Save the Murray Restore the Estuary

8 February 2012

by Susan Myers

On Wednesday night I attended a talk by Dr. Jennifer Marohasy entitled ‘Save the Murray: Restore the Estuary’.

“The Parramatta River empties into Port Jackson that includes Sydney Harbour and Melbourne’s Yarra River empties into Port Phillip Bay.  We don’t expect  Sydney Harbour or Port Phillip Bay to be kept full of freshwater. But we do expect the Murray River to keep Lakes Alexandrina and Albert full of freshwater, even during drought.”   That was one of the points made by Dr Jennifer Marohasy in her address to a packed room at the prestigious Sydney Institute. Listen to the podcast.

Dr. Marohasy described the Lower Lakes as part of a barrier estuary that had its evolution interrupted by the barrages, or as she referred to them in this presentation: ‘sea dykes’.  The Lakes were compared to other barrier estuaries referred to as ‘ICOLLs’ or ‘Intermittently Closed and Open Lakes and Lagoons’. 

Dr Marohasy explained that the ‘sea dykes’ have dammed the estuary making it totally dependent on river flows, stopping the tide, limiting the natural scouring of the sea mouth and interrupting the evolution of the estuary to fully tidal.

Mention was made of the environmental consequences of changing the Lakes to a full time freshwater reservoir, in particular the disappearance of mulloway fish from Lake Alexandrina.

Dr Marohasy disputed the claim that the ‘Lakes have always been fresh’ with reference to the diatom evidence.   Diatoms are microfossils and some scientists claim the particularly species present in the lake sediment over 7,000 years is evidence for a freshwater history.   Dr Marohasy explained there had been three periods in the evolution of the estuary and that this was evident in the diatom data and this included a period when the lakes were fully tidal.

In the end, Dr. Marohasy gave the audience new concepts to consider: that the Lower Lakes have all the characteristics of a barrier estuary in the middle stage of development, that the fossilized evidence of diatoms is open to interpretation, and that the barrages (sea dykes) functioned as a dam during the drought which made the drought worse for the Lower Lakes region.

The lecture was followed by 30 minutes of lively and open-minded Q & A.  Many in the audience, including myself, left thinking that some of the science underpinning the MDBA plan should be challenged, and it is our responsibility to challenge what we think we know to be true about the Lower Lakes. 

The talk was a short preview of her soon to be released work, “Plugging the Murray’s Mouth: The Interrupted Evolution of a Barrier Estuary”.

Dr. Jennifer Marohasy speaking at the Sydney Institute 

8 Feb 2012