Decline and fall

Rivers in sub-humid to arid lowland basins commonly suffer downstream declines in discharge and channel capacity owing to transmission losses and a lack of tributary inputs. The lower basins therefore have a tenuous dependence on upstream flow contributions for ecological health and the provision of ecosystem services. Palaeochannels of these lowland rivers provide a means of investigating the sensitivity of river response to climate-driven hydrological change. The Macquarie River in the Murray-Darling Basin (MDB) of southeastern Australia today breaks down to form the extensive Ramsar Convention-listed floodplain wetlands of the Macquarie Marshes but around 80 palaeochannels are preserved on the alluvial plain recording the evolution of this distributive fluvial system (DFS). Single-grain optically-stimulated luminescence ages from six conspicuous Macquarie palaeochannels yielded minimum age model estimates ranging from 54 ka to 2 ka and, together with observed cross-cutting relationships, constrain the evolution of the DFS over the last 54 ka up to the formation of the modern river and wetland system. The largest of these palaeochannels (Quombothoo, mean age 54 ka) was 284 m wide, more than 12 times wider than the modern river (24 m) and with 21 times greater meander wavelength. A younger group of palaeochannels, Bibbijibbery (125 m wide, 34 ka), Billybingbone (92 m, 20 ka) and Milmiland (112 m, 22 ka), although smaller, were still much larger than the modern river and continuous all the way to the Barwon-Darling River. The mid-Holocene (5.6 ka) Mundadoo channel was four times wider (86 m) and had meander wavelength double that of the modern river, whose earliest sediments are dated to around 5.2 ka. The morphological changes induced by the decline in river discharge were profound and non-linear. The 5.5 ka transition saw a fundamental change in river behaviour, leading to the formation of the ecologically important Macquarie Marshes but also decreased contribution of water and sediment to the Darling River. This trend is similar in timing and scale to the Gwydir River, with its Holocene wetlands.