What you need to know about rivers
David Redfern
Philip Allan Publishers © 2015
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.
Philip Allan Publishers © 2015
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.
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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.
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The drainage basin hydrological
cycle
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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
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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’
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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.
Philip Allan Publishers © 2015
Methods of transportation
Rivers pick up and carry material as they flow downstream. A river may transport material in
four 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.
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Fine particles are
The Hjulström curve
cohesive and difficult
Logarithmic
scale (each
cycle is a tenfold increase)
to entrain
Critical erosion
velocity (CEV). At this
speed river will
entrain particles
Sand is the first
size to be
entrained
Once entrained particles can
be transported at lower
velocities than CEV
Logarithmic
scale showing
wide range of
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particle size
Very fine-grained
clays and muds are
suspended in virtually
still water
Critical settling velocity. At this
speed the river begins to deposit
grains of differing sizes, coarsest
first.
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
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The upper course (1)
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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.
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The middle course (1)
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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).
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A meander on the River Lune
in Cumbria
Photo: Kevin Eaves/Fotolia
Philip Allan Publishers © 2015
The lower course
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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.
Philip Allan Publishers © 2015
Causes of flooding
Physical
Human
Intense
rainfall
All linked to case studies
Population
growth
Prolonged
rainfall
Urban
growth
Causes of
flooding
Snowmelt
Deforestation
Impermeable
rock
Poor
agricultural
practices
Saturated
ground
Physical/
environmental
Human, social
and economic
Impacts of flooding
Casualties
All linked to case
studies
Disease
Damage to
property/
dispossession
Impacts of
flooding
Deposition
of silt
Meander
cutoff/ levee
breach
Loss of
crops/farm
animals, food
shortages
Overall cost/
Insurance
Scale of
flood
Infrastructure/
business damage
Recharge
groundwater
stores
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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