Late Quaternary palaeohydrology of the Macquarie River and the Murray-Darling Basin

More than 40 palaeochannels of the Macquarie River are preserved on its alluvial plain in northern NSW. We developed empirical relationships between modern river gauge discharges and channel dimensions (width; meander wavelength) from the Murray-Darling basin to estimate palaeodischarges of the Macquarie palaeochannels. The largest of these channels carried a peak (bankfull) discharge around 1000 m3s-1, more than 60 times that of the modern river. The same relationship was applied to other MDB palaeochannels. Northern rivers (Gwydir, Macquarie, Namoi) were found to have much higher palaeo to modern discharge ratios than the southern rivers (Murrumbidgee, Murray), with Lachlan river palaeochannels forming an intermediate group. The small, modern rivers in the north all show channel breakdown and wetland morphology in their lower reaches. When all palaeochannels are normalized to the discharges of the modern rivers at the point at which they enter their alluvial plains, the differences between the northern and southern rivers disappear. The northern catchments have seen a threshold transition to channel breakdown and floodouts in the late Holocene. Single-grain OSL ages from three conspicuous, and larger, Macquarie palaeochannels have yielded minimum age model estimates ranging from around 40 ka to around 20 ka. Combining these palaeodischarge estimates of the dated Macquarie channels with published ages for other MDB palaeochannels and our new palaeo-discharge estimates, some trends are evident. Discharge was highest in early or mid MIS3 (except Murray/Goulburn), decreasing in late MIS3, then remaining steady into MIS2 before decreasing greatly in the Holocene. Data in MIS 4 and 5 is too sparse to see any overall pattern. However, the large discrepancy between single grain and single aliquot OSL ages for the Macquarie channels suggests there may be some substantial revision of other palaeochannel ages in the future once single grain techniques are applied to these systems.