by Mike Young, Ret. DEH and horticultural expert
and Ken Jury, Ret Journalist (Marine & Aquatic Ecology)
26 April 2010
This opinion piece is responding to recent statements regarding current river flows to the lower lakes.
Some comments including official ones concerning the amount of fresh water flow for the lower lakes are questionable. Since the rains over the Murray Darling Basin in Queensland and New South Wales, there has been speculation over how much water South Australia will receive from those sources.
There’s been a lot of talk about 400 to 600 gigalitres of flood water earmarked for the lower river and lakes. Generally and mistakenly, the opinion is that it will ‘save us all’ and negate the need to do more than to give thanks.
Some have thought it through and commented how it may not be enough, but it’s better than nothing, while others have dared to recognise that a good percentage of flow into South Australia will not make it into the Lower Lakes, but they concede that some may help.
Albeit, the decision makers have decided they won’t have to commit to a weir (a lock with access), whereas, the structure should have already been built to regulate flows from the river into the lakes, to limit riverbank slumping by maintaining higher river levels and to assist food commitments upstream.
Those in charge omitted to mention that each gigalitre (one km sq x one mtr. deep) of water flowing into the lakes at Wellington, will contain a massive volume of salt from the basin.
Given that salinity levels in the river at Wellington were 1000 parts per million, there would be in excess of 1,000 tonnes of salt dissolved in it.
This current spike of water (say 500-600GL) on its way downstream into South Australia with the lower lakes in mind is likely to contain about 500,000 tonnes of salt when it reaches the lake and this amount of water won’t be enough to fill Lake Alexandrina, even to half capacity.
To deal with the crisis today, river flow would have to be of sufficient volume to both fill the lakes with more than 1000 GL (capacity at least 3,500 GL) plus a similar volume again to enable flushing the contaminated system out to sea to have any chance of removing some of the salt. The Government recognises (Futures Paper) that 3,500 GL are needed in the lakes to do this.
That amount of water is not available in the system and is not likely to be available in the foreseeable future, with no allowances for growing in the food bowl upstream.
The salt from the current and expected flows will sit in the lakes, only to concentrate with no escape, and with protracted drought and next summer’s evaporation, salinity levels will become even more extreme. There is no way to rid the salt from the lakes as they are. To deliver much of South Australia’s ration to the lakes, where some suggest it is sorely needed, is irresponsible. The pending flows should be kept in the river channel as much as possible. Currently, these flows are being denied access to the backwaters and wetlands. With a weir in place, we can exhaust salt out of the river with every flow event, into a tidal lake system and out to sea.
The reality of the situation is that rivers, wetlands, backwaters and billabongs must be restored as a matter of urgency to provide habitat for the freshwater riverland species now in decline. They are in decline because of our determination to deny riverside wetlands of water so we can deliver it via the river into the lakes. Currently, once water passes Lock One at Blanchetown, it is free to migrate into the lower lakes where it is lost from the river forever.
A permanent regulatory weir should already have been built in the vicinity of Wellington, as originally proposed by the State Government, whereby a portion of the current flow could have been retained in the 200km pool between Wellington and Blanchetown, to be used in part to restore riverside wetlands.
We are unable to maintain freshwater wetlands in the lower lakes except for small refuges where Lofty Ranges watersheds enter the lower lakes. Current circumstances reinforce that.
Exhausting salt from the lower lakes is not possible from foreseeable flows. The only option now is to build a permanent weir near Wellington and develop a tidal lower lakes system so that river flow can be used to restore and support a healthy river environment while tidal flow would maintain a biologically productive lower lakes estuarine system. Without the weir, conditions from acid mobilisation will also worsen and it must be remembered that about 1000 GL or so will only top the lakes at barrage height and not allow for necessary flushing.
To say that 400 GL or even 600 GL will fix things is a long way from the mark in this crisis. This promised water is not intended as a single slug and we must remind ourselves that evaporation rates from the lakes per year alone are from about 740 GL to 950 GL per year.