How and why does the water in a river change?
The volume of water passing a given point in a river, any given second is called the discharge.
The discharge of a river changes a lot during the year. It can change in hours due to heavy rain.
HYDROLOGICAL CYCLE
Hydrological cycle – is the transfer of water from the oceans into the atmosphere,
then onto the land and back to the oceans.
Condensation – is the water vapour changing back into water.
Evaporation – is the transfer of water from the sea into the air.
Precipitation – is water falling to earth.
Run-off –is water running over the land to the rivers.
Transpiration – is the transfer of water from plants to the air.
DRAINAGE BASIN SYSTEM
The drainage basin can be described as a system because it has:Inputs where water enters the system through precipitation (rain and snow)
Outputs where water is lost to the system either by rivers carrying it to the sea or
through evapotranspiration which is the loss of moisture directly from rivers or lakes
(evaporation) or from vegetation (transpiration).
Within the system there are stores and transfers (flows).
Stores are places where water is held e.g. in pools and lakes on the surface or in soil
and rocks underground.
Transfers are processes by which water flows, or moves through the system e.g.
infiltration, surface run off, or through flow.
RIVER REGIMES
River regime is the variation in the discharge of water carried by the river at
different times of the year.
Discharge is the volume of water passing on a river at a given time.
Precipitation, evaporation, transpiration, interception and soil moisture content all
influence river regimes in the British Isles. Man can influence river regimes by building
dams and by planting trees.
Relationship between precipitation and run off
There are four ways by which precipitation reaches rivers. They are:a) Falls directly into it.
b) Overland flow
c) Throughflow
e) Groundwater flow
The relationship between precipitation and run off is shown on a flood hydrograph and
it tells us if a river has a high or low flood risk. Lag time is the time taken for the
drainage basin to respond to the rainstorm and produce a flood peak. It is the time
from the heaviest rainfall to peak discharge. The shorter the lag time and the steeper
the rising limb the greater is the flood risk because the water reaches the river
quickly.
Write out the words and meanings on page 108
Precipitation
Interception
Surface Storage
Infiltration
Soil moisture
Percolation
Groundwater
Transpiration
Evaporation
Surface run-off
Through flow
Ground water flow
Water table
Hydrographs
A hydrograph may be used to show how the water flow in a drainage basin (particularly river runoff)
responds to a period of rain.
Rivers that respond rapidly to rainfall have a high peak and a short lag time and are referred to as
flashy. A lower peak and long lag time shows a delayed hydrograph.
This type of hydrograph is known as a storm or flood
hydrograph and it is generally drawn with two vertical
axes. One is used to plot a line graph showing the
discharge of a river in cumecs (cubic metres per
second) at a given point over a period of time.
The second is used to plot a bar graph of the rainfall
event which precedes the changes in discharge.
The scale on the horizontal axis is usually in
hours/days and this allows both the rain event to be
recorded and the subsequent changes in river
discharge to be plotted.
The shape of the hydrograph varies according to a
number of controlling factors in the drainage basin
but it will generally include the following features.
The baseflowof the river represents the normal day to day discharge of the river and is the
consequence of groundwater seeping into the river channel. The rising limb of the hydrograph
represents the rapid increase in resulting from rainfall causing surface runoff and then later
throughflow. Peak discharge occurs when the river reaches its highest level.
The time difference between the peak of the rain event and the peak discharge is known as the lag
time.
The falling limb (or recession limb as it is sometimes known) is when discharge decreases and the
river’s level falls. It has a gentler gradient than the rising limb as most overland flow has now been
discharged and it is mainly throughflow which is making up the river water.
A number of factors (known as drainage
basin controls) influence the way in which
a river responds to precipitation and have
an effect on the shape of the hydrograph.
The size, shape and relief of the basin are
important controls. Water takes longer to
reach the trunk stream in a large, round
basin than it does in a small, narrow one.
Where gradients are steep, water runs off faster, reaches
the river more quickly and causes a steep rising limb.
Prolonged heavy rain causes more overland flow than
light drizzly rain.
Areas of permeable rocks (rocks that allow
water through them) and soil allow more
infiltration and so less surface run off.
Impermeable rocks allow no water through,
decrease infiltration and so more surface runoff.
The way in which the land is used will also have an influence on the hydrograph – vegetation
intercepts precipitation and allows evaporation to take place directly into the atmosphere so
reducing the amount of water available for overland flow while the large number of impermeable
surfaces in urban areas encourages run off into gutters and drains carrying water quickly to the
nearest river.
Factors affecting river discharge/floods
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Amount and type of rainfall – heavy, light, long period
Temperature – hot, very cold
Previous weather conditions Rock type – porous, pervious, impermeable
Land use – deforestation, urbanisation
Rainfall
A lot of rainfall – high river levels.
Little rainfall – lower river levels.
Explanation for this: High amounts of rain saturate the soil (no more water allowed to seep into soil
so lies on surface) and underlying rock. In the case of drizzle, there is time for water to infiltrate the
soil and underlying rock, freeing up space for main rain.
Temperature
This affects the loss of water from the drainage basin and as a result the level of discharge.
Temperature high – greater water loss through evaporation and transpiration, so water levels go
down.
Previous weather conditions
If it has been dry – will take longer for water to reach the river and the amount will be less than if
there had been a number of wet days
Relief
Relief is the shape of the land, steep or gentle slopes etc.
Relief affects the rate at which water runs off the land surface and into rivers.
Steep slopes = encourage fast run-off as the water spills rapidly downwards due to gravity. Little or
no time for infiltration.
Gentle slopes = allow time for infiltration to occur.
Rock type
This is important in determining how much water infiltrates and how much stays on the surface.
In areas were more water is on the surface, the discharge of the river is higher as it reaches the river
fastest.
Rocks can be:
Impermeable (Granite) – allow no water through them, increasing the amount of runoff.
Porous (Chalk) – rock that has pores or spaces between particles to allow water through. This
reduces runoff.
Pervious (Limestone) – allows water to pass through it via vertical joints and horizontal bedding
planes. Again this will reduce run-off.
Land Use
Land use tells us what the land is used for. It is a factor that considers how people change and affect
the land.
Deforestation – cutting down the trees
Urbanisation – expanding towns and building impermeable surfaces (tarmac, concrete)
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If trees are removed, water reaches the surface faster and the trees do not extract water
from the ground. This means more surface runoff, higher river discharge.
Expanding create more impermeable surfaces that do not allow water to seep into the
ground. More surface runoff. This is made even worse by building drains to take the water
away from buildings quickly – and equally quickly into rivers.
Tasks
COPY out Key words and definitions p109.
Do activities Q1, 4A, 5abcd page 107
P109 Write a description of what happened in River Eden from 6th Jan to 9 Jan,
in detail (use Fig B)
Look at fig D. In what months are there more likely to be floods. Explain your
answer and mention figures from the table.
Why do rivers flood?
(see ‘Factors affecting river discharge)
Floods occur when a river bursts its banks if it is carrying too much water that it
can not carry normally.
Flooding is an extreme situation due to high levels of flow.
Bigger floods occur less often and less severe flood events occur more frequently.
Floods are common events. Problems and issues arise when people are affected.
Building on floodplains sees property damaged and lives lost. Floods are a hazard.
Causes of floods
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Amount and type of rainfall – heavy, light, long period
Temperature – hot, very cold
Previous weather conditions Rock type – porous, pervious, impermeable
Land use – deforestation, urbanisation
Snowmelt is a major cause of flooding.
With a lot of rain this also causes a lot of soil erosion that finds its way to rivers as well. This
will rise the water level even more.
(Do Q2 P113)
(DO KEY WORDS p111
(Copy Table D p112)
P116 Q1
Case Study – Bangladesh. Write up the Case study under the following headings:
 Where did it happen?
 When did it happen?
 What caused it?
 Effects (Primary and Secondary)

Responses (Short and long-term)
Frequency and location of flood events
Flooding appears to be becoming an increasingly frequent event.