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The late quaternary evolution of the negro river, amazon, brasil implications for island and floodplain formation

The late quaternary evolution of the negro river, amazon, brasil implications for island and floodplain formation






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    The late quaternary evolution of the negro river, amazon, brasil implications for island and floodplain formation The late quaternary evolution of the negro river, amazon, brasil implications for island and floodplain formation Document Transcript

    • The late Quaternary evolution of the Negro River, Amazon,Brazil: Implications for island and floodplain formationin large anabranching tropical systemsEdgardo M. Latrubessea,T, Elena FranzinellibaUniversidade Federal de Goia´s-IESA-LABOGEF, Campus II, Goiaˆnia, GO 74001-970, BrazilbElena Franzinelli Department of Geology, Federal University of Amazonas, Manaus, BrazilReceived 6 November 2003; received in revised form 20 June 2004; accepted 24 February 2005Available online 26 April 2005AbstractThe Rio Negro has responded significantly in the Late Pleistocene and Holocene to lagged environmental changes largelyassociated with activity during the last glacial in the Amazon basin. On the basis of geological structure, the Rio Negro can bedivided into six distinct reaches that each reflects very marked differential processes and geomorphological styles. No depositsof the Upper Pleniglacial were recognized in the field. The oldest recognizable Late Pleistocene alluvial unit is the UpperTerrace of Middle Pleniglacial age (ca. 65–25 ka) (reach I), tentatively correlated with the oldest terrace identified on the leftbank of reach III. At that time, the river was mainly an aggradational bed load system carrying abundant quartz sand, a productof more seasonal conditions in the upper catchment. The late glacial (14–10 ka) is represented by a lower finer-grained terracealong the upper basin (reach I), which was recognized in the Tiquie´, Curicuriarı´, and Vaupes rivers. At that time, the rivercarried abundant suspended load as a response to climatic changes associated with deglaciation.Since about 14 ka, the river has behaved as a progradational system, infilling in downstream series a sequence of structurallycontrolled sedimentary basins or dcompartments,T creating alluvial floodplains and associated anabranching channel systems.Reach II was the first to be filled, then reach III, both accumulating mainly sand. Fine deposits increase downstream in reach IIIand become predominant in some anabranch islands of the distal reach. The lowermost reaches of the Negro (V and VI) havebeen greatly affected by a rising base level and associated backwater effect from aggradation of the Amazon during late glacialand recent times. Reach V has acted almost entirely as a fine sediment trap. The remarkable Anavilhanas archipelago is theproduct of Holocene deposition in the upper part of this sedimentary basin; however, suspended sediment load declined about1.5 ka, prior to the lower part of this basin becoming infilled.The progradational behavior of the Rio Negro, filling tectonic basins as successive sediment traps with sand in the upperbasins and fines in the downstream ones, illustrates how a large river system responses to profound changes in Late Quaternarybase level and sediment supply. The most stable equilibrium conditions have been achieved in the Holocene in reaches IIb andIIIa, where an anabranching channel and erosional–relictual island system relatively efficiently convey water and sediment0169-555X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.geomorph.2005.02.014T Corresponding author.E-mail address: latrubes@terra.com.br (E.M. Latrubesse).Geomorphology 70 (2005) 372–397www.elsevier.com/locate/geomorph
    • downstream. Reaches IIIb and V never achieved equilibrium conditions during the Holocene, characterised as they are todaywith incomplete floodplains and open water.D 2005 Elsevier B.V. All rights reserved.Keywords: Negro River; Late Quaternary; Holocene; Floodplain; Anabranching; Equilibrium1. IntroductionFloodplains are alluvial systems with a substantialhistory preserved within (Lewin, 1996). In largetropical rivers of South America, extensive areas offloodplain show at least a Holocene history that isusually characterized by more than one episode ofchanged conditions. The architecture of the depositsin these tropical fluvial systems results from tomorpho-sedimentary responses to some dominantcontrol, such as climatic change during the LatePleistocene and Holocene Epochs (e.g., Baker, 1978;Latrubesse, 2003; Stevaux, 1994, Stevaux and Santos,1998), to tectonic changes (Dumont, 1996; Dumontand Fournier, 1994; Iriondo and Suguio, 1981;Latrubesse and Rancy, 2000; Smith, 1986), and to0º50º70º0 400 800 Km0º1 - Solimoes-Amazonas2 - Xingú4 - Madeira6 - Purus7 - Juruá3 - Tapajos5 - Negro8 - Caquetá/Japurá11 - Pastaza12 - Ucayali9 - Putumayo-Içá 13 - MarañónLEGEND19112345678101011111212131310- NapoFig. 1. Location map. Rio Negro basin.E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397 373
    • both (Latrubesse and Kalicki, 2002). They can alsoresult from autogenic adjustments along a river(Latrubesse and Franzinelli, 2002). These depositscan be occasionally or frequently and totally orpartially flooded. For that reason, in big systems, theterm dfloodplainT can be vague and may not necessa-rily define the currently flooded alluvial surface. It canbe difficult to clearly map and the zone of floodinfluence can be very variable and highly dependenton the period of observation. Furthermore, largealluvial rivers have complex alluvial plains and awide variety of channel patterns.In addition to the traditional tripartite classificationof braided meandering and straight channels (Leopoldand Wolman, 1957), multiple-channel patterns(termed anabranching patterns and including anasto-mosing) have been more recently recognized (Rust,1978; Schumm, 1981, 1985; Knighton and Nanson,1993; Nanson and Knighton, 1996; Tooth andNanson, 2000). Anabranching characterizes most ofthe large alluvial rivers of the world (Jansen andNanson, 2004). Anabranching rivers characterise awide range of environments with anastomosingvertical accreting systems common in subsiding orsimilar vertically accreting areas (Makaske, 1998;Smith, 1983, 1986, Smith et al., 1989) and otheranabranching types in stable areas (Nanson andKnighton, 1996; Jansen and Nanson, 2004). Thispaper discusses the adjustments in a large partiallyblocked tributary, the Rio Negro, by aggradation inthe trunk system, the Rio Solimo˜es-Amazon, in themiddle Amazon basin from the Late Pleistocene to theupper Holocene (Fig. 1). A tectonically controlledvalley gradient and a rising base level have producedhydro-geomorphological conditions along the lowerRio Negro that has created spectacular fluvialarchipelagos, the Anavilhanas and Mariua´ archipela-gos (see Figs. 9, 10 and 15).2. Materials and methodsThe authors conducted several expeditions on theriver by boat between 1990 and 2002. Sedimentaryprofiles of banks, channel, and floodplain crosssections and geomorphological descriptions werecomplemented with geomorphologic mapping of theriver using several years of digital Landsat TM 5images, JERS radar images for 1995, side lockingradar mosaics (SLAR) at a scale of 1:250,000 scalefor the years 1971/1972, navigation charts, andtopographic maps. Mapping and measures of channeland floodplain distance and area employed a SPRINGGeographic Information system. Coarser grain sizeswere determined by sieving and fine fractions (silt andclay) by use of an Appiani tube. X-ray diffractionswere performed at the Earth Science Department ofthe University of Modena by grinding of the total finesedimentary fraction. Radiocarbon (14C) ages wereobtained by dating wood detritus and charcoal at theLATYR Laboratory, National University of La Plata,Argentina.3. Regional settingThe regional setting of the Negro river wasexplained by Latrubesse and Franzinelli (1998) andFranzinelli and Igreja (2002). The Negro basin, withan area of more than 600,000 km2, extends throughparts of Colombia, Venezuela, and Brazil (Fig. 1). Theupper basin passes through the Colombian plains andthe Brazilian shield, the river being called the Negroafter the confluence between the Guaı´nia and theCasiquiare rivers, which connect the Negro basin withthe Orinoco basin. Nearly all the basin is covered withtropical rainforest, although savannas cover somemarginal areas of the Colombia Llanos and the flatsareas of Roramia State, Brazil.The climate is dominantly humid tropical with anaverage precipitation higher than 2000 mm/year andincreasing in a northwest direction to reach ~3500mm/year (Radambrasil, 1976). Rainfall less than1800 mm characterizes savanna areas. The upperNegro drains a small area of sedimentary flats ofColombia covered with savanna and Precambriangranitic and granulitic rocks of the Brazilian Shieldcovered with dense tropical rain forest. Regionally,the upper Negro and its tributaries, such as Xie´,Ic¸ana, Vaupe´s, and Curicuriarı´ on the right side,drain a flat area of low relief with heights varyingbetween 60 and 160 m a.s.l. Higher relief isrepresented by numerous inselbergs isolated orgrouped like ranges up to 700 m a.s.l. Somemountains reach 3000 m. The middle and lowerNegro, downstream of Santa Isabel town, crossesE.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397374
    • IaIIIIIIVIbabVVI0100200km68º66º64º62º60º58ºNEGRORIVERBRAZIL3ºVENEZUELAGUYANAAMAZONRIVERBRANCORIVERDEMINIRIVERJAUAPERIRIVERCUIUNIRIVER0º50º70º0400800Km0º1234ABCED0ºSFBAMAFig.2.Reachesandsub-reachesoftheNegroRiver(I,II,III,IV,V,andVI).Numbers1–4indicatethenodalpoints.Insetareas(A),(B),(C),(D),and(E)relateto(Figs.5,9,10,and15).LocalitiesofFig.5:MA,Manaus;BA,Barcelos;SF,Sa˜oFelipe.E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397 375
    • flat-topped hilly areas of Paleozoic rocks of theSilurian Trombetas Formation, which are restricted toa relatively narrow belt in the middle course. Creta-ceous sedimentary rocks of the Alter do Cha˜oFormation and small areas of Tertiary sedimentaryrocks of the Solimo˜es Formation also outcrop. In themiddle reach, the Negro receives some large tributarieson the left side, the largest one being the Branco River.The Branco drains hilly areas of crystalline rocks andtable mountains formed of Precambrian sedimentaryrocks (locally named btepuisQ) from Roraima State(Brazil) and the border between Brazil and Guyana aswell over the savanna plains of the Roraima Lavrado.In the middle course, the river crosses a huge low-gradient plain named the Northern Pantanal.4. Reaches of the Rio NegroOn the basis of geomorphologic style and struc-tural control, the Rio Negro can be divided into sixreaches (Fig. 2). Reach I is the upper basin or sourcearea and is divided in sub-reaches baQ and bb.Q Theformer supplies a large part of the sedimentary loadand the water discharge, and finishes in the con-fluence of the Vaupe´s and the Negro River. The latterextends from downstream of Sa˜o Gabriel daCachoeira to Nodal Point 1 (Fig. 2) on crystallinerocks of the Precambrian Shield that are alignedapproximately along E–W lineaments. The more con-spicuous features along the channel in Reach I arerapids (Fig. 3) and big rocky islands formed of crys-talline rocks outcropping along the channel (Fig. 4).White sand bars are irregularly scattered along theupper course of the Rio Negro, more frequent wherebedrock outcrops. The bars are accumulations of verypoorly sorted sand usually lacking internal sedimen-tary structures. Reach II starts at Nodal Point 1 wherethe anabranching pattern of the river begins in alluvialsediments that form a wide Holocene floodplain.Reach III begins where the river turns to anapproximate NW–SE alignment that coincides withtectonic lineaments and crosses mainly Paleozoic andCretaceous sedimentary rocks. It extends for approx-imately 275 km up to Nodal Point 2, 7 km down-stream of the Rio Branco and Rio Negro confluence,and can be divided in two sub-reaches baQ and bb.Q Itsmost spectacular geomorphic element is the Mariua´archipelago and a huge asymmetrical terrace, whichappear on the left side practically all along the reach.Fig. 3. Rapids to Sa˜o Gabriel da Cachoeira, upper Rio Negro, reach I.E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397376
    • Reach IVextends between Nodal Points 2 and 3 and ischaracterised by the occurrence of some rock outcropsin the channel and because it is a narrow zone. ReachV coincides with a wide channel belt between NodalPoints 3 and 4, crossing Cretaceous rocks of the Alterdo Cha˜o Formation. It is characterised by a trulyremarkable large archipelago of muddy islands namedAnavilhanas. Reach VI runs from Nodal Point 4 to theconfluence of the Rio Negro and Rio Solimo˜es. Thevirtual absence of islands or other distinctive fluviallandforms in the channel is the main characteristic ofthis reach.S 00º 22´w 60º 33´ w 60º 13´S 00º 22´0 5 10KmScale0 5 10KmScalew 55º 10´ w 54º 50´w 55º 10´ w 54º 50´S 00º 30´ S 00º 30´w 60º 33´ w 60º 13´ABFig. 4. JERS images showing rocky islands in areas (A) and (B) of figure, upper Rio Negro, reach I.E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397 377
    • 5. Tectonic controlsThe fluvial belt of the Rio Negro shows strongstructural control (Fig. 5). Evidence of neotectonicactivity in middle Amazonia has been reported byseveral authors who described river alignments,captures, and flower structures indicating transcurrentmovements and faults systems affecting Quaternarystone lines (Sternberg, 1950; Franzinelli and Latru-besse, 1993; Latrubesse and Franzinelli, 2002; amongothers). The main structural features of reaches I andII are approximately ~N–S and ~E–W fracture trends.Big faults control the main valley along all itsextension on reaches III and IV. Very wide valleyareas are abruptly separated structural narrow zones ornodal points where older rocks outcrop. An adequatetectonic model could be that proposed by Franzinelliand Igreja (2002) for reach IV. They suggest a halfgraben system with dextral transcurrent N458W andsecondary faults arranged in echelon in the directionN708E. A third ~E–W set of fractures (Franzinelli andLatrubesse, 1993) produced transtensional basins(rhombochasm) and formed part of a set of dextraltranscurrent faults along the Amazon valley axis(Franzinelli and Igreja, 2002). The big fault thatcontrols the valley on the right side along the lowerRio Negro continues in a NW direction to longitude61838V. It controls the lowermost reaches of the rightside tributaries such as the Unini, Jau´, and Carabinanirivers, which have rocky rapids (Forsberg et al.,2000).Reach III also appears to be a response to a similarhalf graben model, with asymmetrical developmentof fluvial geomorphologic units where the channelruns along the right side of the recent floodplain anda large asymmetrical terrace is located on the leftside, both fluvial units encased in older rocks.Narrow zones separate the main sunken blocks where68º 66º 64º 62º 60ºNEGRO RIVER3ºV E N E Z U E LABRANCO RIVERDEMINI RIVERJAUAPERI RIVER 0ºNormal faults associated to strike-slip faults,half graben-rombochasm featuresAntithetic faultsInferred faultsAreas with significant normal componentsStrike-Slip faultsLineamentsRestraining Zones0 100Fig. 5. Main structural features along the Rio Negro.E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397378
    • archipelagos form. Along the narrow zones, somecritical nodal points were identified (nos. 1, 2, 3, and4 of Fig. 2).6. Fluvial regime and sediment transportWith a mean annual discharge of ca. 29,000 m3/sand a drainage area ca. 600,000 km2, the Rio Negro isthe second largest-discharge tributary of the Amazonafter the Madeira River (ca. 31,000 m3/s) (Filizola,1999), and ranks sixth in the world in water discharge.As defined by Sioli (1984), it is the typical bblackwaterQ river of the Amazon basin, with olive-brown tocoffee-brown colored water and transparencies from1.3 to 2.3 m because of dissolved humic substances.The river transports just 8 Mt/year of suspendedsediment to the Solimo˜es–Amazon (Filizola, 1999),an insignificant quantity of suspended load in relationto the huge water discharge. The bed load iscomposed of white quartz sand. Fine sediments areformed by Kaolinitic clays rich in iron and sandsderived from weathered rocks of the Precambriancrystalline basement or Paleozoic and Mesozoicsedimentary rocks. The sands are supermature andquartz represents more than 98%. The more con-spicuous present-day active landforms along theNegro are sand bars. Differences exist in bedloadsands along the length of the Rio Negro. Fig. 6 showsa bivariate plot of the mean grain size (Mz) versus thestandard deviation (Folk and Ward, 1957) of samplesfrom sand bars.Samples from the upper and middle Rio Negro(reaches I, II, and III) mainly overlap in Fig. 6,while the samples of the lower Rio Negro (reachesV and VI) plot separately. The sands of the UpperNegro are medium sand with sand diametersvarying between 0.38 and 2.81 a and dominantvalues varying from 1 and 1.5 a. The sorting variesbetween 0.47 (well sorted) and 1.07 (poorly sorted).The textural parameters of the middle Negro show abetter homogeneity. Mean size varies between 0.64and 1.75 a, with dominant values between 1.11 and1.32 a. The sorting of this group varies from 0.22to 0.89. This suggests that the upper reachestransport finer, more poorly sorted sand derivedmainly from the Precambrian shield and may besome sand reworked from Paleozoic rocks, whilethe middle and lower area appears to reflect localsand from the Alter do Cha˜o Formation, which iscoarser, slightly better sorted, and mainly from theCretaceous sandstone.XXXXXXXXXXXXXXXXXXXXXX00,20,40,60,81,01,2σ1MZ φ Mean1,0 1,5 2,0 2,5 3,0 φ0,5σ1StandardDeviationXLegendUpper Rio NegroMiddle Rio NegroLower Rio NegroFig. 6. Mean size vs. standard deviation of sand samples of upper (reaches I and II), middle (III), and lower reaches (V–VI) of the Rio Negro.E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397 379
    • ManausManacapuru020040060080010001200140016001/1/19921/9/19921/5/19931/1/19941/9/19941/5/19951/1/19961/9/19961/5/19971/1/19981/9/19981/5/19991/1/20001/9/20001/5/20011/1/20021/9/2002TimeStage:mBarcelos1/7/19921/1/19931/7/19931/1/19941/7/19941/1/19951/7/19951/1/19961/7/19961/1/19971/7/19971/1/19981/7/19981/1/19991/7/19991/1/20001/7/20001/1/20011/7/20011/1/20021/7/2002Time02004006008001000Stage:mSão Felipe020040060080010001/1/19921/9/19921/5/19931/1/19941/9/19941/5/19951/1/19961/9/19961/5/19971/1/19981/9/19981/5/19991/1/20001/9/20001/5/20011/1/20021/9/2002TimeStage:m020040060080010001200140016001/1/19921/9/19921/5/19931/1/19941/9/19941/5/19951/1/19961/9/19961/5/19971/1/19981/9/19981/5/19991/1/20001/9/20001/5/20011/1/20021/9/2002DateStage:cmFig. 7. Hydrograph of daily mean stages for Sa˜o Felipe, Barcelos, and Manaus stations in the Rio Negro, and Manacapuru station in theSolimo˜es River. The data were arbitrary referred to a 0 (zero) value corresponding to the lowest recorded value for each station.E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397380
    • The textural composition of Anavilhanas Islands issilty clay or clayey silt, but sand bars can be observedin reach V as well. The sand diameter of the largelateral sand bars of Praia Grande, Tupe´, and PontaNegra vary between 0.4 and 2.3 a, but commonlybetween 1 and 1.75 a, with sorting ranging from 0.26to 0.67.While the flow gauging network in the Amazonbasin is not particularly detailed, that on the Rio Negrois particularly poor. We analyzed the stage record. Tocompare stage oscillation at three locations along theRio Negro, this paper examines hydrograph data at Sa˜oFelipe, Barcelos, and Manaus gauge stations as well asManacapuru´ gauge station in the Solimo˜es River (Fig.7; for locations, see Fig. 2). Considering that thestations are not related to an absolute bdatum,Q thestage data were arbitrary referred to a 0 (zero) valuecorresponding to the lowest recorded value of theseries for each station. As noted by Meade et al.(1991), stage heights in the lowermost reaches of theLEGENDSANDSTONE, SILTSTONE-TERTIARYSANDSTONE, SILTSTONE-TERTIARYCARBONATITE-CRETACEOUSSANDSTONE, ARKOSE-MIDDLE PROTEROZOICGRANULITE, GNEISS,MIGMATITE, DIORITE-ARCHEANGEOLOGIC CONTACTSAMPLE LOCATIONS OFTHE UPPER TERRACESAMPLE LOCATIONS OFMODERN SEDIMENTSSECTION LOCATION OFTHE LOWER TERRACEPROFILE WITH C DATING14QPIA(pg)1,2...PmrQPIVAUPÉSSERRA DOSSEIS LAGOSSÃO GABRIEL DACACHOEIRA (UAUPÉS)RIVERCuricuriari riverA(pg)MarieriverNEGRORIVERTiquié river134211210QPIQPI0 50 100KmKIKIPmrA++++++ + +++ + +++++14m0LOW WATER LEVEL0??cc.100mLOW WATER LEVEL? ?++++ + + + +++ + +++++14m04cc.100m+++FLOODPLAINLOWER TERRACE SEDIMENTSUPPER TERRACE SEDIMENTS(TIQUIÉ FORMATION)CRYSTALLINE BASEMENTLEGENDB68° 66°0°Fig. 8. Geomorphologic fluvial units and places with radiocarbon dating of the upper Negro River (adapted from Latrubesse and Franzinelli,1998).E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397 381
    • Negro are in agreement with the backwater effect ofthe Solimo˜es–Amazon river that extends as farupstream as Moura, 300 km upriver from the Amazonconfluence. The Manaus station indicates that stagefluctuations in the Rio Negro are in phase with theSolimo˜es–Amazon River stage fluctuations (Stern-berg, 1987; Richey et al., 1989). In Manaus, the riveroscillates up to 15–16 m with an average of ~11 m,whereas 470 km upstream of the confluence atBarcelos (reach III), the average oscillation is ca. 6.6m, while in Sa˜o Felipe (reach I) ~1000 km upstreamof the confluence, the average oscillation is ~7.5 m(Fig. 7). The lowest stages in reach III occur inOctober–November, but downstream, the loweststages occur in February–March. While the rivercontinues to fall in December and January in reach III,downstream (reaches V and VI), the river begins torise (Meade et al., 1991) . These differences in thebehavior of water stage demonstrate how the RioNegro is differentiated into several reaches. The stageoscillation being least at Barcelos is associated withthe downstream reduction of secondary peaks and thesmoothing of the hydrograph, but also is produced bythe reservoir effect of the large tectonic block of reachIII and the existence of the Mariua´ archipelago.The mean surface slope from Sa˜o Gabriel to Sa˜oCarlos was estimated to be 0.000074 (measured by theU.S. Army Corps of Engineers, 1943; in Meade et al.,1991). The mean low-water elevation at Sa˜o Gabriel(990 km upriver from the mouth), however, isestimated to be 55 m above mean sea level and 8 mabove sea level to low water stages at Manaus (Meadeet al., 1991). This would relate to a mean water surfaceslope from Sa˜o Gabriel to Manaus as low as 0.000045.7. The Late Quaternary units7.1. Upper Terrace sedimentsIn the upper Rio Negro, an ancient terrace leveloccurs up to 14 m above the low water level (theUpper Terrace) (Fig. 8). Its sediments, outcroppingalong the Vaupe´s, Tiquie´, and Curicuriarı´ rivers, werenamed Tiquie´ Formation and described by Latrubesseand Franzinelli (1998). They are dominantly sandywith predominant planar cross bedding and troughcross bedding structures. Fine sediments are limited tosome lenses and thin strata just a few decimeters inthickness. Gravels are less abundant and associatedwith the sandy deposits. Iron oxides and organicmatter frequently impregnate the sediments withleaves and stems; even small logs are present. Theuppermost part of the terrace sediments frequently isleached into a white friable sand. The sands are wellsorted. In the Tiquie´ and Vaupe´s Rivers, mean sizevaries between 0 and 1.24 a, while those in theCuricuriarı´ are significantly finer and vary between1.35 and 2.6 a, and quartz is dominant (N95%) in allthree rivers. Radiocarbon dates of logs and organicmatter found in the sediments of Tiquie´ Formationrange between 27 ka BP and N40 ka BP (Table 1).Table 1Radiocarbon dates of the Rio NegroRiver Reach Unit Radiocarbon age(years BP)Sample code LocationTiquie´ I Upper Terrace 27,220F200 Beta 52467 P1, Fig. 8Tiquie´ I Upper Terrace 37,240F520 Beta 52465 P1, Fig. 8Vaupe´s I Upper Terrace Older than 38,450 Beta 52465 P2, Fig. 8Curicuriarı´ I Upper Terrace Older than 40,000 Beta 52464 P4, Fig. 8Tiquie´ I Lower Terrace 13,420F100 Beta 52464 P10, Fig. 8Tiquie´ I Lower Terrace 12,590F120 Beta 52469 P12, Fig. 8Curicuriarı´ I Lower Terrace 4050F60 Beta 52466 P13, Fig. 8Negro III Mariua´ archipelago 3650F90 LP-985 Profile 3, Figs. 9 and 14Negro III Mariua archipelago 1450F70 LP-1000 Profile 5, Figs. 10 and 14Negro III Mariua archipelago 1710F60 LP-1004 Profile 6, Figs. 10 and 14Negro III Mariua archipelago 1330F40 LP-1063 Profile 10, Figs. 10 and 14Negro III Mariua archipelago 1060F40 LP-1036 Profile 12, Figs. 10 and 14Beta,beta analytic; LP,Tritium and Radiocarbon Laboratory—LATYR-UNLP.E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397382
    • The distribution of the Upper Terrace sedimentsis restricted to a narrow zone, only sporadicallyoutcropping along the banks of reaches I and II. Inreaches II and III, a large asymmetric terraceextends along the left bank of the Negro. Eoliandunes formed on the terrace surface by deflation ofalluvial sediments. Thermoluminescence dating indi-cates ages range between 12.6 ka BP and 22.8 kaBP (Carneiro Filho et al., 2002). It is possible thatthis asymmetric terrace correlates with UpperTerrace as described by Latrubesse and Franzinelli(1998) in the upper Negro; however, no absolutedating exists to corroborate this. We examined thestratigraphy of this terrace near the Jufari RiverFig. 9. Fluvial Units of reach IIIa. Note the Upper Terrace (T) on the left side in the JERS image. Numbers indicate profiles of Fig. 14.E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397 383
    • confluence with the Rio Negro where it waspossible to identify outcrops 5–9 m high composedfrom bottom to top with brown-gray silty-clayeysand with orange-yellow mottled, massive gray lightgreen clays with reddish mottle.7.2. Lower TerraceLatrubesse and Franzinelli (1998) described theLower Terrace as 2–4 m high above the low waterlevel in the Upper Rio Negro (Fig. 8). In general, thebase is not visible, but in some locations, it liesdirectly on the crystalline basement rocks. It consistsof fine (clay and silt) massive or finely laminatedoverbank and secondarily by lateral accretion depos-its, very different from the Tiquie´ Formation of theUpper Terrace. Sand is scarce, and the dark colordiagnostic of sediments in the Tiquie´ Formation isabsent. X-ray diffraction analyses show that quartzand kaolinite are dominant. Radiocarbon datingplaced this terrace at between 13.5 ka BP and 4 kaBP (Table 1).7.3. Reach III—the Mariua´ archipelagoThe Mariua´ archipelago is a fascinating array ofvegetated stable islands. It extends 275 km alongreach III where the mean width of the valley here isFig. 10. Fluvial units of reach IIIb. Note the Upper Terrace (T) on the left side in the JERS image. Numbers indicate profiles of Fig. 14. Cross-sections are described in Fig. 11.E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397384
    • 16–18 km. The archipelago can be divided in sub-reaches IIIa and IIIb (Fig. 2) that result from sunkentectonic blocks elongated in NW–SE direction. Thelimit of reaches baQ and bbQ is in the confluence areaof the Demini River with the Rio Negro. In bothareas, the channel pattern is anastomosing withstabilized islands separating the channels. In somelocations, the valley is almost full of sediment, withmuch of the valley forming an impeded drainagesystem of depositional islands, with particularly theleft side of the valley characterised by paleochannelsand narrow opportunistic floodplain channels thatdrain impeded areas of floodplain (Figs. 9 and 10).Downstream, in the distal part of the Mariua´archipelago (Fig. 10), it is more difficult todetermine the position of the main channel becausethe islands are spread across the full valley widthand the river divides in many minor branches. Fromupstream to downstream, the islands change frommore typically sandy cored islands topped withmuddy sediments, to islands where the mudbecomes predominant. A geomorphologic mapdivides the fluvial features into: a) channels (themain anabranching river pattern); b) islands withinthe main anabranching pattern; c) the internalfloodplain; and d) lateral floodplain (Figs. 9 and10). Lateral floodplain includes those alluvial areasnot enclosed by the main network of anabranchingchannels and, therefore, attached to the valley side.Local floodplain channels can be present in thisunit, running mainly along depressed paleochannelsareas. The internal floodplain is formed by bigislands generated by the amalgam of islands featuresand erosional anastomosing channels, which havedissected the floodplain alluvial sediments. Thefloodplain was divided in these three categories,which indicate successional stages in the filling ofthe sedimentary basin. The first stages are islandsand the second stages are amalgamated big islands.The third stage is an area of impeded floodplainformed by the coalescence of big amalgamatedislands that form a flat plain with paleochannels andswamps (lateral floodplain). The fourth stage is anerosional pattern dissecting part of the floodplainarea, reactivating paleochannels, and generatingsome new ones.Within this tectonic prism, the areas of alluvialsediment and remaining water (channels and lakes)have been measured using a Spring-GIS for reachIII baQ and bb.Q The total area for reach III is ~4143km2, being ~1827 km2for sub-reach baQ and ~2316km2for sub-reach bb.Q The valley has more sedi-ment in sub-reach baQ (70.7%) than bbQ (64.4%).Interestingly, the proportion of sediment that formsislands is significantly greater in sub-reach bbQ(35.7%) than in sub-reach baQ (14.5%), The areasof lateral and internal floodplains decrease from56.2% in baQ to 28.5% in bbQ.The upstream part of sub-reach bbQ has accumu-lated more sediments than the downstream part. Thenumber of channels and islands increases from theupstream to the middle part of sub-reach bb,Q with adecrease in the number of islands and an increase inchannel width in the most distal part because, at thisstage, there has been insufficient sediment to fill thelower end of the basin. Part of the lower basin hasbeen filled by a large internal delta built from theBranco River (see Rio Branco below).Two surveyed transects, integrated with bathy-metric data and simplified into two schematic cross-sections, were in reach IIIb (Fig. 11). In bothtransects, the river has two main channels: ~5 to~10 m deep. Correcting depth values for high waterstages, the w/d ratio ranges approximately from 250to 100. The islands vary between 3.5 and 4.5 mheight in relation to the low water level, but at somepoints, the banks can reach up to ~7 m in height.Elongate sand bodies form the core of the islandsand are topped by muddy strata (Fig. 12) with awell-defined contact between. The sand bodiesexhibit tabular cross-beds oriented downstream,generally decreasing in grain size upwards frommedium sand to very fine sand and silt, withoccasional ripple sets present at the top of thesands. The surficial muds are compact, homoge-neous, and light yellow-gray to white-colored.Sometimes, thin horizontal long dark lines oforganic material, in the top few meters of theislands, with fragments of tree trunk and leaves areexposed in the muddy banks. Very wide whitesandy bars, formed mainly by tabular cross-beddingsets, occur at the distal ends of the islands, or asmiddle channel bars (Fig. 13). These are theproducts of active accretion. In the dry season,wide sandy sheets can extend between higher banks,with large dunes and minor asymmetrical rippleE.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397 385
    • + ++Width:km105-5-10-150Depth:m0 2 4 6 8 10 12 14 161+++2105-5-10-150Depth:mWidth:km0 2 4 6 8 10 12105-5-10-150Depth:mWidth:km0 2 4 6 8 10 1234105-5-10-15-200Depth:mWidth:km0 2 4 6 8 10 12 14 16+++ ++++++5015105-5-10-15-20-25-30Depth:mWidth:km0 2 4 6+++++++++++ ++++++++ +++SandFine (clay-silt)Older rocksIgapo forestLow water levelInferred surface´Fig. 11. Schematic transverse sections of reaches IIIb, V, and VI. Heights and bathymetric data have been approximated from topographic maps,navigation charts, and field survey data. Depths in navigation charts are generally referred as below the average minimum stage. For locations, seeFigs. 10 and 15.E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397386
    • marks over the surfaces. The sandy bars in placesreach 3 m or more in thickness.Some stratigraphic sections along the banks ofthe islands are shown in Fig. 14 and Table 1. Profile3 on the left bank shows ~2 m of sandy depositsand includes some granules and fine gravel. In someplaces, the sandy and sandy conglomeratic depositsshow orange iron oxide from incipient induration,although the sediments remain friable. The sandysediments are topped by light gray muddy sedimentsand dark gray to almost black color mud in freshexposures, with abundant organic matter and someFig. 12. Typical banks of Mariua´ Islands, reach IIIa. The sandy core of the island has planar cross-bed stratification in the lower part of theprofile and is topped by fine sediments.Fig. 13. Large sand bar with planar cross stratification in reach IIIa. Many channels of this reach are very shallow because aggradation continuesto be active in an area that acts as an efficient trap for sandy sediments.E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397 387
    • Fig. 14. Selected profiles in reaches III and IV. For location, see (Figs. 9, 10, and 15).E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397388
    • 0510KmScaleS02º00´S02º30´+w61º10´w61º00´+w60º30´S02º00´S03º00´+++w61º10´w60º50´+w60º20´S03º00´S02º30´++++a-Anavilhanas:Proximalareab-Anavilhanas:Distalarea010km010kmI-AlterdoFormation(Cretaceous)III-HoloceneAlluvialSediments(AnavilhanasArchipielago)IV-BlockedvalleysIIIVIVIVIVIVIVIVIVIVIVIVIVIVIVIVIVIVIVIIIIIIa3b4Chao˜Fig.15.JERS-1imageof1995showingthemaingeomorphologicalelementsoftheAnavilhanasarchipelago,reachV.Cross-sectionsaredescribedinFig.11.E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397 389
    • stems and leaves still visible. Weakly organichorizons can be continuous for several tens ofmeters. At one location, radiocarbon dating gave anage of 3650F90 BP.Profile 5 on Ramada Island is formed of 2.0–2.5 mof massive bioturbated muddy sediments that are lightgray in color. Lenses of muddy sediment fill depres-sions on the islands where organic matter concentrates,in particular logs in horizontal position. At the base ofthe bank, a log of 0.30 m diameter radiocarbon wasdated at 1450F60 BP. Abundant thin logs (0.4 m indiameter) in horizontal position in a 2 m thickpaleochannel formed of muddy sand covered by 5 mof gray massive and bioturbated clayey sediments alsowere located on Jararaca Island (Profile 6).Logs in Profile 10 on Calango Grande Island gavean age of 1330F40 BP, and an age of 1060F40 BPwas obtained from a log in Peixe Boi Island (Profile12). Logs were also locates in Profiles 1 and 2.7.4. The Rio Branco and other left side tributariesThe Rio Branco is the main tributary entering inthe lowermost part of reach III of the Rio Negro. Cleargreen-colored water with a yellow sandy bed load andvery little suspended load characterize the Branco,Jauaperi, and Jafari rivers. The yellow sands of theRio Branco enter the Rio Negro remain along theleftmost main channel for ~50 km downstream of theconfluence. The contrasting yellow sands of the RioBranco and the white sands of the Rio Negrodemonstrate that the mixing of bedload may notoccur over very considerable distances, even within asingle channel.But the more important feature of the Rio Brancoin the study area is a delta feature, which penetrateslike a progradational landform inside the Negrovalley (Fig. 10). At present, the delta is an almostnon-active landform because of the low quantity ofsuspended sediment carried by the Rio Branco. Thedelta front is formed by banks up to 7 m high ofbrownish massive clayey horizontally bedded sedi-ment with red to orange mottling (Fig. 14, Profiles13 and 14). The sediments of the delta system alsocan be identified along the Branco River, with aterrace 11–12 m high and made of fine sediments,resting on granulitic gneiss rocks of the Precambrianshield, having formed on the left side. The RioBranco delta acts as an obstacle in the Negro valley,narrowing the channel which at this location is just2 km wide (Fig. 10).7.5. Reach V— the Anavilhanas IslandsThe Anavilhanas Islands are a very complexarchipelago formed of fine sediment (Fig. 15) andcovered by vegetation. They begin in reach IV butextend downstream to reach V where the wider valleyis more favorable to formation. During the high flowseason, these islands are completely flooded, but inthe dry season, it is possible to observe banks that insome islands are very steep. The islands are charac-terized by a somewhat dphantom morphologyT formedby a narrow levee-like deposit of fine sediment, whichsurrounds a semi-circular island head and extendsdownstream as long as two lateral tails. Two mainkinds of islands can be identified in the AnavilhanasFig. 16. Igapo´ forest on one of the Anavilhanas Islands, reach V.E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397390
    • archipelago, divisible on the basis of whether the twolateral tails join to enclose an inner lake (Fig. 15). Inthe more upstream part, the islands are shorter andcompact with large rounded lakes in the head andshort tails, while from the middle to the downstreamreach, the islands are characterized by a more typicalphantom morphology with very long tails often openat the end. A more spectacular example is theAnavilhanas Island, which is 40 km long but whosetails are just ~140 m wide.The banks are steps and can reach ~7 m above lowflow, but in general, the height decreases graduallydownstream. This variation in the height of the banks,from upstream to downstream, is marked by changesin vegetation from an arboreal flooded forest (Igapo´forest) (Fig. 16) to a low shrubby vegetation. Theinternal lake banks are smoother in contrast to themore abrupt outside banks. The islands are totallyflooded during high stages and just the bIgapo´Qvegetation appears above the water.A survey section across a tail displays a sinusoidaltopography with ridges and parallel to the flow. Theinternal structures of the levees seem to be mainlycompact and homogeneous sand, silt, clay, but insome outcrops, a sequence of silt and clayey thinhorizontal beds was observed. Several profiles wereFig. 17. Typical fine sediment banks on the Anavilhanas Islands (levee facies) in reaches IV and V. Similar logs were dated using 14C.E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397 391
    • performed in reaches IV and V. Massive highlybioturbated fine sediment and horizontal strata arecharacteristics (Fig. 17). Charcoal fragments in layerswere observed in the banks, some associated witharcheological artifacts (some pieces of ceramics, forexample), with some logs being present.The area of the prismatic Anavilhanas sedimentarybasin is ~2050 km2. A geomorphologic map identi-fied: a) anabranching channels, b) islands (formed oflevees), c) lakes generated by enclosing levees, and d)dead water areas, formed and partly enclosed bylevees at the downstream end (Fig. 18).The Anavilhanas tectonic block is also an incom-pletely filled sedimentary basin. The mean width ofthe valley ranges from 18 to 20 km. The Spring-GISdata indicate that channels occupy 33.5% of the totalarea, depositional islands (levees) 40.6%, and lakesand deadwater areas 25.9%.Two transects were surveyed and integrated withassociated bathymetric data (cross-sections 3 and 4,Fig. 11). Cross-section 3 shows a more proximal areaof the archipelago and a large density of leveesmorphologies on the islands and a multi-channelpattern formed by narrow channels. The channels are0 10 20 kmS02°30’S02°50’W60°30’W60°30’ChannelsIslandsLakes"Dead waters"Fig. 18. Fluvial units of reach V (for comparison, see Fig. 15).E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397392
    • no more than 12 m deep, ~1500 m wide, and with w/dratios ranging from 20 to 60 (values corrected to highwater stage). In contrast, cross-section 4 shows the endof the archipelago where a big area of bdead water,Q ~6km wide, occurs downstream of the two main tails ofAnavilhana Island. The w/d ratios vary between ~80and N400 (values corrected to higher stages).The Anavilhanas lithology is not compatible withcontemporary conditions in the river, which atpresent transports an insignificant amount of sus-pended sediment. The islands appear to haveresulted from the deposition of elongated levees inwater loaded with suspended sediment. Tricart(1977) suggests a delta model for the Anavilhanasformation as consequence of the Flandrian sea leveltransgression of the middle Holocene. In contrast,Sioli (1991) suggested that the islands are formingat present as a consequence of the sedimentary inputto the Negro by the Branco River and the sedimentscould be being deposited in the Anavilhanasarchipelago. Some problems exist, however, withTricart’s and Sioli’s interpretations regarding timeand environment of formation. The radiocarbondating recorded in the islands varies between ~3.7ka BP and ~1 ka BP, younger than the Flandriantransgression and, therefore, not a product of it. Onthe other hand, the Branco River is a sandy riverwith insufficient suspended load used to produce thedeposition of these giant islands today. The vege-tated islands appear to be relict but to have beenformed earlier in the late Holocene.Four main components are essentials for thedevelopment of the Anavilhanas: a) a sufficient amountof suspended sediments; b) a low energy environment;c) a linear space accommodation in the valley; and d) arising base level (the Rio Solimo˜es–Amazon).The Anavilhanas evolved as a vertical accretion,low-energy system where the levees were limiting acomplex of anastomosing channels systems andgenerating successive channels by avulsion.8. An evolution model for the Mariua´ andAnavilhanas archipelagosThe Rio Negro system has part of its evolutionclosely linked to the behavior of the huge Solimo˜es–Amazon River into which it flows. Latrubesse andFranzinelli (2002) demonstrated that the Solimo˜es–Amazon system experienced high rates of verticalaccretion during the Holocene. While the Amazongenerated bimpeded floodplainsQ by vertical accretion,an autogenic adjustment in the form of a complexresponse to climatic changes in the middle Holocene,the Rio Negro gradually became a blocked valley.This began with the late glacial response of the fluvialsystem and continued during the Holocene trans-gression of the middle Holocene and into the lateIIIIIIVIVVIAMAZON RIVERLATEGLACIAL-MIDDLE HOLOCENE14-4 ka BPMain area of depositionDecreasing sediment availabilityand sedimentation“In rising” base levelIIIIIIVIVVIAMAZON RIVERUPPER HOLOCENE4-1 ka BPTransfering sediments“In rising” base levelincreasing backwater effectMain area ofsandy sedimentationMain area offine sedimentationIIIIIIVIVVIAMAZON RIVERUPPER HOLOCENE1ka BP-PRESENTTransfering sedimentsStrong backwater effect-totally in phaseNegro and Amazon stagesSandy sedimentation andtransfering sedimentsNo fine sedimentation, sediment transfer,lateral sandy “beach” depositionFig. 19. Schematic evolution model for the Rio Negro from the lateglacial to the present.E.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397 393
    • Holocene. A set of marginal levees deposited by theAmazon close to the mouth of the Rio Negro (theXiborema levee complex) also contributed to blockingpart of the Negro and creating a backwater effect inthe latter.An evolutionary model is presented for the RioNegro in Fig. 19. The Lower Terrace, dated at N10 kaBP in the upper Negro, is a product of the response bythe basin to the climatic changes of the late glacial. Atthat time, the upper Negro was a source area ofsediments that were transferred downstream andpartly stored in the upper alluvial plain. This behaviorcontinued during the late to mid Holocene, with thesedimentation progressing downstream and continu-ing to fill the big structurally controlled sedimentarybasins of reach III. By this means, the first largereservoir was mainly filling up with sandy bedload,creating the Mariua archipelago. The core of largesandy islands formed probably initially as sandychannel bars in a progradational manner. Sedimenta-tion has been greatest upstream and less downstreamin reach III. The lowest part of reach III was occupiedby the Rio Branco delta, which severely confined theNegro at this point to a width of just ~1.5–2 km. Atthat time, the Rio Branco carried abundant suspendedload that formed the delta. Reach V was the mainsedimentary trap for suspended sediments.As explained before, four conditions were essentialfor the development of the Anavilhanas: a) sufficientsuspended sediment; b) a low-energy depositionalenvironment; c) sufficient accommodation space inthe valley; and d) rising base level.During the transition from mid to late Holocene(as late as ~1000 BP), these conditions werereached. The Anavilhanas evolved as a verticalaccreting low-energy system where levees restricteda complex of anastomosing channels. The valleywas filled by the general downstream progradationof a multichannel system dominated by verticalaccretional processes. The bphantomQ morphology ofthe islands, outlined by levees and usually infilledwith water, is the product of two narrow levees thatextend many kilometers downstream from anupstream island core. These islands are the sedi-mentary response to river valley expansion andassociated flow deceleration downstream of theNodal Point 3, and a rising base level andbackwater effect produced by alluvial dammingfrom the rising Solimo˜es–Amazon at a time whenit was depositing its impeded floodplains.Sediment transport and floodplain formationoccurred in the late glacial and the Holocene from14 ka BP to approximately 1000 BP, when thesupply of fine suspended sediment to the Anavilha-nas Islands ceased because of re-establishment ofthe rainforest cover and stabilization in the basin.During the entire Holocene, the river has been adisequilibrium system, with sediment deficits createdby deposition initially in reach II and then inreaches III and V (mainly sand in reaches II and IIIand fines in reach V).The river achieved equilibrium conditions fromupstream to downstream, partially filling its valleywith a complex anabranching pattern and then slowlycreating a more efficient corridor for water andsediment with fewer channels over time. The olderfloodplains support fewer river channels and thevalley is almost full of alluvial sediment (as in theupstream sector of reach III). Valley infilling pro-gresses downstream, creating many anabranches andthen more efficient systems formed of fewer channels,until a relatively efficient fluvial belt is created. Thispermits the river to carry sediment further in towardsits aggrading confluence with the Amazon.At approximately 1000 BP, the supply finesediments practically ceased and the sedimentationin the Rio Negro and Rio Branco stopped. TheNegro and Branco headwater areas, however, havecontinued to supply some sand to the Mariua´archipelago (which stored it in the only availablespace—the channels; Fig. 13). Navigation chartsshow that the channels here are very shallow,varying between 0 m and 5 m during lower stage.The sediment data of Fig. 6 show that the Mariua´basin continues to be active in differentiating andtrapping sand, isolating sediment from the nextbasin downstream, the Anavilhanas, which exhibitsa very different grain size distribution.Downstream, the formation of the AnavilhanasIslands and the filling up of the valley ceased whenthe system changed from a river carrying abundantsuspended load to a bblackQ water river withinsignificant suspended load. We can define thelower Negro as a river alluvially dammed by theSolimo˜es–Amazon and forming an incompletefloodplain. While today it carries some sandyE.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397394
    • bedload, it is essentially a huge lake system with itswater level oscillating in phase with that of theAmazon River.9. ConclusionThe Upper Terrace of the upper Negro can betentatively correlated with the older terrace on theleft side of reach III. The proposal age is MiddlePleniglacial (between 60 and 28 kyr BP), assuggested by Latrubesse and Franzinelli (1998). Atthat time, the river was mainly a bed load system,aggradational, and carrying abundant pure quartzsand. The climate was probably tropical with moreseasonal contrasts than today. The river was mor-phogenetically more active as indicated by theexistence of a greater incidence of gravel comparedto the modern sediments.No units of the Upper Pleniglacial were recog-nized in the field. Either a sedimentary hiatusexisted at that time, or alternatively the record isbelow water level. More detailed research especiallyin the upper basin and its tributaries may offer someinformation for that period of time. The late glacialhas a substantial record in the form of a LowerTerrace along the upper basin, which is recognizablein the Tiquie´, Curicuriarı´, and Vaupes rivers. Duringthe formation of the Lower Terrace, the upper RioNegro was a white water river that carried abundantsuspended load as an effect of the global climaticchanges during the deglaciation (Latrubesse andFranzinelli, 1998).Since that time, the river has acted as aprogradational system, creating a floodplain andalso adjusting to structural compartments whichacted as sedimentary basins (Fig. 19). Reach IIwas the first to be filled, then reach IIIa. This ismarked by an erosive large islands pattern and largeareas of aggradation in the valley. Reach IIIb showsan incomplete pattern. The Rio Branco delta filledpart of the downstream zone of reach IIIb, deposit-ing mainly sands but with a relatively thin sheet offine sediment covering the sand. In reach III, finedeposits increase downstream and become predom-inant in some islands at the end of the reach.Reach V acted as a fine sediment trap. TheAnavilhanas archipelago is the geomorphologicproduct of deposition in this Holocene sedimentarybasin. Sediments incompletely filled the valley withmore deposition in the upstream part of the basinand large downstream areas not yet occupied bysediment.Reaches IV and VI were mainly transfer zonesconnecting active sedimentary basins. Reach VI isvery deep, reaching in some points values close to 100m. A cross-section 30 m deep close to Nodal Point 4is shown in Fig. 11.Generally speaking, the river has exhibited pro-gradation of sediment into successive basins in anattempt to achieve conditions close to equilibriumfrom upstream to downstream, filling successively aseries of tectonically formed basins which wereacting as sediment traps (depositing mainly sand inreach III and fines in reach V). Conditions close toequilibrium have been obtained during the Holocenein reaches IIb and IIIa, where a more efficientanabranching channel system, cutting large erosionalislands, transfers sediment and water downstream.Reaches IIIb and V never reached equilibrium duringthe Holocene and today have incomplete floodplains.Additionally, the source of abundant fine sedimentfrom upstream was practically exhausted by approx-imately 1 ka BP. At that time, the Anavilhanasarchipelago dominated by silty–clayey levees stop-ped forming. The lowermost reaches of the Negro (Vand VI) were controlled by a rising base levelbackwater effect caused by alluvial dammingbecause of an aggrading Solimo˜es–Amazon fromthe late glacial to recent times. Today, the backwatereffect extends 300 km up river and the Rio Negrobehaves like a lake in reaches V and VI, with riverstages in phase with those of the Amazon (Sternberg,1987; Richey et al., 1989).The data presented here interpret the evolutionof the Negro system during the late glacialHolocene and describe the formation of anabranch-ing patterns in a large tropical river. They alsodescribe floodplain evolution and the lagged geo-morphic responses of a large river system toclimatic changes that have occurred during the LateQuaternary. Since the climatic changes producedduring the last deglaciation, these large tropicalsystems have not had sufficient time to reachequilibrium conditions along whole the river.Furthermore, minor climatic changes during theE.M. Latrubesse, E. Franzinelli / Geomorphology 70 (2005) 372–397 395
    • Holocene have possibly affected the fluvial styleand produced additional adjustments. Some riversin the basin, such as the Rio Solimo˜es–Amazon,continue to be supplied with sediment from theAndes, filling and modifying floodplains. Sedimentis stored in active structural basins and the excesssediment is transported to the ocean. Rivers such asRio Negro suffered a sudden termination of sedi-ment load from the source areas. Today on the RioNegro, just a small volume of sandy sediments isbeing transferred from the upstream and middletributary rivers. These sediments are mainly beingtrapped in reach III and some sand is beingtransported. The exhaustion in the transport of finesediment to the furthest reaches downstream (V andVI) and halts the development of the remarkableAnavilhanas archipelago.AcknowledgementWe specially like to thank Gerald Nanson for thefundamental and critical review which improvedsignificantly the manuscript.ReferencesBaker V., 1978. 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