Trunk stream evolution from lateral to vertical accretion and channel breakdown in the Holocene Macquarie River, eastern Australia

Channel breakdown and floodplain wetlands are common features of lowland rivers in the Murray-Darling Basin, eastern Australia, in the Holocene (e.g.Gwydir, Macquarie, Lachlan). The lower Macquarie River is characterised by four discontinuous, meandering trunk stream reaches separated by areas of channel breakdown and unchannelised wetlands in the Macquarie Marshes. Channel breakdown is driven by in-stream sedimentation and avulsion, while trunk stream reformation is triggered by steepened floodplain gradients at the downstream ends of floodouts and wetlands.
Morphometric analyses, sedimentology and geochronology of the discontinuous reaches of the Macquarie River indicate that the trunk stream was characterised by lateral migration in the mid-Holocene, with meanders and cut-offs developing until at least ~4.8 ka. More recently (since ~1 ka), low levees have developed over lateral migration features (e.g. scroll bars), reflecting a shift in the dominant style of channel adjustment from lateral to vertical accretion, with periodic avulsion. An upward-fining sediment sequence in a modern point bar also shows a transition to a lower-energy environment in the last ~0.8 ka. Avulsion appears to have increased the degree of longitudinal connectivity in the system by short-circuiting some reaches that were previously separated by channel breakdown and unchannelised wetlands. A pilot study using Uranium-series disequilibrium methods suggests a significant reduction in sediment residence time in the modern channel (58 ± 2 ka) compared with a meander cut-off (153 ± 5 ka) abandoned next to the trunk stream ~4.8 ka. These changes in channel morphology and processes of fluvial adjustment may be caused by a combination of local channel change and catchment-scale change in sediment dynamics and hydrology, and may be reflected in other rivers in the Murray-Darling Basin.