A wetland is any non-marine environment that is, to some extent, water-dominated. Wetlands can be comprised of standing water, such as in a lake; of flowing water, as in a stream or estuary; or merely of water that saturates the soil for significant periods. They may be fresh or saline, and fed by tidal waters, stream flow or groundwater.
Wetlands may be permanent, or last only a few weeks each year, or a few months every decade or so. Wetlands include the riparian zone and floodplains along creeks and rivers and the vegetation growing in them.

What makes a wetland different from an area that is occasionally flooded is that water influences a wetland's whole character. The period over which a wetland is wet shapes its form and determines the plants and animals that live in it. Each type of wetland supports distinctive plants and animals, depending on how often, and for how long, it is flooded, and how salty it is.

MacDonnells  Waterhole

Photo: © Jeff Cole

Biodiversity values
Wetlands are sensitive environments that are rich contributors to biodiversity and have value for production. Wetland health is important for plants and wildlife - especially frogs, fish, insects and other small aquatic animals. Wetlands play a role in filtering sediments, nutrients and pollutants from the catchment before the waters reach marine systems. They are often nutrient-rich environments, and may be highly dynamic, changing in response to rainfall, ground water levels and stream flows. Degradation of wetlands not only disrupts the wetlands and their dependent species, but also has the potential to pollute downstream environments.
Threatened species are found in the full range of Northern Territory wetland environments. Arrowleaf Monochoria grows only in a few swamps and billabongs on the Darwin coastal plain. Purplecrowned Fairy-wrens live in Cane Grass along the edges of streams in the Victoria River District. Gulf Snapping Turtles are found only in rivers that drain into the Gulf of Carpentaria, where they nest in the river banks and feed on fallen figs. Yellow Chats live on the floodplains in the Alligator Rivers region, and Yellow-spotted Monitors on floodplains across the Top End. Australian Painted Snipe visit the full range of wetlands scattered across the north of the continent.
There are also threatened wetland species in the arid zone. Rushes and sedges found on the damp margins of arid zone wetlands or semi-permanent drainage depressions include the threatened Dwarf Desert Spike-rush and Caldwell's Clubrush. Tobermorey Melon grows on cracking clay soils along a  few drainage lines in the Mitchell Grass Downs and Channel Country. Plains Mice also live in cracking clay soils along drainage lines through gibber plains, their population increasing following rain.

South Alligator floodplain and Magpie Gees

Photo: © Jeff Cole

Wetlands and associated run-on areas that collect moisture from the rest of the catchment hold the most reliable supplies of moisture and nutrients in the landscape. They not only support a range of threatened species and other native wildlife, but also attract introduced animals looking to drink and graze on the green plants that abound in wetland environments. High grazing pressure in these fertile pockets, particularly in times of drought, appears to have contributed to the extinction of many arid zone mammals. Rabbits in particular competed for the food resources of many extinct arid land mammals. Other animals, such as camels, have become a significant environmental problem. They compete with native animals for water resources and foul arid zone waterholes. In some cases, this can lead to the destruction of the waterhole.
In the Top End, the most significant wetland pests are buffalo, pigs and feral cattle. Each of these despoils the wetlands in different ways. Buffalo will wade into standing water to graze on aquatic plants. Cattle trample and compact the soft soils around wetlands. Pigs dig through the soils at the edges of drying wetlands, looking for food. As well as directly feeding on bulbs of sedges and lilies and on small crustaceans, pigs destroy the habitat of wetland plants and animals. The disturbance caused by these introduced animals also reduces water quality. Cane toads impact on wetland health, and are considered a particular threat to Yellow-spotted Monitor. The Finke Goby, found only in the upper reaches of the Finke River, is threatened by the spread of introduced fish species, such as the Mosquito Fish.
There are also many weed species that are adapted to the moist, nutrient-rich soils of wetlands, or may thrive in the disturbance caused there by grazing animals. The seeds and fruits of many weeds species, such as Bellyache Bush and Parkinsonia, are spread in floodwaters and deposited along river frontages and in wetlands.
Prickly Acacia has invaded floodplains in the Gulf Country, where it could be a problem for Gulf Snapping Turtle. Mimosa has spread across the floodplains of the Adelaide River, where it threatens the habitat of a range of species, including the Yellow Chat. Noogoora Burr threatens Purple-crowned Fairy-wren habitat along the Victoria River. Para Grass, a productive pasture plant, has become a biodiversity issue across the Top End wetlands, replacing both areas of open water and a diversity of wetland plants, including the threatened Arrowleaf Monochoria.
A number of wetlands in the centre are also threatened by weeds. Couch Grass, which not only replaces other species, but increases fire hazard and intensity, is considered a threat to the wetland plants Swamp Twig-Rush, Caldwell's Clubrush and Dwarf Desert Spike-rush. Buffel Grass is equally destructive of arid zone drainage systems, although its impact is also felt across the broader landscape. Athel Pine is so well established along streams in the Centre that it could be mistaken for a native plant. Not only does it replace native vegetation, it is so water-demanding that it dries out waterholes. As yet, this species is not thought to affect any threatened species directly, but has the potential to do so. Other weeds that could become a problem to threatened wetland species if they become established or expand beyond their current Northern Territory distributions include Rubber Vine, Mesquite, Neem and Parkinsonia. Wetlands are also affected by changes to natural water flows, pollution, vegetation clearance and natural processes such as fire, floods, cyclones and droughts. The most significant threat facing coastal wetlands is saltwater intrusion, due to predicted climate change and associated sea level rise (see the management profile for climate change for further information).

Pig diggings

Photo: © Stephen Garnett         Pigs dig over the soil in receding wetlands, looking for food

One key way to protect wetlands is to manage feral animal populations, particularly large hooved animals that trample plants and soils and destroy wetland areas. The most effective way of managing large feral animals is through aerial shooting operations.
Controlling weeds with pesticides may pollute wetlands and poison threatened species. If not used with care, pesticides can kill non-target plants, or poison aquatic animals. Fertilisers are also a potential problem, especially for species that need clear freshwater, such as the Gulf Snapping Turtle, because they can cause
algal blooms. Fertilisers may increase weed growth, and have been shown to have an adverse impact on Swamp Twig Rush.
A reduction in the water supply is already a problem for some threatened species, and this could become an increasing problem in wetlands as a result of future water extraction and/or climate change. The lower reaches of the Mary River floodplain have been extensively damaged by saltwater intrusion caused by buffalo damage to landforms that provided natural barriers to the sea. Remediation work is underway, but eventual loss of these and other wetlands in the coastal lowlands may be irreversible.
Methods for measuring wetland health are still being developed, and have recently been trialled in the Darwin area. Riparian health checks can be done using Tropical Rapid Appraisal of Riparian Condition, a scheme that assesses condition using measures of vegetation cover, woody debris, weediness, native plant regeneration and evidence of disturbance.
Wetlands will benefit from basic good land management practices - especially weed control, and control or exclusion of stock and pest animals. In some cases, it will be necessary to fence key habitat to protect threatened wetland species, either entire wetlands or long sections of river frontage. When pumping water from a wetland to water livestock, make sure that any excess water is returned to maintain wetland health.
Some wetlands thrive under a regular burning regime, and this is the way many Top End wetlands were traditionally managed by Aboriginal people. Patch burning as a wetland dries out towards the end of the year can increase habitat diversity by reducing the abundance of both native and introduced grasses (including Hymenachne and Para Grass), increasing the amount of open water, and promoting important food plants (water chestnuts and water lilies). However, fire can also be destructive, especially when swamps have been drained or dried out. Fire is also a tool in the battle against some of the problem woody weeds, such as Mimosa.
Wetlands are places of great cultural significance. They contain many story places, are the focus of fishing, hunting and gathering activities of Traditional Owners and many non-indigenous folk, and are places of recreation, respite and spiritual renewal. When well managed, there is no reason for these activities to conflict with wildlife conservation.

Noogoora Burr

Photo: © Gabriel Crowley