LETTERS PUBLISHED ONLINE: 2 NOVEMBER 2014 | DOI: 10.1038/NGEO2282 Sediment supply as a driver of river meandering and floodplain evolution in the Amazon Basin José Antonio Constantine1*, Thomas Dunne2,3*, Joshua Ahmed1, Carl Legleiter4 and Eli D. Lazarus1 The role of externally imposed sediment supplies on the fine-grained Neogene sedimentary rocks of the Central Amazon evolution of meandering rivers and their floodplains is poorly Trough yield an order of magnitude less sediment per river each understood, despite analytical advances in our physical under- year9, and those that drain the Guiana Shield to the north and the standing of river meandering1,2. The Amazon river basin hosts Brazil Shield to the south of the Trough yield the least7. tributaries that are largely unaected by engineering controls As sedimentary deposits attached to the inner banks (convex and hold a range of sediment loads, allowing us to explore the relative to the channel centreline) of meander bends, point bars influence that sediment supply has on river evolution. Here we seem to be the link between changes in externally imposed calculate average annual rates of meander migration within 20 sediment supplies and adjustments in river behaviour. Modern reaches in the Amazon Basin from Landsat imagery spanning theory attributes the meandering behaviour of alluvial rivers to 1985–2013. We find that rivers with high sediment loads instabilities in river flow11 that arise either inherently12 or from experience annual migration rates that are higher than those of disruption of the flow field by bedforms13. Recent lab experiments rivers with lower sediment loads. Meander cutoalso occurs suggest that this latter influence may not be ubiquitous14,butthe more frequently along rivers with higher sediment loads. capacity for point bars to promote meandering by altering the flow Dierences in meander migration and cutorates between field15 has been observed both in the field16 and the laboratory17. the study reaches are not explained by dierences in channel A view of the Rio Mamoré, flowing through the Andean foreland slope or river discharge. Because faster meander migration basin of Bolivia, offers an illustration. The Rio Grande, a tributary and higher cutorates lead to increased sediment-storage of the Rio Mamoré, drains 7.2 104 km2 of the Bolivian Andes and 1 ⇥ space in the resulting oxbows, we suggest that sediment transports about 136 Mt yr− of suspended sediment as it enters the supply modulates the reshaping of floodplain environments foreland basin10. The substantial sediment load of the Rio Grande by meandering rivers. We conclude that imposed sediment has resulted in the enhanced development of point bars on the Rio loads influence planform changes in lowland rivers across Mamoré downstream of the confluence (Fig. 1a) that has instigated the Amazon. a nearly 1.7-fold increase in rates of meander migration (Fig. 1b). Decades of fluvial research have yielded physical insight Observations from the Sacramento River of California, USA, further into river behaviours associated with various channel patterns, from planform controls on meander migration, to mechanisms a responsible for dispersing sediment into the floodplain2,3 and the in-channel dynamics that produce sediment supply from local bank erosion4. By contrast, the role of externally imposed MamoréFlow RiverN sediment supplies in channel planform dynamics—that is, when Rio Grande 5 km sediment supplies are an imposed environmental variable to which b the channel planform must adjust—remains poorly understood. Puerto Ganadero Rio Chimoré −1 −1 (64 Mt yr ) (∼4 Mt yr ) Input into M1 M2 External sources of sediment may derive from mountain building, −1) −1 (∼82 Mt yr ch-w yr glacial and other climatic changes, volcanism, and land uses that −1 0.047 Puerto Villaroel 0.039 ch-wM0 r r (9 Mt yr−1) M1 -w y La Satísima accelerate landscape-scale soil loss. They may also be interrupted − 1 ) Mamoré Rive yr 1 − nde Trinidad 0.066 ch Flow N by water resource developments such as the large reservoirs under Flow 5 Rio Gra69 Mt yr 35 km consideration for many Amazonian tributaries . ∼ ( The Amazon Basin remains a unique opportunity to assess the role of externally imposed sediment supplies in river Figure 1 | Enhanced development of point bars on the Rio Mamoré evolution because of the generally undeveloped nature of its downstream of the confluence with the Rio Grande. a, Landsat image fluvial environments and the range of sediment supplies in its from 2013 of the Rio Mamoré and its confluence with the Rio Grande, various tributary basins. Rivers within the 6.9 106 km2 basin which drains the Bolivian Andes from the southeast. b, Study reaches ⇥ are generally free to modify their forms, and they drain distinct of the Mamoré (M0, green; M1, red; M2, blue), with average annual physiographic provinces, each of which supplies sediment to the meander migration rates provided for each (in units of channel widths channel network at different rates6,7. Rivers draining the highly per year) for the time period 1986–2010. The box shows the location erodible Andean Cordillera yield the greatest amount of sediment of the image in a. Estimates of total suspended sediment (TSS) flux are to the Amazon lowlands, on the order of 100 Mt of suspended shown in brackets. Details of TSS flux estimates can be found in the material per river each year8–10. Rivers with headwaters draining the Supplementary Information. 1School of Earth and Ocean Sciences, CardiUniversity, CardiCF10 3AT, UK, 2Bren School of Environmental Science and Management, University of California–Santa Barbara, Santa Barbara, California 93106, USA, 3Department of Earth Science, University of California–Santa Barbara, Santa Barbara, California 93106, USA, 4Department of Geography, University of Wyoming, Laramie, Wyoming 82071, USA. *e-mail: [email protected]; [email protected] NATURE GEOSCIENCE | ADVANCE ONLINE PUBLICATION | www.nature.com/naturegeoscience 1 LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO2282 a b 0.4 Mean TSS 0.06 Mean MR (Mt yr−1 m−1) (ch-w yr−1) 0.3 0° Va Br 0.2 0.03 Pt 0.1 Na Jt P2 Cu It 0.0 0.00 Hu Ju Shield Trough Andes Shield Trough Andes Ir Uc P1 10° S Ma 0.4 Mean CR Mean NL Xi 0.002 (no. yr−1 ch-w−1) 0.3 (no. ch-w−1) Be M2 Ar M1 0.2 Shield M0 0.001 Central Trough 0.1 Andes–Foreland Basin 0.000 0.0 80° W70° W 60° W 50° W Shield Trough Andes Shield Trough Andes Ar, Araguaia; Va, Vaupés; Br, Branco; Ir, Iriri; Xi, Xingu; P1, Purus1; P2, Purus2; Ju, Juruá; Jt, Jutai; It , Ituí; Cu, Curuca; Na, Nanay; Pt, Putumayo; M0, Mamoré0; M1, Mamoré1; M2, Mamoré2; Be, Beni; Uc, Ucayali; Hu, Huallaga; Ma, Madre de Díos cd0.100 0.00500 0.28 0.43 MR = 0.043TSS CR = 0.0016TSS ρ = 0.73, = 0.53 M1 ρ = 0.66, = 0.51 M1 τB Uc τB Uc 0.050 α < 0.001 M2 0.00200 α < 0.002 Hu Be M0 Be M0 M2 ) 0.00100 Hu 1 − ) Ma 1 − 0.020 0.00050 Ma ch-w Br 1 P1 − Ju Cu Na It Va Na (ch-w yr (ch-w R 0.010 Ar 0.00020 It Ju P1 (no. yr (no. M Cu Ar R Ir Jt C Xi Pt 0.00010 Xi P2 Jt Pt 0.005 Br 0.00005 Va P2 Ir 0.0001 0.0010 0.0100 0.1000 1.0000 0.0001 0.0010 0.0100 0.1000 1.0000 TSS (Mt yr−1 m−1) TSS (Mt yr−1 m−1) Figure 2 | Study sites, river characteristics, and sediment fluxes. a, Locations of study reaches within the physiographic provinces with reach abbreviations defined. b, Mean values for total suspended sediment (TSS) flux (megatonnes per year per metre of channel width), average annual meander migration rate (MR; channel widths per year), average annual meander cutorate (CR) (number per year per channel width of down-valley distance), and total number of observable oxbow lakes (N ; number per channel width of down-valley distance). Error bars represent 1σ about the means. c, M plotted L ± R against TSS flux. d, CR plotted against TSS flux. highlight the role of point bars in facilitating bank erosion in reaches rates of lateral shifting. Cutoffrates (CR) were normalized by reach that experience a net accumulation of bed material18. valley-length, expressed in channel-width units (ch-w). TSS flux Within a given meander, curvature and shoaling of river flow estimates were normalized by RW to account for the ability of wider onto and over a point bar force high-momentum fluid to be channels with greater flows to convey more sediment. We used TSS advected outwards, increasing boundary shear stress along the outer data from the most extensive and systematically collected survey of (concave) riverbank and decreasing boundary shear stress over sediment supplies currently available for the Amazon Basin7. Details the inner-bank bar15. Elevated shear stresses near the outer bank of values from other sources are provided in the Supplementary promote the fluvial entrainment of the bank material primarily Information. TSS flux is the only widely (if sparsely) distributed responsible for bank erosion19, and the displacement of the zone measure of sediment supplies in the Amazon and other large river of maximum shear stress from the inner to the outer bank shifts systems. It includes both sandy bed material, which is the material of the zones of maximum bed-material transport and bank scour bar formation, and silt–clay washload, which is transported through accordingly15. As a result, the bar aggrades, growing laterally and the channel and into floodplains without being sequestered onto vertically20 until its surface is infrequently inundated, promoting the bars.