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Geography notes: Rivers
The drainage basin hydrological cycle
The drainage basin is an area of land drained by a river and its tributaries. The hydrological cycle is
the process by which water is moved within the drainage basin. The drainage basin hydrological
cycle is an open system as water is gained and lost. The system is made up of;
Inputs ­ in the form of precipitation e.g. rain, hail, snow or sleet.
Transfers ­ flows through the system
Stores ­ storages above or below ground
Outputs ­ where the water is lost from the system either by the river carrying it to the sea or
through evaporation.

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Precipitation-This forms a major input into the system, though amounts vary over time and space.
Evapotranspiration ­ the two components of evapotranspiration (evaporation and transpiration)
contribute to form an output of the system. it is also possible to distinguish between the potential
and the actual evapotranspiration. For example, in deserts there is a high potential
evapotranspiration because the amount of moisture that could be lost is greater than the amount
of water actually available.…read more

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Vocab Definition
Transpiration The biological process by which water is lost from a plant through the stomata in its leaves.
Evaporation The process by which liquid water is changed into water vapour and lost directly into the atmosphere from
water surfaces, including vegetation and the soil due to the effects of air movement and the suns heat.
Precipitation Any form of water that falls from the atmosphere onto the earth's land including rain, snow, sleet and hail. This
forms the major input into the system.…read more

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Percolation The slow vertical movement of water through underlying compact soil or rock layers. The rate of percolation
depends on the rock and impermeable rocks e.g. granite does not allow water to percolate.
Groundwater Storage of water caused by constant percolation in the underlying rock.
Groundwater The slow vertical transfer of water through the layers of underlying rock. It provides the main input of water into
flow a river during drought or dry seasons.…read more

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The graph shows the water budget for a town in Ghana and shows
two distinct seasons. A dry season from November to March and
an increasingly wet season from April to October. When
evapotranspiration exceeds precipitation the water stored as soil
moisture storage is used soil moisture utilisation in October and
November. This leads to soil moisture deficit, caused by
evapotranspiration exceeding precipitation. Between July and
September precipitation exceeds potential evapotranspiration
leading to soil moisture recharge, In September, there is soil
moisture surplus.…read more

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Factor affecting Flashy Hydrograph Steady hydrograph
Annotated sketch
Precipitation Prolonged rainfall ­ soil becomes saturated If rain falls gently over a long period of time
therefore infiltration can't occur which it encourages infiltration therefore
leads to an increase of surface run off. This increasing soil moisture storage
decreases the lag time to create a flashy If snow falls it reduces the volume of water
flood. reaching the river because it is stored on
Intense storm ­ rainfall exceeds infiltration the surface.
capacity.…read more

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Soil This hydrograph may have clay soils which This soil may be deep therefore increasing
encourage surface run off due to their platy its soil moisture storage capacity. Thus
structure. They are difficult to infiltrate and there is a longer lag time as water
so little water infiltrates creating a flashy infiltrates into it and is stored.
hydrograph.…read more

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Therefore more energy is lost through friction and the rivers
velocity therefore energy its energy level decreases. However, when the river contains large
quantities of water following heavy rain or snowmelt, it does possess the energy to perform great
amounts of erosion.
There are 4 types of erosion:
Hydraulic action ­ is caused by the sheer power of moving water. It is the movement of
loose unconsolidated material due to the frictional drag of the moving water on sediment
lying on the channel bed.…read more

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Traction ­ large stones and boulders are rolled along the river bed by water moving downstream.
This process operates only at times of high discharge.
Saltation ­ small stones bounce or leap frog along the channel bed. This process is associated with
relatively high energy conditions. Small particles may be thrust up from the bed of the river only to
fall back to the bottom again further downstream.…read more

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There is an increase in the size of the load. This may be due to a tributary bringing in larger
particles, increased erosion along the rivers course or a landslide into the river.
The river floods and overtops its banks, resulting in a reduced velocity on the floodplain
outside the main channel.
Hjulstrom curve
Hjulstroms curve shows the relationship between the velocity of the river and the size of particles
that it can erode, transport and deposit.…read more


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