Comments on the Ecological Case for a Flow Regulator on Chowilla Creek, SA

"September 2006".

Project Number: Ecological Evaluation of proposed flow control structure at Chowilla significant ecological asset - M/BUS/69.

MDFRC item.

Associated reports see (Ecological evaluation of proposed flow control structure at Chowilla significant ecological asset) and (Risk Assessment of Cyanobacteria and Blackwater events in Chowilla Floodplain).

The floodplain at Chowilla, near Renmark, is one of six recognised Icon Sites associated with the River Murray. It is a habitat for river red gum, Murray cod and other well-known native species, and a scenic area worthy of preservation in its own right. For several decades the area has been deprived of water, and in recent years many trees have died. There has been substantial work toward restoration, including exclusion of grazing and an intensive program of pumping. The pumped water has improved tree health and germination, attracted waterfowl and enhanced wetlands, but has benefited only small areas. The outlook for local wetlands and woodlands is bleak unless there is more frequent, more extensive flooding. The ideal, long-term solution would be to restore significant flows to the river. This is unlikely to be realised, despite the 500 GL per annum target of The Living Murray program proposed for 2009. Under present operating conditions, seasonal ‘entitle-ment’ flows to South Australia, determined by irrigation requirements rather than environmental needs, are 3500–7000 ML d-1, and 3900 GL would be required to sustain a flow of 65,000 ML d-1 for 60 days. In future, there is likely to be less water available, as a consequence of global warming. A pragmatic solution therefore must be sought in using smaller flows to best effect, acknowledging that this is a palliative rather than a cure. This paper considers three alternatives. The first, a 'do nothing' option, is unsupportable given the nature, extent and rate of degradation of the Chowilla environment. The second option is to continue deployment of mobile pumps, and the third is to construct a flow regulator on Chowilla Creek. The latter option is preferable because the benefits of flooding would be spread over a larger, more continuous area than could be achieved by pumping water into isolated wetlands and ephemeral creeks. The proposed regulator would operate only for short periods, typically three months in three years, and should not be compared to the permanent weirs on the main channel of the Murray. It would include a series of 6 m bays supporting concrete ‘stop logs’ that could be added/removed to raise/lower water in Chowilla Creek by up to 3 m, forcing water onto the floodplain. Levee banks would provide supplement¬ary options for water-level management. Operated in a flow band of 5000–20,000 ML d-1, the regulator could be used to inundate 28% of the floodplain area, comparable to a river discharge of 65,0000 ML d-1. The maximal pool depth would be 5.2 5.5 m. When the stop¬-logs were removed, the supporting piers would not significantly impede flow. Regional water levels then would be determined by flows in the Murray and operations at Locks 5 6 and existing regulators on Pipeclay and Slaneys Creeks. The proposed regulator could provide a number of environmental benefits within the area affected by the impoundment. These include increased connectivity between riverine and floodplain habitats, improved soil condition, rejuvenation of existing vegetation, establishment of new floodplain and wetland plant communities, enhanced regional biodiversity, increased zooplankton abundance, and additional habitat for small native fish. Nevertheless, operation of the regulator would not be without some risk of negative impacts. These include an increased 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. In general, the regulator would need to be designed and operated to maximise the benefits and minimise negative impacts. To ensure that these goals are met, it is imperative that a comprehensive, scientifically rigorous, long-term monitoring program is implemented, and that the results are subjected to continuing scrutiny by environmental scientists. The degraded state of the Chowilla environment, and the prospect of continued low flows, demand a bold approach. The proposed regulator is such an initiative, and appears to offer more benefit than the mobile pumping option, notwithstanding certain risks. This commentary concludes that the proposal should be further investigated, with regard for measures to offset the risks incurred.