Origin and structure of riverine wetlands in the Murray-Darling Basin: the example of the Macquarie Marshes

John Oxley sailed down the Macquarie River in 1818 until it disappeared into a sea of reeds. He insisted that only the impassable reeds separated him from the inland sea he sought. The Macquarie Marshes, on the Macquarie River of northern New South Wales, are representative of riverine wetlands (marshes) found on many rivers of the Murray-Darling basin. Their widespread distribution and characteristic fluvial forms seem to have escaped notice and previous explanations for the origins of individual marsh systems concentrate on local causes. These mostly rely on damming mechanisms; in the case of the Barmah Forest damming of the Murray River is known to have occurred through fault movement and the Great Cumbung Swamp on the Lachlan River may be the result of trunk stream alluviation and damming (by the Murrumbidgee). By contrast, the fluvial wetlands of the Loddon, mid-Lachlan, Macquarie, Castlereagh, Namoi and Gwydir (and many more smaller streams) share characteristics which cannot be explained by the chance occurrence of local external mechanisms but indicate a common response to hydrological conditions of the Holocene. The Macqaurie Marshes, like other fluvial wetlands of the MDB, result from channel breakdown on a river which suffers downstream decreases in discharge and sediment calibre but a perennial (if erratic) flow. Floodplain wetlands form where ‘breakaways’ (crevasses) deliver water from the main channel on its alluvial ridge to the floodplain. The attenuated discharges of the lower reaches of the river cause these breakaways to be active almost continuously, giving rise to lush wetlands. Some of these breakaways result in avulsion of the main channel, but others seem to exist as minor distributories for decades. These floodplain marshes neighbouring a continuous trunk stream are only a precursor to the eventual complete breakdown of the channel into a semi-perrenial wetland without continuous channels. This pattern appears to be a response to three factors, (1) the predisposition of the channels to form breakaways and avulsions (anastomosis), (2) the greater expanse of low-lying plain allowing dispersal of the distributaries and their flow and (3) the near-perennial flows lead to vegetation clogging channels and trapping sediment.
The Macquarie River undergoes at least four scales of breakdown at a range of temporal and spatial scales. The main area of marsh occurs where the confining palaeochannel tracts diverge leaving a lowlying plain up to 30 km wide. Only the smallest scale of breakdown leads to complete channel breakdown however these account eventually for the whole discharge of the river. These individual marsh areas have a deltaic form and are themselves arranged in series; breakdown is followed by convergence of flows, channel development, breakdown and so on. Sediment trapped by vegetation in breakdowns may be (in part) re-entrained by the reformed channels. One of the curious features of these riverine wetlands, with their total channel breakdown, is that they are not terminal. All the rivers reform into single channels. The stability of these downstream channels may rely on the retention of suspended sediment in the marshes.