Great Sandy Strait

Flaws in the Mary Basin Water Resource Plan

Efforts to minimise the impacts of changes in flow regime in this proposed action have relied on measures to restore the stream flow regime through the setting of environmental flow releases. The current Mary Basin Water Resource Plan (WRP) does not adequately protect the river health and is not endorsed by the Community Reference Panel. It also does not account for linkages between runoff, river water and ground water.

Choosing 85% of average annual flow at the mouth of a river as an adequate figure to maintain the health of a river is flawed. (Burgess & Edwards, 2006). The statistic used should be the median annual flow. The scientific basis of the number 85% has no documented empirical basis (Arthington et al, 2006).

The State Government hydrological model which claims the 85% figure also predicts that with the dam in place, the Mary River will cease to flow to the sea for considerable periods of time, something which would have occurred in it’s natural state only once in the 110 years of data used to formulate the WRP. The WRP now also allows for discretionary interim licences, so the dam operator could legally stop all flows in the Mary River for arbitrary periods of time, as the need arises.

Detailed analysis of the hydrological modelling used in the formulation of the WRP shows that the critical points in the river seem to be in the reach just downstream of the dam—at Dagun Pocket (an important breeding ground for the Australian Lungfish and Mary River Turtle), where the statistics indicate that the flow regime will be severely disturbed, and at the river mouth, where the no-flow regime will be significantly altered from the natural state.

At the river mouth, the modelling suggests that the river will cease to flow to the sea for 9% of the total time under the single large dam scenario. The Annual Proportional Flow Deviation (APFD) statistic for the river mouth is predicted to rise from 0.57 under current conditions to 0.95. Ecological processes at the river mouth are already heavily impacted on by the operation of the Mary River Barrage (See ‘Barrages, Dams and the Mary’s Environmental Flows’) and this further disruption in freshwater flow patterns could have a severe cumulative effect on estuarine processes related to water quality and the limited operation of fishways.

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Inadequate Environmental Flows