Rivers and lakes | Wetlands | Wetlands of the
savannas | Mangrove systems | References |
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Victoria River, NT. Photo: Steven
Tapsall.
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The tropical savannas region is dominated by several large river
systems. Many of these are of considerable length, such as the
Mitchell, Gregory and Leichhardt in northern Queensland, the Daly
and Victoria in the Northern Territory, and the Ord, Fitzroy,
Ashburton, Fortescue and Gascoyne in Western Australia. All of
these rivers have extremely large variations in flow between wet
and dry seasons. This reflects the great seasonal rainfall
variations experienced in the tropical savannas region. The
Mitchell River discharges about 12 cubic kilometres of water every
year. The amount of water that flows in the Mitchell River in
February and March is about 100 times that which flows in July.
This region also has a number of inland flowing rivers. For
example the Paroo, Bulloo, Diamantina and Cooper Creek in western
Queensland do not reach the sea, but drain into Lake Eyre or
dissipate without reaching any other river system.
Rivers and their riparian zones have a fundamental role in the
functioning of ecosystems. The riparian zone is the strip of
vegetation that exists along the banks of rivers, and provides a
zone between the water and the forest, woodland or grassland that
grows next to it. The condition of riparian zones in general around
Australia is declining. This is due to over grazing, exotic weeds,
changed hydrology such as the building of dams, increased
fragmentation or breaking up of the vegetation along the banks,
feral animals and changed fire regimes.
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Wetlands in the Daly River region, NT.
Photo: Ian Dixon.
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Australia has some amazing wetland systems. The term
‘wetland’ refers to just about any area that is wet for
some period of the year, and according to the Ramsar Convention for
Internationally Important Wetlands (see web link to the Ramsar
Convention at right) includes 'areas of marsh, fen, peatland or
water, whether natural or artificial, permanent or temporary, with
water that is static or flowing, brackish or salt, including areas
of marine water the depth of which does not exceed six metres'
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Wetlands play an important role in protecting Australia’s
shores from wave action and reducing the impacts of floods. They
also absorb pollutants and provide habitat for animals and plants.
In Australia, 50% of wetlands have been destroyed since European
settlement, with many more damaged or drained. Those that remain
play a very important role in maintaining Australia’s
biodiversity. Many of these important wetlands exist in the
tropical savannas region. Here you can still find stunning
undeveloped river and wetland systems, often teeming with wildlife,
that are in good ecological condition.
Wetlands in Australia are important in other ways as well. They
purify the water and are important for recreational activities.
They form nurseries for fish and other freshwater and marine life
and, because of this, they are critical to Australia's commercial
and recreational fishing industries.
They also bear historical significance with some having high
cultural value. In particular, many wetland areas throughout
Australia are important to Aboriginal people. Consideration of
these historical and cultural relationships is a fundamental part
of wetland management.
A wonderful array of wetlands are found in the tropical savannas
region. This diversity is created to some degree by the range of
climatic zones in the region. The great hydrological variation;
lots of water in the wet season and nearly none in the dry, also
creates different habitats that support a wide variety of plants
and animals.
Wetland types found in the tropical savannas region include:
- escarpment streams,
- flood basins and plains,
- waterfalls and plunge pools,
- estuaries and seagrass beds,
- lowland permanent or seasonally-flowing streams,
- tidal reaches of streams,
- permanent billabongs or lagoons, and
- mangroves and salt flats.
Vast floodplain wetlands including Melaleuca swamps exist
across the coastal zone of northern Australia which, in comparison
to southern Australia, are largely unaffected by agriculture and
development.
The importance of many of these wetland sites and other types in
this region, 106 in all, is recognised by their listing in the
Directory of Important Wetlands in Australia. Nine sites are also
listed as Ramsar sites of international importance. A number of
sites including the Arafura wetland complex and the Gulf Salt Flats
have also been proposed for registration on the National
Estate.
The wetlands of northern Australia support large populations of
many aquatic fauna species (fish, birds, reptiles) which play an
important role in the nutrition and culture of Aboriginal peoples
and sustain important commercial and recreational fisheries and
tourism. They also play an important role in protecting areas from
flooding, and act as natural filtering systems.
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Mangrove system near Darwin, NT. Photo:
Ian Dixon.
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Mangroves are one type of wetland system common to the tropical
savannas. Mangrove forests cover 750 000 hectares in a
discontinuous pattern around the Australian coastline, but the
majority are found along the coast of northern Australia.
Mangroves form some of Australia’s most important and
widespread coastal ecosystems, growing in the intertidal zone of
tropical, subtropical, and protected temperate coastal rivers,
estuaries and bays. Mangrove forests generally range from
2–10 metres in height, but their structure and height vary
with the environment. In high rainfall areas of far north
Queensland, they may reach 30 metres in height. In some temperate
and highly salty areas on the inland side of tropical mangroves,
trees may only reach 1 metre high, and therefore be too short to be
classified as forest. Mangroves can form dense, almost impenetrable
stands, often dominated by only one or two species.
Tropical mangrove forests are most diverse and widespread, with
the greatest concentration of species, along the north-east coast
of Queensland. The number of species decreases further south due to
lower winter temperatures and from east to west across the tropics
as rainfall decreases.
Mangroves are valued for their unique biodiversity. The total
Australian mangrove flora consists of 40 species from 19 families.
What species you find in a mangrove forest is influenced by
latitude and tidal inundation. White mangrove ( Avicennia
marina ) is the most widespread and common of the species.
Ferns and orchids grow on the trunks and branches of mangroves in
tropical areas.
Mangroves play important roles in the ecology of wetlands and
estuaries. By reducing the speed of currents they can trap
sediments and help to reduce siltation in adjacent marine habitats.
River-borne nutrients and chemicals are also trapped and recycled
within mangroves. Habitat and breeding sites for birds, fish and
other wildlife are also provided by mangroves.
Mangroves have adapted to low oxygen levels in the deep muddy
soil by evolving aerial or breathing roots that grow up through the
water into the air.
- National Land and Water Resources Audit
- Department of Environment and Heritage (Supervising Scientist
Division)
- Australian Bureau of Statistics 2006. 1301.0 Year Book
Australia, 2006
