Saltwater intrusion into the coastal plains of the Lower Mary River, Northern Territory, Australia

The Lower Mary River, Northern Territory, Australia, is flanked by low-lying estuarine and coastal plains. Marked changes in the form and position of the coastline over the past 50 years have been paralleled by a dramatic extension of tidal creeks and the intrusion of salt water into many of the freshwater wetlands of the plains. More than 17 000 hectares of freshwater vegetation have been adversely affected and a further 35–40% of the plains are immediately threatened. A single cause for the onset of tidal creek expansion in the 1930s or 1940s cannot be identified. Instead a combination of several factors, some interrelated, seem to have tipped the balance reverting the system from a predominantly freshwater wetland environment to one dominated by saltwater conditions. The processes of creek network extension, elaboration, widening and reoccupation of palaeochannels are now internally driven and seem likely to continue until an equilibrium state is reached. In the meantime management response is focusing on the construction of barrages and earthen blocks in an effort to limit the intrusion of saltwater upstream. The pattern of tidal creek extension and saltwater intrusion on the Lower Mary River plains, as well as the approaches being taken to the management of the plains, are instructive for other coastal lowland settings in northern Australia. The situation on the Lower Mary River also presents a possible analogue of more widespread salinization that could be anticipated to accompany any future rise in sea level.1998 Academic Press