External impacts of humans
Human impacts within the
park are different fromthose that occur outside the park because
those inside can to some degree be controlled by management.
Those that occur outside the park cannot necessarily be controlled,
despite the fact that they might threaten the park and the rainforest's
continued existence as a viable ecosystem.
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Topographic map
(Click to enlarge) |
In the past, when the Minnamurra Reserve was under the control of Kiama Council
and consisted of the core 20 hectares of natural rainforest, the
external impacts were great on what is one of the last remaining
stands of natural rainforest in the Illawarra. The reserve was surrounded
by properties with working farms, as well as Crown Land that could
be used for a variety of purposes, including logging. Mr. Howard
Judd, manager from 1945 until 1980, realised this was a problem
and that the precious ecosystem required a buffer around it to protect
it to some degree from external impacts. Over time this resulted
in the addition of Crown Land and land donated by local residents
Ellen and Frederick Tieck, which, although it has all been disturbed
through logging and clearing to some degree, acts as a protective
barrier to external impacts. One of the ways in which external impacts
can cross this protective buffer is through the Minnamurra River
system, which passes through the most sensitive section of the rainforest,
bringing with it any external impacts that have already affected
the river upstream. Whilst most of the river upstream from the rainforest
is in the control of NPWS, the headwaters of the river and the swamp
that feeds it are not, and this provides opportunity for powerful
external human impacts to threaten the rainforest ecosystem.
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Aerial photo
(Click to enlarge) |
Looking at the aerial photo and the topographic map, you can see
on the tablelands above Minnamurra a patch of cleared land (AR 8965).
This land is partly owned by NPWS (up to GR 893695 along the road
to GR 902653). However, the land to the north of the road is not
controlled by NPWS and because it contains the headwaters of the
Minnamurra River (and the swamp that feeds it), the activities that
take place on this piece of land have a direct impact on the river
and the rainforest.
Looking at the aerial photo three areas of reddish soil can be
seen with very straight diagonal lines below them. This land is
now being used as a protea (native flower) farm. Note that in
1986, when the topographic map was made, this was not the landuse
and some of this land was uncleared, so in the 11 years to when
the aerial photo was taken (1997) there has been a marked increase
in the external impacts on Minnamurra. Water from the Minnamurra
River is used to irrigate the plants from the dams represented
by the three red patches, and these are fed from the river through
the use of a weir (located 200 metres upriver from the boundary
with National Parks). A weir holds most of the flow of a river
back, making a deeper section from which to pump water a small
flow is allowed to continue through the weir to maintain the river's
flow downstream. This situation has created several human impacts
on the Minnamurra River and, as a result, the rainforest ecosystem.
- The amount of water being taken from the river to irrigate
the protea farm is reducing the amount of water available for
the rainforest. As rainforests are very much dependent on the
supply of water, this is a critical factor that could change
the ecosystems of the river and the rainforest in the long term
to one that is better able to survive in dry conditions.
- The holding back of the river's flow with a weir restricts the volume of water running through the catchment, and may deprive the rainforest of heavier volumes of water that occur with heavy rain. This natural variation in the water flow is part of allowing unimpeded natural change in the ecosystem.
- The clearing of land for the farm as well as for the development
of the dams has induced a soil erosion problem, with the red
volcanic soil from the plateau being found in the upper region
of the Minnamurra Rainforest, in the form of silt. This will
alter the chemistry of the river but will also cause an increase
in turbidity and, where sedimentation occurs, will change the
depth of the river, possibly altering its course and the width
of its banks. All of these things will lead to irreparable change
in the ecosystem of the river as well as having implications
for the riparian rainforest along the banks of the river.
- The reduction in water flow over the escarpment has reduced
the Minnamurra Falls so that they are a less interesting feature
of the rainforest environment (they do not run during drier
periods), and this reduction in flow has reduced the fine water
spray in the gorges through which the waterfall runs, changing
the water supply for various plants that live there. Although
these gorges, because of their rugged nature, have not been
well documented in terms of plants and animals, it is known
that at least one rare species of orchid, Pterostylis pulchella,
exists in this area, and it is vulnerable to a reduction in
the fine spray off the waterfalls.
Recently the protea farm has made some attempt to reduce its
impact on the river (and make better use of its own resources,
saving money) by beginning to collect rainwater off the roof of
its packing shed, which measures 1000 square metres. (This can
be seen on the aerial photo as a large white square). The water
is being collected in a new dam, which is lined with plastic (this
cuts down water lost through seepage) and covered with shade cloth
(this cuts down water lost through evaporation) and this dam is
not being used to store catchment runoff or groundwater. It has
been estimated by Kiama Council that this technique of capturing
rainwater will supply 900 000 litres of water in a drought year,
for the farm, or about 25% of the year's water needs. In a good
year up to 50% of the farm's water supplies could come from the
roof of the packing shed.
This is a big step towards reducing the external impacts, even
though it must be remembered that the construction of yet another
dam exposes more soil to erosion (although the volcanic soil of
the plateau is not highly erodable) and the water which is being
captured off the packing shed is nevertheless water which might
once have ended up in the Minnamurra River. The improvement is
that the farm is removing less water from the river. Ideally,
such a water-dependent development is not well suited to being
located at the headwaters of a river that supplies such a unique
and now very rare ecosystem.
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