Great Sandy Strait

Salinity

Salinity problems have already arisen from previous dams and barrages in the Mary catchment. The commissioning of Borumba Dam increased salinity in downstream waters by 8% as a result of reduction in downstream flows. The construction of the Mary River Barrage in 1979, some 56 kilometres upstream of the Great Sandy Strait, increased salinity in downstream waters by a further 17%.

The condition of the estuarine reach of the Mary River system has been compromised, with a 30 kilometre tidal reach of the river having been isolated by the barrage. Reductions in low and medium flow events will increase salinity levels within the estuarine reach and reduce the quantity of ponded water behind the barrage, and consequently the number of occasions in which it overflows. The natural flushing of the river and mixing of freshwater with the sea water that creates brackish conditions will also be reduced. See ‘Barrages, Dams and the Mary’s Environmental Flows’.

Fishways

While a fishway structure will be incorporated into the dam design, evidence from the ‘state of the art’ fishway on the Burnett River has demonstrated that this measure is not enough to reduce the level of impact below the ‘significant impact’ threshold. Low flows in the river will also impact on the efficiencies of the fishway and the salinity levels at the barrage and thus impact on fisheries through the Ramsar wetlands and Great Sandy Strait. See ‘Ramsar’.

Fisheries

Prior to the construction of the barrage the river supported a thriving Fish Board in Maryborough. The Board had one of the highest product turnovers in Queensland (Maryborough Fish Board, 1980). With the barrage came a drop in fisheries production and the Board became unviable. The combination of the existing barrage and a new barrier to fish passage along the main channel of the Mary River if this dam goes ahead, will further deplete fish populations in the Great Sandy Strait. As well as impacting the Ramsar wetlands, this will have a social impact on the communities downstream who rely on this area for income from tourism related activities. See ‘Industries Threatened by the Dam’.

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The Mary River Tidal Barrage

The barrage, as it is meant to operate. To see it as it is now, take a tour with Steve Burgess.

In recent years very little water has flowed over the Mary River Tidal Barrage. Low rainfall has contributed to this trend and infestations of water weeds like hyacinth, salvinia, even para grass, now choke the area. Attempts are currently being made by the local shire council to control the weeds.