The Idea of a Freshwater driven Estuary

24 April 2010

The Signal Point Riverine Environment Group sponsored a community event in Goolwa with the guest speaker, Dr. Mike Geddes on 22 April 2010. The topic was, "A Freshwater-driven Estuary for the Lower Lakes, Murray Mouth and Coorong".  The subject was introduced as an idea that was neither totally freshwater, nor totally seawater but somewhere in between.

Dr. Mike Geddes of CLAMMecology and Adelaide University, has studied the Lower Lakes, Murray Mouth and Coorong since 1995, he is now semi-retired and living in Goolwa. 

His idea for a 'large connected estuary’ which still needs large flows of freshwater was outlined. 

Dr. Geddes said history shows that the Murray Mouth and Goolwa Channel were largely marine, while further up near Pt Sturt, the waters were estuarine.  Further back by Lake Alexandrina it was mostly freshwater. A gradation of salty to fresh and saline water moving back and forth was the norm.

It is very important to have a river to the sea connection, so that water and organisms can move around, and a big diversity of habitats, marine to fresh, as the salinity varies, 

About estuaries…Estuaries have a range of habitats, and shallow areas around the edges.  Estuaries, being near sealevel ranging with freshwater and with tidal influences. Estuaries are very ‘productive’ ecosystems.  In fact they are the second most productive type of ecosystem when measured by diversity, quantity, and resiliency of the lifeforms it supports.

With an estuary, water levels would be lower than typical Lake Alexandrina and Goolwa "full pool" levels (+.75AHD) and closer to sealevel.  Users around the lakes would need to adapt to changing water levels for recreation. 

How much freshwater do we need? Dr Geddes discussed historic water levels and flows. He said the river had been getting 10,000 GL instead of the small amounts of late. Also that the flows down the river can vary a lot.

Estuarine health involves large volumes of water, but not steady volumes. 

How much water should 'end of the river' get? – What’s natural? 

Dr. Geddes said that 40% of river flow is what SA should get through to the Murray Mouth or to the 'end of the system'.  The amount will vary between dry and wet years. 

What sort of salinity would we see in the estuary? All areas of the estuary should be connected with recent regulators removed. 

A better connection to the ocean and up through barrages is needed.  The barrages should introduce fish passage. SA should manage barrages differently and make them 'transparent'.  Improve connectivity between all parts of the system.

He then discussed what he calls the "Virtual Weir" and explained how it would work. With enough freshwater coming down the river the seawater would be kept out. It should be easy to work out what flows are needed to prevent incursion of salt water. We could also modify water channels to help water flows protect the fresh water needed for city intakes.  Tides coming in and out would maintain Lake Alexandrina at sealevel, and manage the barrages to allow tides in.  

It should be possible to model what salinities would be like throughout the system.  Keep the barrages, so we can still use them to manage targeted salinities. If low flow in river, fresh near Wellington, then brackish to much of Lake Alexandrina.  There would still be fresh water up the smaller river channels. When higher flows happen, at 40% for example, the range of fresh to slightly brackish, in Lake Alexandrina, and more tidal towards Goolwa. When the river floods, the whole system would be more fresh. 

In the South East, the Coorong is struggling and denied flows from Salt Creek due to irrigation.  This should be remedied.

Summary of what should happen:

1. End of system: 40% of average flows as target 

2. Estuarine solution should be given more consideration, this is natural for this part of the world, make the  barrages 'transparent' 

3. Modelling of the system and what it might look like as an ‘open system’ 

4. Barrages are an opportunity to manage system 

5. Mentions the size of the old estuary (just the Coorong) to the new one (if it included Lake Alexandrina) 

6. SE of the Coorong needs help

7. Would solve acid sulphate problems 

8. Connectivity is essential 

9. Higher Ramsar values with estuarine