The IBA includes the whole floodplains of the Adelaide and Mary Rivers. The Adelaide floodplain extends from south of the Arnhem Highway to the river mouth (The Narrows), mangroves at the base of Cape Hotham peninsula (Wilshire Creek area) and includes Chambers Bay. The Mary River Floodplain extends east to Swim Thrings Creek floodplain and Point Stuart, where it abuts the Alligator Rivers IBA. Floodplain wetlands within the site include Lake Finniss, Melacca and Black Jungle Swamps, the artificial Fogg and Harrison Dams, and various billabongs and lagoons. The unusual morphology of the Mary River floodplain contributes to rapid overtopping of levees and inundation of huge seasonal wetlands, even in years of relatively low rainfalls. Chambers Bay is dominated by sand/mudflats with samphire and mangrove or sand-dune coastlines. The IBA includes waterbird colonies references W23, W25, W27 and W46 in Chatto (2000a). The mean annual rainfall at Middle Point Research Station is approximately 1380 mm, mostly falling in December-March.
The waterbird colony on the Adelaide River 15km from its mouth is the largest colony in the Top End, with an estimated 30,000 birds including at least 1500 Great Egret, 6000 Intermediate Egret, 4500 Little Egret, 9500 Cattle Egret (Chatto 2000a). A high count of 33,000-41,000 waterbirds was made at Finniss Lake Lagoon during surveys by Kingsford et al. (2008). Eighty-eight waterbird species recorded on the Directory of Important Wetlands in Australia website database. High counts of waders along Chambers Bay include 2300 Black-winged Stilt, 1960 near threatened Black-tailed Godwit, 1525 Terek Sandpiper, 1650 Grey Plover and 1500 Whimbrel (Chatto 2000b, 2003 and in litt. in Bamford et al. 2008). Coastal mudflats and nearby areas apparently support in the order of 10,000 shorebirds at times and thousands of Oriental Pratincole on the floodplain, before the Wet sets in, and up to 2000 Comb-crested Jacana have been counted at Fogg Dam (August 1983) (DEWHA 2008). Twice, single flocks of more than 2000 Black-tailed Godwit have been observed on swamps 20 km inland from the coast (Chatto 2003). Other species rarely recorded in the IBA include the near threatened Australian Bustard, the restricted-range Yellow-rumped Mannikin and the Australian Little Bittern.
Non-bird biodiversity: A major breeding area for Saltwater Crocodile Crocodylus porosus.
Pressure/threats to key biodiversity
Actions to mitigate the impact of rising sea-levels need to be considered: these floodplains have suffered more than anywhere in Australia from saltwater inundation. Invasion by alien plants and ungulates needs to be controlled.
Conservation responses/actions for key biodiversity
This IBA includes several protected areas: Djukbinj National Park (55,351 ha), Melacca Swamp conservation area (2315 ha), Black Jungle Swamp and Lambell's Lagoon are in a special reserve (4000 ha), Fogg Dam conservation reserve (1854 ha), Harrison Dam conservation area (3267 ha), Marrakai Flora and Fauna Reserve (25,900 ha), part of the east bank of the upper Adelaide River is in a reserve, the Arnhem Highway Protected Area (30,400 ha) includes some floodplain either side of the highway and the proposed Mary River National Park covers 121,015 of wet and dry habitats. The Adelaide and Mary Rivers floodplains are classified as sites of conservation significance by the Northern Territory Government (Harrison et al. 2009; Ward & Harrison 2009). Some of the Adelaide River and all of the Mary River floodplains are listed as nationally significant in the Directory of Important Wetlands.
Several national parks and conservation areas.
Woolner, Koolpinyah and Kerlin Stations, Annaburroo, Marrakai, Opium Creek, Melaleuca and Swim Creek pastoral leases. Protected Areas jointly managed by the NT government and the Limilngan-Wulna people. Traditional Ownership.
Site access / Land-owner requests
Much of this IBA is on private or Traditionally Owned land.