Quaternary palaeochannel morphology as a function of palaeohydrology, SE Australia

The Murray-Darling Basin (MDB) of south-eastern Australia plays host to a vast array of river channels both modern and relict, comprising multiple distributive fluvial systems (DFS). On such systems in the MDB, morphologic and sedimentologic contrasts between contemporary channels and their respective palaeochannels have been utilized as evidence for hydrologic and climatic change throughout the late Quaternary, with prior studies suggesting palaeochannel evolution under a vastly larger discharge regime. This study mapped the bankfull width and meander wavelength of the modern and remnant Murrumbidgee and Macquarie rivers to determine the downstream evolution of (palaeo)channel morphology. Successive downstream channel measurements permitted a regression analysis which gave the approximate bankfull channel size at the apex of the DFS, alongside the rate of channel change with distance downstream. A transfer function was applied to estimate bankfull discharge from the head of the DFS, moving downstream in reaches. Downstream channel trends suggest the surveyed palaeochannels to be of similar planform to their modern rivers – single channel, laterally unconfined and of variable sinuosity – but operating on much larger scales. Comparison between the modern and surveyed palaeochannels suggests palaeochannels were formed under larger and highly variable bankfull discharges that dissipate with distance downstream and show agreement with a temperature-driven model that increased seasonal snowmelt and enhanced runoff efficiency during the last 20 ka to 80 ka.