The mechanism and history of channel avulsion in a floodplain wetland, Macquarie Marshes, N.S.W

The Macquarie River is one of several large inland rivers of south-eastern Australia which experience channel breakdown and distributary development on their lower reaches, forming extensive floodplain wetlands and at times losing all channel definition as flows enter swamps or marshes. Channel breakdown and marsh formation is characteristic of the Holocene drainage systems of lowland interior-flowing Australian rivers that have lower discharges and are mainly dominated by suspended sediment loads. These marsh areas appear to be very dynamic and undergo both frequent growth and abandonment, principally through a process known as avulsion (the sudden abandonment and formation of new channels). These morpho-dynamics continue to have profound effects on the marsh ecosystems and are currently a cause of concern for habitat management and conservation. In order to understand the mechanism and history of avulsion in an inland fluvial wetland system, 210Pb and AMS 14C dating were used in conjunction with sedimentological analysis to document the rate and distribution of sedimentation adjacent to Buckiinguy Break, a distributary marsh feeder channel in the southern Macquarie Marshes. The rates of sedimentation determined from the 210 Pb analysis of six sediment cores show significant trends in the spatial distribution of recent sedimentation in the Buckiinguy marsh system. Recent sedimentation was greatest near the channel margin and decreased with distance across the floodplain, leading to the formation of contemporary levees on the channel margin. Results show that avulsion in this system is related to extensive in-channel and overbank deposition resulting in the formation of an alluvial ridge, which elevates the channel above the surrounding floodbasin. A gradient advantage is thereby created between the channel and floodplain, providing favourable conditions for avulsion and subsequent channel abandonment if flows are preferentially diverted to the lower surrounding floodplain. Differences between the contemporary and long-term sedimentation rates and historical evidence indicates a recent change in sedimentation regime, which is related to avulsion of Buckiinguy Break 70 to 90 years ago, rather than to catchmentscale post-European influences. In-channel and floodplain vegetation promotes sediment deposition out of suspension and therefore also plays a critical role in the avulsion processes observed.