I’m just back home from five wonderful days at the Adelaide Test and a day visiting the western side of Lake Alexandrina, the GoolwaChannel ,Milang, Hindmarsh Island and the so called Murray Mouth. (I prefer to call it the Lake Alexandrina Mouth with, as explorer Sturt said, the Murray Mouth being the entrance to Lake Alexandrina with everything below that being an estuary-click to see photos). I was fortunate to have as my guide one of the very best informed people in the area, in the form of Ken Jury, a qualified marine ecologist who has made a lifetime study of the Lower Lakes, Coorong etc. and has a strong interest in the Murray Darling Basin.
Suffice to say it was a wonderful Test Match accompanied by some tremendous hospitality from my South Australian Cricket Association (and wool industry) friends.
To summarise my day at the Lower Lakes, I can only say I come home greatly enthused by the beauty of the area, enthused by the history and its preservation, and the wonderful water body; yet aghast at the waste of fresh water when it is so obvious that even better environmental outcomes could be achieved with the judicial use of healthy, oxygenated seawater with a concomitant reduction on the demand for upstream fresh water.
When you consider this in the context of the Murray Darling Basin Plan it is simply outrageous that the issue of better management (and upgrading) of the Barrages has not even been properly debated. It is not overstating the “final” position of the Plan to say that it has the supply of fresh water to the Lower Lakes as its prime objective.
There is absolutely no reason why the objectives for the environment and recreation, with irrigation needs and Adelaide water needs covered from upstream fresh water extractions, can’t be achieved with downstream seawater. Studies of tidal inflow movements, even when 80 gigalitres a day had been flowing to the Southern Ocean, have demonstrated the capacity, with Barrage manipulation, to use seawater to continue to hold the Lake level at the usual .75 metres above sea level and thus achieve the basic objective. Furthermore, such a return to a more estuarine situation would have the added environmental benefit of re-introducing the marine biota which contributes so much to the character of estuaries. An additional Lock would almost certainly be necessary to prevent sea water intrusion, in low flow conditions, up the main stem of the river.
Clearly the Barrages need to be “upgraded” to allow more cost effective management including the capacity to make it possible to periodically release substantial surges from the elevated Lake so as to keep the mouth to the ocean open. It will probably also be necessary to excavate much of Bird Island which has built up since the building of the Barrages in the 1930’s. I was surprised to discover that the Barrages are managed by the Murray Darling Basin Authority. I had always assumed that they were managed by the South Australians.
My ‘Australian Little Oxford Dictionary’ defines a “barrage” as “an artificial barrier in a river”. A “weir” is generally accepted to be a barrier which can be over-topped by water flows e.g Hume Weir. The term “dam” is usually used to describe a blockage in a water course to hold back water. It may be overtopped by big flows or have other means of allowing big flows to pass-pipes or spillways. In many areas the word “dam” is used to describe what is also called a “tank” (earthen) which is an excavated hole in the ground to capture water where there is no natural watercourse. The term “sea dyke” has also been used to describe the Barrages. A ‘sea dyke’ is usually used to describe a barrier to seawater intruding over land wanted for other purposes eg in low lying countries like Holland. On the Murray we also have a series of ‘locks’. These may also play the role of a dam, but always with the capacity to allow boats to get through.
So what is the right word to describe the Barrages? They were originally designed to store fresh water, principally for irrigation. A purpose since replaced by the piping of fresh water from upstream. They were built with a beautiful flexibility to change their height as desired by the lifting or adding of concrete panels (Goolwa Barrage). Their construction brought with it the secondary impact of keeping seawater out so in that sense they are ‘sea dykes’. So when one considers all that, the word “barrage” will do me.
To my mind, the two most important principles in water management to deal with Australia’s notorious climate variability are conservation (ie to conserve water from big flows) and flexibility. The Barrages with their modular construction strike me as meeting these principles perfectly, albeit the purpose of conserving water is somewhat different to the usual objective.
One further important factor that my Goolwa visit brought to mind was the huge impact that wind can have on water movement. When the south westerly winds get up, particularly at high tides, they can drive seawater into the Lakes even when there are substantial flows of fresh water coming from upstream. Providing the Barrages can be lowered and raised quickly this water can be captured to maintain the elevated level. In very low flow times it might be difficult to maintain the full .75metre elevation, but with the use of seawater at least the emergence of acid-sulfate soils could be avoided by keeping the lake bed inundated.