https://doi.org/10.1130/G46319.1

Manuscript received 29 March 2019 Revised manuscript received 28 May 2019 Manuscript accepted 29 May 2019

© 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 19 June 2019 The role of sediment supply in the adjustment of channel sinuosity across the Amazon Basin Joshua Ahmed1,2*, José Antonio Constantine3, and Thomas Dunne4 1Energy and Environment Institute, University of Hull, Hull HU6 7RX, UK 2School of Earth and Ocean Sciences, Cardiff University, Cardiff CF10 3AT, UK 3Department of Geosciences, Williams College, Williamstown, Massachusetts 01267, USA 4Bren School of Environmental Science and Management, University of California, Santa Barbara, California 93106, USA

ABSTRACT mented the importance of sediment supplies in Sediment supplies are a fundamental component of alluvial river systems, but the im- planform adjustment through bedform develop- portance of sustained supplies of externally derived sediments for the evolution of meander- ment (Braudrick et al., 2009; Bufe et al., 2016). ing planforms remains unclear. Here we demonstrate the importance of sediment supply In the River Ystwyth (Wales, UK), sediment in enhancing the growth of point bars that influence the rate of sinuosity increase through supplies led to the accumulation of point-bar flow deflections in meander bends. We use an archive of Landsat images of 16 meandering complexes, which intensified the development reaches from across the Amazon Basin to show that rivers transporting larger sediment loads of a sinuous planform despite repeated attempts increase their sinuosity more rapidly than those carrying smaller loads. Sediment-rich rivers to artificially straighten the channel (Lewin, are dominated by downstream-rotating meanders that increase their sinuosity more rapidly 1976). Physically based numerical models have than both extensional and upstream-rotating meanders. Downstream-rotating meanders produced similar results, with enhanced bar mi- appear to establish larger point bars that expand throughout the meander, in contrast to gration and riverbank erosion predicted along extensional meanders, which have smaller bars, and upstream rotating meanders, which are reaches that experience increased bar construc- characterized by deposition over the bar head. These observations demonstrate that the size tion (e.g., Dunne et al., 2010; Schuurman et al., and position of point bars within meander bends influences flow routing and thus controls the 2016). Because point bars exert a strong control dominant direction of meander growth. Rivers with low sediment supplies build smaller point on bank erosion—which determines how sinuos- bars, which reduces their capacity to increase meander curvature and the resulting sinuosity. ity changes—and rely upon sediment supply for their growth, we assess the relationship between INTRODUCTION meanders in reaches with generally high channel meander migration and channel lengthening, and Meandering rivers are characterized by their mobility. Still, a formal evaluation of the nature quantify how they vary with sediment supply. sinuous planforms and tendency to migrate. A of the relationship between meander migration range of studies have highlighted sedimentologi- and channel sinuosity has been prevented by the METHODS cal (e.g., Schumm and Khan, 1972), biotic (e.g., absence of detailed observations across a range The Amazon Basin provides an ideal opportu- Tal and Paola, 2007), valley-slope (e.g., Ouchi, of meandering river settings. nity to evaluate the relationship between sediment 1985), and planform-curvature controls (e.g., Meander migration is principally accom- supply, meander migration, and sinuosity adjust- Hickin and Nanson, 1975) on the evolution of plished through curvature-driven flow divergence ment over a large range of scales in the absence the meandering planform, yet there remains no initiated at the bend entrance. The transverse of engineering influences on channel mobility. quantitative statement on how channel sinuos- water-surface slope facilitates sediment excava- The physiography of the basin leads to systemati- ity should adjust in response to meander migra- tion from the bank toe, while obliquely oriented cally varying sediment supplies throughout the tion. Hooke (1984) defined patterns by which near-bed currents transport sediment inwards to drainage network, at least as measured in total the meandering planform can adjust, but the the point bar (Dietrich and Smith, 1984). Point suspended sediment fluxes (Guyot et al., 1989, multitude of patterns suggested that the plan- bar growth increases the length and curvature of 1994; Latrubesse et al., 2005; Filizola and Guyot, form response to migration is controlled by a bends, increasing the cross-stream centrifugal 2009). We selected 16 meandering reaches for number of variables that might prevent gener- force (Hickin and Nanson, 1975) and enhanc- study, sampling across the physiographic prov- alization. Schwendel et al. (2015) highlighted ing cross-stream velocity by topographic steer- inces of the Amazon (Fig. 1A; Table DR1 in the the sensitivity of river migration to the hetero- ing (Dietrich and Smith, 1983; Legleiter et al., GSA Data Repository1). We ensured that the geneity of floodplain deposits along the Beni 2011). Constantine et al. (2014) observed that reaches had wide valley margins and were free River (Bolivia), documenting that fine-grained sediment fluxes correlated positively with rates from visible evidence of confining elements (e.g., sediment matured in distal parts of the floodplain of riverbank erosion and oxbow lake production, bedrock exposures) as discerned from satellite could affect the migration rates and forms of implying that the construction of point bars is an imagery, although we have no way of assessing influence on meander migration and sinuosity subtler erodibility differences in alluvium of the *E-mail: geomorphicjosh@​gmail​.com adjustment. Empirical observations have docu- kind referred to by Schwendel et al. (2015), which

1GSA Data Repository item 2019289, extended methods, Figures DR1–DR3, and Tables DR1–DR6, is available online at http://www​ ​.geosociety​.org/datarepository​ ​ /2019/, or on request from editing@​geosociety​.org. CITATION: Ahmed, J., Constantine, J.A., and Dunne, T., 2019, The role of sediment supply in the adjustment of channel sinuosity across the Amazon Basin: Geol- ogy, v. 47, p. 807–810, https://​doi​.org​/10​.1130​/G46319.1

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/9/807/4819768/807.pdf by guest on 27 September 2021 A 80° W 70° W 60° W 50° W B ues of individual σ measurements. We selected Extension σ ʂ 0.90–1.05 the reach-median metric to capture broad-scale UD meandering behavior that would otherwise be 0° t1 t0 obscured by inter-meander variability. Pt Na Jt σ= D Cu It U Hu Pu S1 S2 S3 RESULTS AND DISCUSSION 10° S Uc Xi Upstream rotation Downstream rotation Fractional change in sinuosity resulting from Ma σ < 0.90 σ > 1.05 B1M3 B2 M2 channel migration (S*) increases for rivers with B3 M1 t t 1 1 greater rates of sediment supply, a pattern that t0 t0 is consistent for both the annual and decadal Shield (TSS: 0.013± 0.020 Mt m–1 yr–1) Central trough (TSS: 0.019± 0.019 Mt m–1 yr–1) S1 S2 S3 S1 S2 S3 data sets (Fig. 2A). Rivers with TSS fluxes Andes–foreland basin (TSS: 0.24± 0.17 Mt m–1 yr–1) >0.05 Mt m−1 yr−1 increase their sinuosity sig- Figure 1. A: Amazon drainage basin symbolized by physiographic province and labeled with nificantly more rapidly than those with lower reach abbreviations: Xi—Xingu; Pu—Purus; Jt—Jutai; It—Ituí; Cu—Curuca; Pt—Putumayo; sediment loads. This relationship suggests that Na—Nanay; Hu—Huallaga; Uc—Ucayali; B1, B2, B3—Beni; M1, M2, M3—Mamoré; Ma—Madre rivers characterized by high TSS fluxes undergo de Díos. Exact reach coordinates can be found in Table DR1 (see footnote 1). Average annual width-normalized total suspended sediment flux (TSS, in megatons per meter of river channel more rapid meander migration and sinuosity in- width per year) values for each physiographic province are indicated; further information on their crease (Fig. 2B). collection and representation can be found in the Data Repository. Dashed white box indicates Reach-median σ indices positively correlate reaches (excluding M3) resolved by near-annual imagery; remaining reaches are resolved at near with increasing sediment load (Fig. 3A). Median decadal time scales. B: Meander symmetry index (σ) and exemplary scenarios. Clockwise from σ values range from 0.95 to 3.25 (with variabil- top left: σ is ratio of total eroded area produced between first (t0) and final year (t1) of record in downstream (D) versus upstream portions (U) of meander. Each meander is divided through ity described by the interquartile range [IQR] bend apex as characterized by point of maximum curvature (star-ended vertical dashed line) of 0.50–2.46) for the decadal data, and from 1.39 to earliest center­line (t0), and erosional areas are bounded by inflection points (curvature minimum) 3.55 (IQR 0.43–3.81) for the annual data (Table derived from connected reach segments (S1, S2, etc.) along t centerline. Extension occurs when 0 DR4). S* is positively correlated with differences ratio between down- and upstream eroded areas is within range 0.90 < σ < 1.05, downstream rotation is where ratio is >1.05, and upstream rotation is where ratio is <0.90. in meander deformation style (Fig. 3B), with ­rivers characterized by σ >1.50 experiencing, on average, more than four times greater sinuosity

are incorporated into our average effects. Total where S1 and S2 are channel sinuosity at the first change than rivers where σ <1.50. Frequency suspended sediment (TSS) flux data compiled and second time steps, measured as the ratio of distributions of σ values for each reach (Fig. DR1 from various sources by Constantine et al. (2014) reach length to valley length, and t is time. For in the Data Repository) illustrate that most me- were normalized by bankfull channel width, be- the decadal data set where cutoffs were unavoid- anders (36%–76%) rotated downstream (1.05 cause wider channels can convey larger sediment able, we determined S* using a modified relation < σ < 5.0). The percentage of meanders with loads, and used as a proxy for the bed material derived by Constantine and Dunne (2008, their symmetry indices >5.0 increased for sediment- supply to each reach, which was necessary in the equation 4), in which: rich rivers in the basin, with up to 14% of their absence of direct measurements of bed material meanders having indices between 10.0 and 15.0. fL ∆n flux (Table DR2). SM**=+ , (2) Although median values are all >1.0, a fraction of M t Multispectral Landsat images from 1984 1 ∆ meanders within each reach (14%–47%) exhibit to 2014 were used to delineate bankfull chan- where M* is the fractional change in channel σ values <0.90, reflecting upvalley meander mi- nel margins. Reach imagery for three rivers length measured directly from imagery between gration. Most upstream-rotating meanders are in (Mamoré,­ Beni, and Madre de Díos) was ob- time steps, f is the fractional change in chan- reaches with moderate (<0.035 Mt m−1 yr−1) sedi- tained at a near-annual resolution (Fig. 1A), nel length resulting from meander cutoff, L is ment loads (Fig. DR1; Table DR2). Extensional while the remaining reaches had a temporal the characteristic length of a cutoff event in meanders are consistently rare, comprising only

spacing of ~10 yr. The average bankfull channel channel-width units, M1 is the reach length at 5%–24% of meanders across all reaches. In three width of each reach was measured along straight the first time step in channel-width units, and sediment-rich reaches (M1, Hu, and Uc; Fig. sections of river (n > 20 per reach). The bankfull n is the number of cutoff events between time 1A), 18%–24% of the meanders are extensional, channel boundary was delineated by selecting steps. Equation 2 was solved using information which may be explained by the large number of a representative pixel threshold at the channel- derived from our Landsat imagery (Table DR3) temporally clustered cutoff events (2–13), which bank interface using the normalized difference and from Constantine and Dunne (2008). reduces the population of mature asymmetrical vegetation index from composite images. The We developed a meander symmetry index meanders on the reach. boundary was vectorized and used to generate (σ) to ensure a quantitative and repeatable Our results suggest a link between sediment centerlines for each year on record. After Micheli method for describing the direction of plan- supply and meander deformation, driven by sedi- et al. (2004), overlapping centerlines between form geometry change resulting from meander ment sequestration on point bars. Bar growth aug- sequential time steps were then used to calculate migration (Fig. 1B). A symmetry index was ments the curvature-driven components of the

reach-averaged channel migration rates (MR), re- calculated for each meander as defined by the force balance driving sediment transport and bank ported in units of channel widths per year. ratio of the eroded areas created in the upstream erosion (Dietrich and Smith, 1983). Rivers with The near-annual resolution of Landsat imag- versus downstream portions of the meander (see greater sediment loads can deposit this material ery available for the Mamoré, Beni, and Madre the methods in the Data Repository). Large σ as water shoals over the point bar and momentum de Díos rivers allowed us to directly measure values are indicative of downstream rotation, diminishes in the lee, or as material is transported the fractional change in sinuosity resulting from while small values describe more extensional or across the channel by transverse near-bed cur- channel migration (S*) as the following: upstream bend deformation. In total, 281 mea- rents (Dietrich and Smith, 1984; Frothingham surements were made of reach-averaged channel and Rhoads, 2003). Experimental evidence sug- 1  SS−  S* = 21, (1) migration rate (M ) and S*, while 16 measure- gests that point bars grow preferentially in the S  tt R 1  21−  ments of σ were formed from reach-median val- downstream direction as sediment is deposited

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/9/807/4819768/807.pdf by guest on 27 September 2021 –1 ) A 10 n A σ = 1.13

) R A n io B –1 B3 eni –2 M2 Hu 10 M1 B2 Hu Uc S* M2 B3 = 0.00075 B1 M1 F

S *) (y r l Discontinuous –3 Cu o ( Ma M3 Pu 10 B1 w point bars It Jt It Ma B2 Pu Xi Xi Cu Pt M3 Uc 10–4 Na Pt Na Jt

–5 1 km 10 2011 0.001 0.01 0.11 B σ = 2.57 TSS (Mt m–1 yr–1) Meander symmetry index (σ –1 –1 10–1 TSS (Mt m yr ) S* = 0.012 B n ) Growing point bar –1 n –2 B B2 B3 Ri 10 ) (y r –2 o Ben 10 Uc M2 M1 i M3 –3 B1 F 10 Pu Hu low –3 Ma 10 Cu –4 It 1 km 10 Jt Xi 2011 –4 Pt Vegetation Fractional change in channel sinuosity n 10 C σ = 11.36 –5 Na ni 10 Be

Fractional change in o 0.01 0.01 –5 Ri –1 10 channel sinuosity ( S* Meander Migration Rate (MR) (ch-w yr ) us continuo Large r Meander symmetry index (σ) point ba Figure 2. A: Width-normalized total suspended S* = 1.63 sediment flux (TSS, in megatons per meter of Bar age Figure 3. A: Width-normalized total suspended 1989 channel width per year) plotted against frac- 1996 sediment load (TSS) plotted against reach- 1999 tional change in channel sinuosity (S*, per year). 1 km 2004 aver­aged meander symmetry index (σ). Dashed 2013 2009 Solid triangles represent decadal data set, while black line partitions data at extensional mean- open circles represent temporally averaged data der threshold (σ ≈ 1). Bars indicate statistical Figure 4. Meander deformation and point bar points from annually resolved data set. Reach dispersion represented by interquartile range extents for three meanders on Beni River. On names are labeled according to abbreviations of symmetry indices for meanders on each left, subaerial point bar extents are outlined in in Figure 1A. B: Average annual channel migra- reach. Solid triangles represent decadal data black on Landsat imagery converted to dis- tion rate (M , in channel widths [ch-w] per year) R set, while open circles represent annual data play normalized difference vegetation index related to fractional change in channel sinuosity set. B: Reach-averaged meander symmetry (NDVI). On right, extents of point bars in five through time (S*, per year). Solid triangles are index (σ) plotted against fractional change in discrete years are displayed after extraction derived from decadally resolved data set, and channel sinuosity (S*). Reaches are labeled from Landsat imagery; warmer colors indi- open circles indicate S* values calculated for according to abbreviations in Figure 1A. cate younger deposits. Flow direction, me- rivers from annually resolved data set. Statisti- ander symmetry indices (σ), and direction of cal characteristics have been withheld from the bend movement are all annotated. Direction figures, but are available in Table DR6 in the Data and Garcia, 2009b). The role of low sediment of movement was determined by visually Repository (see footnote 1). supply and potentially slow bar growth in the inspecting bend for approximate trajectory initiation of upstream rotation requires further between images. Each bend is prescribed a at the tail or on the margins of the bar (Dietrich examination. However, our observations sug- fractional change in sinuosity (S*) value calcu- et al., 1979; Pyrce and Ashmore, 2005; Braudrick gest that upstream-rotating meanders are associ- lated using the regression equation generated by fitting a power function (S* = 0.0005σ3.33) to et al., 2009). Frequent deposition associated with ated with compound bend development or with the data in Figure 3B; associated arrows indi- greater sediment fluxes should result in faster out- sediment deposition at the bar head (Fig. DR2). cate rates of channel lengthening. Bar areas ward and downstream bar growth, enhancing the Protruding sediment lobes are then generated and meander locations are specified in Table displacement and retention of high-velocity flows that promote localized bank erosion upstream DR5 in the Data Repository (see footnote 1). near the outer bank downstream of the meander of the meander apex, similar to the observations apex. Downstream-rotational meanders experi- of Nanson (1980) and Hagstrom et al. (2018). Channel stability analysis suggests that in low- ence more-sustained curvature-driven transfer of We assessed the low-flow subaerial extent of sinuosity rivers with poorly developed meanders, momentum to the outer bank and have well de- point bars for three meanders on the Beni River point bars are suppressed in favor of migrating veloped secondary circulation cells that facilitate (Fig. 4) to discern whether the direction of plan- alternate bars (Tubino and Seminara, 1990). In the the growth of longer, more developed point bars form geometry change was related to systematic Amazon Basin, this has been observed on rivers that begin at the upstream inflection (Abad and patterns of bar growth. In meanders with high σ with low sediment supplies (e.g., Xingu River), Garcia, 2009a, 2009b). values, bars tended to be longer, more continuous, while more sinuous channels (e.g., Beni River) Upstream-rotating meanders were observed and wider between inflection points. Conversely, are characterized by fixed point bars and high in all reaches and were especially common in extensional meanders had narrower, sometimes sediment loads (Monegaglia et al., 2017). Rivers reaches with low sediment loads. Although re- discontinuous, bars with smaller surface areas enriched with sediment can construct point bars ported less frequently in the literature, upstream- (Table DR5). Our observations are similar to more rapidly, increasing bend length and chan- rotating meanders have been previously docu- those made in the laboratory experiments of Abad nel curvature while facilitating channel migration. mented (e.g., Seminara et al., 2001; Duan and and Garcia (2009b), who observed larger, better- A positive feedback is initiated wherein greater Julien, 2010). Upstream rotation has been at- developed bar forms in downstream-rotational sediment loads encourage bar growth and chan- tributed to spatial variations in the erodibility meanders. They attributed this development to nel curvature, ultimately exceeding the curvature of bank materials (Posner and Duan, 2012; the rapid cross-stream transition of the high- threshold required for the persistence of fixed point Schwendel et al., 2015), to backwater effects velocity core in response to the change in local bars over migrating bars. Rivers in which mean- caused by meander cutoff (Schwenk and Fou- curvature, which facilitated the development of ders are unable to grow as quickly fail to attain the foula-Georgiou, 2017), and to the upstream val- stronger helical­ circulation cells and quicker es- necessary curvature required to establish perma- ley orientation of tightly curved meanders (Abad tablishment of the inner-bank bar. nent fixed bars, therefore limiting sinuosity growth.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/9/807/4819768/807.pdf by guest on 27 September 2021 CONCLUSIONS as a driver of river meandering and floodplain Lewin, J., 1976, Initiation of bed forms and meanders The direction of meander deformation deter- evolution in the Amazon Basin: Nature Geosci- in coarse-grained sediment: Geological Society ence, v. 7, p. 899–903, https://​doi​.org​/10​.1038​ of America Bulletin, v. 87, p. 281–285, https://​ mines how channel sinuosity evolves in response /ngeo2282​. doi​.org​/10​.1130​/0016​-7606​(1976)87​<281:​ to meander migration. The fractional rate of sinu- Dietrich, W.E., and Smith, J.D., 1983, Influence of the IOBFAM>2​.0​.CO;2​. osity change is observed to vary as a function point bar on flow through curved channels: Water Micheli, E.R., Kirchner, J.W., and Larsen, E.W., 2004, of sediment supply for a range of rivers across Resources Research, v. 19, p. 1173–1192, https://​ Quantifying the effect of riparian forest versus doi​.org​/10​.1029​/WR019i005p01173​. agricultural vegetation on river meander migra- the Amazon Basin. This relationship between Dietrich, W.E., and Smith, J.D., 1984, Bed load tion rates, central Sacramento River, California, sinuosity change and sediment supply is mani- transport in a river meander: Water Resources USA: River Research and Applications, v. 20, fested through the rate of alluvial sequestration Research, v. 20, p. 1355–1380, https://doi​ .org​ /10​ ​ p. 537–548, https://​doi​.org​/10​.1002​/rra​.756​. onto point bars. Alluvium is typically seques- .1029​/WR020i010p01355​. Monegaglia, F., Tubino, M., and Zolezzi, G., 2017, tered downstream of the bar apex, facilitating the Dietrich, W.E., Smith, J.D., and Dunne, T., 1979, Flow Dynamics of migrating alternate bars in large me- and sediment transport in a sand bedded mean- andering rivers: Combining remote sensing and down-channel growth of the bar. For upstream- der: The Journal of Geology, v. 87, p. 305–315, theoretical approaches, in Lanzoni, S., et al., eds., rotating meanders, sediment accumulates near https://​doi​.org​/10​.1086​/628419​. RCEM2017—Back to Italy: The 10th Symposium the head of the bar, creating lobes that protrude Duan, J.G., and Julien, P.Y., 2010, Numerical simula- on River, Coastal, and Esuarine Morphodynam- into the channel, locally increasing curvature and tion of meandering evolution: Journal of Hydrol- ics—Book of Abstracts: Trento-Padova, Italy, ogy (Amsterdam), v. 391, p. 34–46, https://​doi​ RCEM2017 Organizing Committee, p. 66. inducing bank erosion upstream of the meander .org​/10​.1016​/j​.jhydrol​.2010​.07​.005​. Nanson, G.C., 1980, Point bar and floodplain forma- apex. Extensional meanders are rare, reflecting Dunne, T., Constantine, J.A., and Singer, M.B., tion of the meandering Beatton River, northeast- an early stage of meander development. The 2010, The role of sediment transport and sedi- ern British Columbia, Canada: Sedimentology, meander symmetry index, which quantifies the ment supply in the evolution of river chan- v. 27, p. 3–29, https://​doi​.org​/10​.1111​/j​.1365​ geometric change in meander growth, is also cor- nel and floodplain complexity: Transactions -3091​.1980​.tb01155​.x​. of the Japanese Geomorphological Union, Ouchi, S., 1985, Response of alluvial rivers to slow related with sediment supply and rates of channel v. 31, p. 155–170, http://​ci​.nii​.ac​.jp​/naid​ active tectonic movement: Geological Society growth. Our results suggest that the evolution of /110007621738/en​ /​ .​ of America Bulletin, v. 96, p. 504–515, https://​ meandering rivers is strongly tied to upstream Filizola, N., and Guyot, J.L., 2009, Suspended sedi- doi​.org​/10​.1130​/0016​-7606​(1985)96​<504:​ supplies of sediment. These are responsible for ment yields in the Amazon basin: An assessment ROARTS>2​.0​.CO;2​. using the Brazilian national data set: Hydrologi- Posner, A.J., and Duan, J.G., 2012, Simulating river the turnover of floodplain sediments, nutrients, cal Processes, v. 23, p. 3207–3215, https://doi​ .org​ ​ meandering processes using stochastic bank ero- and habitats and the creation and preservation of /10​.1002​/hyp​.7394​. sion coefficient: Geomorphology, v. 163–164, alluvial architecture important for recording the Frothingham, K.M., and Rhoads, B.L., 2003, Three- p. 26–36, https://​doi​.org​/10​.1016​/j​.geomorph​ river’s evolutionary history through time. dimensional flow structure and channel change in .2011​.05​.025​. an asymmetrical compound meander loop, Em- Pyrce, R.S., and Ashmore, P.E., 2005, Bedload path barras River, Illinois: Earth Surface Processes and length and point bar development in gravel-bed ACKNOWLEDGMENTS Landforms, v. 28, p. 625–644, https://​doi​.org​/10​ river models: Sedimentology, v. 52, p. 839– We would like to thank Bodo Bookhagen for assis- .1002​/esp​.471​. 857, https://doi​ .org​ /10​ .1111​ /j​ .1365​ -3091​ .2005​ ​ tance with the development of channel extraction Guyot, J.L., Bourges, J., Calle, H., Cortes, J., Hoorel- .00714​.x​. methodologies, Alex Horton for assistance with curva­ becke, R., and Roche, M.A., 1989, Transport of Schumm, S.A., and Khan, H.R., 1972, Experimental ture data generation, Joan Teng for assistance with suspended sediments to the Amazon by an An- study of channel patterns: Geological Society of Landsat imagery collection, and Rolf Aalto for useful dean river: The river Mamore, Bolivia, in Pro- America Bulletin, v. 83, p. 1755–1770 https://​ discussions relating to discharge variability. We also ceedings of the Fourth International Symposium doi​.org​/10​.1130​/0016​-7606​(1972)83​[1755:​ thank T.C. Hales and the reviewers for constructive on River Sedimentation, June 5–9, 1989: Beijing, ESOCP]2​.0​.CO;2​. comments that helped improve the clarity and quality China Ocean Press, v. 1. Schuurman, F., Shimizu, Y., Iwasaki, T., and Klein- of the manuscript . 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