multichannel rivers: their definition and classification

EARTH SURFACE PROCESSES AND LANDFORMSEarth Surf. Process. Landforms 39, 26–37 (2014)Copyright © 2013 John Wiley & Sons, Ltd.Published online 30 April 2013 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/esp.3419

State of Science

Multichannel rivers: their definitionand classificationPaul Carling,1* John Jansen2 and Lyubov Meshkova11 Geography and Environment, University of Southampton, Highfield, Southampton, SO17 1BJ, UK2 Department of Physical Geography and Quaternary Geology, Stockholm University, Stockholm 106 91, Sweden

Received 24 August 2012; Revised 3 March 2013; Accepted 4 March 2013

*Correspondence to: P. Carling, Geography and Environment, University of Southampton, Highfield, Southampton, SO17 1BJ, UK. E-mail: [email protected]

ABSTRACT: The etymology and historic usage of such terms as ‘anabranch’, ‘anastamose’ and ‘braided’ within river science arereviewed. Despite several decades of modern research to define river channel typologies inclusive of single channels and multiplechannel networks, typologies remain ill-conditioned and consequently ill-defined. Conventionally employed quantitative planformcharacteristics of river networks possibly cannot be used alone to define channel types, yet the planform remains a central part of allmodern classification schemes, supplemented by sedimentological and other qualitative channel characteristics. Planform characteris-tics largely have been defined using non-standardized metrics describing individual network components, such as link lengths, braidingintensity and bifurcation angles, which often fail to separate visually-different networks of channels. We find that existing typologiesremain pragmatically utilitarian rather than fundamentally physics-based and too often fail to discriminate between two distinctiveand important processes integral to new channel initiation and flow-splitting: (i) in-channel bar accretion, and (ii) channel avulsionand floodplain excision. It is suggested that, first, if channel planform is to remain central to river typologies, then more rigorous quanti-tative approaches to the analysis of extended integral channel networks at extended reach scales (rather than network components) arerequired to correctly determine whether ‘visually-different’ channel patterns can be discriminated consistently; and, second, if suchvisually-different styles do in fact differ in their governing processes of formation and maintenance. A significant question is why do somany seemingly equilibrium network geometries possess a large number of anabranches in excess of predictions from theoreticalconsiderations? The key research frontier with respect to initiating and maintaining multichannel networks remains the understandingand discrimination of accretionary-bar flow splitting versus avulsive processes. Existing and new knowledge on flow splitting processesneeds to be better integrated into channel typologies. Copyright © 2013 John Wiley & Sons, Ltd.

KEYWORDS: channel typology; channel classification; anastomose; anabranch; braiding

Introduction

Multichannel networks have been recognized cartographicallyat least since the 16th century (Figure 1). Yet despite severaldecades of research in the modern era, significant uncertaintyremains as to how to systematically describe the variety of riverstyles that arise wherever water and sediment conveyance isdivided between multiple independent channels (Makaske,2001). In particular, there is ongoing confusion concerningthe descriptors ‘anastomosing’ and ‘anabranching’ rivers (Northet al., 2007) with both terms now commonly used to describeequilibrium multichannel networks of apparently similar plan-form (Rosgen, 1994; Nanson and Knighton, 1996; Heritageet al., 2001). Despite attempts to standardize the terminologyby ascribing ‘anastomosing’ to the low-energy end of a genericspan of ‘anabranching’ styles (Nanson and Knighton, 1996;Nanson and Gibling, 2004), the two terms often are usedinterchangeably (Bridge, 2002, p. 147). Some have usedthe two terms to define different non-equilibrium phases of

the development of a given channel–network (Taylor andWoodyer, 1977; Brown et al., 1995) or as a descriptor of non-equilibrium networks (Kleinhans and van den Berg, 2011) thatare transitional between meandering single-thread and braid-ing systems (Knighton and Nanson, 1993; Eaton et al., 2010).Thanks to the influential tripartite scheme of Leopold andWolman (1957), the term ‘braiding’ is well established; yeteven here the waters are muddied since few of their braidingchannels are truly braiding and many would be considered tobe anabranching in keeping with modern terminology (Nansonand Knighton, 1996; Eaton et al., 2010).

Given the disarray, improved process-based classification isunlikely to be achieved by building on past typologies withoutconsiderable reflection. Current descriptors are not sufficientlysensitive to formative differences and visually dissimilarmorphologies may be ascribed similar quantitative planforms.While it is true that planform similarity, between systems, mayreflect convergent evolution of forms via differing processes, itshould be the aim of a process-based classification to promote

Figure 1. Da Vinci’s carefully executed map of the braided ArnoRiver, west of Florence (1502). The dots visible between the channelsat the topmost bifurcation represent recent river sedimentation andreflect da Vinci’s interest in barhead evolution, river bank erosion andchannel migration. This consideration of a river network was paralleledby comparisons with networks of superficial veins visible on humanarms. The Royal Collection © 2011, Her Majesty Queen Elizabeth II. Thisfigure is available in colour online at wileyonlinelibrary.com/journal/espl

27MULTICHANNEL RIVERS: THEIR DEFINITION AND CLASSIFICATION

mechanistic understanding of how different river styles areformed. Process is defined herein in the sense advanced byChurch (1996) as: ‘lower-level’, i.e. at small scales classicalmechanical deterministic theories hold, while at larger scales‘higher-level’ contingent, exogenous factors play an increased

Table I. The extent of anabranching in the world’s five largest rivers

Five largest rivers (in terms of meanannual discharge)

Upstream–downstreamlimits of anabranching

Amazonas-Solimões-Ucayali Contamana–GurupáCongo Kibombo–TshumbiriOrinoco Ventuari–Barrancas deltaChangjiang Yichang–Chongming DaoPadma-Jamuna-Brahmaputra Pasighat–Meghna delta

aDefined as hosting at least one large alluvial island (≥1 km) per unit length

Copyright © 2013 John Wiley & Sons, Ltd.

role. Here we advance an approach that relies less on planformtypology but rather on improved discrimination of formativeprocesses and morphologies that are depositional versus thosethat are erosional. Such an approach is consistent with effortsto unite surface processes, the concern of geomorphologists,with that of sedimentologists whose main consideration lieswith sedimentary products of the geological record. Finally,the motivation for this paper in part stems from the rapidlygrowing interest in the world’s largest rivers (Gupta, 2008),which essentially all exhibit extended multichannel reaches(Table I), making this the dominant pattern for large rivers(Nanson and Gibling, 2003; Jansen and Nanson, 2004;Latrubesse, 2008).While there is currently no comprehensive ex-planation for why the world’s largest rivers aremultichannelled, aprocess-based classification is a constructive opening act forsubsequent studies that will attempt to solve this outstandingresearch problem. We recognize that deltaic networks are partof river systems, but the distributive networks of deltas areexcluded from consideration in this paper.

Origins of the Terms ‘Anastomosing’ and‘Anabranching’

We begin with a focus on etymology that might seem pedanticto some but we regard it as informative to probe the origins andhistoric usage of words when considering how classificationschemes have evolved, especially when usage is contested.‘Anastomosis’ derives from the Latin ‘anastomōsis’, which hasan earlier Greek origin, and originally referred to an opening,outlet, or junction of one (water) body with another. EarlyEnglish usage dates from 1615 and it was specifically appliedto river networks by Jackson (1834) after whom Burton, theexplorer, invoked its original sense as ‘a meeting of waters’ in1859 and then more specifically, in 1876, to describe themultichannel Zambezi River (Burton, 1876). Burton also usedthe verb form ‘to anastomose’ meaning ‘to rejoin’. The firstdefinition of ‘anastomosing-channel’ as a planform distinctfrom classic braiding systems was provided by Schumm(1968) to mean a multichannel suspended-load-dominatedsystem with large, stable islands between channels that areexcised within the neighbouring floodplain. The islands, incontrast to bars (in braiding systems), do not flood duringinbank flow conditions, a distinction dating back to Jackson(1834). Jackson, an English geographer, introduced the term‘ana-branches’ in 1834 (Figure 2) as a contraction of ‘anasto-mosing branches’, referring to defined channels that leave themainstem and then re-enter downstream, with non-floodingislands formed of floodplain material separating the individualbranches (Jackson, 1834). Thus ‘anabranching’ and ‘anasto-mosing’ are grammatically synonymous. Other variant defini-tions of ‘anabranch’ have been used subsequently: adistributary that terminates in ‘sandy soil’ (Jackson in Porter,1913), i.e. a ‘floodout’, and as a ‘blind’ channel with no outlet

Anabranchinglength (km) a

Total alluviallength (km)

Alluvialanabranches (%)

3480 3910 891880 1980 951120 1330 841330 1330 1001000 1000 100

of river equivalent to five channel widths.

Earth Surf. Process. Landforms, Vol. 39, 26–37 (2014)

Figure 2. Extract fromdefinition diagram of Jackson (1834; his Figure 6).The letter ‘C’ is used to clearly indicate single-channel links (anabranches)in the anastomosed network. The key uses the phrase ‘Anadel Branches’as a contraction of ‘anastomosing-deltoidal-branches’ as the figure illus-trates a river feeding a delta, with the receiving water-body to the right.However, elsewhere in the text ‘anastomosing-branches’ is contractedto ‘ana-branches’ and defined as in the figure, to give the modern term‘anabranches’. Jackson provides examples of multichannel networks inthe headwaters of the Indus. This figure is available in colour online atwileyonlinelibrary.com/journal/espl

28 P. CARLING ET AL.

(Riley, 1975; Harwood and Brown, 1993; Brown et al., 1995;Partridge, 2008). Thus ‘anabranch’ historically has had at leastfour meanings: a single channel in a network (Jackson, 1834); afloodout (Jackson in Porter, 1913); a blind distributary (Brownet al., 1995); and as a fugitive channel in a multichannelnetwork, quite remote from the parent river course (Hills,1940; Brown et al, 1995; Carling, 2009). Nanson and Knighton(1996) admit all four definitions within their classification of‘anabranching’. Mayhew (2009) argues that an anastomosedchannel differs from an anabranch channel in that it may dividefurther to form distributaries of its own, but this definitionclearly is at odds with Jackson’s original definition. Evidentlythe term ‘anastomosis’ has historical precedence and ‘anasto-mose’ is commonly used as a verb whereas ‘anabranch’ is acontraction and a recognized noun (from 1834); the use of‘anastomosis’ as a noun describing a conjunction is recordedas rare (Oxford English Dictionary, 1989). Thus while it remainsgrammatically correct to state that ‘an anastomosed river con-tains many anabranches’, scientific usage has led to differencesarising in the fluvial community for no good reason. For exam-ple, Nanson and Gibling (2003, 2004) advance a definition ofanabranching systems that includes the subset ‘anastomosing’due to presumed common usage and preclude the use of eitherterm in descriptions of braiding rivers (see also Ashmore, 2003).


While standardization of a process-based terminology is to bewelcomed (Nanson and Knighton, 1996), it is preferable thatthe original grammatical definitions are clearly recognizedalong the way.

The Roots of a Terminological Divide

In the earlier literature (c. pre-1950) ‘anabranch’was often usedto describe a single-channel link in networks of interlinking wa-terways. The Oxford English Dictionary (1989) records the termas especially used in Australia, beginning in 1847 and 1849.Morris (1898) noted the English origin, detailed below, and Mill(1898) commented that the term ‘had been entirely forgotten bythe geographers of Europe’ but noted that, at that time, theexpression was still used in Australia to describe ‘side chan-nels’. Stamp (1961) described its Australian use from 1920but regarded the term as obsolete. Fairbridge (1968) correctlyreports the original use of the term as a noun (describing a sec-ondary channel in a network), but incorrectly as deriving fromdescriptions of the Murray and Murrumbidgee systems in east-ern Australia (Hills, 1940); although, the use of the term ‘anas-tomosis’ to describe multichannel systems was also prevalentin Australia at this time (Hills, 1940; Whitehouse, 1943). Ratherthan ‘anabranching’, ‘anastomosing’ (and related terms) wasused simultaneously and widely by UK and North Americanscientists, with the noun-form ‘anabranch’ reserved for describ-ing single channels within an anastomosing network (Stamp,1961; Fairbridge, 1968; Schumm, 1968). Although the north–south dichotomy in usage is not clear-cut, the conflation ofterms that started in the 19th century, was perpetuated in the20th century due to the differing approaches of geomorpholo-gists and sedimentary geologists (North et al., 2007), who bothhave contributed to the development of river classification. Theformer have concentrated on developing distinctive planformdescriptions, qualified by process and product to be inclusiveof variant styles (Nanson and Knighton, 1996). The latterhave focused on developing type-stratigraphic models formultichannel systems and until recently tended to see theAustralian ‘anabranching’ examples as anomalous specialcases (Miall, 1996, p. 33), preferring North American anasto-mosed models as standard, thereby rejecting the term‘anabranch’ by default. It is fundamentally undesirable thatthe disciplines of fluvial geomorphology and sedimentary geol-ogy should develop different channel typologies unless thereare over-riding considerations. If a single typology is to emergeit must come with a predictive capacity to explain surfaceformative processes along with their sedimentary products. Akey issue to be reconciled is that of scale (Brierley, 1996).While geologists generally have been concerned with system-scale river typologies to explain the subsurface alluvial stratig-raphy, geomorphologists have been interested in reach-scalediscrimination based on specific erosional and depositionalprocesses. The reach scale (reach length< 100 channel widths)has worked quite well for single channels (Ferguson, 2008) butless well for multichannel systems as, at larger length scales,network structure is probably determined by landscape history(Church, 2008). The disjuncture between planform and thesediments below was eloquently outlined by Brierley andHickin (1991), and so challenged the veracity of assumed rela-tionships between planform and the controlling processes. Thequestion still stands: is the notion of a distinctive river patternuseful for inferring process-based sedimentary associationsand, in particular, is the classic schema (straight, meandering,braiding (multichannel); Leopold and Wolman, 1957) devel-oped and applied at the appropriate scale to form the basisfor a trans-disciplinary classification?



The Process-basis for Separating‘Anastomosing’ and ‘Braiding’

As a means of side-stepping the terminological confusion,some authorities apply the term ‘multithread channel’ to in-clude both ‘anastomosing’ (sensu Schumm, 1968) and ‘braid-ing’ river planforms (Schumm, 1968; Church, 2002; Gurnellet al., 2009). ‘Braided’ means to ‘twist’ or ‘be interwoven’ fromthe Middle English ‘braiden’ from Old English ‘bregdan’ toweave – the origin and the metaphor is evident. The use of‘braided’ for describing river planforms can be traced at leastto Peale (1879) although it probably has an earlier vernacularorigin. Jackson’s (1834) first use of ‘anastomosis’ was inclusiveof braiding rivers, as with Peale (1879) and Chamberlin andSalisbury (1909). Given that the adjective ‘anastomose’ is adirect synonym for ‘braid’, the separation of these two termshas a pragmatic, rather than grammatical basis; a point raisedby Bridge (2002, p. 149) against anastomosing being used todefine an alternative channel type. We will return, below, tothe question of whether this distinction is process-based. Theterms were used interchangeably throughout the 1960s and1970s (see references in Miall, 1977; Rust, 1978), especiallywithin the USA (Schumm, 1968) as until the late 1970s,single-thread and braiding channels were the only widelyrecognized channel types (Miall, 1977). The term ‘anastomos-ing’ was used by Rust (1978; see also the precedents: Smith,1973; Miall, 1977) to specifically define high sinuosity braid-ing; the term ‘braiding’ being reserved by Rust for low sinuositybraiding systems. In the schemes of Leopold and Wolman(1957), Schumm (1968), and Rust (1978) channel networksconsisting of bars or islands are both subsumed within their def-inition of braiding systems. Rust (1978) considered it ‘to be awaste of a good term’ to use ‘anastomosing’ as a synonym for‘braiding’ and that it deserved a special usage. As recently as1996, Miall (1996, p. 15) recommended that the term ‘anasto-mosing’ be applied to stable channel networks of low to highsinuosity and ‘braiding’ be restricted to unstable low sinuositychannels – note Miall makes no reference to unstable, highsinuosity systems.Numerous field studies since the 19th century had indicated

that braiding rivers are unstable and subject to bar-splitting, andFriedkin (1945) had used flume experiments to demonstrate thatchannel and bar instability characterize braiding streams. In suchstreams, chute development, or flow splitting around new bars,seems to be critical for initiating a braiding planform (Ashmore,1991). In contrast, despite precedence (David and Browne,1950, p. 11), it was not until the 1970s that several studies (Smith,1973; Smith and Putnam, 1980; Smith and Smith, 1980; Carson,1984) recognized a style of channel network that was more sta-ble, although subject to avulsions. The term ‘avulsion’ commonlyis used, as here, to refer to sudden formation of a new channeloutwith the existing channel (Jones and Schumm, 1999). In con-trast bar-evolution, as detailed above, is related to flow-splittingprocesses within an existing channel. Partitioning of sediment-laden flow around mobile bars, in principle, may be describedmechanistically, which may explain the many attempts to modelthe processes numerically (reviewed by Kleinhans et al., 2013). Incontrast, avulsion might be considered a higher-level process,and thus more difficult to predict (Slingerland and Smith, 2004)and model (Siviglia et al., 2013), as it is contingent on theinherited local boundary conditions such as the stratigraphy ofthe floodplain, the riparian vegetation association and the pres-ence or absence of neighbouring palaeochannels (Heritageet al., 1999b; Taylor, 1999).Thus, a common theme began to emerge towards the end of

the 20th century such that ‘anastomosing’ should be applied to


relatively stable channel networks with non-flooding islandsand ‘braiding’ to unstable channels with bars that are regularlyinundated, for reasons of expedient discrimination. TheNanson and Knighton (1996) scheme, more or less conformswith this idea treating ‘anastomosing’ as the type-1 low-energy mud-organic group, and maintaining ‘anabranching’as the generic term for all non-braiding multichannel styles. Inthis respect, and contrary to Leopold and Wolman (1957) andRust (1978), the key classical reference remains that ofSchumm (1968) who clearly identified channel stability as anetwork characteristic requiring the adjective ‘anastomosed’to distinguish stable networks from the channel instability andflooding bars of braiding networks (Schumm, 1968; contrasthis Plates 2 and 3).

Bridge (2002, p. 147) noted that many prior definitions ofbraiding included flow around suites of bars or islands. Notethat braiding systems with unvegetated bars can be definedonly during low flows when the bars are exposed; during highflows, including bankfull, many braiding systems appear as asingle, low-sinuosity channel that might exhibit only a fewexposed bars (Chitale, 1970; Kellerhals et al., 1976; Thorneet al., 1993). In contrast, braiding systems dominated by accre-tionary (perennially vegetated) islands (Leopold and Wolman,1957; Schumm, 1968) that do not flood during high flows,are evidently multichannel forms at both low and high flowconditions. These observations raise an important point.Braid-bars are given diagnostic prominence in many studiesdue to their role in flow-splitting, although field studies gener-ally do not consider their development during formative highflows when most bars are concealed. The modern tendencyto view channels separated by islands as distinct from braid-bar systems, is predicated on the assumption that island-dominated rivers are formative at flows close to or greater thanbankfull, as has recently been shown in northern Australia(Jansen and Nanson, 2010). Nonetheless, classification basedon development processes in dynamic mode must be, in ourview, the preferred approach.

Possibly for the reasons alluded to above, Bridge (2002)reserved the term ‘anastomosing’ for networks of channelsincised into areas of floodplain, and ascribed as ‘braiding’ thosenetworks consisting of accretionary islands developed frombars within the flow field (Bridge and Demicco, 2008,p. 382). In the view of Bridge (2002, p. 314) ‘anastomosed riv-ers are intimately associated with avulsions’; that is, vertically-accreting and therefore non-equilibrium settings. However,where does this leave multichannel networks that are neitheravulsion-dominated nor non-equilibrium, some of which areincised into ancient alluvium or even bedrock? For instance,a substantial list of apparently equilibrium systems has beendocumented in Australia by Nanson and colleagues (Wendeand Nanson 1998; Jansen and Nanson, 2004, 2010; Toothet al., 2008). If the focus is on the planview channel pattern,rather than process, there is no logic in excluding these froman anastomosing class.

In the 1970s several studies of multichannel networksfocused on fine-grained, low-energy systems (Smith, 1973;Smith and Putnam, 1980; Smith and Smith, 1980; Carson,1984) and the term ‘anastomosed’ was employed by analogywith the suspension-dominated streams described by Schumm(1968). In a seminal paper, Nanson and Knighton (1996)reviewed and revised the classification of multichannelnetworks other than classic braiding rivers. Recognizing sixtypes of multichannel network under the generic term‘anabranching’, they noted that their type-1 was usually termed‘anastomosed’ in the original descriptions of the type localities(Smith and Smith, 1980; Rust, 1981). Importantly, Nanson andKnighton (1996) included (i) channel networks that contain



islands excised from floodplains and (ii) networks containingislands formed by within-channel accretion initially as mid-channel bars (see Jackson, 1834 for the first distinction of theseisland classes). Thus in the highly-cited Nanson and Knightonclassification, networks developed predominately by erosion(e.g. avulsion) and networks resulting from flow-splitting viaaccretionary islands are grouped together under ‘anabranching’.Given the distinctive difference in formative processes of thesetwo networks the use of the common appellation firmly placesthe emphasis on the simple-planform multichannelled natureof both types of networks. We discuss below the sustainabilityof this position.

Key Issues

Of the many problems noted in the foregoing section we canraise three main issues that require some resolution for animproved approach to identifying and classifying multichannelrivers: (1) multichannel networks characterized by in-channelislands developed via accretion may be regarded as eitherbraiding, anastomosing or anabranching – depending onclassification scheme; (2) non-avulsive networks incised intoindurated sediments, ancient alluvium or bedrock have noclassification (Bretz, 1923; Garner, 1966; Heritage et al.,1999a; van Niekerk et al., 1999; Heritage et al., 2001; Toothand McCarthy, 2004; Nanson et al., 2005; Meshkova andCarling, 2012; Meshkova et al., 2012). Such channels are spe-cifically excluded from Nanson and Knighton’s (1996) schemedue to the lack of well-determined examples, although a simpleextension would permit their inclusion; (3) the use of the terms‘braiding’, ‘anabranching’ or ‘anastomosing’ as descriptors ispragmatic rather than grammatically derived (Bridge, 2002)and fails to discriminate incontrovertibly without supplemen-tary qualification. Discrimination based upon arbitrary spacing(Kellerhals et al., 1976), size or shape (Osterkamp, 1998;Wyrick and Klingeman, 2011) classification of islands is notdefensible (Makaske, 2001) as there is no physical basis forsuch a division. With the exception of Nanson and Knighton(1996), the reach-scale sedimentation and vegetation controlson channel planform are not prominent in historic attempts todefine system-scale planform patterns. One might expect thatdistinction through consideration of sedimentation styles medi-ated by vegetation associations (Millar, 2000) should constitutean important component of any classification (Schumm, 1968;Hickin, 1984). It was possibly the work of Doeglas (1962) thatfirst considered the relationship between braiding channelpattern and sedimentation style. Yet, Bridge (1985) and Fielding(2008) have noted that sedimentation style alone is not defini-tive in determining larger-scale (geological focus) channelpatterns. Sedimentation style at the reach-scale (geomorpho-logical focus) should have an influence on channel typethrough the ratio of bank strength to incident flow power. Forexample, non-cohesive banks should lead to broad shallowchannels, for which bar theory would predict braiding (Crosatoand Mosselman, 2009), while cohesive banks should impedebank retreat thus encouraging vertical aggradation, leading toavulsion (Kleinhans et al., 2013). It scarcely needs to be statedthat such a proposition, although seemingly simple to test(Braudrick et al., 2009), has not resulted in any clarity withrespect to planform topologies, possibly for reasons advancedby Van den Berg (1995). Several experimental studies havedemonstrated the importance of vegetation in effecting channelform (Gran and Paola, 2001; Tal and Paola, 2007, 2010) andconsiderable effort is being expended to model the interactionsof vegetation and channels (Perona et al., 2009). Yet, the effectof vegetation on the development of distinctive reach-scale


planforms remains elusive (Millar, 2000), although possiblyfundamental (Millar and Quick, 1998; Eaton et al., 2004;Nanson and Gibling, 2004; Jansen and Nanson, 2010)but scale-dependent (Eaton and Giles, 2009). Neverthelesssuch controlled experiments hold much promise to exploreseveral fundamental issues about river planform development(Braudrick et al., 2009).

The problem with planform

Evidently planform alone is, in many cases, not sufficient to dis-criminate process-based differences amongst rivers (Makaske,2001), although consideration of the planform is probably anecessary first step (Gregory, 1985). Planform analysis, except-ing a few notable exceptions, has remained simplistic: countingof channels, number (or angle) of bifurcations, link lengths,etc., coupled with qualitative descriptors of the planform style.The basis of this approach is that the fundamental channel pat-tern is an emergent (visual) and normative property (Phillips,2011) of a few key system components (Kleinhans and vanden Berg, 2011). However, many authors do not define howthey derive their metrics, and summary tables in textbookspertaining to river classification include metrics derived by var-iable means that strictly are not compatible. Such an approachto defining a class for a given river encounters potential diffi-culty when channel style varies rapidly downstream introduc-ing a significant issue of spatial variation. For example, manymultichannel rivers locally exhibit a single channel for shortdistances (Western et al., 1997; Jansen and Nanson, 2004).An example of the spatial switch, at the reach-scale, betweensingle and multiple channels arises with ‘wandering rivers’(Church, 1983; Carson, 1984); a channel style that some viewas transitional between single thread and braided (Nansonand Knighton, 1996). Similar issues of the spatial scale at whichclassification is developed have been noted in respect ofmultichannel networks that exhibit ‘straight’ single-channels,sinuous single-channels or are braided within individual links,such that the reach-scale planform differs from the system-scale pattern (Schumm, 1968; Makaske, 2001).

The three failures of channel classification

Despite several decades of research into channel pattern char-acterization, there is no agreed classification and no agreedterminology; rather what has emerged is a notional sense thatowes more to pragmatism than to physically-based principles.If one accepts that there are key planform ‘river types’ thatmay be defined within a welter of variation (that might proveto be continuous rather than process specific), the key issuesin respect of the failure of channel classification are essentiallythree-fold.

(1) First, it is evident that channels cannot be classified by plan-form alone and yet ill-determined planform types remain atthe heart of all current classification schemes. Note that the‘straight’ class of alluvial channel is nonsensical when con-sidering natural rivers which all have a degree of curvature,if not sinuosity, unless the channels are aligned by geologicstructures, entrenchment, etc. (Nanson and Croke, 1992).The differing arbitrary divisions in the sinuosity metricbetween ‘straight’ and ‘meandering’ adopted historically(Rust, 1978 cites 1.3, 1.5 and 1.7) have no process basis.Meandering rivers ‘cut-off’ through time, switching from‘straight’ to ‘meandering’ as they reduce and then increasetheir sinuosity without any shift in governing processes



(Martinsen, 1983; Stølum, 1996; Hooke, 2004). This consider-ation applies equally to the planview of individual links withinnetworks of channels. Thus sinuosity-based definitions ofclassic equilibrium classification schemes of equilibrium(time independent) forms are not useful for the reason that theyare actually time-dependent. Yet it is important to identifytransience systematically, not least because most systems willbe transient to some degree (Egozi and Ashmore, 2009). Thusan appreciation of planform alone without process insight isinadequate. However improved process discrimination, suchas has been obtained through the development of bar theory,brings its own problems as noted immediately below.

(2) Second, geomorphology as with other natural sciencesis fraught with measurement difficulties (Beven andWesterberg, 2011), which mean that process is usually trickyto determine robustly. Reach-scale planform processchanges are largely discharge driven and yet discharge israrely measured directly, often not near the channel-style‘type location’ and it is unclear how to represent the variationin the hydrograph through the annual cycle for differentclimatic zones, for example. It is often the case that not alot is known about the geology, structure and tectonic historyof many river catchments and the lag and relaxation timesassociated with erosion, sediment transfer and depositionare poorly characterized, such that depositional styles maybe blurred in space and time (Jerolmack and Paola, 2010).River planforms are thus contingent phenomena (Kleinhanset al., 2005) in that some low-level processes ‘readily’ canbe describedmechanistically (e.g. sediment transport arounda bar-head; Kleinhans et al., 2012) while other, higher-levelprocesses (more difficult to elucidate; e.g. avulsion), areconstrained by local and inherited boundary conditionswhich might be site-specific; this point is returned to belowand in the Conclusions. In practice this means that data usedto construct planform style models may be actually counter-productive and therefore ‘disinformative’ (sensu Beven andWesterberg, 2011) such that unconstrained uncertainties inthe data may lead to false inferences. The geomorphologicalcommunity has a poor record of properly treating andreporting data uncertainty. Generally it is not possible tocheck the uncertainty for older data sets (usually containingsmall total numbers of observations) used to constructmodels of channel behaviour. It is preferable that such disin-formation will eventually be rejected through the normalprocess of scientific enquiry as new, better-determined datasets become available. Yet there are important counter-examples, such as the classic Leopold and Wolman (1957)empirical discharge–slope discriminator of meandering tobraiding, which remains popular, even though in modernterminology few of their braiding channels are truly braiding(Nanson and Knighton, 1996; Eaton et al., 2010). Unfortu-nately, although modern Earth-science data are oftenchecked for aleatory error, there is little consideration givento the epistemic errors that arise and become embeddeddue to disinformation which may exist in the best-defineddata sets. Expert opinion often is applied to consider therobustness of channel style models; a process subject todifferent individuals interpretations, biases and utilities(Milner et al., 2013). In some respects the current models ofchannel styles are utilitarian (being found to be useful to abroad audience) and pay deference to historical precedence(Phillips, 2011) rather than subject to rigorous hypothesistesting. So as to refute a utilitarian approach in favour of a sci-entific approach, Hickin (1993) suggested that the adoptionof a distinctive anastomosing river facies model was prema-ture and the adoption of same should await more extensiveconsideration of river styles.


(3) Channel network metrics derivation and use are not robustand additional rigour is required. In addition to improvedprocess discrimination comment was made in the Introduc-tion as to the need for improved planform characterisation.With respect to derivation, the assembly of very large datasets of quantitatively, rigorously-defined network patternsimilarity (that include uncertainty) is now possible usingremote-sensing technology (Gupta et al., 2013) and it willbe important that these data are consistently obtained usingstandard methodology (within a Geographic InformationSystem platform) if meaningful comparisons between riverreaches are to be made (Molloy and Stepinski, 2007;Thommeret et al., 2010, Bailly et al., 2011). Specifically, met-rics are highly sensitive to river stage for some rivers (van derNat et al., 2002; Luchi et al., 2010b). Such an approachshould supersede the currently limited and simplistic extrac-tion of planform metrics (such as those in Table I!).

With respect to use of network metrics, there are cases inwhich multiple parameters have been proposed to define rivertype (see Makaske, 2001 for additional comment) so as to limitthe detail in the descriptions that define the river type; whereasothers have employed as few as two parameters to which addi-tional parameters are then added when the initial discriminationis found to be insufficiently robust (see Ferguson, 1987 forreview). The former approach is a form of causal reductionismad absurdum, which does not necessarily lead to a parsimoniousexplanatory model, the latter is a form of upward modelling(Sivapalan et al., 2003) whereby the model complexity isincreased in response to deficiencies in reproducing observationsat different levels. However, the two approaches have notconverged on any single causal model of river types. Throughout,important aspects of multichannel networks have yet to receivedetailed attention; for example (i) the presence of a predominantchannel within a subsidiary network (Foufoula-Georgiou andSapozhnikov, 2001; Makaske, 2001; Egozi and Ashmore, 2009;Kleinhans et al., 2012; Zolezzi et al., 2012), and (ii) althoughmultichannel networks can have sinuous individual linking chan-nels, often the individual links are relatively ‘straight’, of variablelengths and the networks may exhibit very acute confluenceangles (including obtuse junctions) yielding rectilate patterns.These obvious characteristics may reflect discriminatory pro-cesses or (dis)equilibrium. These rectilate patterns visually appearto be distinct from the more smoothly-sinuous patterns in classicbar-braiding systems (in which links tend to be very short) andmight suggest that rectilate-channel networks are transitionalbetween single-channelmeandering systems and classic braidingsystems (Eaton et al., 2010).

Equilibrium and nonequilibrium inmultichannel rivers

A key issue is how many individual links in a channel networkare required for efficient conveyance in an equilibrium system?Crosato and Mosselman (2009), Huang and Nanson (2007) andEaton et al. (2010) have determined from theory that stablechannel networks have few channels. Eaton et al. (2010) statethat ‘stable anabranch channels exhibit two or possibly threechannel threads, but no more’ whereas Huang and Nanson(2007) argue for three to four channels as being stable (includ-ing consideration of the important issue of the aspect ratio ofchannel sections). Despite some contradiction between thesetwo theoretical conclusions the key observation is the smallnumber of channels that theoretically constitute a stable chan-nel network (Bolla Pittaluga et al., 2003; Kleinhans et al.,2008). Braided networks in particular appear very dynamic(Egozi and Ashmore, 2009; Bertoldi et al., 2010) varying the



number of channels through time. If these theoretical deriva-tions are correct, key questions are then: ‘Why do so manymultichannel networks exhibit many more channels than four?and ‘Why do networks exist in which one channel is dominantover subsidiary channels? Can these four-plus channel systemsbe considered transitory (Kleinhans and van den Berg, 2011;Kleinhans et al, 2013) even though they persist through time(Knighton and Nanson, 1993) or are they equilibrium forms(Jansen and Nanson, 2010)? In addition, there are a large num-ber of detailed descriptions of multichannel networks whichclearly have planforms that are visually (qualitatively) distinc-tive and differ from classic patterns (e.g. sandy ridge-forms ofWende and Nanson, 1998, and Tooth and Nanson, 1999).Such studies have illuminated the range or continuum of chan-nel topologies. Thus prior recognition of distinctive patterns,whether by eye or through mathematical network description,has been somewhat biased by an assumption of system equilib-rium and an intuitive appeal to low-order physical processes atthe local scale (e.g. bifurcation point) to supply nomologicalunderstanding. Although remote-sensed imagery increasinglyis in common use to explore planform (Henshaw et al.,2013), relatively few people have considered the mathematicalnetwork properties of river networks from a distance; i.e. from afew kilometres above. At that general scale, a local controlwhich might induce the higher-level behaviour alluded to byKleinhans and colleagues (2005), becomes local-noise withina network and the low-level behaviour reduces to a simpleconsideration of the deformation at the reach-scale of a homo-geneous sedimentary medium. If significant differences inholistic network patterns are then detected, considerationcould be given to higher level behaviours at the regional scale(e.g. tectonics) or system-scale differences in the low-levelbehaviour, such as (time-variable) energy expenditure and totalsediment flux including the influence of unsteady sediment fluxon planform (Kleinhans et al., 2012).

Figure 3. Classification of channel planform based on normative data withoto be developed within alluvial floodplains although it is acknowledged that bnot included in this diagram but see Meshkova et al., 2012 for a perspective.)to fully braided channels and not as a separate channel planform category. Bbile, unvegetated bars that are inundated during near-bankfull flows such thseparated on the basis of process: in the case of the braided channel (islands)in anastomosed systems, avulsion across floodplain or other terrain leads to thcases the islands are not inundated during near-bankfull flows.


Towards improved planform-based classification

Figure 3 is an attempt to represent what is widely agreed uponin terms of defining channel types via planform; its simplicity isprovocative and it is not intended to represent a detailed typol-ogy of the continuum of channel form that is evident (qualita-tively) in the large number of detailed narrative studies ofchannel types across the world but rather to emphasize whatlittle is actually known today concerning planform typologyin any genetic, reductionist and quantitatively discriminativesense. The more comprehensive divisions of channel types,such as the excellent proposal of Nanson and Knighton(1996), may be viewed as convenient descriptive summariesof the variability observed in nature but river-science drawsno closer to quantitative genetic understanding of this variabil-ity than was expressed by Schumm in 1968. ConsideringFigure 3, it might be argued that only the single thread systemsare well-characterized. The braiding systems are well-studiedand generally believed to be well-characterised but in fact, asnoted above, there is no clear incontrovertible distinction ofthese networks vís-a-vís other multichannel networks, andthere is no comprehensive explanation of the determinantsgoverning channel division from either a formative or mainte-nance perspective.

Note that in Figure 3, braiding channels can be consideredas single-channels with bars that are inundated during bankfullflows and as multichannel networks in those cases where ac-cretionary islands remain emergent during bankfull flows.Wandering channels are not viewed as a separate channelform, but as intermediate forms between single-thread sinuouschannels and braiding (accretionary bars) channels, or betweensingle-thread sinuous channels and braiding (accretionaryislands) channels. However, it has to be acknowledged thatthere is a complex interaction in wandering channels betweenchannel curvature, channel width, bars and chute development

ut local, site-specific qualification. Single channels are considered hereedrock-confined and bedrock-constrained channels exist. (The latter are‘Wandering’ channels are regarded as transitional from single channelsraided channels are regarded as single channels, characterized by mo-at a single channel occurs during high flows. Multiple channels can be, accretionary alluvial islands cause channel splitting into anabranches;e development of anabranches and multiple-channel networks. In both



which appear to provide the process controls on planform(Luchi et al., 2010a, 2010b). The term ‘multiple channel’(or ‘multichannel’) is adopted as a class of channel for whichthe nomenclature carries no historic connotations. Despitethe historic precedence for use of anastomosis, ‘braiding’ isnow a term embedded in the literature. Consequently, as thereis a need to distinguish between braiding (accretionary islands)from other multichannel networks, the term ‘anastomosed’ isused herein to define all those networks that are not braidingbut are incised channel networks generally due to avulsionacross floodplain or other terrain. The rationale for separatingthese two networks (braiding v. anastomosing) is found in thedifferent process leading to channel bifurcations; the formerdue primarily to in-channel alluviation and the latter due to‘out-of-channel’ incision of new channels (Church, 2006).The previous statement is based on the definition of avulsiongiven above (Jones and Schumm, 1999) although the character-ization of avulsion remains controvertible with examples ofavulsive processes being reported in braiding systems (Zanoniet al., 2008; Bertoldi et al., 2010). Given the historic prece-dence of ‘anastomose’ over ‘anabranch’, the latter term mightbetter be reserved as a general descriptor of any link withinan anastomosing or braiding network although this conclusionis at variance with the highly-cited Nanson and Knighton(1996). As suggested by Jansen and Nanson (2004), multichannelnetworks span a continuum of equilibrium to nonequilibriumstates. It seems that equilibrium systems possess a maximumaround two to four channels, with stability and the likelihood ofequilibrium geometries declining as the number of channels in-creases above four. As noted above, some ideas for why theseconfigurationsmay be stable, especially the aspect ratios of chan-nels, have been advanced byHuang andNanson (2007), Nansonand Huang (1999) and Eaton et al. (2010). In contrast, Kleinhanset al. (2012) have argued from theory and modelling, that multi-ple channels are transient phenomena and such systems evolvetowards a single-channel unless further avulsions occur. Furtherresearch would address the issue of seemingly (dynamic?)equilibrium geometries for networks with large numbers ofanabranches. Single channels can develop into braiding systemsand vice versa and it is possible to speculate that anastomosingchannels can evolve from single channel networks inclusive ofbraiding systems (see Nogoa River; Amos et al., 2008; Crokeet al., 2011; Jansen et al., 2013).Kleinhans et al. (2005) have argued that most Earth science

problems are ‘underdetermined’; that is, there are insufficientnon-ambivalent data to construct tenable explanations.Although this is often true, it more often the case that there isno problem in collating ample data if we only knew what datato collect and how to process such data! Harrison (2001) hasargued that geomorphology ‘needs to acknowledge unknow-ability’ but the problem is that it can never be known that anything is unknowable. Rather, most explanations in the disci-pline are a combination of lower-level (physics-based) causalrelationships and narrative accounts not because of a dearthof data but from a dearth of understanding of higher-level emer-gent phenomena. The narratives are used to detail thoseaspects which cannot currently be reduced to causal relation-ships. Well-structured narratives may lead to further insight thatleads to causal relationships being identified, but this is rarelythe case. Causal relationships, usually expressed as mathemat-ical functions, are the traditional goals of reductionist sciencebut in most cases they require commentary to amplify theirmeaning such that the scientist achieves a qualitative insightfulunderstanding of the quantitative function. In this respect thenarrative accompanying any complete model should explainand amplify the causality and not be a shoddy apology forincomplete reductionism.


Thus, ‘The path to more realistic models seems long andtortuous. . .’ (Beven, 2011) and in this respect it is often arguedthat patterns, such as multichannel river planforms, are emer-gent phenomena which cannot be analysed using a reduction-ist approach (Harrison, 2001) as exogenous factors createcontingency. To counter such an argument, although it maybe true that the microscale of sediment transport cannot berelated to channel switching (Church, 1996), there do seem tobe defined drivers for the development of bifurcations whichsingularly may occur due to local variations in the system, butotherwise appear to occur generically in many multi-channelsystems. Thus the controls on bifurcation in a reductionist sensewill explain why divided channels occur even if the detailedpattern of every and each multichannel river remains emergent.However the key point is that we still have not determined withany degree of robustness, if the planform patterns of differentsystems do differ in any meaningful manner.

Perhaps it is time to take fresh approaches to advance chan-nel classification? The confusion in the use of terminology, asexamined herein, embodies the deep epistemic problems ofriver classification that focuses strongly on the simplest plan-form style. It is highly likely that additional studies of individualrivers will increase our knowledge of the diversity of the riversof the world but will not assist significantly in improving plan-form typologies due to the diverse approaches and philoso-phies embedded within individual studies. These epistemicproblems are at the heart of the failure of classification but aredue mainly to the lack of understanding of the processes andboundary conditions that should define behavioural conditionsfor different multichannel systems, whether considered at areductionist or emergent viewpoint. Determinant thresholdsthat separate behaviour domains, and allow channel patternsto be classified, are ill-determined and consequently subjec-tive. Different classifications can be compared but there areno clear quantitative, robust mechanisms for rejection of anyone model.

Ways Forward

Despite the caveat concerning the inadequacy of planformalone to characterize river types it is evident that a consistent,objective approach is needed to define planform (Luchi et al.,2010a) and to determine any normative differences betweenriver planforms, before other river characteristics are consid-ered. An alternative first approach would be for river scientiststo research and subsequently agree upon a common methodol-ogy to apply to selected multichannel rivers so as to characterizetheir planform network patterns; thus seeking to discriminate thesystems consistently within an existing, or new typology. Theseexamples could include classic case study examples, e.g. the Co-lumbia River (anastomosing), the Ganga (meandering-braiding),Waimakariri (braiding), some of which have been used tobenchmark river typologies. Of course an explanation for whycounter-examples defy classification should then be pursued.Counter-examples, or exceptions, perform an important functionbecause classification based upon case studies (the calibrationdata sets) will include sources of uncertainty (including epistemicerrors) that differ from the sources of uncertainty related to casestudies included in a verification process (Beven et al., 2011).As well as mainly qualitative or semi-quantitative studies on thestratigraphy and vegetation types (Bertoldi et al., 2011), for exam-ple, remote-sensing andGIS can enable large, high-resolution andconsistent quantitative planform and hydraulic geometry data setsto be derived that can be interrogated with topological andgeometric statistical network analyses (Ridenour and Giardino,1991; Torres Filho et al., 1994; Rubinov and Sporns, 2010),



thereby limiting epistemic error. Many channel networks mayappear to be differentiated, but prove to be self-affine, and thusself-similar and fractal (Nikora et al., 1996; Foufoula-Georgiouand Sapozhnikov, 2001; Paola and Foufoula-Georgiou, 2001;Walsh and Hicks, 2002) when rescaled using anisotropic trans-formation (Ijjasz-Vasqueza et al, 1994). If a rigorous comparisonof channel planform networks showed that classically ‘different’channel planforms are in fact similar in network topology, ashas been shown for some braiding rivers (Lane, 2006), then thiswould be a spur to focus on determining the controlling low-level processes (Church, 2006) epitomised in bar theory(Struiksma et al., 1985; Johannesson and Parker, 1989) or, incontrast, to elucidate the higher-level emergent properties ofriver networks. These two levels of attack are not necessarilyantagonistic. River networks manifestly exhibit generic low-levelbehaviour which may amplify into more complex networks(Crosato and Mosselman, 2009) and they may simultaneouslyexhibit generic high-level behaviours as all are subject to the lawsof physics as well as to contingency, complexity and emergence.The dynamic scaling treatment due to Foufoula-Georgiou andSapozhnikov (1998) has implications for the selection of spatialand temporal scales of treatment (Egozi and Ashmore, 2009)and similar approaches may identify the appropriate scales ofinvestigation beyond the reach-scale.In other disciplines, network analysis and graph theory have

been used to explore neural, arterial and transportation net-work function and structure including the evolution of morecomplex (emergent) networks from simple low-level behav-iours (Jarrett et al., 2006; Adamatzky and Jones, 2010; Jones,2010). As noted by Foufoula-Georgiou and Sapozhnikov(2001), application of such techniques goes beyond those thathave commonly been used by geomorphologists to date, butthe results should be capable of plausible physical interpreta-tion and subsequent use in classification of river networks(Nikora, 1991; Puenta and Sivakumar, 2003; Doeschl et al.,2006; Kleinhans et al., 2013). Quantification of channel param-eters can include consideration of longitudinal spatial seriesrepresenting rivers with apparent reach-scale morphologicalchanges downstream (Shen et al., 2010). These consistent datasets can be used to condition classification in a traditionalsense, based upon pattern analysis but with a greater consider-ation of the scale relationships that exist between reach-scaleand catchment-scale functions. Thus extended network analysismay be required, as our preliminary study of suite statistics of tra-ditional network components of reaches of wandering and anas-tomosing alluvial and bedrock-constrained rivers failed todiscriminate the channel patterns (Meshkova and Carling,2013). However, iteration would also be required as the data alsocould be used to test processmodels and threshold concepts suchas avulsion versus bar-evolution switching points that controlnew channel initiation. With reference to Figure 3 it is evidentthat determining the conditions under which (i) avulsion or (ii)flow splitting around accreting bars mediate channel division isthe key process challenge underpinning any advance in planformclassification as these processes are fundamentally germane todetermining all river styles.

Acknowledgements—Gerald Nanson is thanked for constructivelycritical comment on a draft manuscript. The detailed comments of theeditor and two reviewers: Maarten Kleinhans and an anonymousreviewer helped shape the arguments more fully.

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multichannel rivers: their definition and classification

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