The Barwon River, New South Wales : a study of basin fill by low gradient stream in a semi-arid climate.
Creators: name: Taylor, Graham Murray
Publisher: name: The Australian National University
Year: 1976
Subjects: subject: Sedimentation and deposition
subject: Barwon River (Qld. and N.S.W.)

description: 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. These multiple channel complexes show that streams in the basin have been similar to the present ones over an extended period. Estimates of discharges of the former channels reveal that discharge varied with maxima up to three times those of the present and that sandy phases usually carried the greater discharges. The channel deposits of these streams (ancient and modern) form restricted sandy concave lenses (Vauxhall Formation) partially filled with sandy muds (Barokaville Formation) and both are included in and overlain by dark muds (Walgett Formation). The Walgett Formation represents the floodbasin or "overbank" deposits of all the channel complexes. During the Late Cretaceous and Palaeogene the Upper Darling Basin received little or no sediment and the basement was undergoing intense chemical weathering under the influence of dominantly warm and humid climates with rainforest vegetation. Silcrete formed at or near the surface during this time. This was followed by a short arid spell in the late Miocene when the Cumborah Gravel was deposited. The Cumborah Gravel is a thin (less than 4m), widespread sandy conglomerate unconformably overlying the altered Cretaceous sediments. Warm humid conditions again prevailed in the early Pliocene and chemical weathering and formation were again widespread. This was followed by a marked deterioration in climate and the retreat of the dense rainforest vegetation from the area. This together with an uplift of the Eastern Highlands was the impetus for commencement of erosion and consequent deposition in the Upper Darling Basin. Three major fluviatile formations are recognised. The Come- By-Chance Formation, a fluvial interbedded sand and mud unconformably overlies the Cumborah Gravel. It represents the distal facies of a prograding fluvial sheet deposited under climatic conditions similar to the present. This is overlain apparently conformably by the Warrena Formation, a sandy unit representing the proximal fluvial facies which prograded out over the plains during the ?early Quaternary. This is disconformably overlain by the Walgett Formation. The mineralogy of the valley-fill reflects the earlier history of the area, the earlier sediments being kaolinite-rich and the later ones montmorillonite-rich. The kaolinite was produced from the deeply weathered Mesozoic and Palaeozoic basement and Cainozoic basalts, however as erosion continued the weathered profiles were stripped and weathering under the cooler drier climates of the Pliocene and Quaternary produced mainly montmorillonite. A brief examination of erosion^r ates shows that at present between 0.04 and 0.01 mm year- are being removed from the Eastern Highlands, and that about 9 5% of this is being deposited in the Upper Darling Basin. These erosion rates have varied during the Pliocene and Quaternary, although the magnitude of the variation is unknown.
description: v.1. Text -- v.2. 7 folded maps in binder