Spillage sedimentation on large river floodplains

Active deposition across the floodplains of large rivers arises through a variety of processes; collectively these are here termed ‘spillage sedimentation’. Three groups of 11 spillage sedimentation styles are identified and their formative processes described. Form presences on large river floodplains show different combinations of active spillage styles. Only some large floodplains have prominent levees; some have coarse splays; many have accessory channel dispersion and reworking, while still‐water sedimentation in lacustrine environments dominates some lower reaches. Infills are also commonly funnelled into prior, and often linear, negative relief forms relating to former migration within the mainstream channel belt.


Introduction
The world's largest rivers and their floodplains drain a significant proportion of the earth surface (Fielding et al., 2012, their Figure 3) with modern terrestrial This article is protected by copyright. All rights reserved.
sedimentary basins covering 16% of the current continental area excluding passive margin settings (Nyburg and Howell, 2015). These are major global sinks for sediments, nutrients, organics and pollutants (Allison et al., 1998;Aufdenkamp et al., 2011;Syvitski et al., 2012). Despite recent advances in quantifying large river dynamics (Nicholas, 2013;Schuurman et al., 2013) and describing channel patterning (Latrubesse, 2008;Ashworth and Lewin, 2012), much less progress has been made on understanding the diversity and deposits of the largest river floodplains (Dunne and Aalto, 2013) and detailed descriptions on the floodplain architectures and facies models "are not yet viable" (Latrubesse, 2015). These 'large' river floodplains are here defined as the zone stretching up to 100 km in width that borders large rivers, themselves typically greater than 1 km wide, and composed of multiple and complex negative relief assemblages that are intermittently flooded (Lewin and Ashworth, 2014a). Whilst the area of floodplain inundated in catastrophic floods may be extensive, and includes new avulsed courses beyond the current elevated channel, this flooding may only constitute a limited proportion of some very extensive alluvial spreads (Syvitski and Brackenridge, 2013), and floodwaters may rarely return to the main channel when spread across a large megafan surface (Weissmann et al., 2015, their figure 18).
Large floodplains are rarely passive recipients of diffuse overbank sediments across tabular relief (Scown et al., 2015). Instead, large river floodplains have an intricate 'depositional web ' (Day et al., 2008) with an array of linked depressions and channels that may both span and connect significant expanses of ponded water (Assine and Soares 2004;Bonnet et al., 2008;Trigg et al., 2012;Lewin and Ashworth, 2014a). Deposition in and beyond this undulating topography takes a variety of forms. The range of out-of-channel sedimentation processes alongside larger rivers may collectively be characterized as spillage phenomena (Fig. 1), active at flood-stage, and particularly, but not exclusively, bordering main channels or channel belts raised above valley floor level to create strong lateral elevation contrasts (Assine et al., 2014). This may arise because of relatively narrow channel belt aggradation within wider valley floors, or because main-channel sedimentation has only partially occupied larger subsiding basins or drowned Pleistocene valley forms excavated in relation to lower base levels. The troughs occupied by the lowgradient lower reaches of most large rivers reflect forms inherited from long histories that still are present at floodplain levels as well as in elevated terraces or buried sediments (Latrubesse, 2015). Flood-prone realms include long-standing lake-filled depressions and linear forms left by past river migration. By contrast, spillage sedimentation is less characteristic of large, radial, distributive fluvial systems (or 'DFS' as termed by Hartley et al., 2010;Weissmann et al., 2010) that are commonly found in foreland basins and build through a sequence of mostly in-channel depositional lobes as anabranches migrate back and forth across a megafan surface (Weissmann et al., 2015).
The sequential process of sedimenting extensive channel-side negative relief (Lewin and Ashworth, 2014a) can be both varied and active without the intervention of main channel shifting. Palaeochannels deprived of their former mainstream flow discharges, and lateral accretion swales also cover the ground in part, and these direct later overbank processes. Equally, floodplain heterogeneity of forms and nearsurface sediments can have feedback constraints on, and opportunities for, channel mobility by creating extra diversion potential at points of lower bank profiles linking to the relief beyond (Ashworth and Lewin, 2012). They can also form impediments to This article is protected by copyright. All rights reserved. channel migration as strings of less erodible sediment 'plugs' (Schwendel et al., 2015).
Here we consider current spillage phenomena, on a metre to multi-kilometre scale, along large rivers as a holistic class, but one involving eleven sub-groups of process-related forms. The objective is to initiate discussion of the striking global variety of large-river spillage sedimentation, and the worldwide geographies of spillage elements. Rather than focusing on one or other of the process groups as historically identified, and almost entirely examined in case studies, (e.g., levees or crevasse splays at particular sites), spillage is treated as a complex process class so as to map all the activities visible on imagery that indicate the spatial presences and absences of process groups. This paper is organised to: (i) describe the types, modes and diversity of spillage sedimentation on large river floodplains; (ii) illustrate the prevalence of spillage sedimentation styles along the world's largest river; (iii) consider the global presences of spillage phenomena on large rivers in general; and (iv) explain the combined roles of both active hydro-morphological processes and inherited forms in determining spillage sedimentation types and their preservation.

Spillage forms
Spillage styles may be sorted into three groups and eleven sub-types. The eleven identified here, broadly following previous studies of individual features (cf., Fryirs and Brierley, 2013;Wheaton et al., 2015, their Table 4), are given in Table 1 together with listings of previous research on each type. They are also presented in diagram form in Figure 2 together with the codes adopted in this paper.

Floodplain and spillage deposition
Deposition from spilled sediment at flood stage occurs in three broad circumstances.
As on smaller streams (Lewin and Hughes, 1980), there can be a systematic and sequential set of relationships between inundation extent and floodplain morphology showing hysteretic relationships with mainstream flood stage. At first, extra-channel flow deceleration may lead to near-mainstream deposition especially of coarser material. Secondly, as floodplains fill, negative relief elements may continue to channel sediment-laden water for longer distances. This may be via accessory channels, by backing up tributaries which themselves may also be independently feeding in sediment, and via new channels actively created by crevasse flow. Finally, a variety of inherited forms, many of long-standing, may guide and pond water; this ponded water may give near still-water sedimentation of fines over longer periods.
Floodplain water bodies, with different and mostly lesser sediment content, may also be present quasi-independently through groundwater rise, local precipitation, and tributary input (Mertes, 1997). Thus mainstream spillage may be into lacustrine environments, or is even buffered against entry flow by already-high water levels (Lewin and Ashworth, 2014a). Floodplain water levels finally recede through evaporation, water table lowering, and return flows to the main channel where waning flow may also lead to channel margin deposition of residually transported fines. In totality, spillage sedimentation may broadly occur in these three broad domain groupings.

Sedimentation at main channel margins
The first grouping consists of deposits adjacent to major channels. These range from those dispersed only short distances in rapidly overflowing water as levees (MSa) and discrete bank top splays of well sorted, coarser material (MSb), to channel margin slackwater deposits and waning-flow forms on top of bedforms and islands initiated during earlier flood flow peaks (MSc). Island growth by spillage is facilitated by vegetation development on non-migrating bars (Latrubesse, 2015), with sedimentation in flows that have become partly disconnected and may have been deflected by woody vegetation (Mardhiah et al., 2015;Wintenberger et al., 2015).
The Mekong has particularly prominent levees, but also finer sediment accreting as channel-side ramps between set back levees that rise up to 15 m above bed level (Wood et al., 2008). Elsewhere, where sedimentation rates have been measured beyond channel belts (Törnqvist and Bridge, 2002) and across levees (Aalto et al., 2008), they drop off quite sharply away from the channel. In braided systems and meandering ones where there is lateral channel movement, material may continue to deposit in patches (Bätz et al., 2015), or river migration may largely re-erode such material returning it to the channel in the short or medium term. Not only is levee identification difficult in the rock record (Brierley et al., 1997), but present distributions along large rivers are also geographically restricted.
Isolated bank-top patches of coarser material commonly occur on braided and to a lesser extent on meandering channel systems (e.g. Ritter, 1975;Ferguson and Werritty 1983). Figure 3A shows a reach of the braided Zambezi River with active This article is protected by copyright. All rights reserved. sedimentation in the form of bar-top splays (MSb) at the barheads. Figure 4A and B shows a reach of the Jamuna (Brahmaputra) before and after a single monsoon season. The channel has been transformed, with a kilometre of bank recession, new migrating bars, and trimmed islands. But there is also a prominent overbank splay in part being funnelled into a prior palaeochannel (labelled MSb in Fig. 4B). Figure 3B shows freshly deposited sediment on a bend of the Mississippi after the extreme floods of 1993 (see also Gomez et al., 1997). Material has been dumped on the outer bend in levee-like form, but also across the inner point bar, funnelling between the lateral accretion ridges previously produced during point bar growth. Chute development upstream (natural or artificial), in this case as in others (Zinger et al., 2011), may have injected an abnormal volume of eroded floodplain sediment that is then transferred out of channel. Elsewhere in the Mississippi delta system, overbank fine sedimentation has been shown to be episodic over centennial to millennial timescales, with high rates for a while, but then switching to alternative sites (Shen et al., 2015). The authors relate this to autogenic control. This arises from elevation build up so that sedimentation shifts in location to alternative sites that by contrast may have undergone lowering by compaction.

Sedimentation in secondary linear systems
A second grouping involves secondary linear dispersion of sediments further away from main channels into what may be called the perirheic zone (Mertes, 1997). This may be via quasi-independent secondary channels (SLd, see Fig. 3D) generally carrying finer sediment (Lewin and Ashworth 2014a), with some developing scroll This article is protected by copyright. All rights reserved. bars of their own (Rozo et al., 2014). Others are shorter crevasse channels that either have subaerial splays (SLe), or sedimentation in standing water via channelized flows to create positive features such as deltas and linear subaqueous channels across shallow lakes (SLf). Some crevasses may be precursors to avulsion (Slingerland and Smith, 2004), but many exist for multiple seasons injecting mixed sediment loads for several kilometres across normally dry floodplains. Chen et al. Where unconstrained, subaerial splays may involve migration of radiating channels and fan-like forms; loads may also disperse in deltaic features following deceleration in lacustrine environments. Here there may be a co-existing hierarchy of bifurcations, delta foresets and subaqueous levees (see Kleinhans et al., 2013 for a review and discussion of contrasting bifurcations in fans and deltas). Splay deposits may spread across wetlands, extending and evolving their morphology to guide later infilling (Toonen et al., 2015). Figure  Prior forms, relicts from earlier processes, may pond rather than drain floodwaters, so that a range of tie channels (Day et al., 2008) or chains of connector channels may erosively develop to link them. They may also extend erosively as This article is protected by copyright. All rights reserved.  (Grenfell et al., 2012).
Here lateral migration of a bend has been followed by the formation of a sequential set of mid-channel bars with linear void 'channels' between (labelled PFh). These are being infilled by spillage, both blocking off the linear depressions from upstream (much as meander cutoffs may be plugged at the upstream end) and also extending into and narrowing them. In other cases, chute dissection of the point sediments once in place has been documented (Grenfell et al., 2012;2014). 'Point bar' relief on large rivers is not necessarily created by ridge and swale growth simply by attachment one by one at the inner bank margin, but also by island attachment (cf. Rozo et al., 2014) and later stage excavation.
Lacustrine sedimentation rates are difficult to estimate meaningfully for the diverse settings and lake sizes associated with the floodplain of large rivers . Drago (2007) reports average sedimentation rates in the El Tigre Lake (31 0 41'S, 60 0 40'W) adjacent to the Rio Paraná of 32 g m 2 d -1 over a two year period with between 80-99% composed of inorganic material. Whereas Maurice-  suggest for the Amazon an average of 1.6 mm yr -1 (+/-23%).
These are data for individual lakes on large rivers and therefore may not be widely representative.

Anthropogenic transformations
All the above groups are grossly affected by anthropogenic activity. Major global rivers have been engineered for hundreds or thousands of years such that breach flows and minor channels are now artificially constrained in some form (Zhuang and Kidder, 2014). Elimination both of palaeochannel and active anabranches as detrimental to agricultural potential or river navigation, bank-breach sealing and bank stabilization (Hohensinner et al., 2014;Klasz et al., 2014;Džubáková et al., 2015), and new floodplain drainage worksall can create new styles of spillage sedimentation in constrained rivers. Levee growth may be increased with catchment settlement and soil erosion (Funabiki et al., 2012). By contrast, large dams, with discharge regulation and sediment storage, have decreased the supply of water and sediment to larger rivers downstream (Syvitski and Kettner, 2011), and thus the potential for sediment spillage. Lateral channel constraint on the formerly multichannel Danube has led to the 'natural' pioneer growth of a levee over some 100 years in the absence of channel movement (Klasz et al., 2014). A 'great acceleration' of urban and industrial development since c.1945 has led to the increased riverine export of pollutants globally. The quality of spillage sediments on many large rivers has been altered for some centuries as well as their quantity (Middelkoop, 2000).
Millennia of accelerating transformation has affected the most longest-used floodplains such as the Indus (Syvitski and Brackenridge, 2013) or the Hwang He (Yellow River) where Zhuang and Kidder (2014) observe that transformations date from at least the third millennium BCE. Other rivers like the Amazon are, at least as yet, to be less greatly modified.

Spillage along the Amazon floodplain
Although there have been several recent studies describing the geomorphology and relief of the Amazon River floodplain (e.g. Latrubesse and Franzinelli, 2002;Rozo et al. 2012;Trigg et al., 2012;Lewin and Ashworth, 2014b) no study has yet systematically identified, interpreted and quantified the type and prevalence of spillage sedimentation that contributes to floodplain development and aggradation.
Nor has there yet been a survey of the constituents of a large river floodplain over 100s of km and down a long profile. Unlike many other large rivers, the Amazon presents the opportunity to quantify floodplain morphology and spillage prevalence for a catchment almost untouched, for now, by major dam construction.

Characteristics of the Amazon Basin
The Amazon River has the largest drainage basin in the world of 6x 10 6 km 2 ( Fig. 5A) that covers about 5% of the land on Earth (Filizola and Guyot, 2004). The Amazon is more than 3000 km long, with a mean annual discharge of nearly 210 x 10 3 m 3 s -1 (Park and Latrubesse, 2015), and has three of the world's largest tributariesthe Madeira, Negro, and Japurá (Fig. 5B). The centre of the Amazon basin is dominated by Tertiary and Quaternary lacustrine and alluvial deposits of sand and silts, at times weathered to clays, and dissected into a landscape of short 0.05-0.5 km hillslopes (Mertes and Dunne, 2007). The active Holocene floodplain The gradient of the Amazon River varies from 0.000040 to 0.000017 along the 1700 km (Fig. 6A). Mertes et al. (1996) and Dunne et al. (1998)  This article is protected by copyright. All rights reserved.
The study area was divided into nine floodplain 'blocks' (Fig. 5B), spaced at 200 km intervals downstream. Each floodplain block is 50 km long (25 km either side of the 200 km spacing points) and included both a SRTM DEM and a Landsat 8 satellite image. Each block was loaded into Global Mapper TM and polygon features were drawn manually around each spillage feature using the classification scheme in Table 1, together with associated water elements (e.g. main river channel, accessory channels, floodplain water bodies).  (Fig. 6B). The Amazon appears to oscillate within its Holocene floodplain with a preference for the floodplain to be on the left bank in the upstream portion of the study reach and on the right once the Purus tributary enters at ~750 km (Fig.   6B). A similar observation of large-scale channel belt oscillation was made for the Brahmaputra by Thorne et al. (1993) where it was suggested the wavelength of the sinuous braid-belt scaled with the valley width rather than that of the main channel. Figure 7 shows the relief on the floodplain that is typically 5-20 m in between expansive (up to 10 km wide) lakes and channels. SRTM data is accurate only to +/-10 m in the vertical and cannot distinguish between a vegetation canopy and bare floodplain surface. But the floodplain relief is remarkably consistent along each cross-section and in most cases it is relatively easy to define active floodplain width on a morphological basis (see Fig. 7, shaded area).  Table 1 were identified in the imagery partly because individual elements were too small to identify and then represent as polygons. Diffuse overbank spreads (PFk) also cannot be readily distinguished from imagery. Spillage types PFh and PFj that are associated with meander bend evolution and scroll bar growth are here amalgamated into one group called 'point bar complexes' (PBC). If there were no spillage sedimentation forms, or no unambiguous evidence for spillage, the floodplain was classified as UF or 'undifferentiated floodplain'. Inevitably, some of the UF will be older fragments of floodplain relief and spillage sedimentation that has been covered by organic-rich fine material and subsequently masked by vegetation.

Spillage sedimentation along the Amazon floodplain
The frequency of occurrence of different spillage types on the Amazon floodplain over the 1700 km study reach is summarized in Table 2 and Figures 9A-B. There is no consistent trend in the change in MSa, MSb, MSc or SLd downstream and neither of these spillage form elements constitutes more than 5% of the floodplain, though levee sedimentation (MSa) is prevalent in all floodplain blocks.
Bank top splays (MSb) are mostly absent for much of the river, but in particular in the downstream reaches, which is expected given bank top splays are likely to be made up of mostly coarser sediment. This article is protected by copyright. All rights reserved.
As Figure 8A shows, the upper reaches of the Amazon floodplain are dominated by recent and Holocene scroll bar formation (PBC) but this mode of sedimentation and potential for spillage decreases consistently downstream so that only 4% of the floodplain is scroll bars at 1600 km (Table 2, Fig. 9). The downstream decrease in frequency of PBC in the floodplain correlates with a reduction in percentage area of total channel change (r 2 = 0.60) as measured by Mertes et al. (1996Mertes et al. ( , their figure 8, p. 1097) that is attributed to the transition from actively migrating sinuous main and floodplain channels to a more confined and straight channel system.
There is a steady increase in the presence of water bodies and therefore the potential for ponded lake sedimentation (PFi) from up to downstream (Table 2, Figures 9A-B). Note, water bodies are taken as an indicator of potential PFi, though the former is a topographic feature, whilst the latter describes the process of infilling/spillage. The downstream increase in the presence of water bodies is the inverse of the situation for PBC described above. This probably reflects the progressive downstream change from a laterally mobile channel and reworked floodplain to spillage into water-filled voids left by a relatively immobile main channel and topography developed during the time of lower Holocene sea level. Water body area increases exponentially downstream (r 2 = 0.93) and Mertes et al., (1996) note that the water bodies also become rounder downstream. With an increase in PFi downstream, there is a corresponding increase in the presence of inland delta formation (SLf). Around 40% of the Amazon floodplain either shows no discrete spillage forms, is masked by forest, or forms are too small/local to allow quantification from the imagery. This article is protected by copyright. All rights reserved.

Diversity and presence of spillage forms on large rivers
To compare the range of spillage possibilities globally, Table 3

Main channel margins
Only a few large rivers without rapid lateral migration have prominent levees (MSa), notably the lower Mekong where the lengthy mountain course exits to cross a broad depression. Other rivers may have sets of smaller levees stacked laterally where rivers oscillate from side to side (middle Amazon; Latrubesse and Franzinelli, 2002).
The presence on the Mississippi of both levee and migration swales is unusual and perhaps surprising. Levees depend on the short-distance dispersion of bed materials as well as finer suspended sediment diffusion. Bed material splays (MSb) along braided channels may form diffuse patches along 100s of metres of channel bank (Jamuna, Fig. 4B). Splay material may also be fed through crevasses or along palaeochannel depressions to much greater distances laterally. In anabranching systems, sedimentation capping bars and islands (MSc) is very common. This article is protected by copyright. All rights reserved.

Secondary linear systems
Channelized dispersion (SLd) is important on multi-channel reaches with prominent secondary channels (Orinoco, Yenisei, Volga, Lena, Paraná). Secondary branches with limited lateral mobility disperse a generally finer fraction of mainstream sediment loadings considerable distances across basins and container valleys. Others have multiple meandering channels (Ob, Volga) each of which has been laterally active.
Splays are particularly prominent on the Mekong, as previously noted, with high (also breached) levees. Upstream of the delta, many smaller ones are artificial, spreading cultivable material at flood stage without overtopping levees and their strings of settlement. As previously demonstrated, crevasse breaches leading to lake delta sedimentation (SLf) are a feature of the lower Amazon.
Linear dispersion can also relate to presently or formerly active avulsing main channels. Chen et al. (2012) and Syvitski and Brackenridge (2013) have emphasized the extreme importance of avulsion processes along the Yellow and Indus rivers.
Over a longer timespan, Morozova (2005) pointed to the possible role of avulsion in affecting the fortunes of Mesopotamian cultures, whilst the Holocene Mississippi was characterized by avulsive relocation of its meander belts (Saucier, 1994).

Prior form following
Linear and localized infills in swales and palaeochannels (PFh, PFj) are common on over half of the reaches examined (Table 3); these are on middle reaches at steeper gradients reflecting previous lateral channel migration. This article is protected by copyright. All rights reserved.
The contrasting feature alongside distal reaches of many large rivers (Amazon, Magdalena, Mississippi, Paraná, Yangtze) is that they have quasipermanent floodplain water bodies Ashworth and Lewin, 2012) and sedimentation here may be in the form of diffuse lentic (still water) sedimentation (PFh), floodplain deltas (SLf) and channels developed by subaqueous levee extension (Fig. 3C).
It is also striking that, even where channel banks are not continuous (e.g., the Lago Cabaliana at Manacapuru on the Amazon, 3 0 17'S, 60 0 38'W), there exists a hydraulic barrier to sediment spillage. This is because water elevations on the floodplain are already high; sedimentation thus depends very much on existing floodplain water levels at the time of mainstream high sediment loadings. Conditions may be such that, largely free of mineral contamination, extensive in situ organic sedimentation can be generated in floodplain waterbodies or wetlands.

Changing river stage
The important role of stage levels may be illustrated by spillage on the lower Amazon ( Figure 10). Park and Latrubesse (2015) show peak river sediment concentrations in October-January ahead of peak water discharges in May-June; lake sediment accumulation occurs during this flood rise (Maurice . Figure   10A shows a single crevasse through a levee with a sediment plume entering the sediment-free Rio Negro that joins the Amazon some 10 kms further downstream. At a higher stage (Fig. 10B), multiple trans-levee overflows are operating with the diffuse spread of sediment. Lake and tributary levels relative to river levels are crucial in the spillage process. Park and Latrubesse (2015) showed (their Figure 7) the varying relationship between inundation extent and the sediment concentration in inundated areas, with low concentrations at peak flow in May-August, whilst Maurice 2007) demonstrated the complex nature of the balance between input, accumulation and also channel-return for lake sediment. The areas actually flooded on large rivers, and the cumulative process of spillage sedimentation, is often intricate and complex (Gan et al., 2012).

Dryland rivers
Dryland rivers may exhibit different spillage forms because they often experience downstream diminution of flows such that spillage is represented by floodout splays as water is lost (Tooth, 1999;. Some 60% of the Earth surface's modern basin area has an arid climate (Nyberg and Howell, 2015) and 18% of continental land has endorheic drainage, both to small intermontane basins and to very large ones such as Lake Eyre in Australia (1,200,000 km 2 ), the Aral (1,549,000 km 2 ) and Caspian (3,626,00 km 2 ) Seas in Asia, and Lake Chad in Africa (2,434,000 km 2 ). Present day individual and ephemeral rivers are not generally in the 'large' category, but earlier Holocene conditions with greater runoff produced greatly expanded lakes and some large overflow rivers like the drainage of Chad through to the Atlantic via the Benue.

Indeed recent work by Skonieczny et al. (2015) suggests that 'African Humid
Periods' from at least 245 ka, and most recently 11.7-5.0 ka, triggered the reactivation of an ancient river system, the Tamanrasett, which may have ranked as the 12 th largest drainage basin worldwideyet today no major river exists in the area. This article is protected by copyright. All rights reserved. Some large rivers lose most of their flow, and sediment, in dryland basins along their courses, as in the Inner Niger Delta in Mali (Figs 11A-B). Here a distributary channel system operates during the wet season despite water loss at the margins of the channel network (Fig. 11C). Sediment spillage takes the form of low relief (typically <4 m) levees (MSa) that border individual distributaries, or a mosaic of bank-top splays (MSb) that are formed by overbank sedimentation from distributary channels that migrate back-and-forth across a series of narrow (<600 m) active deposition zones. In the dry season (Fig. 11D) the spillage forms are exposed as a series of sinuous 'tentacles' of sedimentation (see also Tooth and McCarthy, 2007). Table 4  This article is protected by copyright. All rights reserved.

Conclusions
Out-of-channel sedimentation and dispersion along the world's largest rivers are both complex and geographically variable. Using remote sensing imagery and from detailed mapping of floodplain sedimentation types it has been shown that out-ofchannel sedimentation processes alongside large rivers can be divided into eleven Globally there is considerable spillage variety such that the Amazon should not be taken as 'typical'. In addition to simple planar spread, the active 'overbank domain' in different large river reaches may or may not include: island build-up; levees; bank top splays; crevasse channels extending to splay fans; accessory channel sedimentation; deltas and linear channels prograding across wetlands and This article is protected by copyright. All rights reserved. ponded lakes; scour through internal drainage connections and network extension created following flood flows; lateral accretion infills; and lacustrine sedimentation.
Spillage sedimentation diversity on large river floodplains has been underrepresented in alluvial depositional models. In certain places, the totality of these processes currently contributes more to fills now being preserved than channel accretion. Lower reach main rivers that show limited lateral activity within container valleys or basins leave spillage to dominate floodplain sedimentation processes.