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What you need to know about rivers
David Redfern
The River Helmsdale at Kildonan
Photo: Michael Redfern
Philip Allan Publishers © 2015
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.
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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.
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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
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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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Photo: Kevin Eaves/Fotolia
A meander on the River Lune in Cumbria
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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.
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
Philip Allan Publishers © 2015
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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