Geomorphic environments, drainage breakdown, and channel and floodplain evolution on the lower Macquarie River, central-western New South Wales

The Macquarie River of eastern Australia is an inland draining perennial system whose lower reaches flow across a large, low-gradient alluvial plain without tributary input. Failure of the Macquarie to sustain its channelled meandering course results in channel breakdown and the development of a broad distributary pattern characterised by a network of discontinuous anabranching and distributary channels and associated permanent and semi-permanent wetlands. The location of the channel breakdown region results from a combination of channel hydrology and sediment load. However, at the local scale, the complete breakdown into unchannelised marshes is influenced by the degree of topographic confinement produced by surrounding paleo-alluvial ridges. Several distinctive marsh channel types are identified on the lower Macquarie, each playing a different role in the process of channel and floodplain evolution. In general, new marsh channels are initiated by avulsion and incision into the floodplain surface and then gradually aggrade until a new avulsion results in abandonment, typically over a time-scale of ~100 a. Floodplain development occurs as a series of spatially discrete sediment lobes, each of which are deposited in permanent swamps immediately downstream of the channel breakdown. The swamps trap almost all of the sediment delivered to them. Despite this, low suspended sediment concentrations result in slow rates of floodplain accretion. Lobe accretion is terminated by channel avulsion through low levees at the channel margin. Avulsion and channel breakdown at the local scale are driven by the characteristic nature of the dense in-channel reed vegetation which chokes the streams and leads to in-channel aggradation, reduced fluvial efficiency and frequent overbank flows that ultimately lead to the development of new channels.19 page(s