Episodic fluvial activity of inland rivers in southeastern Australia: Palaeochannel systems and terraces of the Lachlan River

Abstract

Inland rivers in southeastern Australia preserve a long record of surface palaeochannels, accompanied by alluvial terraces that reflect changes in water and sediment discharges from the southeastern highlands through much of the last glacial cycle. A compilation of ages on rivers of the southern Murray-Darling Basin together with new dates from the Lachlan River suggest that the fluvial response to climate change has been regionally consistent and responsive to climate change on timescales <10 ka. For much of the last glacial cycle, inland rivers were mixed-load, sinuous single and anabranching channels that carried bankfull discharges of between 3 and 11 times present day rivers. Incision of the upper alluvial plains and valleys occurred in the lead up to the coldest part of the last glacial cycle. This was followed at 34 ka by the synchronous development of large meandering channels that may have been associated with lower evapotranspiration from alpine highland catchments accompanied by higher precipitation. Smaller channels and declining lateral channel sedimentation suggests arid climates during the last glacial maximum and late glacial period with seasonally large, actively migrating channels being maintained in rivers dominated by snowmelt runoff from the Australian Alps. During the Holocene, sensitivity to recorded changes in precipitation amounts and variability differed between catchments with some rivers constructing larger or more actively migrating channels in the early to mid-Holocene.