Saltwater Intrusion and Morphological Change at the Mouth of the East Alligator River, Northern Territory

• Published in: Journal of coastal research Vol. 22; no. 1; pp. 137 - 149
• Main Authors: Winn, K. O., Saynor, M. J., Eliot, M. J., Elio, I.
• Format: Journal Article
• Language: English
• Published: Lawrence, KS Coastal Education and Research Foundation (CERF) 01.01.2006; Coastal Education and Research Foundation; Allen Press Inc
• Subjects: Aerial photography; Alligator; Alligator Rivers Region; Aquatic reptiles; Beach ridges; Brackish; Coastal plains; Creeks; Desiccation; Dry season; Earth sciences; Earth, ocean, space; East Alligator River; Eolian processes; Exact sciences and technology; Floodplains; Floods; Floodwater; Fresh water; Freshwater; Geomorphology, landform evolution; Kakadu National Park; Marine and continental quaternary; Melaleuca; morphological change; Mud flats; National parks; Plant morphology; Point Farewell; Rivers; Saline water; Saline water intrusion; Saltwater; Saltwater intrusion; Sea level rise; Sediments; Surficial geology; THEMATIC SECTION; Thematic Section: Australian Coastal Geomorphology, 1984–2004; Water levels; South Pacific; Coral Sea; West Pacific; Australia; Northern Territory Australia; Alligator River National Wildlife Refuge; ISEW, Australia, Northern Terr., Alligator Rivers Region; Australia, Northern Terr., Alligator Rivers Region; morphological change; Australasia; vegetation; Alligator Rivers Region; modern; East Alligator River; cheniers; retention; rivers; saltwater intrusion; salt; aerial photography; salt-water intrusion; chemically precipitated rocks; evaporites; coastal environment; desiccation; floodplains; sedimentary rocks; Kakadu National Park; coastal plains; interpretation; Pacific Ocean; landform evolution; sea level; national parks; Point Farewell
• ISSN: 0749-0208; 1551-5036
• DOI: 10.2112/05A-0011.1

Morphological changes associated with saltwater intrusion of a small salt-affected floodplain in the vicinity of the mouth of the East Alligator River were examined through interpretation of aerial photography and ground surveys. The floodplain, in the Alligator Rivers Region of the Northern Territory, Australia, includes mudflats, tidal creeks, upper and lower floodplains, and a freshwater basin impounded by cheniers. Significant morphological change has occurred since 1950, with the tidal creek extending 4 km inland. By 2000, bare saline mudflats on the coastal plain had undergone a ninefold increase, and 64% of Melaleuca spp. forest had been lost. Saltwater intrusion and associated morphological change over time appears to have been driven by drier-than-average monsoonal conditions, low-frequency and low-intensity cyclonic events, and above-average ocean water levels experienced since 1950 and particularly since the mid-1980s. Loss of vegetation on the lower coastal plain was facilitated by expansion of the tidal creek. As a result, deflation of sediment followed sediment desiccation in the dry seasons. Arguably, the deflation contributes to basin formation and promotes favourable conditions for continuation of tidal-creek development. In turn, this promotes favourable conditions for subsequent retention of floodwater and continuation of tidal-creek development. There is interplay between the two processes. Historical records and field observations indicate that the floodplains may grow in elevation, responding to slight rise in sea level, through alternation of their freshwater and saltwater states. In sequence, the change occurs as a result of shallow basin formation by aeolian processes, saltwater intrusion by tidal creeks, basin infilling by tidal creeks and river deposition, and reestablishment of the freshwater body at a slightly higher level.