The Chowilla Creek environmental regulator is being proposed by the SA MDB NRM Board with the aim of restoring a sequence of inundation to the Chowilla floodplain that more closely resembles the natural conditions under which the plants and animals have evolved. An environmental regulator would be designed and operated specifically for environmental management and would only be operational for small periods, typically for three months, on average one year in three. It would maximise flooding from low flows and would not be operated during large natural flood events (>50,000ML/day). The design would include sophisticated fishways to enable fish passage for large, medium and small-bodied native fish. The regulator would provide much needed water to the floodplain to address the severe ecological decline that has been observed over a number of years, largely due to reduced flooding and elevated saline water tables. The combined impacts of over-allocation of water from the River Murray system and prolonged drought across the Murray-Darling Basin have resulted in an acceleration of this decline over the past few years.
The Chowilla Creek environmental regulator would enable water level variation within the Chowilla floodplain between 16.30mAHD, representing normal upper pool level at the downstream Lock and Weir No. 5 and 19.87mAHD, representing the top of weir level at Lock and Weir No. 6. Thus the new regulator across Chowilla Creek would be used to regulate water levels within the Chowilla floodplain over a maximum range of approximately 3.6m.
The environmental regulator would enable the flooding of between 5580ha and 9000ha (or 30 and 50%) of the floodplain, depending on the prevailing river conditions and influence an even larger area through lateral groundwater freshening. This would enable the maintenance and improvement of 66% of the Red Gum community and 33% of the Black Box community. It would also inundate large areas of other floodplain habitats including 83% of wetlands and water courses, 63% of River Coobah woodlands and 85% of grasslands (CSIRO 2005). The area inundated during a 10,000ML/day event with the regulator operating at 19.87mAHD would be similar to a natural 65,000ML/day flood. However, the inundation extent caused by the regulator would be slightly greater in the western section of the floodplain and reduced in the eastern section when compared to a natural flood event.
In addition to the vegetation benefits, the proposed regulator would provide a number of environmental benefits within the inundated area. These include increased connectivity between riverine and floodplain habitats, freshening of groundwater systems, improved soil condition, rejuvenation of existing wetland habitats, establishment of new floodplain and wetland plant communities, enhanced regional biodiversity, increased zooplankton abundance, increased habitat and breeding opportunities for water birds and frogs, and additional habitat for small native fish (Brookes et al 2006).
The proposal has been costed at $28.02 million, which includes the Chowilla Creek regulator, fishways, ancillary by-pass regulators, blocking banks and other works.
Risks
Engineered solutions to ecological problems are not without risk. A detailed ecological risk assessment process has been undertaken and a number of issues have been identified as potential risks. These include the potential for cyanobacterial blooms, invasion by weeds, reduced lotic or flowing water habitats, interrupted fish passage, decrease in large-bodied native fish populations and increases in populations of common carp.
The scale and mitigation of these risks is fundamentally related to the operating regime introduced. It is intended that the regulator would be operated to maximise ecological benefits and minimise negative impacts. It is recognised that most risks can be mitigated to a satisfactory level by avoiding frequent operation of the regulator under low flow conditions. An adaptive management approach will be applied in the development and refinement of the operating regime.
Most risks, including the risk of reducing high velocity fish habitat and increasing river salinity can be avoided or mitigated in several ways including: avoiding frequent and/or prolonged operation of the regulator at times of low flow (<10,000ML/day QSA); operating at lower than the maximum regulator height during periods of low flow; and reducing the duration of operation during periods of low flow. Ecological risks are significantly reduced or eliminated when the regulator is operated at higher flows (eg. >30,000ML/day QSA).
Following operation of the regulator, a small downstream salinity increase will occur. It is expected that salt will flow off the surface of the floodplain at the commencement of floodplain inundation, whilst salt from groundwater can be expected to enter the anabranch on recession. Peak salinity increase in the order 50 to 100 EC can be expected depending on the regulator recession rate and flow in the River Murray. Assessment of salinity impact over the MDBC 1975-2000 benchmark indicates a long-term increase of 4.5 EC at Morgan assuming operation one year in three. Salinity impact has been the focus of considerable assessment and further work is underway.
Major salt accessions from Chowilla tend to occur following very large floods. The proposed environmental regulator cannot be used during the highest flows (>50,000ML/day flow to SA because it would be overtopped) nor can it replicate the same inundation extent, as experienced during a large natural flood, thus major salt accessions will not result from operation of the regulator. Salinity inflow to the River Murray and Chowilla anabranch system is not regarded as posing any risk ecologically, or to downstream water users.
Operation of the regulator will also result in elevated groundwater levels under certain sections of the floodplain and may increase salt accumulation in non-flooded areas. This process also occurs during natural high flow conditions but may contribute to tree health decline in small sections of floodplain. Approximately 52ha of currently healthy woodland is expected to experience a significant decline in condition as a result of long-term operation of the regulator.
URS (2006). Chowilla Management Options (140Mb)
URS (2007a). Concept Design for the Chowilla Environmental Regulator (78.3Mb)
URS (2007b). Chowilla-Higher Levels of Inundation (17.5Mb)
Hollis, B., Herbert, T. and Mollison, D. 2008. Chowilla Creek Environmental Regulator: Investment Proposal. Report prepared for the SA MDB NRM Board (4.2Mb)
Brookes, J.D., Baldwin, D., Ganf, G., Walker, K. and Zampatti, B. (2006) Comments on the Ecological Case for a Flow Regulator on Chowilla Creek, SA. Report to DWLBC (874kb)
Brookes, J.D., Baldwin, D., Wallace, T., Burch, M. (2007) Ecological evaluation of proposed flow control structure at Chowilla significant ecological asset (1.2Mb)
CSIRO (2008) Groundwater, Surface Water, Salinity and Vegetation Responses to a Proposed Regulator on Chowilla Creek. Report prepared for the SA MDB NRM Board
Howe, B. Yan, W. and Stadter, M. (2007) Groundwater Impact Assessment of the Proposed Chowilla Regulator using the Chowilla Numerical Groundwater Model. Report for DWLBC (6.3Mb)
Nicol, J. (2007a). Risk of pest plant recruitment as a result of the operation of Chowilla environmental regulator. South Australian Research and Development Institute (Aquatic Sciences) (821Kb)
REM (2007). Groundwater and Salinity Impacts of Raising Lock 6 Weir Pool. Resource and Environmental Management, South Australia.
Watertech (2007). Lock 6 Weir Pool Raising: Surface Water Impacts on Lindsay Island (475kb)
Mallen-Cooper, M., Koehn, J., King A., Stuart I., Zampatti B. (2007) Risk assessment of the proposed Chowilla regulator and managed flood regimes on fish (4.7Mb)
Gippel, C.J. and Anderson, B.G. and Andersen, S. 2008. Evaluation of the impacts of operating proposed infrastructure on geomorphology of the Chowilla Floodplain. Fluvial Systems Pty Ltd, Stockton. Report prepared for the SA MDB NRM Board (5Mb)