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Human Modifications of natural processes in the Rakaia River Catchment – A summary
Process
Fluvial
Processes
Examples
Fluvial processes occur
in the channel of the
Rakaia River and its
tributaries, and on the
slopes outside them. In
both cases fluvial
erosion, transport and
deposition occur.
The question is – how
have people modified
(stopped, slowed,
increased, or some other
change) these fluvial
processes?
Modification
There is withdrawal of water for irrigation from the
Rakaia river. Although this is controlled withdrawal
of water decreases water volume. The volume of
water in any river has a direct effect on the “work” a
river does – with a lower volume there will be less
fluvial erosion, transport and deposition.
There are two major HEP schemes (Lake Coleridge
and Highbank). Water is diverted into Lake Coleridge
from the Harper, Acheron and Wilberforce rivers
increasing the level of the lake. This is bringing extra
sediment into the lake (fluvial deposition). Where this
water enters the Rakaia river it increases the river’s
volume. Water from the Rangitata River is added to
the Rakaia via a diversion canal. This discharge may
at times account for 20% of water in the Rakaia at the
SH1 bridge. These are all examples of human
modification of the drainage network which change the
fluvial processes of the Rakaia river.
There are a variety of measures taken in the Rakaia
Catchment area to control the flooding of the river and
its tributaries. Stop banks can be seen at Cleardale
farm – located on the alluvial fan of Little River where
it enters the main Rakaia Valley. As a result there has
been increased fluvial deposition in the river channel
and this must be regularly dredged to remove the
alluvial deposits and keep the river within the
stopbanks instead of spreading across the alluvial fan
when it floods. Containing the river also means that
when it floods the volume of water is increased –
resulting in increased fluvial erosion. It combat this
(as it would cause loss of farmland) trees have been
planted on the river banks and deflection goynes used.
These measures can also be seen along lower course of
the Rakaia River near the sea.
Fluvial slope processes have also been changed.
People have introduced stock and changed the
vegetation of the area for farming. There are also
recreational and transport uses. All of these uses have
increased erosion and are now more controlled in an
effort to stablise erosion. Examples of soil erosion
resulting from fluvial slope processes can easily be
observed on the steep slopes of the Mt Hutt Range.
Slope processes have been increased by people
removing vegetation creating larger loads in rivers,
and increased deposition in the lower part of the river
on the Canterbury Plains e.g Rakaia Island.
Tectonic –
folding and
faulting
Climate
Processes
Vegetation
Succession
Wind Processes
– aeolian
Folding and Faulting are
still active processes and
have created the high
relief of the catchment
e.g. Southern Alps, Mt
Hutt Range
Rainfall processes occur,
cold temperatures are
also essential for the
freeze/thaw – frost action
which creates scree
slopes
This is the process of
vegetation starting on a
bare slope and
developing to the climax
vegetation.
Fluvial erosion is
constantly removing
vegetation and creating a
bare surface to be
colonised by vegetation.
– but have people
changed this process?
People can not really change the process – they do
modify the results of it (the relief) in a minor way e.g.
road cuttings
Wind processes include
wind erosion, transport
and deposition
People do change this by planting shelter belts of trees
and either adding or removing vegetation cover – this
alters the processes.
People can not really change these processes – they
can modify the results (e.g.control water flow after
rainfall, snow moved to new location)
People have interrupted this process – they have
changed the vegetation cover of the area. Forest was
removed to plant grassland for farming. There are
many examples of introduced species of vegetation
which would not be found in the are if people had not
introduced them. Shelter belts of introduced trees are
common on farms throughout the area. Gorse and
broom are found in uncultivated areas, including the
on river flats outside the current river channel. People
also disturb vegetation when making roads. The
introduction of browsing animals e.g. deer has also
affected the vegetation.