These areas form important nurseries for many fish including barramundi. Kakadu's wetlands, including floodplains, billabongs, rivers, coastal and estuarine areas, are recognised internationally as being significant for migratory birds.
Kakadu's almost 500 square kilometres of coastal and estuarine areas, most lined with mangrove forests coast and the creeks and river systems under tidal influence (extending about 100 kilometres inland) make up this landform. The shape of the estuaries and tidal flats varies considerably from the dry season to the wet season. During the dry season tidal action deposits silt along the river beds and banks. During the wet season the river beds are eroded by the floodwaters and large quantities of fresh and saline water flow out across the tidal flats, where silt is deposited. Large silt loads are also carried out to sea, some of the silt being deposited as a nutrient-rich layer on the sea floor, contributing to the muddy waters that characterise Kakadu's coastline.
The estuaries and tidal flats are home to an array of plants and animals adapted to living in the oxygen-deficient saline mud. The dominant habitats are mangrove swamps and samphire flats. Where freshwater springs occur along the coasts and river banks, isolated pockets of coastal monsoon rainforests form, as at Manngarre.
Estuaries and tidal flats can be seen at West Alligator Head and in the lower sections of the South and East Alligator Rivers.
Eastern great egret
Among the birds restricted to the estuarine and tidal areas of the park are the chestnut rail, the collared, or mangrove, kingfisher, the broad-billed flycatcher, the black butcherbird, the mangrove gerygone, or warbler, and the red-headed honeyeater. During the wet season egrets, ibises, herons and cormorants nest in large colonies in the mangrove tree tops.
Mangroves are also used by the black and little red flying foxes which congregate in colonies or camps sometimes containing thousands of individuals. Mangrove monitors and estuarine crocodiles often wait below the camps to eat fallen flying foxes.
On the sandy beaches of West Alligator Head and Field Island, flat-back turtles haul themselves out of the water to lay their eggs in darkness. Other animals of interest in the coastal area are the white-bellied mangrove snake and other marine snakes such as Darwin's and Hardwick's sea snakes. The dugong, or sea cow, is a large marine mammal that feeds on seagrass beds.
The shady coastal monsoon pockets offer plenty of vegetation that orange-footed scrubfowl can rake up into large mounds. The birds lay their eggs in the mounds, using the heat from the rotting vegetation to incubate the eggs. The mounds are used by succeeding generations: some mounds in the park are well over 100 years old.
Also living on the forest floor are the colourful rainbow pittas, which can sometimes be heard scratching through the leaf litter for insects. They are also often seen on the walking track to Maguk.
Fruit from the fig trees that grow in the forest provides food for the rarely seen rose-crowned fruit dove. The figbird, oriole and migratory Torres Strait pigeon also eat these figs.
Kapok flower ready to disperse its fluffy seeds | Pete Hill
Mangroves are common along the banks of tidal creeks and rivers. Thirty-nine of the 47 Northern Territory species of mangrove occur in Kakadu. Mangroves are important for stabilising the coastline and serve as feeding and breeding grounds for many animals, including fish such as barramundi.
Like other plants growing in estuaries and tidal flats, mangroves must be able to cope with oxygen-deficient soils and periodic inundation by salt water. Mangroves use a range of mechanisms to cope with these conditions. Some species, such as the grey mangrove, have roots projecting through the soil (pneumatophores); others, such as the spider mangrove, have an amazing raised root system-like the legs of a spider-to help with oxygen intake. A number of mangroves are able to exclude salt through specialised filters in their roots or through salt glands in their leaves.
On the tidal flats behind the mangroves, hardy succulents (samphire), grasses and sedges grow. Isolated pockets of monsoon forest grow along the coast and river banks. These forests contain several impressive trees, among them the banyan fig, which can be recognised by its large, spreading aerial roots, and the kapok tree, which has a spiny trunk, large, waxy red flowers and pods full of cotton-like material.
- Banyan Ficus virens An-borndi
A large spreading tree with aerial roots and large prop roots from the major branches. It is a strangler fig with edible fruits and makes a great shade tree.
- Beach hibiscus Hibiscus tiliaceus
A small tree, five to eight metres high, commonly found in monsoon pockets along the coast and river banks. It has a large yellow flower with a dark maroon centre.