Saltwater intrusion and mangrove encroachment of coastal wetlands in the Alligator Rivers Region, Northern Territory, Australia
Supervising Scientist Report 191
SM Cobb, MJ Saynor, M Eliot, I Eliot & R Hall
Department of the Environment and Water Resources, 2007
- SSR191 - Saltwater intrusion and mangrove encroachment of coastal wetlands in the Alligator Rivers Region, Northern Territory, Australia (PDF - 2404 KB)
The aim of research reported here was to determine the spatial extent and rate of saltwater intrusion in the Alligator Rivers Region, in the eastern half of Van Diemen Gulf in Northern Australia, and link the findings to similar surveys of the western Gulf. This required examination of the tidal creek networks to identify their network patterns and growth rates. An additional aim was to identify and describe morphology in the vicinity of the headwaters of the tidal creeks to determine their potential association with parts of the estuarine reaches of rivers subject to different tide versus flood discharge relationships. The potential for interplay between large magnitude meteorological and oceanographic physical processes to drive changes on the floodplains is also briefly examined.
The rate, spatial extent and geomorphological character of saltwater intrusion in the Alligator Rivers Region have been determined from an interpretation of aerial photography available at the time the research was undertaken in 1998. The research documents coastal change associated with saltwater intrusion of the Alligator Rivers Region, and completes descriptions of tidal creek and mangrove growth in streams debouching into the southern waters of Van Diemen Gulf. The progress of tidal creek extension and mangrove encroachment of the Wildman, West Alligator, South Alligator and East Alligator Rivers of the Alligator Rivers Region was reconstructed from aerial photographs for the years 1950, 1975, 1984 and 1991 and mapped at a scale of 1:100 000.
Growth of tidal creek networks occurred in the eastern rivers of the Alligator Rivers Region, particularly the Wildman River and South Alligator River. Changes in the spatial characteristics and distribution of the tidal creeks and mangroves indicate that the saltwater reach has significantly expanded along extending creeks since 1950 in a manner similar to that reported from the western Gulf coast. Expansion of tidal creek networks occurred through a combination of headward extension and tributary development. The most vigorous rates of extension were along the low-lying palaeochannel swamps of the South Alligator and East Alligator Rivers. Mangrove colonisation in an upstream direction has increased for the four river systems examined.
It is possible the semi-enclosed basin of Van Diemen Gulf, with its deep basin and broad nearshore shallows along the southern coast, amplifies the effects of northerly monsoonal winds, particularly strong northwesterlies, on water levels along the southern shore. Drier than average wet seasons with strong onshore north westerly winds would be associated with above average water levels in the southeastern Gulf and enhancement of tidal activity on the flood plain surface. If this is so, then the changes observed along the southern shore would be geographically restricted and subject to reversal with a return to more average and higher rainfall conditions. The gradual, sustained increase in northerly winds over the historical period supports this argument. Additionally, areas of saltwater intrusion at Point Farewell and Kapalga show evidence of Melaleuca spp regrowth. This may indicate that saltwater intrusion has occurred as part of the natural variability of the wetlands and the processes driving it are contributing to raising the elevation of the floodplains through splay deposition at the headwaters of the tidal creeks. However, these propositions cannot be tested without detailed analysis of the post 1991 satellite imagery and aerial photography now available coupled with long-term sea level observations from the Gulf, closer identification of the processes involved and examination of the patterns of vegetation regrowth at a site level.