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Australian National University (1976)

The Barwon River, New South Wales : a study of basin fill by low gradient stream in a semi-arid climate

Taylor, Graham Murray

Titre : The Barwon River, New South Wales : a study of basin fill by low gradient stream in a semi-arid climate

Auteur : Taylor, Graham Murray

Université de soutenance : Australian National University

Grade : Doctor of Philosophy (PhD) 1976

Résumé
The Barwon River, northern New South Wales, flows through semi-arid alluvial plains with a gradient of 5 x 10⁻⁵ . Its discharges are highly variable and irregular, varying between 0 to 1,500 m³s⁻¹ . The system is essentially anabranching with a series of highly sinuous, deep and narrow trunk streams. The Barwon and its tributaries (the Namoi, Gwydir, Castlereagh and Macquarie Rivers) in their lower reaches all carry around 80 percent of their load in suspension. The rivers are at present depositing laminated sediments with suspended-load characteristics, on benches at bends and along straight reaches within the channel. The deposits typically contain parallel laminae (flat and wavy) , which drape previous deposits and conform to the contours of the banks. These deposits accrete both vertically and, more importantly, laterally. They develop on both sides of the river, both in straight reaches and in bends. These laminated sediments commonly overlie and grade downwards into sandy bed-load deposits forming near the bottoms of channels. Bed-load activity during the suspended-load phase is restricted to low parts of the channel ; however, during early phases of development of these channels, bed-load is dominant. These bed-load channels contain sandy sediments exhibiting features consistent with their development by lateral accretion, as is common for meandering bed-load streams. Such sediments underlie the laminated sediments (and their associated sand base) at various points along the Barwon. They can be shown to be related to an ancestral channel of the Barwon. These observations provide the basis for a conceptual model of suspended-load channel development. During suspended-load phases, a stream may become restricted by lateral and vertical deposition from opposite banks, by vegetation growth in the channel, by reversal of bed gradients over long reaches and by debris clogs. Once this restriction occurs, an anabranch may develop to bypass the restriction. This bypass, during initial stages, will be relatively straight and small. As the bypass increases in size, sand is eroded from the surrounding sediments and is in part collected in the channel, and as the anabranch develops it gradually behaves as a bed-load channel. Sediments accumulate by lateral accretion and suspended-load is flushed through the channel, not depositing, except in overbank situations. Gradually sinuosity increases, reducing gradients, and this, together with the restriction which occurs at the point of entry of the anabranch to another channel, effectively dams the anabranch. Damming causes the initiation of suspended-load activity and causes an increase in sinuosity and reduces gradient which results in a "fixing" of the bedload material by clay drapes. The anabranch hence gradually develops into a suspended-load channel similar to its parent. The relict channel below the new offtake is gradually plugged by muds, and although it may carry water for long periods, it becomes a minor factor, only flowing at high flood stages. Many such channel complexes are preserved across the alluvial plains of the Uoper Darling Basin. Up to twelve channel complexes are recognised, four, which are directly related to the modern system, show evidence of bed and suspended-load phases. Some of the earlier channels contain sand only. Others, although recognisable on the surface, are too incomplete to yield much data.

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