Braided rivers: perspectives and problems

Braided rivers: perspectives and problems

C. S. BRISTOW 1 & J. L. B E S T 2

1 Research School o f Geological and Geophysical Sciences, Birkbeck College, Gower Street, London W C I E 6BT, UK

2 Department o f Earth Sciences, The University, Leeds, West Yorkshire LS2 9JT, UK

Abstract: Progress towards a fuller understanding of the dynamics and deposits of braided rivers demands an interdisciplinary approach to a host of unresolved problems. Although many advances have been made within recent years in interpreting the mechanics of flow, transport of sediment and sedimentary architecture of braided rivers many key issues remain to be addressed. In particular, several areas demand attention: the mechanisms of braid bar initiation; confluence-diffluence dynamics, the nature of sedimentary facies over a range of grain sizes and the influence of flow stage and aggradational regime upon the depositional architecture over a range of channel scales. This paper focuses upon these issues and highlights several areas of fruitful future interdisciplinary collaboration.

Braided rivers form important topics of study for many scientists and one of the primary aims of this volume is to bring together work from many disciplines in an integrated approach to braided rivers. For the geomorphologist braided fluvial systems are abundant within upland and pro- glacial settings and are agents of considerable erosion and sediment transport. For engineers the high rates of sediment transport, deposition and erosion combined with frequent channel shifting and rapid bank erosion may pose considerable design problems both to within- channel structures, such as bridge piers (e.g. Mosley 1982a; Sutherland 1986) and braidplain edge constructions such as roads and railways Finally, for the geologist braided rivers form important agents of deposition that have been responsible for the accumulation of many sedimentary sequences that form valuable aquifers, hydrocarbon reservoirs and sites for heavy mineral accumulation. Because of these abundant and diverse applications, knowledge of the mechanics and deposits of braided rivers is vital within many areas and yet, when compared to the wealth of literature upon meandering systems, they have been comparatively under- studied. This may, in part, be due to the diffi- culty of measuring flow, sediment transport and morphology in the rapidly shifting braided river environment. Future progress in understanding the mechanics and morphology of braided rivers demands interdisciplinary collaboration and calls for a more integrated approach across the sciences than may have been present until comparatively recently. This paper highlights some specific areas upon which our knowledge of

braided rivers may be fruitfully extended by adopting such an interdisciplinary scope.

Zones of flow convergence and divergence

Braided rivers are characterized by 'having a number of alluvial channels with bars and islands

.between meeting and dividing again, and pre- senting from the air the intertwining effect of a braid' (Lane 1957). The division and joining of channels are essential features of braided rivers and, whilst the bars within these rivers have received attention from both geomorphologists and sedimentologists, the areas of flow convergence and divergence have not been incorporated into braided river depositional models. The flow dynamics and morphology of channel confluences have been studied by several researchers (e.g. Mosley 1976, 1982a; Best 1986, 1987, 1988; Best & Roy 1991; Roy & Roy 1988; Roy & Bergeron 1990; Roy et al. 1988), and recent attention has highlighted the abundant confluences within braided rivers (Ashmore 1982; Ashmore & Parker 1983; Klaassen & Vermeer 1988). However, the link between flow convergence and the downstream division of flow has been neglected, despite the fact that this transition is the area which may be of fundamental importance to the development of braid bars (e.g. Ashmore 1991; Ashworth et al. 1992). Although some depositional models of braided rivers are beginning to recognise and incor- porate confluence scour and fill (Cowan 1991; Bristow et al. 1993; Bridge this volume; Huggen- berger this volume; Seigenthaler & Huggen- berger this volume), areas of flow divergence

From Best, J. L. & Bristow, C. S. (eds), 1993, Braided Rivers, Geological Society Special Publication No. 75, pp. 1-H.

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2 C. S. BRISTOW & J. L. BEST

are less well understood and ignored in these models. In general, flow divergence is associated with flow deceleration and sediment deposition and, once deposition has been initiated, the sediment accumulation is likely to promote further flow division, deposition and bar formation. Divergent flow may also impinge on the bank at an increased angle leading to bank erosion, channel widening and a local increase in available sediment, all of which are likely to lead to the development of a new braid bar (Carson 1984; Thorne et al. this volume). The deposits of diffluence areas may therefore form the foundations of braid bars but there are no known descriptions of these deposits and their internal structure. It is possible that some sedimentation may occur by vertical or even upstream accretion in the diffluence areas in braided rivers (Ashmore this volume) and these must be mani- fested within the sedimentary record. Bed shear stress has been shown to increase in shallow flows over bar tops (Cheetham 1979) where a coarse bed armour may form. In coarse-grained braided rivers the bar heads may be characterized by coarse-grained sediments that are imbricated or laminated (Bluck 1979). There is a clear need for both an understanding of the fluid dynamics of the diffiuence zone and how this may influence braid bar initiation and internal structure.

The influence of flow stage

The planform appearance of braided rivers can change radically with flow stage (see fig. 2 in Thorne et al. this volume). Indeed some authors (Doeglas 1962; Miall 1977) have proposed that fluctuations in discharge are a pre-requisite for braiding although this may often be discounted as has been demonstrated by the modelling of gravel-bed braided planforms in constant discharge scaled flume experiments (e.g. Ashmore 1982, 1991). Bluck (1979) suggests that bars may disappear at high flow stages, reforming as discharge falls, and similar observations have been reported by Smith (1974), Carson (1984) and Gupta & Dutt (1989). This may imply that some braided rivers act as single channels at bankfull stage and only adopt a characteristic braided pattern on the falling stage. However, these observations appear to be fairly unusual and most braided rivers retain their bars at both high and low flow stage (Krigstrom 1962; Coleman 1969; Smith 1970; Cant & Walker 1978; Collin- son 1970; Church & Jones 1982; Bridge et al. 1986; Bristow 1987a). Where bars exist for periods of time in excess of a single flood event they will experience a complex history of

erosional and depositional modification related to changes in stage. At higher flow stages when the largest volumes of sediment are transported, the channels are often scoured, bars may be reduced in height or in some cases completely eroded. However, during falling stage maximum deposition occurs as discharge and flow competence are reduced. Channel beds aggrade, the high stage bedforms may be modified and new bars may be formed or enlarged as sediment is deposited. As discharge continues to fall, bars may become emergent and dissected by low stage channels. Additionally, the nature of the falling limb recession (rate and length of recession) will be important not only in the reworking of higher stage sediments, but also in the deposition and spatial distribution of the finer grained sediments (silts and clays) which may constitute discontinuous permeability barriers within braided alluvium. Classification of emergent areas based on their low stage appearance may be deceptive and care needs to be taken in determining which areas are bars, scaling with channel width, and those which are partially dissected bars or stranded collections of bars (Church & Jones 1982; Bridge 1985). Little data exists for the comparison of bar and channel morphology at different flow stages (but see Mosley 1982b) and this is an area in which controlled and correctly scaled flume models combined with field studies may contribute greatly to our understanding.

Channel hierarchies

The presence of a hierarchy of channels within braided rivers was first suggested by Williams & Rust (1969), who described three orders of channel in addition to a series of levels within the river which represented active and inactive parts of the channel system. These orders and levels of bar deposit may also be adjusted to the dominant discharge of the alluvial system (see Thorne et al. this volume). In the scheme proposed by Williams & Rust (1969) the entire river and active channels were termed the 'composite stream channel' and the 'stream channel' respectively adding two additional levels to the hierarchy. This system was modified by Bristow (1987a) to a three fold hierarchy and Bridge (this volume) suggests additional modifications to this view. If one accepts that the river can operate as a single entity with channels within it and that there may be different scales of channels which depend upon total discharge and discharge fluctuations, then a threefold hierarchy of channels is required. The first order comprises the whole river (see fig. 2 of Thorne



PERSPECTIVES AND PROBLEMS 3

et al. this volume, for an image of the Brahmaputra River in full flood). Second order channels are the dominant channels within the river whilst third order channels are primarily low stage features which modify the bars deposited by the second order channels. Lower order channels may also exist which modify the third order bars. One implication of viewing braided rivers as hierarchical systems is that individual ancient braided river sandstones should also show a hierarchy of channel dimensions, whereas stacking of a single channel river is more likely to produce similar sized sandbodies. The presence of different magnitude channels within a single sandbody may therefore be an indication of braiding. However, caution is required for, if the third order channels are dependent on stage changes, then the presence of several scales of channel could be due primarily to changes in stage. Therefore, a hierarchy of channel sandbody sizes is not an a pr ior i indicator of a braided river but is a feature most likely to occur in braided rivers with fluctuating discharges.

Grainsize influences upon braiding

Within the geological and geomorphological literature there has been a long held distinction between gravel-bed braided rivers and sand-bed braided rivers. However, many natural gravel bed rivers include those with bedloads of sand, granule, pebble, cobble and even boulder grade material while fine-grained sand-bed braided rivers are held to contain less than 25% gravel (Bluck 1979). It is rare for a river to have only one type of bed material, and most rivers have a range of bed and bank material types. Although some small-scale alluvial bedforms show a clear grain size controlled stability (e.g. ripples, lower stage plane beds, particle clusters) many larger scale bar forms and the braided channel pattern appear similar in rivers of widely differing grain size. There has been little quantitative work comparing the planform characteristics of gravel and sand-bed braided rivers even though their apparent similarities may suggest important common processes (Fig. 1). Comparison of confluence scour in the sand-bed Brahmaputra River, for example, reveals similar relation- ships between confluence angle and scour depth to those found in gravel-bed rivers (Klaassen & Vermeer 1988). In a recent review of the differences between gravel and sand-bed rivers Simons & Simons (1987) concluded that 'gravel- bed reaches of a river system exhibit totally different morphological characteristics and in general, they will be less responsive to modest

changes in discharge and discharge duration than a sand-bed river'. While a clear distinction may be required when calculating sediment transport, there appears to be more morpho- logic similarity between sand and gravel bed rivers than differences between them. The depositional styles of gravel and sand bed braided rivers will directly affect the facies models that must be used to interpret ancient deposits. Investigation of the correlation between the classic sand-bed braided river depositional models (e.g. Cant & Walker 1978; Collinson 1970) with those derived for gravel- bed braided alluvium (e.g. Bluck 1979; Steel & Thompson 1983; Ramos & Sopena 1983; Ramos et al. 1986; Smith 1990) remains a high priority, both in terms of evolving realistic facies interpretation and in the feedback these schemes may provide into understanding the similarities and differences in depositional process between these rivers.

Braided channel morphology and scale

Studies of braided channel dynamics and deposits have ranged across at least five orders of channel size from small laboratory models (scaled field dimensions c. 2-20m channel width), to natural braidplains several kilometres in width (e.g. Ashworth et al. 1992; Warburton et al. this volume) to the largest alluvial rivers such as the Brahmaputra (Coleman 1969; Bristow 1987a; Thorne et al. this volume) which have braidplain widths of up to 20 km, individual channel widths of several kilometres and maximum scour depths of up to 50 m. The issue of scaling depositional form and formative process across this range of braided channel sizes is rarely addressed yet is central when applying results and models from one channel size to a system of a completely different magnitude. Superficial examination (Fig. 1) often reveals a gross similarity to the appearance of braided systems of widely differing size, yet data is required, both on the planform and cross- sectional characteristics, to substantiate or refute this apparent similarity. The self- similarity of form across a range of scales or the scale dependent nature of the geometry of braided rivers has several fundamental applications. First, self-similarity across scales of braiding may shed light upon the processes inherent in causing braiding, bar formation and growth. Second, when applying models of braided alluvial architecture deduced from one system to another of a completely different size (for instance, in braided alluvial reservoir heterogeneity models) it is essential to know



4 c . s . BRISTOW & J. L. BEST

(a)

(b)

Fig. 1. Two scales of braided river. (a) Sunwapta River, Alberta, Canada with a one kilometre wide braidplain, 20-50 m wide channels and predominantly gravel bedload. Flow is from left to right. (b) The Congo (Zaire) River with a braidplain width of approximately 8 km and a sand bed. Flow is from right to left.




which geometrical attributes are scale invariant or scale dependent. Data on the morphology of the largest braided rivers is now becoming both more available and of sufficient resolution (e.g. the Flood Action Plan Studies upon the Brahmaputra, see Thorne et al. this volume) to enable this task to be tackled. The links between scale dependence and flow process will then pose the next research goal.

Facies models of braided alluvium

The vertical profile models of braided alluvium presented by Miall (1977, 1978) were based on the deposits of modern braided rivers and were therefore suitable models for the interpretation of ancient braided river deposits, given the proviso that these models only encompassed a certain range and type of depositional setting. However, the vertical profile is not unique and Jackson (1978) and Bridge (1985) have pointed out the convergence of coarse-grained braided river profiles and coarse-grained meandering river models. This realization led to the development of architectural analysis (Allen 1983) where lateral profiles are used to assess the geometry of deposits and reconstruct the original depositional channel form. This

approach became modified by Miall (1985) into architectural element analysis which switched the emphasis from the use of geometry as a primary discriminant to consideration of a hierarchy of bounding surfaces and the com- position of depositional elements. The use of architectural element analysis is ably demonstrated by Miall (this volume) although the mechanics of the approach have been criticised by Bridge (in press).

Braided rivers may be envisaged as a series of channel segments which divide and rejoin around bars in a regular or repeatable pattern. The divisions, channel bends and confluences have characteristic forms and fluid dynamics which can be used to construct an improved model of braided river sedimentation. The evi- dence comes from both theoretical models of braid evolution where braided channels may evolve from straight channels with alternate bars (Bridge 1985), field studies of channel migration in modern rivers (Bristow 1987a) and statistical analysis of braiding parameters. Field studies of the low sinuosity braided Calamus River show that the channels on either side of a braid bar have flow characteristics similar to two curved channel segments (Bridge et al. 1986). From these observations it is predicted that braid bars,

Fig. 2. Satellite image of bars in part of a braided section of the Amazon River. Note the paired accretion topography on the braid bars illustrating that braid bars may accrete laterally on both sides. Braidplain is approximately 10 km wide.



6 C. S. BRISTOW & J. L. BEST

as well as migrating both downstream and across channel, may also accrete laterally in a manner similar to point bars in meandering rivers (Bridge this volume). This is confirmed by studies of accretion patterns in recent braided river deposits (Bristow 1987a) and the presence of paired accretion topography can be clearly seen in the satellite images of braided rivers such as parts of the Amazon River (Fig. 2).

In scale models of gravel bed rivers Ashmore (1991) has shown that braiding can be accom- plished in four ways: accumulation of a central bar, chute cut-off of point bars, conversion of transverse unit bars to mid-channel braid bars and dissection of multiple bars. Preliminary observations indicate that similar braiding processes may also operate in sand bed braided rivers. It may now be possible to adopt a new approach to the development of braided river depositional model where form is just as important as fabric. Furthermore, the processes and their morphological expression may be indepen- dent of scale (see above) and common to braided rivers with a range of grainsizes. As a result these morphological elements may be useful tools for the recognition and interpretation of ancient braided river deposits. Cartoons of morphological elements from common braiding processes are shown in Fig. 3. The relative pro-

portions of the morphological elements may be derived from models of this type or from studies of modern rivers. In the Brahmaputra River, Bristow (1987a) found that exposed new mid- channel bars comprised 13% of deposits by area over a six year period, while lateral accretion to the bank and mid-channel bars amounted to 49%, upstream and downstream accretion were 7.5% and 15.5% respectively and the remaining 15% was formed through channel abandon- ment ~. However, additional channel fill and confluence scour elements which are submerged at low flows are missing from this analysis and should be incorporated into more complete models (see below). It can be deafly seen that lateral accretion will be an important component of braided river deposition although as Bridge (this volume) points out there is a continuum between upstream, through lateral to downstream accretion which may be difficult to resolve at outcrop without measuring the orien- tation of the accretion surfaces relative to palaeoflow.

Aggradation and preservation

A particular weakness of existing braided river facies models arises because they are largely based on sections measured from exposed bars

Fig. 3. Catoons of depositional morphology based on braiding processes derived from scale models of gravel bed braided rivers (Ashmore 1991) and observations of the Brahmaputra River :Bristow 1987b).




or river banks (Miall 1977, 1978; Williams & Rust 1969; Smith 1970; Cant & Walker 1978; Bristow this volume). It is possible to conjecture that bar top sequences should be preserved after avulsion or migration but in many cases the sandbody may be dominated by the deeper channel deposits as the upper, bar-top deposits are removed through subsequent erosion. Huggenberger (this volume) argues that low rates of aggradation will lead to the preferential preservation of the lower topographic (deeper) parts of a river. Studies in the Brahmaputra River (Klaassen & Vermeer 1988) indicate that the deepest natural scours occur at channel bends and confluences. Confluence scour and fill may therefore form an important element of the braided alluvium (e.g. see Huggenberger this volume, who describes junction deposits as a dominant depositional element of the Pleisto- cene Rhine gravels). Recent models of confluence sedimentation have been presented by Bristow et al. (1993) and are reviewed by Bridge (this volume). The scour associated with channel bends, nodal constrictions and obstacles should also not be ignored (Salter 1993).

It is also important to consider the short-term

deposition rate as opposed to the long-term basin subsidence/aggradation rate in shaping what is preserved in the ancient deposit. Most alluvial sedimentary sequences represent long periods of non-deposition or erosion punctuated by rapid and short-lived depositional events. This is amply illustrated with reference to scour and fill at the Ganges-Brahmaputra confluence over a period of 12 years (Fig. 4) which shows periods of rapid incision and deposition at this site, with up to 9 m of deposition at the confluence occurring in one year. Hence, although we may calculate long-term basin aggradation/ subsidence rates, it is the manifestation of the 'geologically instantaneous' processes (e.g. scour fill, bedform migration, anabranch avulsion) that will dominate the sedimentary fill of the channels. Bentham et al. (this volume) also highlight the importance of aggradation rate in determining the architecture of braided alluvial deposits and that it is a common mis- conception that braided alluvium does not contain appreciable quantities of fine grained sediment, this being dependent also upon the sediment supply, climate and tectonic regime. Preservation within braided rivers may be

Fig. 4. Scour and fill at the junction of the Ganges and Brahmaputra rivers, Bangladesh. This plot, showing data collected over a 12 year period, shows the high rates of scour and fill present at this junction. Scour is given as an actual depth and as a relative depth, ds, through division of the actual scour depth by the mean upstream channel depths. Double arrows indicate the approximate flood peaks in 1985 and 1986.



8 C.S. BRISTOW & J. L. BEST

viewed as a function of the frequency of avulsion of the channel belt, the rate of lateral migration of the alluvial system and aggradation rate (Bridge & Leeder 1979). A schematic diagram of avulsion, migration and aggradation rate controls upon braided alluvial architecture (Fig. 5) illustrates that while the preservation of overbank and bar-top sediments increases with aggradation rate the nature of the channel deposits may not change appreciably. However, an increase in either the frequency of channel- belt avulsions (initiated either through major floods or tectonic events) or the rate of migration may radically alter the sandbody geometry, a feature revealed in alluvial simulation models (e.g. Bridge & Leeder 1979). Avulsion frequency appears to increase with high rates of sediment accumulation (Bridge & Leeder 1979), since this is most likely to lead to rapid establish- ment of local gradients favouring avulsion. The preservation of form may also be associated indirectly with a high rate of accretion produced under conditions of rapid aggradation which favour more frequent avulsion. Instantaneous avulsion of a river may result in complete form preservation. In reality this is an unlikely event and, through a combination of migration and

aggradation, channel deposits will become re- worked, superimposed and stacked to form multilateral and multistorey sandbodies. Multi- channel braided rivers are almost certain to form multilateral/multistorey sandbodies. Preserva- tion style within the channel will be a function of the local rate of aggradation and the size and sequence of bedforms/scour surfaces that affect any particular spatial location.

Conclusions: the economic importance of braided rivers

Braided alluvial deposits form substantial hydrocarbon reservoirs (see Martin this volume), sites for the deposition and accumulation of heavy minerals (see Smith & Minter 1980; Slingerland & Smith 1986; Karpeta this volume) and important sand and gravel reserves. One central problem within all of these applications is an understanding of the internal heterogeneity of the braided alluvial architecture both in terms of sandbody connectivity, shale intercalations and the depositional controls upon subsequent diagenesis. Within all of these fields several topics demand urgent atten-

Fig. 5. A schematic diagram illustrating the preservation of braided river depositional morphology as a function of aggradation rate, lateral channel migration and channel-belt avulsion. The preservation of isolated channel sandbodies preserving morphology requires frequent avulsions as well as rapid aggradation.




tion in relating the depositional process to the preserved sediment. This is especially important for coarse-grained braided rivers where the recognition of depositional elements, especially in core, becomes far more problematical. Several key issues, which relate to some of the points raised previously, are evident.

(i) What depositional elements comprise braided alluvium and how is their relative abundance, three-dimensional geometry and spatial distribution influenced by both autocyclic (e.g. hydrograph type, local sediment transport) and allocyclic (e.g. aggradation rate, active tectonics) controls?

(ii) How does the depositional architecture of coarse and fine grained braided rivers differ? How widely is lateral accretion preserved in relation to downstream, upstream or vertical accretion?

(iii) Do the geometrical characteristics and sedimentary structures of braided alluvium show any scale dependence?

In addition to these more geologically related applications, braided rivers will continue to form zones of major threat to human life and settle-

ment as well as posing regions of significant engineering complexity. The issues outlined above overlap directly with some of the pressing needs which can be highlighted for extension of our process knowledge base.

(i) What processes control the dispersal of sediment and bed/bank erosion at both channel confluences and diffluences?

(ii) What is the feedback between flow division, braiding, local sediment yield and channel shifting/bank erosion?

(iii) What controls the spatial and temporal variability of sediment transport at both the channel and braidplain scale?

(iv) What variables influence the depth and distribution of scour at key sites (bends, junctions, flow diversions)?

It is clear that progress towards possible solutions to these broad goals will only be achieved through interdisciplinary collaboration and the desire to cross traditional discipline boundaries in tackling these complex issues.

We would like to thank P. J. Ashworth, J. S. Bridge and T. Salter for the useful and stimulating con- versations on some of the topics included in this paper which have helped shape our views on these issues.

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Braided rivers: perspectives and problems

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