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Dr Michael Douglas discusses riparian issues in northern Australia. The videos are part of Savanna Walkabout - a learning module focusing on biodiversity conservation.

EnviroNorth > All Regions > Waterways and wetlands

Waterways and wetlands

Rivers and lakes | Wetlands | Wetlands of the savannas | Mangrove systems | References |  

Rivers and lakes

Victoria River web

 Victoria River, NT. Photo: Steven Tapsall.

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.

Wetlands

Wetlands in Daly River

 Wetlands in the Daly River region, NT. Photo: Ian Dixon.

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' .

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.

Wetlands of the tropical savannas region

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.

Mangrove systems

Darwin mangroves

 Mangrove system near Darwin, NT. Photo: Ian Dixon.

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.

References

  • 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

More Information

This links to a comprehensive Mangroves Education Kit (external website).