Late Quaternary changes in flow-regime on the Gwydir distributive fluvial system, southeastern Australia

•We use 30 single grain OSL dates to derive a chronology of hydrological change.•Two palaeochannel systems are shown to dominate the surface of the Gwydir megafan.•The ‘Coocalla’ and ‘Kamilaroi’ systems date from 43–34 ka to 19–16 ka respectively.•Synchronous river response across SE Australia in the late Quaternary is indicated.

Ages for large palaeochannels of the Gwydir distributive fluvial system (DFS) in northern New South Wales, Australia have been determined using single grain optically stimulated luminescence. Two palaeochannel systems have been found to dominate; the here named Coocalla (43–34 ka) and Kamilaroi (19–16 ka) which have inferred palaeodischarges 25–100 times the bankfull discharges of nearby channels of the contemporary Gwydir system, which appears to have been established during the Mid-Holocene. This scale differential is very much larger than that reported for other catchments in southeastern Australia, and reflects both a decline in catchment runoff through the Last Glacial cycle and the adoption of a distributary pattern sometime after 16 ka. Actual decline in catchment runoff, determined by comparing estimated palaeodischarge with contemporary flows upstream of the DFS where flow is confined to a single channel, indicate contemporary discharge to be 0.1 times and 0.25 times that of the Coocalla and Kamilaroi, respectively.The chronology presented here shows periods of increased discharge in the Gwydir to be more or less coincident with those observed elsewhere in the Murray Darling Basin. Although no evidence of a ‘Gum Creek’ fluvial phase (from 35 to 25 ka) was found, the Coocalla and Kamilaroi palaeochannel systems broadly conform in age to ‘Kerarbury’ and ‘Yanco’ fluvial phases on the Murrumbidgee and Murray systems. This synchronicity with more southern catchments supports the hypothesis that La Nina – like conditions were semi-permanent for much of the Last Glacial cycle with moisture derived largely from the western Pacific Ocean.