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Braided Rivers: Perspectives and Problems

Braided Rivers: Perspectives and Problems

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Braided : perspectives and problems

C. S. BRISTOW 1 & J. L. BEST 2 1 Research School of Geological and Geophysical Sciences, Birkbeck College, Gower Street, London WCIE 6BT, UK 2 Department of 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, of and sedimentary architecture of braided rivers many key issues remain to be addressed. In particular, several areas demand attention: the mechanisms of braid initiation; -diffluence dynamics, the of sedimentary over a range of grain sizes and the influence of flow stage and aggradational regime upon the depositional architecture over a range of scales. This paper focuses upon these issues and highlights several areas of fruitful future interdisciplinary collaboration.

Braided rivers form important topics of study for braided rivers may be fruitfully extended by many scientists and one of the primary aims of adopting such an interdisciplinary scope. this volume is to bring together work from many disciplines in an integrated approach to braided Zones of flow convergence and divergence rivers. For the geomorphologist braided fluvial systems are abundant within upland and pro- Braided rivers are characterized by 'having a glacial settings and are agents of considerable number of alluvial channels with bars and and . For engineers .between meeting and dividing again, and pre- the high rates of sediment transport, senting from the air the intertwining effect of a and erosion combined with frequent channel braid' (Lane 1957). The division and joining of shifting and rapid erosion may pose con- channels are essential features of braided rivers siderable design problems both to within- and, whilst the bars within these rivers have channel structures, such as piers (e.g. received attention from both geomorphologists Mosley 1982a; Sutherland 1986) and braidplain and sedimentologists, the areas of flow con- edge constructions such as and railways vergence and divergence have not been incor- Finally, for the braided rivers form porated into braided depositional models. important agents of deposition that have been The flow dynamics and morphology of channel responsible for the accumulation of many sedi- have been studied by several mentary sequences that form valuable , researchers (e.g. Mosley 1976, 1982a; Best 1986, hydrocarbon reservoirs and sites for heavy 1987, 1988; Best & Roy 1991; Roy & Roy 1988; accumulation. Because of these abun- Roy & Bergeron 1990; Roy et al. 1988), and dant and diverse applications, knowledge of the recent attention has highlighted the abundant mechanics and deposits of braided rivers is vital confluences within braided rivers (Ashmore within many areas and yet, when compared to 1982; Ashmore & Parker 1983; Klaassen & the wealth of literature upon meandering Vermeer 1988). However, the link between flow systems, they have been comparatively under- convergence and the downstream division of studied. This may, in part, be due to the diffi- flow has been neglected, despite the fact that this culty of measuring flow, sediment transport and transition is the area which may be of funda- morphology in the rapidly shifting mental importance to the development of braid environment. Future progress in understanding bars (e.g. Ashmore 1991; Ashworth et al. 1992). the mechanics and morphology of braided rivers Although some depositional models of braided demands interdisciplinary collaboration and rivers are beginning to recognise and incor- calls for a more integrated approach across the porate confluence scour and fill (Cowan 1991; sciences than may have been present until com- Bristow et al. 1993; Bridge this volume; Huggen- paratively recently. This paper highlights some berger this volume; Seigenthaler & Huggen- specific areas upon which our knowledge of 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. Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

2 C. S. BRISTOW & J. L. BEST are less well understood and ignored in these erosional and depositional modification related models. In general, flow divergence is associated to changes in stage. At higher flow stages when with flow deceleration and sediment deposition the largest volumes of sediment are transported, and, once deposition has been initiated, the the channels are often scoured, bars may be sediment accumulation is likely to promote reduced in height or in some cases completely further flow division, deposition and bar forma- eroded. However, during falling stage maximum tion. Divergent flow may also impinge on the deposition occurs as and flow com- bank at an increased angle leading to bank petence are reduced. Channel beds aggrade, the erosion, channel widening and a local increase in high stage may be modified and new available sediment, all of which are likely to lead bars may be formed or enlarged as sediment is to the development of a new (Carson deposited. As discharge continues to fall, bars 1984; Thorne et al. this volume). The deposits may become emergent and dissected by low of diffluence areas may therefore form the stage channels. Additionally, the nature of the foundations of braid bars but there are no known falling limb recession (rate and length of reces- descriptions of these deposits and their internal sion) will be important not only in the reworking structure. It is possible that some of higher stage , but also in the may occur by vertical or even upstream accre- deposition and spatial distribution of the finer tion in the diffluence areas in braided rivers grained sediments ( and clays) which may (Ashmore this volume) and these must be mani- constitute discontinuous permeability barriers fested within the sedimentary record. shear within braided . Classification of emer- stress has been shown to increase in shallow gent areas based on their low stage appearance flows over bar tops (Cheetham 1979) where a may be deceptive and care needs to be taken in coarse bed armour may form. In coarse-grained determining which areas are bars, scaling with braided rivers the bar heads may be charac- channel width, and those which are partially terized by coarse-grained sediments that are dissected bars or stranded collections of bars imbricated or laminated (Bluck 1979). There is a (Church & Jones 1982; Bridge 1985). Little data clear need for both an understanding of the fluid exists for the comparison of bar and channel dynamics of the diffiuence zone and how this morphology at different flow stages (but see may influence braid bar initiation and internal Mosley 1982b) and this is an area in which structure. controlled and correctly scaled models combined with field studies may contribute The influence of flow stage greatly to our understanding. The planform appearance of braided rivers can Channel hierarchies change radically with flow stage (see fig. 2 in Thorne et al. this volume). Indeed some authors The presence of a hierarchy of channels within (Doeglas 1962; Miall 1977) have proposed that braided rivers was first suggested by Williams fluctuations in discharge are a pre-requisite for & Rust (1969), who described three orders of braiding although this may often be discounted channel in addition to a series of levels within the as has been demonstrated by the modelling of river which represented active and inactive parts -bed braided planforms in constant dis- of the channel system. These orders and levels of charge scaled flume experiments (e.g. Ashmore bar deposit may also be adjusted to the domi- 1982, 1991). Bluck (1979) suggests that bars may nant discharge of the alluvial system (see Thorne disappear at high flow stages, reforming as dis- et al. this volume). In the scheme proposed by charge falls, and similar observations have been Williams & Rust (1969) the entire river and reported by Smith (1974), Carson (1984) and active channels were termed the 'composite Gupta & Dutt (1989). This may imply that some channel' and the 'stream channel' respec- braided rivers act as single channels at bankfull tively adding two additional levels to the hier- stage and only adopt a characteristic braided archy. This system was modified by Bristow pattern on the falling stage. However, these (1987a) to a three hierarchy and Bridge (this observations appear to be fairly unusual and volume) suggests additional modifications to most braided rivers retain their bars at both high this view. If one accepts that the river can and low flow stage (Krigstrom 1962; Coleman operate as a single entity with channels within 1969; Smith 1970; Cant & Walker 1978; Collin- it and that there may be different scales of son 1970; Church & Jones 1982; Bridge et al. channels which depend upon total discharge and 1986; Bristow 1987a). Where bars exist for discharge fluctuations, then a threefold hier- periods of time in excess of a single event archy of channels is required. The first order they will experience a complex history of comprises the whole river (see fig. 2 of Thorne Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

PERSPECTIVES AND PROBLEMS 3

et al. this volume, for an image of the changes in discharge and discharge duration in full flood). Second order than a -bed river'. While a clear distinction channels are the dominant channels within the may be required when calculating sediment river whilst third order channels are primarily transport, there appears to be more morpho- low stage features which modify the bars logic similarity between sand and gravel bed deposited by the second order channels. Lower rivers than differences between them. The order channels may also exist which modify the depositional styles of gravel and sand bed third order bars. One implication of viewing braided rivers will directly affect the facies braided rivers as hierarchical systems is that models that must be used to interpret ancient individual ancient braided river deposits. Investigation of the correlation should also show a hierarchy of channel dimen- between the classic sand-bed braided river sions, whereas stacking of a single channel river depositional models (e.g. Cant & Walker 1978; is more likely to produce similar sized sand- Collinson 1970) with those derived for gravel- bodies. The presence of different magnitude bed braided alluvium (e.g. Bluck 1979; Steel & channels within a single sandbody may therefore Thompson 1983; Ramos & Sopena 1983; Ramos be an indication of braiding. However, caution is et al. 1986; Smith 1990) remains a high priority, required for, if the third order channels are both in terms of evolving realistic facies inter- dependent on stage changes, then the presence pretation and in the feedback these schemes may of several scales of channel could be due provide into understanding the similarities and primarily to changes in stage. Therefore, a differences in depositional process between hierarchy of channel sandbody sizes is not an these rivers. a priori indicator of a braided river but is a feature most likely to occur in braided rivers with fluctuating discharges. Braided channel morphology and scale Studies of braided channel dynamics and Grainsize influences upon braiding deposits have ranged across at least five orders of channel size from small laboratory models Within the geological and geomorphological (scaled field dimensions c. 2-20m channel literature there has been a long held distinction width), to natural braidplains several kilometres between gravel-bed braided rivers and sand-bed in width (e.g. Ashworth et al. 1992; Warburton braided rivers. However, many natural gravel et al. this volume) to the largest alluvial rivers bed rivers include those with bedloads of sand, such as the Brahmaputra (Coleman 1969; , , and even Bristow 1987a; Thorne et al. this volume) which material while fine-grained sand-bed braided have braidplain widths of up to 20 km, individual rivers are held to contain less than 25% gravel channel widths of several kilometres and (Bluck 1979). It is rare for a river to have only maximum scour depths of up to 50 m. The issue one type of bed material, and most rivers have a of scaling depositional form and formative range of bed and bank material types. Although process across this range of braided channel sizes some small-scale alluvial bedforms show a clear is rarely addressed yet is central when applying controlled stability (e.g. ripples, lower results and models from one channel size to a stage plane beds, particle clusters) many larger system of a completely different magnitude. scale bar forms and the braided Superficial examination (Fig. 1) often reveals appear similar in rivers of widely differing grain a gross similarity to the appearance of braided size. There has been little quantitative work systems of widely differing size, yet data is comparing the planform characteristics of gravel required, both on the planform and cross- and sand-bed braided rivers even though their sectional characteristics, to substantiate or apparent similarities may suggest important refute this apparent similarity. The self- common processes (Fig. 1). Comparison of con- similarity of form across a range of scales or fluence scour in the sand-bed Brahmaputra the scale dependent nature of the geometry of River, for example, reveals similar relation- braided rivers has several fundamental appli- ships between confluence angle and scour depth cations. First, self-similarity across scales to those found in gravel-bed rivers (Klaassen & of braiding may shed light upon the processes Vermeer 1988). In a recent review of the differ- inherent in causing braiding, bar formation and ences between gravel and sand-bed rivers growth. Second, when applying models of Simons & Simons (1987) concluded that 'gravel- braided alluvial architecture deduced from one bed reaches of a river system exhibit totally system to another of a completely different size different morphological characteristics and in (for instance, in braided alluvial reservoir general, they will be less responsive to modest heterogeneity models) it is essential to know Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

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

(a)

(b) Fig. 1. Two scales of braided river. (a) Sunwapta River, , 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. Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

PERSPECTIVES AND PROBLEMS 5 which geometrical attributes are scale invariant approach became modified by Miall (1985) into or scale dependent. Data on the morphology of architectural element analysis which switched the largest braided rivers is now becoming both the emphasis from the use of geometry as a more available and of sufficient resolution primary discriminant to consideration of a (e.g. the Flood Action Plan Studies upon the hierarchy of bounding surfaces and the com- Brahmaputra, see Thorne et al. this volume) position of depositional elements. The use of to enable this task to be tackled. The links architectural element analysis is ably demon- between scale dependence and flow process will strated by Miall (this volume) although the then pose the next research goal. mechanics of the approach have been criticised by Bridge (in press). Facies models of braided alluvium Braided rivers may be envisaged as a series of channel segments which divide and rejoin The vertical profile models of braided alluvium around bars in a regular or repeatable pattern. presented by Miall (1977, 1978) were based on The divisions, channel bends and confluences the deposits of modern braided rivers and were have characteristic forms and therefore suitable models for the interpretation which can be used to construct an improved of ancient braided river deposits, given the model of braided river sedimentation. The evi- proviso that these models only encompassed a dence comes from both theoretical models of certain range and type of depositional setting. braid evolution where braided channels may However, the vertical profile is not unique and evolve from straight channels with alternate bars Jackson (1978) and Bridge (1985) have pointed (Bridge 1985), field studies of channel migration out the convergence of coarse-grained braided in modern rivers (Bristow 1987a) and statistical river profiles and coarse-grained meandering analysis of braiding parameters. Field studies of river models. This realization led to the develop- the low braided Calamus River show ment of architectural analysis (Allen 1983) that the channels on either side of a braid bar where lateral profiles are used to assess the have flow characteristics similar to two curved geometry of deposits and reconstruct the channel segments (Bridge et al. 1986). From original depositional channel form. This these observations it is predicted that braid bars,

Fig. 2. Satellite image of bars in part of a braided section of the . 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. Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

6 C. S. BRISTOW & J. L. BEST as well as migrating both downstream and across portions of the morphological elements may be channel, may also accrete laterally in a manner derived from models of this type or from studies similar to point bars in meandering rivers of modern rivers. In the Brahmaputra River, (Bridge this volume). This is confirmed by Bristow (1987a) found that exposed new mid- studies of accretion patterns in recent braided channel bars comprised 13% of deposits by area river deposits (Bristow 1987a) and the presence over a six year period, while lateral accretion to of paired accretion topography can be clearly the bank and mid-channel bars amounted to seen in the satellite images of braided rivers such 49%, upstream and downstream accretion were as parts of the Amazon River (Fig. 2). 7.5% and 15.5% respectively and the remaining In scale models of gravel bed rivers Ashmore 15% was formed through channel abandon- (1991) has shown that braiding can be accom- ment ~. However, additional channel fill and con- plished in four ways: accumulation of a central fluence scour elements which are submerged at bar, chute cut-off of point bars, conversion of low flows are missing from this analysis and transverse unit bars to mid-channel braid bars should be incorporated into more complete and dissection of multiple bars. Preliminary models (see below). It can be deafly seen that observations indicate that similar braiding pro- lateral accretion will be an important component cesses may also operate in sand bed braided of braided river deposition although as Bridge rivers. It may now be possible to adopt a new (this volume) points out there is a continuum approach to the development of braided river between upstream, through lateral to down- depositional model where form is just as import- stream accretion which may be difficult to ant as fabric. Furthermore, the processes and resolve at without measuring the orien- their morphological expression may be indepen- tation of the accretion surfaces relative to dent of scale (see above) and common to braided palaeoflow. rivers with a range of grainsizes. As a result these morphological elements may be useful tools for and preservation the recognition and interpretation of ancient braided river deposits. Cartoons of morpho- A particular weakness of existing braided river logical elements from common braiding pro- facies models arises because they are largely cesses are shown in Fig. 3. The relative pro- 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). Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

PERSPECTIVES AND PROBLEMS 7 or river banks (Miall 1977, 1978; Williams & deposition rate as opposed to the long-term Rust 1969; Smith 1970; Cant & Walker 1978; basin /aggradation rate in shaping Bristow this volume). It is possible to conjecture what is preserved in the ancient deposit. Most that bar top sequences should be preserved after alluvial sedimentary sequences represent long or migration but in many cases the periods of non-deposition or erosion punctuated sandbody may be dominated by the deeper by rapid and short-lived depositional events. channel deposits as the upper, bar-top deposits This is amply illustrated with reference to scour are removed through subsequent erosion. and fill at the -Brahmaputra confluence Huggenberger (this volume) argues that low over a period of 12 years (Fig. 4) which shows rates of aggradation will lead to the preferential periods of rapid incision and deposition at this preservation of the lower topographic (deeper) site, with up to 9 m of deposition at the con- parts of a river. Studies in the Brahmaputra fluence occurring in one year. Hence, although River (Klaassen & Vermeer 1988) indicate that we may calculate long-term basin aggradation/ the deepest natural scours occur at channel subsidence rates, it is the manifestation of the bends and confluences. Confluence scour and 'geologically instantaneous' processes (e.g. fill may therefore form an important element of scour fill, migration, the braided alluvium (e.g. see Huggenberger avulsion) that will dominate the sedimentary fill this volume, who describes junction deposits as of the channels. Bentham et al. (this volume) a dominant depositional element of the Pleisto- also highlight the importance of aggradation rate cene ). Recent models of con- in determining the architecture of braided fluence sedimentation have been presented by alluvial deposits and that it is a common mis- Bristow et al. (1993) and are reviewed by Bridge conception that braided alluvium does not (this volume). The scour associated with channel contain appreciable quantities of fine grained bends, nodal constrictions and obstacles should sediment, this being dependent also upon the also not be ignored (Salter 1993). sediment supply, and tectonic regime. It is also important to consider the short-term Preservation within braided rivers may be

Fig. 4. Scour and fill at the junction of the Ganges and Brahmaputra rivers, . 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. Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

8 C.S. BRISTOW & J. L. BEST viewed as a function of the frequency of avulsion aggradation, channel deposits will become re- of the channel belt, the rate of lateral migration worked, superimposed and stacked to form of the alluvial system and aggradation rate multilateral and multistorey sandbodies. Multi- (Bridge & Leeder 1979). A schematic diagram channel braided rivers are almost certain to form of avulsion, migration and aggradation rate multilateral/multistorey sandbodies. Preserva- controls upon braided alluvial architecture tion style within the channel will be a function (Fig. 5) illustrates that while the preservation of the local rate of aggradation and the size and of and bar-top sediments increases sequence of bedforms/scour surfaces that affect with aggradation rate the nature of the channel any particular spatial location. deposits may not change appreciably. However, an increase in either the frequency of channel- belt avulsions (initiated either through major Conclusions: the economic importance or tectonic events) or the rate of migra- tion may radically alter the sandbody geometry, of braided rivers a feature revealed in alluvial simulation models Braided alluvial deposits form substantial (e.g. Bridge & Leeder 1979). Avulsion fre- hydrocarbon reservoirs (see Martin this quency appears to increase with high rates of volume), sites for the deposition and accumu- sediment accumulation (Bridge & Leeder 1979), lation of heavy (see Smith & Minter since this is most likely to lead to rapid establish- 1980; Slingerland & Smith 1986; Karpeta this ment of local gradients favouring avulsion. The volume) and important sand and gravel preservation of form may also be associated reserves. One central problem within all of these indirectly with a high rate of accretion produced applications is an understanding of the internal under conditions of rapid aggradation which heterogeneity of the braided alluvial archi- favour more frequent avulsion. Instantaneous tecture both in terms of sandbody connectivity, avulsion of a river may result in complete form shale intercalations and the depositional con- preservation. In reality this is an unlikely event trols upon subsequent . Within all of and, through a combination of migration and 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. Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

PERSPECTIVES AND PROBLEMS 9 tion in relating the depositional process to the ment as well as posing regions of significant preserved sediment. This is especially important engineering complexity. The issues outlined for coarse-grained braided rivers where the above overlap directly with some of the pressing recognition of depositional elements, especially needs which can be highlighted for extension of in core, becomes far more problematical. our process knowledge base. Several key issues, which relate to some of the (i) What processes control the dispersal of points raised previously, are evident. sediment and bed/ at both (i) What depositional elements comprise channel confluences and diffluences? braided alluvium and how is their relative (ii) What is the feedback between flow abundance, three-dimensional geometry division, braiding, local sediment yield and spatial distribution influenced by both and channel shifting/bank erosion? autocyclic (e.g. type, local (iii) What controls the spatial and temporal sediment transport) and allocyclic (e.g. variability of sediment transport at both aggradation rate, active tectonics) the channel and braidplain scale? controls? (iv) What variables influence the depth and (ii) How does the depositional architecture of distribution of scour at key sites (bends, coarse and fine grained braided rivers junctions, flow diversions)? differ? How widely is lateral accretion It is clear that progress towards possible preserved in relation to downstream, to these broad goals will only be upstream or vertical accretion? achieved through interdisciplinary collaboration (iii) Do the geometrical characteristics and and the desire to cross traditional discipline of braided alluvium boundaries in tackling these complex issues. show any scale dependence? We would like to thank P. J. Ashworth, J. S. Bridge In addition to these more geologically related and T. Salter for the useful and stimulating con- applications, braided rivers will continue to form versations on some of the topics included in this paper zones of major threat to human life and settle- which have helped shape our views on these issues.

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