Is the Murray River flowing back to the future?
The South Australian Department of Environment and Water is reportedly excited that the Lower Lakes and the Coorong are “heading back towards their natural best”.
The Murray-Darling river system has highly variable water flows and is ecologically adapted to prolonged drought. On his expedition in the 1830’s, Thomas (Major) Mitchell reported brackish water in the river which is an indication that its background hydro-geological complexity has been known if not well understood since Europeans arrived.
Soil loss and nutrient-rich runoff from grazing and cropping contribute to the nutrient load in the rivers. A lot of these nutrients, especially phosphorus, is stored in river sediments from where it enters the water column and ‘fuels’ algae blooms, depending on light and temperature.
Surface waters, including streams and lakes are a reflection of their catchment conditions. Sometimes environmental effects occur a long way from the cause and long after events such as floods which transport the bulk of nutrient loads into and within rivers. Nuisance weed and algae growth (eutrophication) of adjacent lakes and wetlands will not be prevented by a minor increase in river flows. Wetlands actually benefit from seasonal wetting and drying.
The natural forces we have released may not be completely reversible but remedial action must be based on a good understanding of the problem. The idea that rivers are like dunnies and a good ‘flush’ now and then will fix things up is a furphy. It’s a bit like trying to cure diabetes with a drink of water.
The legacy of our ecocidal culture is plain to see in many parts of Australia. I said that occasionally ‘flushing’ a river is like trying to cure diabetes with a drink of water. Taking the analogy further, rivers are the ‘veins’ of the landscape, carrying suspended and dissolved matter towards the sea from where the water is ‘pumped’ around again.
The purpose of returning more water to the rivers is to restore and maintain ecological values. But the fact that stretches of the rivers return to a degraded condition not long after flooding shows that increased flow by itself, natural or otherwise, will not restore the river to long-term good health.
Random returns of nutrient-rich irrigation runoff water to the rivers might actually ‘trigger’ algae blooms if environmental conditions happen to be favourable. Storing return water for later release in a coordinated way to imitate natural seasonal flooding would be better value for money than the present ad-hoc arrangements.
Max Thomas, Dip. Agric. (retired) worked in the public sector and in private consulting on a range of land, water and waste management projects. He prepared guidelines for irrigation with recycled water for EPA Victoria and developed a number of Environmental Management Systems in the water industry.
