Saltwater intrusion and morphological change at Point Farewell, Alligator Rivers Region

Environment Australia, 2001

Internal Report 387
Winn KO
Supervising Scientist Division

About the report

Since 1950 the coastal freshwater wetlands of the Alligator Rivers Region in the Northern Territory have undergone significant morphological change in response to the invasion of saltwater through the landward extension and expansion of tidal creeks. The aims of this thesis were to characterise the contemporary morphology of a small catchment at Point Farewell, a salt affected site in the Alligator Rivers Region, and to describe the recent morphological changes that have taken place since saltwater invaded the freshwater basin. Emphasis is placed on identifying the potential processes driving the morphological change, specifically oceanographic and meteorological conditions which have previously been neglected.

The research was conducted in two parts and was designed to document past and present morphological changes associated with saltwater intrusion. The contemporary morphology was mapped from the 1997 aerial photographs (scale 1:5000) and characterised following ground-truthing in 2001. Processes initiated by saltwater intrusion and currently contributing to morphological change were determined from information collated from the aerial surveys, field observations and laboratory analyses. Morphological changes that have occurred following signs of saltwater invading the freshwater basin were reconstructed from aerial photographs for the years 1950, 1975, 1984 and 1997 at a scale of 1:15,000. A series of morphological maps were then created in a GIS environment. Climatic and oceanographic data for the time span of the aerial photography were analysed to determine the conditions that promoted rapid saline intrusion and subsequent morphological change at Point Farewell.

Seven main morphological units and a number of sub-units characterise the contemporary morphology. The main morphological units identified were: (1) tidally-influenced unit; (2) upper coastal plain; (3) lower coastal plain; (4) tidal creek unit; (5) freshwater basin unit; (6) chenier unit; and (7) Koolpinyah unit. The processes of saltwater intrusion, freshwater retention and Aeolian sediment transport were observed to be currently contributing to morphological change. Saltwater intrusion through expansion of the tidal creek on the lower coastal plain has facilitated loss of vegetation and subsequent deflation of sediment following sediment desiccation in the Dry season. The lowering of the coastal plain surface by aeolian transport processes promotes favourable conditions for continuation of tidal creek development.

The catchment at Point Farewell has undergone significant morphological change since 1950. The area occupied by bare saline mudflats of the coastal plain has increased by 906%, 64% of the Melaleuca forest has been lost and the tidal creek has extended 4km inland. Saltwater intrusion and consequent morphological change over time appears to have been driven by climatic and oceanographic conditions experienced since 1950. The process of landward extension and expansion of the tidal creek appears to be related to the build up of threshold-exceeding events specifically Wet season floods, stronger than average monsoonal activity, storm surges and very high tides.