Cutting down, back and out: assessment of channel erosion in a sensitive floodplain wetland

Processes of erosion influence channel formation and evolution in rivers and floodplain wetlands. However it is often difficult to determine the role of different erosion processes such as channel incision (cutting down), bank widening (cutting out) and knickpoint retreat (cutting back) when they co-occur in a dynamic fluvial system. Knowledge of the degree and severity of erosion is necessary for river and wetland management. In floodplain wetlands like the Macquarie Marshes, New South Wales, erosion plays a pivotal role in the formation of channels and avulsion (relocation of channels on the floodplain), with some potentially adverse impacts on the surrounding wetlands, such as reductions in flooding. We assessed the type and rate of erosion processes in an active channel that drains a wetland with high ecological value using a combination of topographic surveys, erosion pin surveys and fixed-point time-lapse photography. Bed incision, bank widening and knickpoint retreat occurred together during a flood pulse over a period of three months. The spatial patterns of erosion were variable, but topographic surveys were the best method for assessment of changes in bank morphology. The maximum rate of channel widening recorded was 2.6 mm d⁻¹ (8.7 % change) and the maximum rate of channel incision was 0.9 mm d⁻¹ (25.3 % change). Channel erosion is likely to pose a serious threat to wetland ecological values if thresholds of channel and wetland stability are breached, leading to extensive channelisation and drainage. Knowledge of erosion processes, the broader spatial patterns of erosion and the effects of environmental flows are important to inform water and land management strategies in sensitive floodplain wetlands.