Australia is distinctive in that there are few permanent wetlands due to high evaporation rates and low rainfall. Most wetlands on the continent are intermittent and seasonal. Common features of floodplains are waterholes and lagoons called billabongs that retain water seasonally or permanently, providing important habitat for many animals at different times of the year. Floodplain wetlands tend to be sites of extraordinary biological diversity of waterbirds, native fish, invertebrate species, aquatic plants, and microbes. Key drivers of this biodiversity are the lateral connectivity to the river of the floodplain wetland and the unpredictable flows that create wide ranges of temporally and spatially different aquatic ecosystems.
Humid coastal areas are drained by short, perennial streams, while much of the streamflow in the rest of the country is intermittent or nonexistent because of low and unreliable rainfall, high evaporation, and flat topography. Even under these conditions, forested wetlands can be found throughout Australia, but they can only be classed as true forests in the wettest localities. The largest area of floodplain forested wetland (over 60 000 ha) occurs on the Murray River. Floodplain forests are generally composed of Melaleuca or Eucalyptus species, but they cannot survive very long periods (>5 months) of flooding. If flooding exceeds several weeks during the growing season, forest canopy cover declines to between 10% and 70%, creating open woodlands.
Tropical floodplain wetlands are found across northern Australia, covering an estimated 98 700 km . Vegetation of these wetlands has been mapped at various scales, but there are few specific or long-term analyses of the distribution or successional changes of the plants. The Ord River floodplain in northern Australia encompasses approximately 102 000 ha and is a large system of river, tidal mudflat and floodplain wetlands that supports extensive stands of mangroves, large numbers of waterbirds, and significant numbers of saltwater crocodiles. In southeastern Australia, the Murray-Darling river system drains 14% of the continent and contains the greatest amount of floodplain wetlands on the continent.
In recent years, floodplain areas have undergone considerable change because of animal (buffalo, pigs, cane toads) and plant (mimosa, salvinia, paragrass) invasions, changes in fire regimes, water resource management, and saline intrusion. Dams and the cumulative impact of diversions and upstream river management have turned many floodplains into terrestrial ecosystems. The effect of this change in flooding has not been well studied and data exist only for a fraction of the area affected. Floodplain loss will continue until there is a better understanding of the long-term ecological effects of dams and diversions.
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