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