Depositional processes along a very low-gradient, suspended-load stream: the Barwon River, New South Wales

Sedimentary Geology, 22 (1979) 97--120

97

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

DEPOSITIONAL PROCESSES ALONG A VERY LOW-GRADIENT, SUSPENDED-LOAD STREAM: THE BARWON RIVER, NEW SOUTH WALES

K.D. WOODYER 1, G. T A Y L O R 2 and K.A.W. CROOK 2

1 Division o f Land Use Research, CSIR O, Can berra, A.C.T. (Australia) 2 Geology Department, Australian National University, Canberaa, A.C.T. (Australia) (Received January 4, 1977; accepted January 2, 1978)

ABSTRACT Woodyer, K.D., Taylor, G. and Crook, K.A.W., 1979. Depositional processes along a very low-gradient, suspended-load stream: the Barwon River, New South Wales. Sediment. Geol., 22: 97--120. The stratigraphy underlying four Surfaces, within and adjacent to a very low-gradient (5" 10 -s) channel, is described relative to present depositional processes. The two lower surfaces within the channel (called benches) are formed by suspended-load deposition. This is indicative of the low energy gradients (<1.0 • 10 -4) and Froude numbers (<0.1) which characterize within-channel flood waves. On the one hand there is the phenomenon of suspension of the medium sands found in the benches and on the other hand the phenomenon of deposition of the fine suspended load (80--95% < 2 pro). The higher bench is identified as the present flood plain. This level is flooded about once every year on the average. The two higher surfaces are relics, although some deposition occurs on them. Models are presented which see the past phases of river development and deposition as implicit in the hydraulic system. It is recognized that climatic change may play an additional role. Since similar streams and associated paleochannels characterize vast alluviated plains in Australia, these findings have wide application, particularly in relation to paleoenvironmental studies of ancient fluviatile sediments.

INTRODUCTION

Few data are available on the sediments, depositional structures and processes that occur within and adjacent to the channels of very low-gradient suspended-load streams. These streams are characteristic of vast areas of alluvial plains in Australia (Pels, 1964; Bowler, 1967; Schurnm, 1968, Riley, 1973). Such data are valuable for paleoenvironmental studies of ancient sediments. They also contribute to an understanding of the processes that culminate in major channel avulsion, in which the river selects a new channel leaving the old channel as an anabranch. Moreover, these data provide the background necessary to understand the significance of depositional benches

98

within the channel and the frequency with which their surfaces are inundated (Kilpatrick and Barnes, 1964; Leopold et al., 1964, p. 467; Woodyer, 1968). In addition, data from the vicinity of active stream channels provide a base line from which interpretation can proceed outwards to encompass the broad alluvial tract that borders the channels of these very low-gradient streams. Points of interest in this context include the modes of deposition and up-building of the alluvial tract and the age relationships and hydrological significance of various surfaces within the alluvial tract. Recognition of the contemporary flood plain from among these surfaces is one of the questions at issue. This paper describes the sediment and strutures occurring along a 43-km section of the Barwon River. downstream of the Namoi junction (Figs. I and

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2). Data were obtained from surface sites within and adjacent to the channel and from trenches cut in the banks parallel and normal to the channel. GENERAL SETTING

The catchment area of the Barwon River at Dangar Bridge (Fig. 2) is 139,000 km 2. The river rises near the Queensland--New South Wales border at an elevation of 1140 m and falls to 120 m at the Namoi River junction near Walgett. Stream gradients are very low. Upstream from Walgett to Collarenebri (Fig. 1) the general bed gradient is about 13" 10 -s compared with a gradient downstream to Brewarrina of only 5 • 10 -s. This latter gradient is essentially maintained to the Southern Ocean, 2700 river km downstream. The Barwon River flows through alluvial plains and all its tributaries traverse old alluvial plains for a distance of 150--200 km. There are few constraints on the system by way of outcrops of resistant rocks. However, occasional pedogenic calcretes and silcretes form local bars and bank constraints. The plain adjacent to the study section is sparsely vegetated (Fig. 3). Near the stream, river red gum (Eucalyptus camaldulensis Dehn.) grows thickly

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Fig. 3. Vertical aerial photograph (supplied by the New South Wales Lands Department) from Dangar Bridge downstream to cross-section 19 (on hairpin bend) showing the present river channel, remnants of the ancestral stream (cf. inset 1 on Fig. 2), the relatively featureless plain (Walgett Formation and Surface) and the Vauxhall Formation and Surface which occupies much of the points where the river red gum grows thickest.

a n d e x t e n d s d o w n o n t o t h e b e n c h e s o n t h e p o i n t s (Fig. 4). V a r i o u s grasses and h e r b s g r o w o n t h e b e n c h e s and surfaces a d j a c e n t t o t h e s t r e a m . T h e n o x ious w e e d N o o g o o r a b u r r (Zanthium pungens Wallr.) g e r m i n a t e s a n d grows t h i c k l y on t h e b e n c h e s a f t e r s u m m e r floods. Its i n t r o d u c t i o n , p r o b a b l y late in t h e 1 9 t h c e n t u r y , m a y have increased t h e d e p o s i t i o n rate. T h e ti-tree (Melaleuca linariifolia Sm.) b e c o m e s e s t a b l i s h e d in t h e c h a n n e l o f t h e s t r e a m (Fig. 5). T h e m e a n a n n u a l discharge ( 1 8 8 6 - - 1 9 7 1 ) o f 2 . 1 5 8 " 109 m 3 at D a n g a r Bridge (Figs. 2 a n d 3) r e p r e s e n t s an average c a t c h m e n t r u n - o f f o f 15 m m . T h e a n n u a l r u n - o f f f o r t h e s u b - c a t c h m e n t s increases t o w a r d s t h e G r e a t Dividing R a n g e in t h e east f r o m 9.5 m m f o r t h e M o o n i e R i v e r t o 72 m m f o r t h e G w y d i r River.

101

Fig. 4. Point at cross-section 18 looking downstream showing Wanouri Surface (Wn) (lower bench) and Vauxhall Surface (V) (terrace). The Dangar Surface (D) (upper bench) is seen on the left of the photograph just downstream of the point.

T h e flow in t h e B a r w o n is generally low and flow has ceased 22 times in t h e years b e t w e e n 1 8 8 6 and 1971 f o r periods o f u p t o 9 m o n t h s (Fig. 6). F l o o d s are irregularly r e c u r r e n t and p r o l o n g e d . Major f l o o d s are n o t seasonal. F l o o d peaks take 6 0 - - 1 2 0 days t o travel the 1 9 0 0 k m f r o m Walgett t o the V i c t o r i a n b o r d e r {Fig. 1). During m a j o r floods t h e river i n u n d a t e s vast areas o f t h e alluvial plain, and the waters n e a r Walgett are t y p i c a l l y 3 0 - - 6 0 k m wide. A m a x i m u m w i d t h o f 100 k m has been r e c o r d e d .

102

Fig. 5. A. Ti-tree b e n c h f o r m e d b e h i n d s c r e e n o f trees at scross-section 21. B. Ti-tree on a low-level ti-tree b e n c h s h o w i n g aerial r o o t s covered w i t h m u d .

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THE STUDY REACH

In the study reach the Barwon River traverses a broad plain on which there is a maze of channels, the traces of former streams. The present channel is deep and narrow with bank slopes of 19--68 ° (Figs. 4, 5, 7). The average depth is 9.5 m and the width range is 52--110 m. The sinuosity (Schumm, 1963) is 2.3 and the average width/maximum depth ratio (Schumm, 1960) is 8.0. The channel has n o t shifted significantly in plan since 1848 (Lands Department of New South Wales, 1878, 1880; Department of Services and Property, 1969). However, local erosion of the banks occurs, especially by caving of the recent point deposits following floods and b y erosion at the outside of bends and at some places along straight reaches. At the sharper bends erosion of the outside bank is more marked. This rate of erosion has not been monitored although a report of 3 m in 10 years has been given (A. Duncan, pers. comm.) for the bend upstream of cross-section 30 * The bed material coarser than 63 #m varies between 5 and 92% by weight. There is more sand in straight reaches than in the pools at bends. In the bends the percentage of sand increases inwards towards the point. There is usually a sandy point deposit low in the channel (Figs. 7 and P of Fig. 8) extending upstream or downstream depending on local variations of flow and channel morphology. Dunes, ripples and sand sheets, which are exposed on the bed at low stage, progress downstream during medium to high stage. Dunes observed have not exceeded 14 cm in amplitude and their wavelength varies from 4.6 to 50 m.

* The positions of cross-sections 1-38 (Fig. 2), established for a continuing study of channel stability, are used to locate features described.

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105

The bulk of the sediment load is carried in suspension. Suspended-sediment concentrations are n o t high, being 140--500 ppm near bankfull stage and 30--50 ppm at low stage. Averaged over two years, these concentrations represent a discharge of 1100 tonnes of suspended sediment per day or a b o u t 400,000 tonnes per year. At all stages below bankfull 80--95% of the suspended solids by weight is smaller than 2 pm. Despite this high percentage of clay the water clears after several months at very low stage. The fine suspended sediment (<=37 pm) reflects the sluggish nature of the stream. While flow is confined within the channel, energy gradients are < 1 . 0 • 10 -4, mean velocities < 0 . 7 m sec-' and Froude numbers <0.1. In general, only mild surface macroturbulence is seen and conditions appear to be borderline for sand suspension. The Barwon River on the basis of this evidence is a suspended-load stream of a stable to slightly eroding type according to Schumm's (1968) classification. STRATIGRAPHY

The stratigraphic relationships of the sedimentary units, adjacent to the river channel (Table I), are shown in Figs. 2 and 7. The Walgett Formation is exposed in the outside bank at bends and in small gullies and excavations away from the river channel. It underlies an extensive surface (the Walgett Surface: Fig. 3) into which the younger units are inset. The sediments of this formation have been strongly pedogenised producing deep-profiled gray to brown self-mulching clay soils which crack deeply. Gypsum, carbonate and manganese concretions and disseminations occur low in the profiles. Primary sedimentary structures are absent. The sandy clays were probably interlaminated sand and clay before pedoturbation homogenised them, destroying the primary structures. TABLE I Stratigraphic units adjacent to the river channel Stratigraphic unit

Characteristics

Surface name

Barokaville Formation

Member 2

Interlaminated thin sands and muds presently being deposited

Wanouri, Dangar

Member 1

Cross-bedded fine sands with mud interbeds presently being deposited as points sands Disconformity

Vauxhall Formation

Fine sand with thin indistinct mud laminae and mud balls

Vauxhall

Disconformity Walgett Formation

Heavy cohesive cracking clay and sandy clay

Walgett

106 The almost uniform very fine grain size of the Walgett Form at i on indicates that it represents the finest fraction of the suspended load of the stream. The marked pedogenesis and disconformable relations with younger stratigraphic units could be taken as evidence that accumulation of the Walgett F o r m a t i o n has ceased. We do not believe this is so. Much of the Walgett Surface is inundated during major floods (about once every fifteen years on the average) by slowly moving water. Small accretions of suspended load occur, but the rate of accretion is slow enough to allow pedogenic processes to homogenise the new material. The soils are therefore of the cumulative ty p e o f Nikiforoff (1949). The extensive plain of the Walgett Surface is broken by ancient channels, or prior streams (Butler, 1950) known locally as 'warrambools' and by ancestral streams (Pels, 1964) close to the present channel. The savannah-woodland t y p e vegetation consists o f black box (Eucalyptus largiflorens F. Muell.) as the d o m i n a n t species with coolibah (Eucalyptus microtheca F. Muell.) as a co-dominant together with various shrubs and grasses. Adjacent to the Barwon channel the elevation of the Walgett Surface varies from R.L. * 131 m upstream to 129.8 m downstream in the study reach. The Vauxhall Formation is disconformably inset into the Walgett Formation and has a ma xi m um width of 1.2 km. It forms a terrace (the Vauxhall Surface: Figs. 3 and 4) which occupies most of the area below the Walgett Surface (Fig. 2) and borders the present channel and abandoned channels such as the Vauxhall Ox-bow (Inset 1 on Fig. 2). In m any places the Vauxhall F o r m a t i o n displays ridge-and-swale topography, a feature normally associated with laterally accreting sandy poi nt bars, and here regarded as a relic. A detailed study of the Vauxhall F o r m a t i o n has not been carried out because of the lack of outcrop. It is a sandy shoe-string deposit with minimal mud, dune and ripple cross-lamination, plane-parallel fine lamination and some erosional contacts between laminae. These features strongly suggest bed-load deposition in and adjacent to a channel. The present stream channel repeatedly cuts across the trend of the Vauxhall F o r matio n , indicating t ha t the f or m a t i on is related to an earlier channel, or ancestral stream (Pels, 1964), remnants o f which remain as ox-bows, e.g. opposite cross-sections 18 and 19 (inset 1 on Fig. 2; Fig. 3). This channel was considerably wider than that of the present stream. The elevation of the Vauxhall Surface varies from R.L. 129.3 m upstream to 126.6 m downstream in the study reach. It is flooded a b o u t once every two years on the average. The river red gum is the major species growing on this surface. The Vauxhall F o r m a t i o n is overlain by the Barokaville F o r m a t i o n in some places adjacent to the present channel. Elsewhere it is either exposed at the surface or is overlain by a thin veneer of the Walgett F o r m a t i o n which is still slowly accreting. * R.L. indicates reduced level in terms of New South Wales standard datum.

107

The Barokaville Formation is intimately related to the present channel. It overlies the Walgett Formation disconformably and the Vauxhall Formation gradationally or disconformably and is exposed in the banks and benches of the present channel, although excavation is generally needed to reveal sedimentary structures. Member 1 of the formation consists of fine sand units 10--15 cm thick with 2--4 cm mud interbedded which become more frequent towards the top. The sands have cross-lamination dipping both downstream and upstream, the latter indicating reverse flow in the channel (Taylor et al., 1971). Clay balls are abundant in the sands, particularly high in the sand bodies. They are concentrated at the base of sand beds. The muds show parallel lamination which conforms to the underlying sand surface. When cracked by desiccation, these muds may be partially eroded to form clay balls. Member 1 contains no major erosional breaks, b u t is n o t everywhere present. It occurs as sandy point deposits and is presently being formed at sites 13A, 19A and 38A, near cross-sections 13, 19 and 38 (Fig. 2). At these sites Member 1 is partly covered, conformably, by Member 2. At other places (e.g. cross-section 18) Member 1 does n o t outcrop, being completely covered by Member 2. The sandy point deposits are dominantly a product of bed-load deposition. They occur no higher in the channel than bed-load m o v e m e n t is possible. This level depends, to a large extent, upon turbulence. The point sands, therefore, develop low in the channel as narrow crescentic deposits to a maximum height of 2.5 m above the thalweg. These point sands accrete during the highest energy phases of the flood wave. During the falling stages m u d drapes are deposited from suspension over the sand bodies, armouring them against later erosion. Since mud deposition is n o t inhibited as the sandy point deposit builds up, these deposits have more mud beds in their upper parts. Member 2 of the Formation consists of intterlaminated sand and m u d which superficially resembles the flysch-type laminites II of Lombard (1963) (Fig. 9). The sands vary in thickness from 0.1 to 16 cm, averaging 5 cm. Grain size is less than 500 pm with mean size between 88 and 125/~m. They rarely contain more than 10% clay. The m u d d y laminae ranges in thickness from 0.1 to 14 cm, averaging 2 cm, and mostly contain more than 20% sand. Pure clay laminae are rare and are mostly thin (<0.5 cm). In contrast to Member 1, cross-lamination is rare in Member 2. However, graded bedding is c o m m o n (Fig. 9). The grading is normal, reverse, or threefold from sand to mud and back to sand or mud/sand]mud. It can be postulated that the coarser sediment is suspended b y the steeper energy gradients preceding the flood peak and the finer sediment alone remains available for deposition following the flood peak. This could give rise to reverse grading. Thus, the grading of sediments may reflect the form of the flood hydrograph. Threefold grading m a y result from a bimodal flood hydrograph or a hydrograph with a temporary plateau. However, there is no direct evidence

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from sediment sampling o f such changes in the size of the suspended sediment for within-channel flood rises. An alternate hypothesis relates the suspension of coarser sediment to the launching of the sand particles into suspension from the crests of dunes and ripples (Moss, 1972). Moss found that in flume studies the zone of temporary suspension of coarser particles reached the water surface over a dune bed compared with about half way to the surface over a rippled bed. The possible combination of plane bed, rippies and dunes at different times and at different distances upstream, in combination with different hydraulic conditions, could produce the various types of grading observed. Wavy lmmination is c o m m o n . It forms by deposition over and around vegetation or laminae disturbed by h o o f marks and mud cracks. Possibly it also

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Fig. 10. A tracing of the more muddy sedimentation units within Member 2 of the Barokaville Formation at cross-section 19A. The locations of the composite log in Fig. 9 are shown. forms by repeated swelling and contracting of montmorillonite-rich mud laminae. Once waviness appears it persists through several depositional units gradually dying out upstream in the section. Erosional contacts between interbeds are rare. They are local features due to scour around major obstructions to flow, e.g. larger trees. Small mud balls in some laminae probably originated from sun-cracked mud deposits on benches and banks. Stratification in Member 2 is c o m m o n l y not horizontal {Fig. 10) but dips towards the present channel at up to 36 ° . Contorted and disrupted laminae are formed by slip in the steeply dipping parts. The member is thickest close to the channel and wedges out as it ascends away from the channel (Fig. 7). Topographically high parts of the member display somewhat disrupted lamination due to pedoturbation. The grading, uniformity of lamination and the geometry of Member 2 strongly suggest suspended-load deposition with some local reworking of material as bed load. The member accretes both vertically and laterally and is presently being formed. Topographically the Barokaville Formation displays two distinct features, the Dangar and Wanouri Surfaces {Figs. 4, 7 and 8); both surfaces being underlain by Member 2. Although their elevations vary considerably downstream, they are mappable entities {Fig. 2, insets). The Dangar Surface varies in elevation from R.L. 128.4 m upstream to 125.1 m downstream in the study reach and is flooded about once every year on the average. The Wanouri Surface varies from R.L. 126.3 m upstream to 122.6 m downstream and is flooded 2--3 times every year on the average. The river red gum extends down onto both these surfaces. Most of the present regeneration of this species appears to be confined to these two surfaces. BENCHES OF MEMBER 2 OF THE BAROKAVILLE FORMATION The Barokaville Formation is a within-channel deposit which expresses the present hydraulic regime of the stream. The topographic surfaces associated

111

with it are composed of benches, defined here as actively accreting fiatt o p p e d bodies of sediment occurring along the banks of a stream channel (cf. Kilpatrick and Barnes, 1964; Leopold et al., 1964, p.467; Woodyer, 1968). The internal structure of Member 2 of the Barokaville Formation (Fig. 10) in particular shows that these benches are not steps formed by slumping of the banks. Some benches however, may be built upon a foundation of slumped bank material. The benches are generally narrow because lateral channel m o v e m e n t is restricted. Most benches in the study reach belong to one of three categories: point benches, concave-bank benches and ti-tree benches -- each of which is distinguished by location, form, and to some extent by sediment properties. Point benches

In plan these benches are arcuate, although they may also extend some distance upstream or more usually downstream of the point as a narrow parallel-sided bench. Their sizes vary considerably. Generally, the lower bench does n o t extend more than 70 m along the channel or exceed 9 m in width. In one case the upper bench extends about 1.2 km and varies in width from 6to llm. Two levels of point benches occur on the inside of bends, the higher Dangar Surface and the lower Wanouri Surface (Figs. 4 and 7). When both surfaces are present they may be separated by a distinct riser or the separation may be more gradual. In some cases the Wanouri Surface may be represented only by a slight bevel. Elsewhere either surface may be missing. The point benches are formed primarily by suspended load sedimentation of Member 2 of the Barokaville Formation upon the dominantly sandy bedload deposit (Member 1) which is exposed on the sandy sloping points (P of Fig. 8). Concave-bank benches

At sites 19B and 38B (insets on Fig. 2) benches have formed along the outside or concave bank just upstream of the apex of the bend. Both sites are upstream of hairpin bends (Woodyer, 1970). This association with a hairpin bend contrasts with that noted by Carey (1969) along the lower Mississippi River, where 'concave or eddy accretions' are associated with bends of a b o u t 90 °. At these very sharp bends on the Barwon River, channel expansion (related to former channels) favours the development of reverse flow and provides the space for deposition. Elsewhere space is not available because of the negligible downstream migration. The larger of these t w o concave-bank benches (Fig. 8A and B) is 160 m long and 29 m at its widest part. The benches are as much as 1.5 m higher at the downstream end (H of Fig. 8B) than upstream. Behind this elevated part a trough (T of Fig. 8B) skirts the bank. At the upstream end these benches

112 slope gently down from the bank in contrast to their horizontal attitude at the downstream end. Woodyer {1975) gives a more detailed description of these benches.

Ti-tree benches Ti-tree benches develop in straight reaches where ti-trees become established in the bed of the river during low-fiow periods. The ti-tree can endure long periods o f flooding and, while floods remove trees nearest the thalweg and keep the channel open, trees close to the bank often survive. In this way a screen o f tree trunks, trailing branches and trapped logs plus a mat of aerial roots is f o r med {Fig. 5A and B) behind which sediments are deposited against the bank. These benches, whose levels are extremely variable, can be seen at all stages of development along straight reaches of the river. At an early stage o f development t hey may form islands near the bank at low to medium river stage. The best developed ti-tree bench occurs at cross-section 21 (Fig. 5A) and is about 156 m long by 4 m wide with a front slope of 30 ° .

Unclassified benches Only two bench occurrences in the study reach, namely the benches at cross-sections 14 and 37, do not fit the above categories. At site 14 there is a well-defined bench in a straight reach. Although there are a few ti-trees present th ey do not appear to have played a significant role in bench formation. It is apparent that the bank against which the bench has form ed has been eroded and the channel widened. At cross-section 37 (site 37B on inset 2 on Fig. 2), which is on the inside bank just upstream o f a bend, the bench is almost semicircular with the straight side towards the river. In this case, also, erosion o f the bank has widened the channel, and although a few ti-trees are present this is not a typical ti-tree bench. The bench at cross-section 37 may be similar to scour-bars described by G upt a and Fox (1974).

Distinctive sediment properties An imp o r tan t difference between the sediments of the point, ti-trees and concave-bank benches is the percentage of mud in Member 2. The sand/mud ratio is highest for poi nt benches on sharp bends, where m ud laminae are thinnest, and is lowest for concave-bank benches (Table II). The ratio for the ti-tree benches is intermediate between the ratios for concave-bank benches and point benches on sharp bends. The low sand/mud ratio for the concavebank bench is attributable to the thicker {ca. 7.0 cm) mud laminae. Although few hard data are presented, field observations confirm the trends. In general, the sediments of the different types of benches are essentially m u d d y , although, surprisingly, even the higher ti-tree benches contain some fine sand at high levels. Ti-tree bench sediments also contain a fairly high percentage o f organic m a t t e r which gives them a porous texture.

113 RA TE S AND PROCESSES OF BENCH SEDIMENTATION

The time required to deposit Member 2 of the Barokaville Formation, underlying the lower bench at cross-section 18 (Fig. 4), was estimated by determining the age of the larger river red gum growing on the bench (Fig. 4). Trenching revealed t h a t this tree became established on the top of Member 1 of the Barokaville Formation so that the age of the tree gives a m a x i m u m age for the base of Member 2. The age of the tree in 1969 was estimated at 116 years by using an average growth rate of 7.6 mm per year. This is the average growth rate for the period 1919--1969. The 1919--1920 wood was identifiable because the river discharge for this year was the lowest (only 2.3% of the average) in the period 1886--1969 and was followed by two years of above-average discharge. On the basis of this estimated age, the minimum mean rate of accretion of Member 2 at this site approximates 28 m m per year. This rapid rate of accretion is supported by evidence provided by the discovery of a polythene bag (endorsed with the manufacturer's name) under 460 mm of undisturbed sediment at cross-section 14 (unclassified bench) during October 1969. This type of bag was produced from 1953 through 1961. This gives a mean rate of accretion of 27--57 mm per year. Bench height relative to flood regime affects the rate of accretion, which decreases with increasing bench height. Thus, a higher rate of accretion is likely on the bench at cross-section 14 because the bench surface is lower than that at cross-section 18. These rates are high, especially in view of the fineness of the suspended sediment and its moderate concentrations (<500 ppm). However, it must be remembered that deposition is restricted to certain favourable localities within the channel. The thickness of sediment deposited by individual floods on small metal pegs driven flush with the lower bench surface was measured at cross-section 14 and on the point bench near site 38A. Sediment budgets show that during floods only about 0.3 ppm of suspended sediment was deposited from the total flow over these lower bench levels. Little is known about the processes of sedimentation on benches. The sedimentary evidence indicates that the laminae owe their overall character to deposition from suspension. Three aspects are discussed. The first concerns the bed-load/suspended-load character of the laminae. The second concerns the high content of fine to medium sands in point-bench sediments (and to some extent in the ti-tree bench sediments) considering the sluggish nature of the stream flow. The third concerns reverse flows which appear to favour deposition of muds. Hitherto, in fluvial sedimentation studies, lateral and vertical accumulation within the channel has been attributed to bed-load deposition and overbank deposition has been considered solely the result of suspended sediment accumulation. However, Member 2 of the Barokaville Formation has been formed within the channel by sediments brought to the site as suspended

114 load and its overall geometry and structure reflect deposition from suspension. Cross-lamination in some laminae o f Member 2 indicates bed-load movement. Moreover some thin laminae of fine sediments (4 pm mode) have elongation-function curves (Moss, 1962, 1972) characteristic of bed-load deposits. This suggests that either some of the sediments on reaching the depositional surface behaved as bed load or subsequent more energetic flows moved the surface sediments to impart bed-load characteristics. In this cont e x t Green (1974) found bed-load features in fine sand dispersed by pedogenic processes through clay forming the flat far flood plain of a prior stream. It is clear that the distinction between bed-load sediments and suspended-load sediments cannot be drawn on the basis o f within-channel and overbank zones. The high c o n t e n t of fine to medium sand in Member 2 of the Barokaville F o r m a t i o n and in the sediments of the ti-tree benches is surprising in view of the fineness of the suspended sediment sampled. Some 80 water samples, from different depths at several verticals on two cross-sections and at selected sites, taken during the passage o f within-channel flood waves contained sediments no coarser than 37 pm. Overbank floods with greatly increased energy gradients and turbulence carry the sandy bed load in suspension. However, these flows short-cut most of the points and flow velocities through many bends are reduced. The conditions in the bend are then more conducive to the deposition of mud. Further, the n u m b e r of sand laminae in Member 2 of the Barokaville F o r m a t i o n at cross-section 18 greatly exceeds the n u mb er of over-bench floods that occurred during its formation. Hence sand deposition must occur from within-channel flood waves and more than one sand-depositional episode may be associated with one flood wave. Failure to sample sand in suspension suggests that it is restricted in space and time. Under these borderline conditions sand suspension is most likely to occur from dune crests as shown by the flume observations of Moss (1972). There is some evidence (Table II) to suggest t hat the sand/mud ratios for point-bench sediments increase with increasing bend sharpness. Given a suspension mechanism, the increasing turbulence associated with increasing bend sharpness would keep sand in suspension longer and cross-currents would favour transport and deposition on point benches o f the sharper bends. The third aspect of sedimentation on benches m e n t i o n e d above relates to the association of mud deposits with reverse flows over concave-bank benches. Examination of sediment profiles and sand/mud ratios (Table II) indicates that sand is seldom in suspension over concave-bank benches. This is probably because the sand moves in towards the point as it enters the bend (Vogel and Th o m ps on, 1933) and forms point deposits. Towards the concave bank the channel deepens and the bed is mainly in cohesive silts and clays. Normally, only fine suspended sediments are drawn into the reverse flow over the concave-bank bench in spite of marked macroturbulence (boils and small vortices) along the interface between the main stream and the

115 T A B L E II Sand/mud ratios of bench sediments Bench type

Site or cross-section

B e n c h level

Sand/mud ratio , 3

Concave Concave Ti-tree

38B 19B 21

----

0.2 0.1,1 (1.3,1) (1.2 , 2 0.3 0.5 1.5 1.3 1.5 3.7 0.6

( Point bench Point bench Point bench Point bench Point bench Point bench Unclassified

36 13A 37A 18 19A 38A 37B

upper lower lower lower lower upper --

Unclassified

14

--

0.9

B e n d angle , 4 (degrees)

R o c k in bed

hairpin hairpin _

downstream -_

150 120 120 80 hairpin hairpin --

--upstream --downstream opposite

--

--

) )

, l Including organic material. ,2 Excluding organic material. , 3 D e t e r m i n e d b y m e a s u r e m e n t s a t r e p r e s e n t a t i v e s e c t i o n s in t r e n c h e s a n d e r o s i o n c u t tings. , 4 B e n d angle is i n t e r n a l angle so t h a t a h a i r p i n b e n d a p p r o x i m a t e s 0 °.

reverse flow. The rate of aggradation on concave-bank benches appears to be very rapid, 48--64 mm per year (Woodyer, 1975). However, the concavebank benches occur below the nearby lower point-bench levels so that rates of accretion are likely to exceed those at higher levels. Nevertheless, it is apparent the reverse flows are very effective in depositing fine suspended sediment. Carey (1969) reports 2.4 km downstream migration and reverse flow accretion in 80 years in the low flow channel of the Mississippi River. These deposits have a higher fine silt and clay content than any other alluvial deposits except back swamp clays. It seems that reverse flows play an important role in the deposition of muds to form concave-bank benches (Woodyer, 1975). Reverse flows occur also over point benches, and over the elliptically shaped bench at site 37B (inset 2 on Fig. 2). At site 37B the sediment profile is very m u d d y and the sand/mud ratio is 0.6 (Table II). At cross-section 36 (a point bench) the bend is very gentle, and presumably bend turbulence is slight, so that reverse flow plays a correspondingly more important role than turbulence in deposition at this point. This probably explains the low sand/mud ratio (0.3). Reverse flow has been observed over the point bench at site 13A where the sand/mud ratio is 0.5. The high ratio of (1.5) for site 37A (on the point bench downstream of the bend), with the same bend angle as cross-section 13A, is probably due to turbulence created by a rock outcrop near site 37B.

116 BENCH--FLOOD-PLAIN RELATIONSHIPS W o o d y e r ( 1 9 6 8 ) r e p o r t e d the w i d e s p r e a d o c c u r r e n c e o f t h r e e b e n c h levels in N e w S o u t h Wales. He identified t h e m b y relative elevation at gauging stations w h e r e all three levels were present, as the ' l o w ' *, ' m i d d l e ' and 'high b e n c h e s ' . T h e s e gauging stations i n c l u d e d the t h r e e s t a t i o n s on the p r e s e n t s t u d y reach (Fig. 2), n a m e l y D a n g a r Bridge, Walgett (cross-section 18) and C o m b a d e r y (cross-section 37) - - gauging s t a t i o n s 4 2 2 0 0 1 , 4 2 2 0 9 9 and 4 2 2 0 0 8 , r e s p e c t i v e l y (Australian Water R e s o u r c e s Council, 1971). T h e b e n c h e s at D a n g a r Bridge and Walgett are t y p i c a l p o i n t benches. Ti-tree and c o n c a v e - b a n k b e n c h e s were n o t c o n s i d e r e d b y W o o d y e r (1968). Because o f t h e lateral stability o f t h e B a r w o n , b e n c h e s are e x p r e s s e d best at the p o i n t s o f bends. T h e u p p e r and l o w e r p o i n t benches, w h i c h c o n s t i t u t e the D a n g a r a n d W a n o u r i Surfaces in this s t u d y reach, c o r r e s p o n d r e s p e c t i v e l y w i t h the high and m i d d l e b e n c h levels r e p o r t e d b y W o o d y e r (1968). W o o d y e r ( 1 9 6 8 ) e x a m i n e d t h e t h r e e b e n c h levels in N e w S o u t h Wales in t h e light o f claims ( W o l m a n and L e o p o l d , 1957; D u r y et al., 1 9 6 3 ) t h a t the active flood-plain level is i n u n d a t e d a b o u t o n c e a y e a r o n the average, a freq u e n c y which is c l a i m e d to be a p p r o x i m a t e l y c o n s t a n t f o r a wide range o f e n v i r o n m e n t s . He c o n c l u d e d t h a t in m a n y cases t h e insignificant high b e n c h , and n o t the a p p a r e n t f l o o d plain above, was t h e active f l o o d plain in t e r m s o f this m e a n y e a r l y e x c e e d a n c e . In t e r m s o f m e a n e x c e e d a n c e f r e q u e n c y the D a n g a r S u r f a c e (or u p p e r b e n c h ) is e x c e e d e d on the average a b o u t o n c e a y e a r and the V a u x h a l l S u r f a c e (or a p p a r e n t f l o o d plain) a b o u t o n c e every t w o years. If this were t h e o n l y evidence, the V a u x h a l l S u r f a c e c o u l d n o t be r e g a r d e d as a terrace as distinct f r o m the active f l o o d plain t. H o w e v e r , the V a u x h a l l F o r m a t i o n consists o f d e p o s i t s o f an ancestral s t r e a m , r e m n a n t s o f w h o s e c h a n n e l o c c u r n e a r cross-section 18 (inset 1 on Fig. 2). This, plus the f a c t t h a t t h e V a u x h a l l F o r m a t i o n d i s c o n f o r m a b l y underlies the c o n t e m p o rary Barokaville F o r m a t i o n at cross-section 18 (Fig. 7), indicates t h a t it is n o t related to the p r e s e n t s t r e a m regime. On t h e o t h e r h a n d , l a m i n a e are clearly discernible in M e m b e r 2 o f the Barokaville F o r m a t i o n on b o t h b e n c h levels. T h e s e l a m i n a e are generally m o r e distinct b e l o w t h e l o w e r Wanouri * The low bench reported by Woodyer (1968) is not distinctly developed in the study reach. However, it probably corresponds to the low-level point sands (Figs. 7 and 8) over which the point-bench deposits prograde. This finding accords with Riley's {1973) observations that the low bench is not easily identified along the sand-bedded streams of the Namoi and Gwydir catchments within the Barwon catchment. On the other hand Riley finds that along cohesive-bedded Namoi and Gwydir streams the low bench is easily identified. t Gary et al. (1972) define flood plain as 'the surface or strip of relatively smooth land adjacent to a river channel, constructed (or in the process of being constructed) by the present river in its existing regime and covered with water when the river overflows its bank at times of high water. It is built of alluvium carried by the river during floods and deposited in the sluggish water beyond the influence of the swiftest current. A river has one flood plain and may have one or more terraces representing abandoned flood-plains.'

117

Surface than the upper Dangar Surface as would be expected from the relative frequency of inundation. The Dangar Surface is selected as the flood plain because it is the highest substantial body of sediment deposited by the present stream regime. T h e present regime of flood-plain formation is not associated with a meandering channel and point-bar formation. The sandy point deposits which are forming are not typical point bars with a bar and swale topography a n d M e m b e r 2 of the point-bench sediments is characterized by suspended-load deposition, not bed-load deposition. The resultant flood plain does not fit the typical concepts of flood plains, being a narrow discontinuous surface within the stream channel. MODELS FOR SEDIMENTATION BY VERY LOW-GRADIENT, SUSPENDED-LOAD STREAMS

From the data obtained on the studied reach of the Barwon River, several points can be advanced concerning some spatial and temporal aspects of sedimentation by very low-gradient streams: (1) The present channel and its adjacent tract (Barokaville and Vauxhall Formations), and inferentially the channel complexes of the prior streams (warrambools) elsewhere in the Barwon alluvial plain, each preserve a record of hydraulic regimes that decrease in intensity with time. This record comprises three stages. The first is incision of a new channel into the alluvial plain (Walgett Formation), the second a bed-load channel with most of the suspended load being bypassed (Vauxhall Formation) and the third is of channel stabilisation with concomitant within-channel deposition (Barokaville Formation) culminating in channel evulsion. (2) In the third stage the channel becomes very sinuous with low gradients and stream velocities, which together with a low friction factor favours within-channel deposition. Constriction of the channel leading to evulsion may occur in a number of ways, for example: Ca) by lateral and vertical growth of benches, (b) by local base levels causing damming effects, (c) by vegetation growth within the channel during prolonged periods of low flow, and (d) by debris clogs. (3) Abandonment of a stage 2 channel due to decreasing hydraulic regime during the evolution of the channel tract or of a stage 3 channel by terminal evulsion is followed by encroachment of the channel related deposits by suspended-load deposits (Walgett Formation). This is exemplified by the clay plug infillings of abandoned channels in the Vauxhall Formation and Surface (inset 1 on Fig. 2; Fig. 7). (4) Under the existing hydraulic regime, sediment accumulation is greatest within the stream channel, forming.the Barokaville Formation with its associated benches.

118

(5) Sediment continues to accumulate at a slow rate over the alluvial plain (Walgett Formation and Surface) away from the channel tract by suspendedload deposition during major floods. (6) The sedimentation processes are conducive to strong differentiation of sediment according to grain size within the alluvial plain. Within the channel the differentiation is vertical with the deposits becoming finer upwards. This is superimposed upon a more marked lateral differentiation parallel to the channel tract. The coarser sediments predominate in the channel tract while the finer sediments predominate further out. MODELS

These points provide a partial conceptual framework for integrating the data into sectional and overall conceptual models for the accumulation of alluvial deposits through the agency of streams such as the Barwon River. In these rivers the three stages of channel development are: (1) an erosional phase, (2) a stable phase, including erosion and deposition, and (3) a final phase of deposition and channel infiUing. In any one region along the channel tract all three stages of development may be active at any one time. In the present tract of the Barwon River it is difficult to identify the original erosional phase; however, the Wanouri Creek (Fig. 2) is probably one such example. During this erosional phase the new channel is cut through the older alluvium and in the initial stages it is relatively straight and free of bed load. The sand eroded from the alluvium, however, gradually accumulates within the new channel. When an adequate bed load is available the channel enters the second stage. With continued development the channel also taps bed load from the parent channel. The second-stage channel differs from the first in that it is wider, shallower and is laying down bed-load deposits similar to those described by Allen (1970). Accumulation of sand on the inside of bends increases the sinuosity and decreases the gradient, velocity and turbulence of the flow. The suspended-load carried by the parent channel is carried through the stage one and two channels but because of the nature and hydrology of those channels it is n o t deposited. The suspended material only begins to deposit during the third phase of development. The transition from the second to the third phases is initiated by damming of the second-stage channels as they enter channels in a later stage of development than themselves. This impedance restricts bed-load activity and hence increases sinuosity with a resultant drop in gradient which promotes suspended-load deposition on the banks and hence increases channel stability. The impedance to flow gradually moves upstream by a damming effect causing an upstream migration of suspended-load deposition. As suspended-load deposition advances up the channel the point sands within the bed of the previous stage (Vauxhall Formulation) become draped, intermittently with mud laminae (Member 1 of the Barokaville Formation).

119

This effectively armour-plates these deposits and inhibits their erosion. Hence they gradually increase in thickness and in mud content until a level is reached where bed-load material is n o t deposited on them b u t well-laminated suspended load begins to accumulate (Member 2 of the BarokaviUe Formation). Once channel stabilisation occurs, lateral growth of benches, or one of the other obstructions to flow (see point (2) above), develops, gradually constricting the channel until abandonment occurs. The relict channel is filled with black self-mulching clays which after pedogenesis b e c o m e indistinguishable from the Walgett Formation. The sedimentary record of this evolution is summarised in Fig. 7. There are obvious similarities between the nature and history of development of the Barwon and other streams in eastern Australia (Pels, 1964, 1966; Bowler, 1967; Schumm, 1968). Pels discusses the relationships between the modern and ancestral Murray River channels and attributes three separate phases to climatic oscillations. Bowler (1967) comments on this and earlier arguments along similar lines, viz. 'there is little agreement on the specific nature of the changes involved . . . Controversies on the role of climate, local geomorphic histories and synchroneity of events over wide areas arise largely from lack of stratigraphic d e t a i l . . . ' The phases referred to by Pels (1964, 1966) and others need n o t necessarily be related to climatic shifts. The models, based on detailed stratigraphic, hydrologic and sedimentologic studies of the Barwon River and its ancestors, show that these phases can be generated within the hydraulic system itself. While n o t denying the potential role of climatic change, the authors see it as an external and additional factor to the basic mechanisms inherent within hydraulic systems similar to that of the Barwon River. ACKNOWLEDGMENTS

The authors gratefully acknowledge the assistance and support given by the following individuals and institutions: The Survey Branch, Department of Services and Property, New South Wales Water Conservation and Irrigation Commission, Mr J. Nugent,. Shire Engineer and Mr M. Curl, Assistant Shire Engineer of the Walgett Shire Council, Mr and Mrs A. Duncan, Mr and Mrs E.S. Fuller, Mr and Mrs R. Hannaford , Mr and Mrs N. Keane, Mr and Mrs D. Olsen, Mr and Mrs H. Sanderson and Mr S. Weate. The assistance of Professor J. Ovington, Australian National University, and Dr J. Davidson of the Department of Forests, Port Moresby, Papua New Guinea, for help in the tree dating, is gratefully acknowledged. K.D. Woodyer and G. Taylor are responsible for the field work and data interpretation. K.A.W. Crook contributed in the form of discussion and advice during preparation of the manuscript. REFERENCES Allen, J.R.L., 1970. Physical Processes of Sedimentation. G. Allen and Unwin, London. Australian Water Resources Council, 1971. Stream gauging information, Australia 1969. Department of National Development, Canberra, A.C.T.

120 Bowler, J~M., 1967. Quaternary chronology of Goulburn valley sediments and their correlation in southeastern Australia. J. Geol. Soc. Aust., 14: 287--292. Butler, B.E., 1950. A theory of prior streams as a causal factor of soil occurrence in the Riverine Plain of south-eastern Australia. Aust. J. Agric. Res., 1: 231--252. Carey, W.C., 1969. Formation of flood plain lands. J. Hydraul. Div. Am. Soc. Civ. Eng., 95 (HY3): 981--994. Department of Services and Property, 1969. Map entitled 'Walgett, C.S.I.R.O. River Investigation'. Scale 1,000 ft/inch. Negative No. 15028, Department of Services and Property, Sydney, N.S.W. Dury, G.H., Hails, J.R. and Robbie, M.B., 1963. Bankfull discharge and the magnitude frequency series. Aust. J. Sci., 26: 123--124. Gary, M., McAfee, Jr., R. and Wolf, C.L. (Editors), 1972. Glossary of Geology. American Geological Institute, Washington, D.C. Green, P., 1974. Recognition of sedimentary characteristics in soils by size-shape analysis. Geoderma, 11 : 181--193. Gupta, A. and Fox, H., 1974. Effects of high-magnitude floods on channel form: A case Study in Maryland Piedmont. Water Resour. Res, 10: 499--509. Kilpatrick, F.A. and Barnes, H.H., 1964. Channel geometry of piedmont streams as related to frequency of floods. U.S. Geol. Surv. Prof. Pap., 422-E. Lands Department, New South Wales, 1878. Plan of Barwon River drawn from Surveyor G.B. White's pencil plots of 1848. Scale 20 chains/inch. Reference No. R132 r, New South Wales Lands Department, Sydney, N.S.W. Lands Department, New South Wales, 1880. Survey of Barwon River by Surveyor C.W. King. Scale 20 chains/inch. Reference No. R298, New South Wales Lands Department, Sydney, N.S.W. Leopold, L.B., Wolman, M.G. and Miller, J.P., 1964. Fluvial Processes in Geomorphology. Freeman, San Francisco, Calif. Lombard, A., 1963. Laminites: A structure of flysch type sediments. J. Sediment. Petrol., 33: 14--22. Moss, A.J., 1962. The physical nature of common sandy and pebbly deposits, Pt. I. Am. J. Sci., 260: 337--373. Moss, A.J., 1972. Bed-load sediments. Sedimentology, 18: 159--219. Nikiforoff, C.C., 1949. Weathering and soil evolution. Soil. Sci., 67 : 219--30. Pels, S., 1964. The present and ancestral Murray River system. Aust. Geogr. Stud., 2: 111--119. Pels, S., 1966. Late Quaternary chronology of the Riverine Plain of southeastern Australia. J. Geol. Soc. Aust., 13: 27--40. Riley, S.S., 1973. The Development of Distributary Channels with Special Reference to Channel Morphology. Thesis, Univ. Sydney, N.S.W. (unpublished). Schumm, S.A., 1960. The shape of alluvial channels in relation to sediment type. U.S. Geol. Surv. Prof. Pap., 352-B. Schumm, S.A., 1963. Sinuosity of alluvial rivers on the Great Plains. Bull. Geol. Soc. Am., 74: 1089--1100. Schumm, S.A., 1968. River adjustment to altered hydrologic regimen -- Murrumbidgee River and paleochannels, Australia. U.S. Geol. Surv. Prof. Pap., 598. Taylor, G., Crook, K.A.W. and Woodyer, K.D., 1971. Upstream dipping foreset crossstratification: Origin and implications for paleoslope analysis. J. Sediment. Petrol., 141: 578--581. Vogel, H.D. and Thompson, P.W., 1933. Flow in river bends. Civ. Eng., 3: 266--268. Wolman, M.G. and Leopold, L.B., 1957. River flood plains: some observations on their formation. U.S. Geol. Surv. Prof. Pap., 282--C. Woodyer, K.D., 1968. Bankfull frequency in rivers. J. Hydrol., 6: 114--142. Woodyer, K.D., 1970. Discussion of 'Formation of flood plain lands', by W.C. Carey. J. Hydraul. Div. Am. Soc. Civ. Eng., 96 (HY3): 849--850. Woodyer, K.D., 1975. Concave-bank benches on the Barwon River. Aust. Geogr., 13: 36--40.