Kooragang Wetland Rehabilitation Project
To assess and enhance the fish and fisheries habitats of Kooragang Island with a view to applying the lessons learned to the rehabilitation of other degraded estuarine habitats in NSW.
Non Technical Summary:
The Kooragang Wetland Rehabilitation Project (KWRP) was initiated in recognition of the role played by estuarine wetlands in supporting food chains, modifying hydraulic events and maintaining water quality, and that in combination these factors contribute significantly top human welfare. One of the first studies initiated within the ambit of the project was the preparation of a simple descriptive model of the wetland habitats of Kooragang Islands. The geographical extent was subsequently expanded to include estuarine wetlands on the northern shore of the lower Hunter River. Hexham Swamp has not yet been assessed.
Old maps were analysed within a geographic information system (GIS) to extract information about changes to the amount of land and water in the estuarine portion of the Hunter River over the past 200 years. Aerial photographs were analysed to determine changes in area of mangrove and saltmarsh over the past 40 years.
Examination of the historical record suggests environmental change with the lower Hunter River was encompassed within four major periods: an early development stage (1796 - 1895), mid development stage (1896 - 1950), late development stage (1951 - 1989) and rehabilitation stage (1990 - present). In the early development stage, wetlands were primarily altered by the agricultural and shipping needs of the colonists at Newcastle, whereas the mid and late development stages were characterised by surges in industrial expansion. From the 1980s there has been an increase in community interest in wetland conservation, and it is convenient to assign the commencement of the rehabilitation stage to the washout of a culvert on a tidal creek on Kooragang Island in April 1990. This incident, among other things, re-established full tidal flow onto what had once been wetland, and outlined the implied potential of simple and inexpensive rehabilitation techniques.
Analysis of historical maps shows the progressive consolidation of the complex of islands in the mouth of the Hunter River into one land mass. Most of the consolidation came about by dredging and spoil disposal. The area of land now present is approximately 20% larger than shown on the naval chart of 1801.
The number of islands in the study zone has decreased from twenty-one to six, and in turn the length of shoreline has fallen from 154 km to 121 km. Much of the existing shoreline has been modified by breakwaters, retaining walls and bank protection works.
The construction of roads, railways and agricultural structures such as levees and drains has modified the flow of water across the study site. Flow within wetlands has been modified by water crossings (e.g., bridges, culverts and fords), floodgates and levees.
Analysis of aerial photographs from each of the decades beginning in the mid 1950s and up to the mid 1990s showed a decrease in the amount of open water in the study area. Three hundred and forty four hectares (13%) of open water was lost, most of it due to reclamation.
Seagrass was not identified in the aerial photos and has not been seen along the foreshores of the lower Hunter River for at least the past three decades. It is likely to have been present at some earlier stage as it occurs in all other analogous rivers in NSW. The cause of its demise is not known.
The areas of mangrove increased substantially from the mid 1950s to the mid 1990s, from 1310 ha to 1711 ha. This was in spite of the loss of over 200 ha during the development of the industrial site on Kooragang Island and the loss of another 40 ha along Trosby Creek. Some of the increase was into the main and secondary channels of the Hunter River, particularly in the Tomago/Fullerton/Stockton region, but also on Kooragang Island. Much of this expansion occurred upslope of existing mangrove stands and seemingly at the expense of saltmarsh. The reasons for the expansion are not clear, but may be related to change in soil salinity levels brought about by periods of enhanced rainfall and/or change in tidal levels in the estuary.
In contrast to the gain in mangrove, there was a long-term loss in the area of saltmarsh; of the 2133 ha present in 1954, 1,428 hectares (67%) was gone by 1994. Most of the loss was in the Tomago/Fullerton/Stockton region (871 ha), followed by Kooragang Island (508 ha). The small amount present at Throsby Creek was completely gone by the mid 1970s. The rate of loss has since stabilised on Kooragang Island and along the South Bank, but not at Tomago/Fullerton/Stockton. Drainage works, reclamation and infiltration by mangrove are assumed to be the main reasons for these losses.
The distribution of wetland vegetation in the lower Hunter River is controlled by the amount of substrate that is tidally inundated. Over past millennia the rise and fall of sea level has been the primary determinant of these conditions, modified by sediment redistribution after major floods. In the post colonial era, catchment clearing and small scale works to exclude tide and drain wetlands initially played a role in modifying the area under mangrove and saltmarsh, but large scale reclamation for industrial facilities in the middle portion of the 20th century has had a greater impact. Mangrove species move quickly onto any newly available and suitable substrate, including where tidal flushing has been re-established, and this appear to have a competitive advantage over saltmarsh.
There have been a number of disturbances to the wetlands of the lower Hunter River over the past 200 years, but the impact on fauna is still little understood. In regard to the fish community, two types of impact are considered likely, change in the number of species and/or change in the abundance of fish present. Assessing these changes is difficult, due to a lack of historical data for the Hunter River at any time in the recent past, much less 200 years ago. A separate study would be needs to investigate this issue.
A number of recommendations arise out of each of the chapters in this report and are consolidated below.
Chapter 2. History of change in wetland habitats on Kooragang Island
- There is still more to be learned about the nature and evolution of the wetlands of the lower Hunter River. Incentives should be offered to further study habitat changes that have occurred there
Chapter 3. Analysis of change in landforms and drainage in the lower Hunter, 1801-1994
- To provide a fuller understanding of change in the estuarine portion of the Hunter River, modifications to the Hexham wetlands over the past 200 years should be assessed and integrated with the results of this study.
- To assess the extent of change caused when overland or tidal flow is enhanced or reduced, a formal monitoring program should be initiated.
- To confirm the loss of open water along the tidal portion of the river, Manley's (1963) investigations should be updated.
Chapter 4. Analysis of change in estuarine vegetation of the lower Hunter River, 1954-1994
- While aerial photographs of the lower Hunter Valley were taken prior to 1954, neither the originals nor copies were found. If available, they should be analysed to provide further assessment of historical change.
- To complement this study, an assessment of the change in distribution of open water, mangrove and saltmarsh at Hexham Swamp should be commissioned.
- The discrepancy between Buckney's (1985) and our assessment of the areas of mangrove and saltmarsh present in the late 1970s and early 1980s, particularly as it relates to tracking short term changes in the distribution of estuarine macrophytes should be further investigated.
- To redress the loss of saltmarsh, potential sites for re-establishment should be identified.
- To assist in projecting the long term recruitment of mangrove, data on sedimentation rates within the lower Hunter River should be updated.
- To assist in projecting the long-term dynamics of wetland vegetation, investigations of changes in mean sea level on Kooragang Island and elsewhere in the lower Hunter River should be initiated.
- To facilitate future planning of rehabilitation efforts, a fine scale elevation model of Koorgagang Island and other relevant wetland sites should be constructed.