Geomorphic analysis of channel change and erosion processes contributing to avulsion in the Macquarie Marshes, Australia

Geomorphic processes such as erosion play a critical role in the development of existing river channels, as well as the formation of new channels by avulsion in multi-channelled systems. The Macquarie Marshes is an anastomosing and distributary system that supports extensive floodplain wetlands through overbank flooding and channel erosion poses a threat to these sensitive ecosystems. Four sites within the Southern Macquarie Marshes were investigated to identify the dominant patterns, processes and rates of erosion that contribute to channel change and to establish an evolutionary sequence of channel development. Channel incision and narrowing were the dominant processes measured together with knickpoint retreat at the three less mature sites, while channel widening and stabilisation were characteristic of the most mature site. Significant differences in channel morphometrics including width, depth and cross sectional area were found at the four sites using data from 2008 and 2014. The sites and their erosion processes fit into a geomorphic and evolutionary sequence: (1) knickpoint retreat and incision in a small swamp outflow channel (Buckiinguy); (2) incision and widening in a large swamp outflow channel (Willancorah); (3) minor incision and widening in the upper and lower reaches of a shallow, continuous marsh channel (Pillicawarrina); and(4) minimal erosion and channel stabilisation in a well-established, continuous marsh channel (The Breakaway). Bank strength is a key factor that contributes to erosion potential in the system, and sediment moisture content affects bank strength. Other factors (organic content, sand, silt and clay content, dry bulk density) have very weak relationships with bank strength. A trend was found where more mature sites have higher bank strength, probably related to the extent of past erosion. A conceptual geomorphic model shows how erosion affects the development and formation of new channels during avulsion, and the effects on longitudinal and lateral connectivity in systems like the Macquarie Marshes, by redistributing water and sediment within the wetlands. The formation of channels by erosion plays a critical role in the evolution of anastomosing and distributary systems.