Channel bifurcation and adjustment in multi-channelled floodplain wetlands
Multi-channelled rivers and floodplain wetlands are geomorphologically dynamic systems that provide vital resources and habitat. River bifurcation is a critical element of channel formation and adjustment in these systems. Bifurcations may be stable or unstable, leading to anastomosing reaches and also wholesale avulsions that create new channels. The division of discharge, stream power and sediment load between channel branches may be uneven and change over time. Depending on the spatial patterns of branches and the associated processes of channel adjustment, either continuous or discontinuous channels will occur that define the structure of floodplain wetlands into the future. New evidence from the Macquarie Marshes, a large, low energy, floodplain wetland system in semi-arid Australia, demonstrates the importance of bifurcations and return points leading to maintenance of channel capacity despite loss of flow to the floodplain. Estimates of bankfull discharge and unit stream power suggest that channels are able to effectively transmit water and sediment to downstream reaches until a threshold is crossed whereby channels become increasingly inefficient and decline in size rapidly downstream. This ultimately leads to channel breakdown where channels cannot be maintained and where water floods out onto alluvial surfaces at channel termini. The historical trajectory of channel behaviour in hyper-avulsive reaches of this system showed fluctuations in channel capacity adjustment from 1992 to 2018, including large variations in bankfull width, depth and cross-sectional area, as well as bankfull discharge and unit stream power. Erosion is central to the formation of bifurcations and avulsion processes in the Macquarie Marshes. Analysis of erosion risk showed that despite channel ranking according to likelihood and consequence of channel change, few sites experienced significant changes between 2012 and 2018. Overall, understanding the biophysical character and behaviour of multi-channelled floodplain wetlands is important for water resources and environmental management.