Unstable, eroding banks do not provide secure burrow
sites for platypus. Unfortunately, many Australian waterways currently suffer
from severe erosion. This is true both in urban areas, where runoff from
hard surfaces like roads and roofs can vastly increase the volume and speed
of water flows after storms, and in farming areas where protective vegetation
has often been cleared right to the edge of the water. In Victoria alone,
over 25,000 kilometres of stream courses are believed to be either actively
gullying or vulnerable to erosion because of soil compaction and loss of
plant cover.
In places where stream or river banks are actively eroding,
structural works involving the use of heavy machinery are sometimes needed
to help stabilise the damaged section of waterway. These works may create
a direct barrier to further erosion, encourage sediment to be deposited
along an eroding surface, or reduce the rate of water flows after storms.
While stream stabilisation works often look quite ugly
immediately after completion, they can also fundamentally help to improve
the quality of natural habitats along waterways and should be regarded by
conservation-minded persons in this light.
The effect of stream improvement works on the behaviour
of radio-tagged platypus has been studied along a tributary of the Yarra
River in Melbourne's eastern suburbs.
Approximately 20% of the banks and channel along a stretch
of badly eroded waterway (1.1 km long) were remodelled using heavy machinery
over a two month period. While local platypus continued to use the area
affected by construction activities both during and after the completion
of works, some burrows were destroyed - with the number of burrows damaged
by construction activities directly proportional to the length of bank directly
affected by the work. However, because of the large number of platypus burrows
found along the stream (with one burrow recorded on average in each 100 metres
of channel), this was not considered to be a major problem for the animals.
What is the best way to undertake
stream improvement works with respect to platypus?
In order to be as "platypus-friendly" as possible, we
recommend that special consideration be given to the following issues when
carrying out stream or river restoration works:
* Except in cases of emergency (for example, if failing
banks threaten a home or other major structure) waterway improvement activities
should be initiated and carried out as part of an integrated catchment improvement
plan.
* Avoid lining substantial sections of the banks or
channel with concrete or other materials which effectively "seal off" natural
habitats so they are unavailable for use by platypus and other freshwater
species.
* Take steps to control soil loss at the work site and
minimise any associated increases in water turbidity downstream - blanketing
sediments can be particularly harmful to aquatic invertebrates (an important
food source for the platypus).
* Ensure that construction activities affecting the
bank are followed as soon as possible by an appropriate revegetation program,
including understorey plants as well as trees.
* Whenever possible, provide sections of vertical bank
rising one metre or more above the water as habitat for platypus burrows.