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September 13 2002

The greatest River Murray flood eclipses 1956 levels

Described as the greatest catastrophe in South Australia’s history, the 1956 River Murray flood is the largest ever recorded in our state.  Now UniSA researchers are causing more than a ripple with a new wave of investigations revealing that a prehistoric River Murray flood of much greater magnitude surpassed the biggest flood on record.  

With records dating back only to recent times, honours students Amanda Sulley and Derek Snowball from the School of Environmental and Recreation Management undertook research, using the Black Box gum (Eucalyptus largiflorens) to interpret the pre-1840 salinity and flood history of the river. 

While data that can extend the flood records is valuable, little work on these important phenomena has been undertaken in Australia. 

Amanda and Derek’s research, funded by UniSA’s Division of Information Technology, Engineering and the Environment, uncovered valuable data that more fully reflects the Murray’s flooding history, according to Adjunct Professor Robert Bourman who supervised the study with Ms Joan Gibbs and Dr Fleur Tiver from the School. 

To establish the reliability and timing of pre-historic floods, known as palaeofloods, the researchers looked at the distribution of the Black Box Gum in the River Murray valley..

“Black Box is considered to be a reliable biological indicator of past flood levels because it grows in distinct horizontal lines on the River Murray floodplain. Its seeds germinate in the debris deposited on the floodwater fringes of the riverbank,” Professor Bourman said. 

“Radiocarbon dating of samples collected from existing gums revealed that the trees were of a modern age, with establishment in the last 250 years. This gives us an indication of the possible timing of the pre-historic flood of around the year 1750. 

“The researchers also undertook a survey to obtain the heights of individual trees at their bases. This showed that the palaeoflood reached a maximum height on the River Murray at Overland Corner of 18.01 metres, making it greater than the largest flood on record, rising 2.11 metres above the 1956 flood height. 

“Having measured the cross-section of the river, they applied the Manning Equation to determine the discharge of the prehistoric flood. This was estimated to be 7,686 cubic metres per second, almost double the discharge of the 1956 flood, which measured 3,950 cubic metres per second,” Professor Bourman said. 

“Given the calculated discharge and proposed age of the flood, the students were able to calculate when on average a palaeoflood might return. This was measured at 1000 years. This means that every year there is a 1000 to one chance of a flood of that magnitude occurring, as it certainly will, at some time in the future. 

“The findings of this research have major implications for the wider community. They show that houses, shacks and other structures even above the 1956 level are at risk.” 

Professor Bourman believes that policies should now be put in place that limit construction of permanent structures within the flood prone area from the river channel extending to the palaeoflood level, to avoid possible damage and costs caused by extreme flooding. 

So what’s the good news on floods? 

Flooding should not be seen as disastrous for the River Murray but as an important part of the natural system, and as a partial solution to salinity, according to Ms Gibbs. “Extreme floods are nature’s way of flushing out the salt,” she said. 

“A healthy river should have salinity levels of less than 25,000 parts per million. Medium level flows of water through the River Murray are needed to control salinity levels. 

“Sadly, the River Murray has high salinity levels averaging 34,000 parts per million caused by the removal of 80,000 to 100,000 megalitres of water each day for irrigation. This is severely affecting vegetation, causing salt stress and destroying the river’s aquatic ecosystem. 

“Amanda studied the effects of varying salinity and flooding levels on the seed germination, root growth rates and seedling development of three Black Box Gum varieties. This was necessary to establish the conditions required for the gums to germinate and grow,” Ms Gibbs said. 

“Her research showed that while adult trees were tolerant of saline conditions, juvenile trees were very salt intolerant. The Black Box seeds germinated in different saline conditions but even at low saline levels the seedlings were not healthy until flushed regularly with fresh water. Flooding is essential for juvenile Black Box trees to grow, to flush salt from their roots.  

“If you think that the prehistoric River Murray flood was the greatest catastrophe in South Australia’s history, the River Murray’s high salinity levels could be an even greater catastrophe affecting all of us in the near future,” Ms Gibbs said. 

Media contact: Geraldine Hinter (08) 8302 0963 or 0417 861832





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