Sediment dates with implications for the age of the conversion from palaeochannel to modern fluvial activity on the Murray River and tributaries

Dates for sediment deposition have been obtained for a number of abandoned channels with modern features on the Upper Murray, Kiewa, Ovens and Goulburn Rivers, and from palaeochannel deposits on the Upper Murray River just west of Albury. The dates from modern channels on the Upper Murray are problematic, but suggest modern fluvial activity had begun by 5-9 ka. Two dates from the Ovens and Kiewa Rivers are in agreement with this. West of Albury, a palaeochannel point bar returned an age of 7.4-11.6 ka, and a source bordering sand dune ˜22.5 ka. Sand beneath the floodplain near Albury was aged 7.3 ka, and was not deposited at the same time as underlying gravels, where two embedded trees returned ages of 12 ka. It therefore cannot be established if the sand deposit was associated with either modern or palaeochannel activity. Therefore, on the Upper Murray River the transition from palaeochannel to modern fluvial activity most likely occurred between 7 and 10 ka. In contrast, two dates from an abandoned channel on the Goulburn River indicates that modern channels were active on this river before 11-13 ka. This suggests that palaeochannels were active later on the Upper Murray than on the Goulburn River, and compared with previous work, possibly later than on the Murrumbidgee River and the middle Murray River near Echuca. As upper catchment areas were revegetated following the LGM, discharge or sediment supply may have declined first in lower altitude catchments, like the Goulburn, and last in the high-altitude Upper Murray catchment, creating an asynchronous transition from palaeochannel to modern fluvial activity in subcatchments of the southeast Murray drainage basin.