Downstream declines in discharge and stream power are inherent features of many Australian lowland rivers that experience channel breakdown and floodplain wetlands on their lower reaches. The Macquarie Marshes are an alluvial system of anastomosing and discontinuous channels with wetlands on the lower Macquarie River. While contemporary processes of avulsion in the system are relatively well understood, this research investigated mechanisms of channel discontinuity (i.e. channel breakdown and reformation) and channel adjustment in the southern Macquarie Marshes. Morphometric analysis of discontinuous reaches of the Macquarie River using digital terrain data derived from a light detection and ranging (LiDAR) survey was coupled with sedimentological analysis and optically stimulated luminescence (OSL) dating of a late Holocene meander cut-off to determine the characteristics and timing of channel adjustment. Sinuosity declined from X to XX, channel depth declined from X to XX, and cross sectional area declined to less than 10 m2 in the breakdown zone of each reach. Channel reformation occurs where floodplain gradients allow small tributaries to rejoin and form another section of the trunk stream. Sediment deposited in the breakdown zone of each reach forms a lobe-shaped splay with a steepened downstream end, which can instigate knickpoint retreat and reconnection of reaches of the river. Overall, there has been a shift from a combination of lateral and vertical accretion as a means of channel adjustment to a dominance of vertical accretion with periodic avulsion in the system. The cut-off was actively meandering 1.25 ± 0.199 ka, so this shift has occurred within the last ~1000 years and is unrelated to European settlement and disturbance in the catchment. Long-term patterns of river adjustment can provide insight into the response of systems like the Macquarie Marshes to future changes in sediment regime or hydrology related to human impacts or climate change.
|Title of host publication||15th Australian and New Zealand Geomorphology Group Conference|
|Subtitle of host publication||program and abstracts|
|Number of pages||1|
|Publication status||Published - 2 Dec 2012|
|Event||Australian & New Zealand Geomorphology Group Biennial Conference (15th : 2012) - Bundanoon, NSW|
Duration: 2 Dec 2012 → 7 Dec 2012
|Conference||Australian & New Zealand Geomorphology Group Biennial Conference (15th : 2012)|
|Period||2/12/12 → 7/12/12|
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Wetlands in Drylands: conservation through environmental research, citizen science and global engagement
Tim Ralph (Participant)
Impact: Science impacts, Environment impacts, Policy impacts, Society impacts