allan publishers © 2015

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