Some Palaeoflood Indicators in the River Murray Valley of South Australia

The 1956 River Murray flood is the largest recorded South Australian event (3,956 cumecs, R.I. 160 years). The greatest flood previously observed (3,690 cumecs) was in 1870. However, palaeoflood indicators reveal floods exceeding that of 1956. Fluvial sediments intercalated with Aboriginal occupation layers reveal a flood peak, some 3,000 years ago, exceeding the 1956 level. Moreover, large logs of Eucalyptus camaldulensis dated at ~ 2,500 years BP, and washed into a cave some 8 m above normal river level, suggest palaeofloods at least as great as that of 1956. Eucalyptus largiflorens is an excellent palaeoflood indicator as its seeds germinate prolifically at the top fringe of floodwaters. A former flood level indicated by E. largiflorens reveal a flood approaching twice the discharge (7,350 cumecs; R.I. 1,000 years) of the 1956 flood. Radiocarbon dating of heartwood of E. largiflorens from this flood level is compatible with a major flood occurring about 1760. Statistical analysis of a flood record > 100 years, utilising the log-Pearson Type III distribution, is compatible with palaeoflood data. Some can be calibrated with major flood events of the Burdekin River, recorded as organic fluorescent layers in corals of the Great Barrier Reef. The palaeoflood data has extended the record of flood events well beyond historical time, providing useful information on the magnitudes of low frequency flood events. Regulatory structures and water abstraction have reduced the impacts of small floods, but dangers from large infrequent events remain.