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Channel floodplain geomorphology along the solimões-amazon river, brazil
 

Channel floodplain geomorphology along the solimões-amazon river, brazil

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    Channel floodplain geomorphology along the solimões-amazon river, brazil Channel floodplain geomorphology along the solimões-amazon river, brazil Presentation Transcript

    • 1089 ABSTRACT Across the cratonic landscape of Brazil the Solimões-Amazon River transports to its delta plain 1240 Mt of suspended sediment derived from Andean erosion and reworks another 3200 Mt of floodplain sediments. Distribution of these sediments has resulted in a variable along-stream pattern of geomorphology. The upstream reaches are characterized by sedi- ment erosion in the main channel and deposi- tion in floodplain channels that are an order of magnitude smaller in discharge than the main channel. Sediment deposition in and mi- gration of the floodplain channels erases ox- bow lakes of the main channel and yields an intricate scroll-bar topography that forms the boundaries of hundreds of long, narrow lakes. In contrast, downstream reaches are charac- terized by channels restricted by stabilizing, long-term, levee building and floodplain con- struction dominated by overbank deposition. Overbank deposition buries the scroll-bar topography, resulting in a flat floodplain cov- ered by a patchwork of large, more equant, shallow lakes. On the basis of estimated rates of recycling of floodplain sediments, the mod- ern floodplain of the Brazilian Amazon could have been recycled in <5000 yr, and is recycled more rapidly in the upstream than the down- stream reaches. The cratonic interior is inter- rupted by structural arches that bound in- tracratonic basins. Four of these arches cross the valley of the main river system at intervals of several hundred kilometres and impart a tectonic imprint on the channel-floodplain ge- omorphology at this spatial scale. Structural arches appear to exert a primary influence by promoting entrenchment of the river as it passes through zones of deformation, thus re- stricting channel movement. For example, as the river crosses the Purús arch, the valley narrows to <20 km compared to an average of ≈45 km, the water-surface gradient decreases, sediment is deposited, and yet the rate of channel migration is negligible. Hence, the ef- fect of the arches is to create a landscape where, on the spatial scale of hundreds of kilo- metres, the river is confined and entrenched in its valley, is straight, and is relatively immo- bile. Local valley tilting apparently unrelated to the arch structures also imprints the geo- morphology. In particular, a tilted valley in the upstream reaches appears to have caused avulsions which have left behind the only large-scale, oxbow-type features on the Brazil- ian Amazon River floodplain. INTRODUCTION The continental-scale Solimões-Amazon River drains the Andes and crosses a humid craton to theAtlantic Ocean (Fig. 1) while traversing struc- tural arches. The structural arches impart a tec- tonic imprint on the riverine landscape at wave- lengths of hundreds of kilometres. Local valley tilting (Tricart, 1977) and apparent fracture pat- terns (Sternberg and Russell, 1952) also have influenced the development of the channel-flood- plain system. Across the craton the river carries 1240 Mt/yr of sediment and reworks≈3200 Mt/yr of floodplain sediment (Dunne, Mertes, R. H. Meade, and J. E. Richey, unpub. data). Deposition and erosion of these sediments have resulted in different channel patterns and the construction of a complex floodplain, the várzea, which is a mo- saic of deposits (Wolman and Leopold, 1957) from the main channel, anabranches, floodplain channels that have discharges an order of magni- tude lower than the main channel (Mertes, 1985, 1990), and nonchannelized overbank flows. As the Amazon River enters Brazil it is known as the Solimões River until downstream of the Negro River confluence (Fig. 1). The entire sys- tem will be referred to as theAmazon River. Over the length of 3000 km of the Amazon River in Brazil we have compiled data on the geomorphic characteristics of the channel and floodplain from remote sensing images and navigation charts. Analysis of navigation charts, aerial photographs, and travel diaries dating to the middle of the nine- teenth century indicates that different rates and styles of channel behavior dominate in different reaches of the river. The surface morphology of the floodplain as measured by the geometry and percent cover of scroll bars, overbank deposits, lakes, lake deposits, and floodplain channels also changes in a downstream direction, reflecting variation in the depositional and erosional history of each reach. Interpretation of these geomorphic data in the context of the tectonic setting allows for interpretation of the sediment budget in the context of basinwide hydrology and hydraulics (Dunne, Mertes, R. H. Meade, and J. E. Richey, unpub. data). In addition, we provide here a process-based interpretation of the current devel- opment of the channel-floodplain system and a framework for anticipating future patterns of de- velopment in the context of global-scale changes in climate, sea level, tectonics, and land use. BACKGROUND This study of processes currently active along 3000 km of a continental-scale, tropical alluvial river requires considering basin-wide processes such as tributary effects and interactions between reaches and the long-term effects of climate change, base-level change, and tectonics. For ex- ample, Potter (1978) pointed out that continental- scale tectonic deformation controls the physio- graphic setting of large rivers, in that most large rivers are located in structural lows or continental Channel-floodplain geomorphology along the Solimões-Amazon River, Brazil Leal A. K. Mertes Department of Geography and Institute for Computational Earth System Science, University of California, Santa Barbara, California 93106-4060 Thomas Dunne School of Environmental Science and Management and Institute for Computational Earth System Science, University of California, Santa Barbara, California 93106-5131 Luiz A. Martinelli Centro de Energia Nuclear na Agricultura, 13400 Piracicaba, SP, Brazil GSA Bulletin; September 1996; v. 108; no. 9; p. 1089–1107; 19 figures; 1 table.
    • rifts, and flow into oceans off trailing edges of continents. In addition, patterns of uplift and sub- sidence in drainage basins of the Mississippi River watershed (Schumm, 1986; Schumm et al., 1982; Merritts and Hesterberg, 1994), in streams of the Los Angeles basin (Bullard and Lettis, 1993), along the Yangtze River (Lian-yuan, 1982), and along the Rio Grande (Ouchi, 1985) correlate to changes in channel gradient and channel pattern. Data from flume experiments showed that uplift and subsidence cause changes in channel pattern (Ouchi, 1985). Potter (1978) not only drew attention to the di- rect influence of plate tectonics on the locations of continental-scale rivers, but suggested that some basinwide impacts could result from climatic and sea-level change. Schumm (1993) concluded that base-level or sea-level changes in a large alluvial river would frequently have only local effects due to the river’s ability to adjust slope, planform, width-depth ratios, or roughness, thus confirming the observations of Leopold and Bull (1979) and Merritts et al. (1994) concerning the limited extent of depositional wedges related to base-level changes in small river systems. Schumm (1993) also suggested that climatic changes or tectonic de- formation could affect an entire river system, de- pending on the scale and trend of the perturbation. Amazon River Basin Area of Study. The scope of this paper is the Brazilian reach of theAmazon River that extends ≈3000 km from Vargem Grande near the Peru- Brazil border to theAtlantic Ocean (Fig. 1).At its most downstream gaging station, Óbidos, the Amazon River has a water discharge of 180 000 m3/s (Richey et al., 1989) and a drainage area of 4.6 million km2. The sedimentary deposits of the main-stem Amazon alluvial plain between the Peru-Brazil border and the Atlantic Ocean cover an area of ≈64 400 km2, according to an estimate by Ca- MERTES ET AL. 1090 Geological Society of America Bulletin, September 1996 Figure 1. General lithologic zones and structural features in the Amazon basin. The approximate locations of the axial traces of four in- tracratonic arches that traverse the axis of the main Amazon channel are marked: from east to west, they are Iquitos, Jutaí, Purús, and Gu- rupá arches. Distances downriver of Iquitos are listed in Table 1. Map adapted from Sioli (1968, 1975b), Klammer (1984), and Caputo (1984). Lower map shows study reach of Amazon River. Locations are shown for large tributaries and primary sampling stations for water and sed- iment discharge. Distances downriver of Iquitos are listed in Table 1.
    • CHANNEL-FLOODPLAIN GEOMORPHOLOGY, SOLIMÕES-AMAZON RIVER Geological Society of America Bulletin, September 1996 1091 margo (1954; as described by Irion, 1976a). Sippel et al. (1992) calculated this area to be ≈92 400 km2, on the basis of a planimetric mea- surement of the valley area between topograph- ically higher (10–15 m) terraces. The terraces parallel both sides of the river valley and are composed of unconsolidated fluvial and lacus- trine deposits and the Tertiary Barreiras For- mation, which consists of unconsolidated sands, silts, and clays, with scattered conglomerate lenses and crude stratification (Radambrasil, 1974, 1977, 1978). Irion (1976c), Tricart (1977), Iriondo (1982), and Klammer (1984) suggested that the main- stem floods do not inundate the entire alluvial plain. Using Iriondo’s (1982) interpretation of landforms visible on radar images (Radambrasil, 1972) and our estimates from remote sensing data of the distance that sediment-rich main-stem wa- ters travel into the alluvial plain (Mertes et al., 1995), we estimate that between Vargem Grande and Óbidos, ≈40 000 km2 of floodplain and 4000 km2 of islands are inundated by direct flood- ing from the Amazon River water. The remainder of the alluvial plain between Vargem Grande and Óbidos (≈20 000 km2) comprises deposits that may remain dry or are flooded by small tributaries, ground-water seepage, or rainfall. Structural Setting. The pattern of the drain- age network of the Amazon Basin has been in- fluenced by structural controls since the origin of the basin (Caputo, 1984; Potter, 1978). On the basis of stratigraphic evidence from deep cores, it has been determined that from west to east in Brazil, the main Amazon valley is a 6000-m- deep sag in the crust between northern and south- ern crystalline shields (Guiana and Brazil shields, Fig. 1) that leads into a graben in the eastern half of the basin (Caputo, 1984). The structural sag may have first developed in response to Paleo- zoic rifting (Caputo, 1984) that was reactivated during the Triassic separation of South America and Africa (Potter, 1978). Although the Amazon trough has been present since at least the Paleo- zoic, significant eastward flow of water may have commenced with the Miocene uplift of theAndes (Caputo, 1984). This picture of mountains bordering a conti- nental-scale basin is interrupted by structural features that bound intracratonic basins. Räsä- nen et al. (1987), Dumont et al. (1991), Dumont and Garcia (1991), and Kalliola et al. (1993) re- viewed the structural character of the sub- Andean foreland in western Amazonia (Fig. 1) and considered the PeruvianAmazon basin to be an active zone of thin-skinned fold and thrust movement related to Andean tectonics. From near the Peruvian border to the Atlantic Ocean, four structural arches (Fig. 1) lie transverse to the mainAmazon trough; these are (from west to east) the Iquitos, Jutaí (Carauari), Purús, and Gu- rupá arches (Fig. 1). Information on the Brazil- ian arches is scarce and is typically restricted to oil exploration documents that are not published. The presence of the arches in the central Ama- zon valley has been suggested to explain a dis- continuous stratigraphy along the main Amazon trough. The stratigraphy is based on data from sediment cores and seismic surveys and shows that the formations in each basin pinch out as they approach the structural highs (Caputo, 1984, 1991). From the limited data it appears that the struc- tural arches represent intraplate tectonics (Big- arella, 1973; Caputo, 1984, 1991; Putzer, 1984). For example, Soares et al. (1978) concluded that the Brazilian intercratonic basins passed through a five-stage cycle, and varying rates of uplift and subsidence controlled deposition and erosion. They described the Tertiary sequence as residual deposits preserved in “Quaternary relict basins of slight subsidence” (Soares et al., 1978, p. 190). The stratigraphy of deposits in interven- ing basins indicates that the Jutaí and Purús arches have been areas of relief since the Paleo- zoic (Caputo, 1984), and the Iquitos and Gurupá arches first evolved during the Mesozoic (Putzer, 1984; Dumont et al., 1991). The Iquitos arch seems to mark the cratonic boundary and is probably the result of lithospheric flexure due to the loading of the Andes (Caputo, 1991). Du- mont et al. (1991) concurred with this view and also concluded that, after alternate periods of up- lift and subsidence, the Iquitos arch has been up- lifted since the beginning of the Quaternary, re- sulting in terraces 30 m above present river floodplains. Caputo (1991) presented evidence for substantial tectonic deformation for the basins between the Iquitos and Purús arches and south of the Jutaí arch. Although no detailed analysis was provided, Caputo (1991, p. 249) suggested that the “stresses that generated the structures along the Solimões megashear [also generated] other similar structures at distances far to the east in the Amazon basin.” It is not known whether the Brazilian arches are currently deforming. However, Dunne, Mertes, R. H. Meade, and J. E. Richey (unpub. data) found a consistent pattern of steepening channel gradient (Fig. 2B) as the main stem of the Amazon River crosses the approximate po- sitions (Fig. 1 and Table 1) of the three eastern arches. The water-surface gradient decreases as the river passes into the intervening basins. Val- ley gradients could be steeper across the arches because of subsidence in the basins on either side in response to the sediment loading in the basins, which is on the order of thousands of me- tres (Soares et al., 1978; Caputo, 1984). Recent uplift of these arches could also be the cause of the steepening, as has been suggested for the Mississippi River (Schumm, 1986), although such an effect would presumably require expo- sure of less-erodible material at the river bed. Although not measured, the arches may be un- dergoing active deformation. For example, the structures may have been active since the Mio- cene, because theAndean orogeny has altered the structural character of the entire Amazon basin. The impact of this orogenic loading may extend east of the Iquitos arch in the form of reactivation of structural features that originally developed in response to earlier plate movements and atten- dant stresses. Continental deformation on the scale of 1000 km from subduction zones (Mitro- vica et al., 1989) and first flexural nodes of the lithosphere on the scale of hundreds of kilome- tres from mountain belts (Karner and Watts, 1983) have been predicted from models of sub- duction and isostatic adjustments to loading. Stratigraphic and structural data for the Sabine uplift, a structural arch in eastern Texas and northern Louisiana, show that it was reactivated several times, resulting in ≈0.01 mm/yr of uplift since the mid-Cretaceous (Nunn, 1990). Possible mechanisms for reactivation and therefore con- tinued uplift of cratonic arch structures through changes in the continental lithosphere include thermal rejuvenation, new horizontal stresses, long-term viscous relaxation, or new extensional faulting (Nunn, 1990). Local tilting and fracturing of the valley floor along the Amazon main stem and floodplain in Brazil that are apparently not associated with the TABLE 1. MEASURED DISTANCES ALONG THALWEG DOWNRIVER FROM IQUITOS, PERU Site Distance name (km) Iquitos 0 São Paulo de Olivença 740 Vargem Grande 863 Santo Antônio do Içá 891 Xibeco 1051 Tupé 1248 Jutica 1528 Itapeua 1704 Anorí 1885 Manacapurú 2031 São José do Amatari 2228 Madeira River confluence 2300 Paurá 2474 Parintins 2631 Óbidos 2750 Iquitos Arch ≈–100* Jutaí Arch ≈700 Purús Arch ≈2000 Gurupá Arch ≈3200 Notes: Arch distances based on approximate location of axial trace; arches extend tens of kilometres upstream and downstream from trace. Locations shown in Figure 1. *Upriver.
    • MERTES ET AL. 1092 Geological Society of America Bulletin, September 1996 Figure 2. (A) Schematic illustration of observed along-stream pattern of channel-floodplain geomorphology and valley width. Brackets with “tilt” indicate approximate extent of tilting deformation of valley. JA—Jutaí arch, PA—Purús arch, MR—Madeira River confluence, and GA—Gurupá arch. (B) Low-water channel gradient based on Seasat radar altimetric data reported by Guzkowska et al. (1990). Data col- lected between July 27 and August 9, 1978, with a reported precision of tens of centimetres. The orbit paths for the 15 elevation data with the highest accuracy were replotted on the Radambrasil (1972) images and Operational Navigation charts for Peru. Corrected river distances were combined with the elevation data to compute water-surface gradients (Mertes, unpub. data). The data show the abrupt decrease of the water-surface gradient as the river crosses from the sub-Andean foreland into Brazil and the irregular pattern as it crosses Brazil. (C) Net channel storage of coarse and fine sediment based on sediment budget for channel reaches (Dunne, Mertes, R. H. Meade, and J. E. Richey, unpub. data). Positive values indicate deposition and negative values indicate erosion.
    • arches also affect the development of the channel- floodplain system. Tricart (1977), Iriondo (1982), and Iriondo and Sugio (1981) interpreted the presence of tilted drainage networks, beheaded streams, and patterns of undated alluvial deposits to be evidence for tectonic activity along the Amazon valley. In particular, Tricart (1977) sug- gested that the progressive development of ter- races between the Japurá River confluences and Itapeua (Fig. 1) has been caused by the southward tilting of the northern valley. The tilting has caused the main stem of the river to lodge against a scarp along the southern edge of the floodplain. Tricart also proposed that the presence of enor- mous lakes at the mouths of severalAmazon trib- utaries (Negro, Tapajós, Xingu) is in part due to subsidence along major faults. Drainage networks oriented in a northeast and northwest direction may also reflect deep-seated basement fracturing that has continued to disturb the overlying sedi- mentary rocks (Sternberg and Russell, 1952). Inferences that structure and tectonic activity are influencing the channel-floodplain systems in the Amazon basin have been based primarily on sparse stratigraphic and geomorphic evidence. Sternberg and Russell (1952) listed the dates of several earthquakes that have been reported in the basin since the seventeenth century. Without pre- cise geodetic or seismic measurements it is not yet possible to prove that there has been recent tectonic movement. However, our interpretation of tectonic influence is consistent with either past or continuing deformation. Recent Geologic History. The Amazon low- land is dominated by a vast alluvial plain sur- rounded by Quaternary and Tertiary terraces. The western Amazon basin in Peru was de- scribed in detail by Salo et al. (1986), Räsänen et al. (1987, 1990, 1991), Dumont and Garcia (1991), Dumont et al. (1990, 1991), Neller et al. (1992), and Kalliola et al. (1991, 1992, 1993), who discussed the relations among long-term patterns of river migration, floodplain develop- ment, tectonics, and forest diversity. For the Brazilian Amazon region, Sternberg (1960, 1975), Sioli (1951), Tricart (1977), Irion et al. (1983), and Johnsson and Meade (1990) de- scribed in some detail the construction of the floodplain, island development, floodplain chan- nels, and channel migration. Several reviews of hypotheses related to the Quaternary evolution of the eastern Amazon basin are those of Sombroek (1966, 1984), Tri- cart (1977), Putzer (1984), Mertes (1985), Räsä- nen et al. (1990, 1991), Tuomisto et al. (1992), and Kalliola et al. (1993). In general, the hy- potheses concerning the evolution of the land- scape range from tectonic control (Iriondo and Sugio, 1981; Iriondo, 1982) to a mix of climatic and tectonic control (Tricart, 1975, 1977, 1985), or to dominance by glacio-eustatic fluctuations during the Tertiary and Quaternary (Sioli, 1957, 1968, 1975a, 1975b, 1984; Irion, 1976a, 1976b, 1976c, 1978, 1979, 1982, 1984; Klammer, 1984; Müller et al., 1995). Most of these hypotheses are based on sparse data measured from the field, maps, and radar images (Radambrasil, 1972), and comparisons with apparently analogous glacio-eustatic effects around the world. METHODS Channel width, sinuosity, island area and perimeter, number of islands, and the radii of curvature of main-stem bends, floodplain chan- nel bends, and floodplain scroll bars were mea- sured on radar images (Radambrasil, 1972) for 100-km-long reaches of channel. These images have a 1:250 000 scale, 16 m pixel resolution, and were recorded during low-water periods in the years 1971 and 1972. Channels and islands narrower than 1 mm (250 m) on the images were not included in the measurements. Channel width was computed as the average width of a single channel in reaches where the river has only one channel. Sinuosity was computed as the main channel thalweg length over the valley length. Floodplain width is the average distance across the alluvial surface. The boundary of the alluvial surface was defined as the innermost boundary of either Quaternary or Tertiary ter- races designated Apf, t, and Tb on Radambrasil sheets (Radambrasil, 1974, 1977, 1978). The circularity index for islands is the ratio of the measured total island perimeter per section to the calculated total island perimeter per section if all the islands were circles of the same area. The radius of curvature of a bend was measured as the radius of the smallest circle containing the arc of the bend. On the modern channel, the arc was measured at the center line of the channel; on the floodplain the arc was measured at the center line of the levee. Data on the lakes were provided by S. Sippel (1994, personal com- mun.), and the methods for their measurement were described by Sippel et al. (1992). The low-flow water depth was computed as the average per reach of all of the depths on the most recent editions (1970, 1979, and 1980) of Brazilian Navy navigation charts (Marina do Brasil, 1970–1980). These depths were origi- nally measured at different river stages from 1967 to 1979 and were converted to low-water depths for the charts. The depths are well dis- tributed along the channel length, although in some reaches more measurements were taken in shallow sections, because the intent of the maps is to help pilots avoid shoals during low water. CHANNEL-FLOODPLAIN GEOMORPHOLOGY, SOLIMÕES-AMAZON RIVER Geological Society of America Bulletin, September 1996 1093 Figure 3. Example of channel migration on the Amazon River, 1971–1972 to 1980. Migra- tion of this bend was documented from an overlay of 1972 Radambrasil image SA-19-ZB and 1980 Brazilian Navy navigation chart number 4112B (Marina do Brasil, 1974, 1977, 1978). Maximum migration at the apex of this bend for this time period was ≈140 m/yr or 3%/yr of the average channel width.
    • Because of this sampling bias the computed av- erage depth may underestimate the average low- water depth in some reaches. Channel changes were measured from two sets of maps (see example in Fig. 3). For the section of the river from Vargem Grande to Manacapurú (Fig. 1), channel and bar margins were traced from radar images recorded in the low-flow sea- son of 1971–1972 (Radambrasil, 1972). The radar images were projected to a scale of 1:100 000 to match the scale of the 1979–1980 edition of 1:100 000 Brazilian Navy navigation charts (Marina do Brasil, 1970–1980). These charts are new editions based on the Radambrasil images and ship surveys and include revisions through 1979 and 1980. These two sets of data were the most accurate available for this reach of the river. For the reaches of the river downstream of Manacapurú, an overlay was constructed of radar images from the Radambrasil project and of revised 1980 editions of 1970 and 1972 naviga- tion charts. The revised 1980 editions of the navi- gation charts include revisions for channel move- ments that were current as of 1980. The 1970s editions of the Brazilian Navy navigation charts (Marina do Brasil, 1970–1980) were originally based on aerial photography and ship surveys, but not on radar images. Therefore, measurements of channel change for the sections of the river down- stream of Manacapurú are considered less accu- rate than the data for the reaches upstream of Manacapurú, because there is a greater possibility that the measurements include apparent differ- ences in the channel that actually are due to dif- ferences in the mapping techniques. The channel change values were computed as the total land area eroded or deposited, divided by the total channel area between the low-water banks of the main river channel in 1971–1972, and divided by the approximately 8 yr during which change occurred. It was not possible to assess errors as- sociated with paper shrinkage and other distor- tions for both sets of overlays. Therefore, to cor- roborate the quantitative analysis, a qualitative analysis of overlays of historical maps and more recent remote sensing images was done. To characterize bank geometry and the texture of floodplain sediments, field measurements were made at 20 sites along the main channel from Vargem Grande to Óbidos during low water in 1988 and 1991. Banks were classified as one of three types on the basis of the morphometric char- acteristics of the bank. Steep, nonvegetated, cut banks were considered erosional. Steep, well- vegetated banks were considered stable. Low-gra- dient banks with grass cover were considered de- positional. Elevation measurements were made with a tape and Abney level along a transect per- pendicular to the channel, using the water surface of the main channel as the reference elevation.The accuracy of the surveying method was estimated to be ±30 cm over 200 m with an elevation change of several metres, based on one resurveyed tran- sect. At most survey points the soil beneath the duff layer was sampled with a 2-cm-diameter soil auger. The sample core was 29 cm long; the top 4 cm and bottom 3 cm were discarded. Samples were air dried and returned to the laboratory for standard particle-size analysis using sieve weights for sand fractions (see Mertes and Meade, 1985, for a detailed description of method) and hydrom- eter analysis for silt and clay (American Society for Testing and Materials [ASTM], 1984). Most of the results are shown in figures where the reference distance is kilometres downstream from Iquitos. Many of the data were measured along reaches of ≈100 km in length. We normal- ized these data by reach length to compare dif- ferent reaches. ANALYSIS OF RESULTS Channel Features The Amazon River in Brazil is typically an anastomosing river (usage, Knighton and Nanson, 1993). Over a length of 2500 km the average depth at low water increases from 10 to 20 m, and the single-channel width increases from 2200 m to an average of 4500 m; the maximum is 6000 m (Fig. 4). The width-depth ratio (W/D) does not change significantly downstream; the average is 210, and the smallest values for the reachesarebe- tween Jutica and Itapeua,≈1500 km (W/D = 180). The greatest values for the reaches are between Anorí and Manacapurú, ≈1900 km (W/D = 260). To determine the relationships among river depths, bank heights, and maximum flood heights, we used the height of the river banks from the bank surveys.The surveyed bank heights were ad- justed to a low-water surface reference in order to compare the bank heights to each other and to the maximum flood height measured at nearby gaging stations. The maximum flood height was also ref- erenced to the low-water surface. Figure 5 shows the results from these calculations, where typical bank heights are 11 m in the upstream reaches, 10–11 m in the middle reaches, and 7–8 m in the downstream reaches. The maximum flood height decreases from 12 to 13 m in the upstream and middle reaches to 7.5 m at Óbidos (2750 km). Fig- ure 5 also shows the difference in height between the top of the bank and the maximum measured flood. This difference is 1.5 m in the reaches be- tween Tupé and Itapeua (1248–1704 km) and the reaches between São José do Amatari and Par- intins (2228–2631 km). In the other reaches the freeboard is <1 m. These data on bank and maxi- mum flood heights indicate that the banks are rel- atively lower in the reaches between Jutica and Itapeua (1248–1704 km) and between São José do Amatari and Parintins (2228–2631 km). There- fore, in these reaches, the maximum floods top the banks with relatively deeper flows than in other reaches. The texture of the bank material was found to be more variable along any one transect than along the entire study reach (Mertes, unpub. data). Figure 5 shows the downstream trend in the percent sand of both the erosional and de- positional banks, based on averaging data across transects. The percent sand ranges from ≈10% to nearly 20%. As the sand fraction var- ies, the clay fraction most consistently varies, MERTES ET AL. 1094 Geological Society of America Bulletin, September 1996 Figure 4. Average depth and width of Amazon River main stem for low water stage. JA— Jutaí arch, PA—Purús arch, MR—Madeira River confluence, and GA—Gurupá arch.
    • and the silt fraction remains relatively constant at 65% for depositional banks and 70% for ero- sional banks. The most significant trend is an in- crease in the sand proportion downstream of the Madeira River confluence at ≈2300 km. The along-channel consistent width to depth ratio and homogeneous bank texture do not cor- relate with the variation in sinuosity of the river (Fig. 6). In contrast, based on Student’s t statistic (Zar, 1974) for a linear regression, there is a pos- itive correlation between floodplain width and sinuosity, which has probability P <0.001. The maximum sinuosity of 1.7 at 1400 km was mea- sured in the reach that also has the maximum valley width of nearly 80 km. Both upstream and downstream of these maximum values the sinu- osity and valley widths decrease. The river is nearly straight for 350 km from Jutica to Anorí (1528–1885 km), but then increases in sinuosity downstream as the width of the alluvial valley increases downstream from Manacapurú (2031 km). The narrowest reach of the valley, averaging <20 km, is ≈100 km long in the vicin- ity of Manacapurú. The asymmetry of the flood- plain, that is, the proportion of the floodplain on either side of the river, also changes from up- stream to downstream (Fig. 6) without any clear correlation to floodplain width. In the reach be- tween Tupé and Itapeua (1248–1704 km) virtu- ally 100% of the floodplain is north of the river. Figure 7A shows a general downstream de- crease in number of islands per kilometre per reach; there is some increase at 2700 km. As the number of islands decreases, the size of the islands per kilometre increases somewhat less dramati- cally. The size of the islands is significantly (Stu- dent’s t statistic yieldsP <0.05) and positively cor- related with the channel width (Fig. 7B). The values for sections 13 (2100 km), 14 (2200 km), 15 (2300 km), and 20 (2900 km) are unusual in that they contain islands of immense size. For ex- ample, Careiro Island in section 13 is nearly four times larger (430 km2) than most of the other islands in the entire study reach. The total island area per each reach of ≈100 km (Fig. 7C) gradually increases until the reach that contains Careiro Island (2100 km), decreases downstream of this reach, and then in- creases rapidly in the last few downstream reaches.As the total island area increases, the to- tal perimeter of the islands remains near an aver- age of ≈240 km until downstream of the Madeira River confluence (2300 km), where the total perimeter increases simultaneously with the dramatic increase in island area. The rela- tionship between total island area and island perimeter is such that, as the islands increase in size, they also appear to become slightly more circular in shape as shown by the pattern of cir- cularity values in Figure 7D.Values >>1, as seen in the reaches between Xibeco and Tupé (1051–1248 km), indicate that the islands are more elongate than circular. Although the pat- tern is not entirely consistent, there seems to be an increase in circularity from the upper reaches to about Manacapurú (2100 km), and then the circularity decreases again as indicated by an in- creasing value for the index. Downstream of Óbidos (2750 km) the islands abruptly become more circular and become elongate again down- stream of 3000 km. Historical Change Figure 8 shows the magnitude of channel change for the years between 1971–1972 and 1979–1980 for the main channel betweenVargem Grande and Óbidos. The values were computed as the total land area eroded and deposited, di- vided by the total water surface area between the banks of the main river channels, and divided by the 8 yr over which the change occurred. In gen- eral, the percentage of erosion recorded is higher than deposition. This pattern does not imply that CHANNEL-FLOODPLAIN GEOMORPHOLOGY, SOLIMÕES-AMAZON RIVER Geological Society of America Bulletin, September 1996 1095 Figure 5. Ovals indicate bank height with respect to low water, based on topographic surveys of banks, and rectangles indicate the maximum flood height based on the difference between the lowest and highest river stage at main-stem gaging stations (data from Departamento Nacional de Aguas e Energia Elétrica—DNAEE) along the Amazon River. Triangles show average per- centage of sand across transects on erosional (ero) banks compared to those across depositional (depo) banks. JA—Jutaí arch, PA—Purús arch, and MR—Madeira River confluence. Figure 6. Channel sinuosity, floodplain width, and location of floodplain relative to main channel of Amazon River. Sinuosity equals thalweg length/valley length for each section. Per- cent northern equals percentage of floodplain north of main channel. JA—Jutaí arch, PA— Purús arch, MR—Madeira River confluence, and GA—Gurupá arch.
    • MERTES ET AL. 1096 Geological Society of America Bulletin, September 1996 volumes of erosion exceed those of deposition, because the mapping of new bar features does not incorporate differences in elevation between new bars and eroding banks or the sediment being de- posited on older floodplain surfaces. The total rate of channel change generally decreases from up- stream to downstream until it increases again downstream of the Madeira River confluence (2300 km), only to decrease again in the reaches just upstream of Óbidos. A maximum rate of channel migration of ≈140 m/yr, or ≈3% of the average channel width, was recorded at the apex of the bend shown in Figure 3. In addition to the along-stream patterns in channel change rate, three distinct types of chan- nel change have been observed.A migrating main channel bend (Fig. 3) is most often found in the upstream reaches. Migration of floodplain chan- nels was also observed throughout the study area for the period between 1971–1972 and 1979–1980 and was previously described by Sternberg (1960). The third type of change that was observed is change in islands.The percentage of total channel change that was due to changes in islands is shown in Figure 8. In most reaches, even reaches with migrating bends such as shown in Figure 3, >50% of the channel change occurred as changes in islands. In the reaches of the river at ≈1400 km and 1600 km, changes in islands ac- counted for >90% of the total change. In order to verify the 8 yr record of channel change, older maps and records were qualita- tively compared for evidence of channel change (Figs. 9, 10, and 11). Only fragments of maps could be pieced together due to inaccuracies in orientation and scale. Figure 9 shows the pattern of long-term channel change between Xibeco (1051 km) and Tupé (1248 km). The migration of the bends in this reach follows the scroll-bar floodplain, which probably represents an area of recent channel migration. The rate of erosion of the terrace along the south bank near Fonte Boa can be calculated from historical records. Both Herndon (1853, p. 248) and Bates (1962) re- ported that Fonte Boa was 2 mi (3.2 km) away from the main Amazon channel, on the banks of a floodplain channel named Cayhiar-hy. To quote Bates (1962, p. 386), “. . . arrived at Fonte Figure 7. (A) Number of islands and average island area per kilometre of river length,Amazon River. JA—Jutaí arch, PA—Purús arch, MR— Madeira River confluence, and GA—Gurupá arch. (B) Relation between island area per kilometre and average single-channel width, Amazon River. Numbers ascend in order of reaches shown in Figure 7A from upstream to downstream. Linear regression shown as thick solid line and thinner lines indicate range of regression based on the standard error of the slope of the linear regression. (C)Total island area and island perime- ter per reach of the Amazon River. JA—Jutaí arch, PA—Purús arch, MR—Madeira River confluence, and GA—Gurupá arch. (D) Circularity index values were calculated by computing island radius (ra) from the total measured island area (A, Fig. 7C) for each reach as if all of the islands were circles (ra = [A/π]0.5). From this radius a new “circular” perimeter was calculated (Pcir = 2πra). The measured total island perimeter (Pobs, Fig. 7C) per section was divided by Pcir to get the “circularity” index per reach. Values near 1 indicate more circular and higher values indicate more elongate islands. JA—Jutaí arch, PA—Purús arch, MR—Madeira River confluence, and GA—Gurupá arch. A C B D
    • Boa; a wretched, muddy, and dilapidated village, situated two to three miles [from] . . . the main Amazons.” As of 1984 the village church had to be moved before it slid into the water (R. H. Meade, personal observation). The rate of mi- gration of the main channel would therefore have been ≈3000 m divided by 120 yr, yielding 25 m/yr or 1% of the channel width per year. These long-term rates are within the range of the short-term rates measured in this reach. Recently acquired Landsat images (1988) and U.S. Space Shuttle photographs (various dates, 1988– pre- sent) indicate that the observed migration esti- mated from both the long-term and short-term records is continuing. In contrast to Figure 9, Figure 10 shows mini- mal change between Manacapurú (2031 km) and São José do Amatari (2228 km) over a 20 yr period. This corresponds to our lower, short-term rates, and also to the minimal rates of change es- timated by Sternberg (1960). The 14C dates of levee deposits on Careiro Island suggest that the island has been at its present location for hun- dreds of years (Sternberg, 1960), and that the only significant change in this area since the early 1900s has been the development of a flood- plain channel directly south of Careiro Island at its westernmost tip. Figure 11 shows that the long-term rate of channel change is slightly greater in the reaches of the river downstream of the Madeira River confluence (2300 km) compared to the rates near Careiro Island. This pattern of increased channel migration in the sections downstream of the Madeira River confluence was also mea- sured from the short-term record. The high rates of channel change measured for the upstream reaches are comparable to rates reported for other reaches of the Amazon River and for other large, alluvial rivers. Salo et al. (1986) reported rates of 12 m/yr for a 13 yr period on meander bends of the Manu River (a tributary of the Amazon River) in Peru. On the basis of an analysis of Landsat images dating from 1979 to 1983, Kalliola et al. (1992) re- ported rates of migration as high as 400 m/yr, or 37% of the channel width, for theAmazon River in Peru. Drago (1990) reported rates ranging from <50 m to >200 m per year on sections of the Parana River in Argentina, based on records from the early 1900s to the present. The maxi- mum annual rate of lateral migration measured in our study, 140 m or 3% of the channel width (Fig. 3), compares with the Kesel et al. (1974) report of 40.5 m/yr, or 4% of the channel width, of lateral migration for a bend on the Mississippi River. Our average rates of channel change are even within the range of 0.1%–5%/yr reported by Lewin et al. (1977) for gravel-bedded streams in England. CHANNEL-FLOODPLAIN GEOMORPHOLOGY, SOLIMÕES-AMAZON RIVER Geological Society of America Bulletin, September 1996 1097 Figure 8. Channel change in Amazon River during the years between 1971–1972 and 1979–1980. The channel change is reported as the land area reworked per year over 8 yr, that is, eroded, deposited, or total change, expressed as a percent of the total active channel area which was measured as the area between the main channel banks in each reach. Shaded boxes at top of graph show the percentage of the channel change that was due to island movement. JA—Jutaí arch, PA—Purús arch, and MR—Madeira River confluence. Figure 9. Channel change since 1853 between Xibeco and Fonte Boa. The 1853 map was drawn by Herndon (1853) as he traveled the Amazon River by canoe, and should be regarded as very approximate. Even when standing in his canoe, Herndon would not have been able to perceive the true degree of curvature of the river. His horizon would have allowed him to see only a small segment of any given bend at any time, and his portrayal is likely to have under- estimated river curvature. The ca. 1940 map was drawn by a Brazilian Army detachment supposedly in 1934 as part of reconnaissance for the conflict of Leticia, according to the Insti- tuto Brasileiro de Geografia e Estatística (IBGE in Rio de Janeiro), and was supposedly based on trimetrigon aerial photographs. However, in 1942–1943 the U.S. Army Air Corps also took trimetrigon aerial photographs of the Amazon River to draw maps for aerial reconnaissance (Natural Resources Unit, 1966). The 1942–1943 maps were printed by the U.S. Defense Map- ping Agency, St. Louis, Missouri, and they are almost exact replicas of the 1934 maps. It may be that both sets of maps were based on the same set of photographs, hence the date ca. 1940 has been applied to these maps. The 1971–1972 map was traced from radar images from Radambrasil (1972).
    • Floodplain Features TheAmazon floodplain is a complex mosaic of lakes and lake deposits, floodplain channels, scroll bars,and overbank deposits.According to Sippel et al. (1992),11% of the 92400km2 of the main-stem floodplain in Brazil is covered by 6510 lakes >100 m across, not including the largest ria lakes (tributary mouthbay; usage of Sioli, 1957). Like is- lands,lakesbecomerounderdownstream(Figs.7D and 12C). However, unlike island area, which showedacorrelationtoincreasedwidthoftheriver, lake area (Fig. 12A) does not show a significant correlation to floodplain width (Fig.12B),suggest- ing that the processes of lake formation are decou- pled from the processes controlling the floodplain width. In particular, in the upstream reaches (c, 1095 km, and d, 1357 km, in Fig. 12B) where the floodplain is widest, the lakes are small and are confined to narrow swales between scroll bars. In contrast, the larger lakes tend to be in the down- stream reaches, regardless of floodplain width, and have less-well-defined boundaries. The largest lakes are just downstream from Óbidos (2750 km) and are as much as 65 km across (Melack, 1984). The along-stream pattern in number of lakes is also irregular. The number of lakes increases between 1000 km and 1700 km (approximately Xibeco to Itapeua), decreases in the downstream direction to São José doAmatari (2228km),increases again af- ter the Madeira River confluence (2300 km), and then the lakes disappear just downstream from the Tapajós River (2900 km) (Klammer, 1984). The Amazon floodplain is also covered by a MERTES ET AL. 1098 Geological Society of America Bulletin, September 1996 Figure 10. Channel change since 1952 (map from Sternberg, 1960) to 1971–1972 (Radambrasil, 1972) between Manacapuru and São José do Amatari. Mixed floodplain rep- resents areas covered equally by scoll-bar topography and rounded lakes. Figure 11. Channel change since 1900 between São José do Am- atari and Paurá. The 1900 map was drawn by members of the British Telegraph Agency in association with maintenance of tele- graph lines between Manaus and Parintins (map supplied by H. O. Sternberg). The 1971–1972 map was traced from radar images of Radambrasil (1972). Mixed floodplain represents areas covered equally by scoll-bar topography and rounded lakes.
    • dense network of floodplain channels. In our work we have distinguished floodplain channels from the main channel and its anabranches by defining floodplain channels as channels that have approximately an order of magnitude smaller discharge than the main channel flow and are typically not active during low-water pe- riods. Figure 13 shows the width and depth dis- tributions for 105 of these channels measured on the low-water 1980 navigation charts for reaches of ≈100 km. Generally, the width of the largest floodplain channels increases in the downstream direction from 800 to 2000 m, although the smallest floodplain channels (100 m wide) were observed in every reach. Depths were estimated over tabular sills at the entrance to the floodplain channels, on the assumption that these sills con- trol the flow entering the channel. The depth showed no correlation to downstream distance. As the main channel, anabranches, and flood- plain channels migrate, scroll bars are left behind on the floodplain surface. Figure 14 shows typical scroll-bar patterns found in the upstream reaches between Vargem Grande and the Japurá River confluences. The radii of curvature of the scroll bars decrease toward the center of the scroll bar complex (a–d in Fig. 14). In the downstream reaches the scroll bars tend to be longer, farther apart, and straighter than the upstream bars. Empirical relationships between the width of an alluvial channel and the radii of curvature of bends in that channel have been documented by several authors (see Richards, 1982, for a review). We used the radii of curvature of channels and floodplain features to help identify the places where the floodplain scroll-bar features were de- posited. Downstream of the Madeira River con- fluence the scroll bars are less distinct and tend to be straight along their preserved length, thus yielding infinite radii of curvature; they were not included in this analysis. Figure 15A is a fre- quency plot of the radii of curvature of all the ac- tive main channel bends and bends in floodplain channels upstream of the Madeira River conflu- ence. The mean plus one standard deviation for the main channel is 6.7 + 3.1 km, and for the floodplain channels it is 1.9 + 0.7 km. According to the Mann-Whitney test, a nonparametric statis- tical ranking test, these two populations are sig- nificantly different at a confidence level >99.9%. The frequency distribution of radii of curva- ture for a representative sample of outer flood- plain scroll bars (i.e., the features labeled “a” in Fig. 14) and inner floodplain scroll bars has been plotted (Fig. 15B). The mean plus one standard deviation for the outer floodplain scroll bars is 6.2 + 2.6 km, close to the values for the main channel bends. The mean value of the radii of curvature of the inner floodplain scroll bars is 3.2 + 2.6 km and appears to incorporate values that scale with both the floodplain and main channels. The outer scroll bars were originally formed by the main channel, but many of the inner scroll bars were deposited in the bends of floodplain channels, which are significantly smaller than the bends of the main Amazon channels. DISCUSSION The Amazon River and floodplain in Brazil display an along-stream trend in channel-flood- plain geomorphology and channel behavior (Fig. 2A), largely controlled by transfer and stor- age of sediment from the Andes to the coast. Dunne, Mertes, R. H. Meade, and J. E. Richey (unpub. data) have developed a sediment budget for the BrazilianAmazon River that includes the following inputs: bed load, suspended sediment CHANNEL-FLOODPLAIN GEOMORPHOLOGY, SOLIMÕES-AMAZON RIVER Geological Society of America Bulletin, September 1996 1099 Figure 12. (A) Number of floodplain lakes per kilometre of river length and average lake area (data reported by Sippel et al., 1993). JA—Jutaí arch, PA—Purús arch, MR—Madeira River confluence, and GA—Gurupá arch. (B) Floodplain lake area versus floodplain width. Reaches shown in Figure 12A are labeled alphabetically in or- der from upstream to downstream (lake data reported by Sippel et al., 1993). (C) Ratio of fetch (maximum length) to width of Amazon- ian floodplain lakes. Low values indicate more equant lakes and high values indicate longer, narrower lakes (calculation made from lake data reported by Sippel et al., 1992). JA—Jutaí arch, PA—Purús arch, MR—Madeira River confluence, and GA—Gurupá arch. A B C
    • load, sediment from tributaries, and sediment contributed by erosion of bank material. The outputs include bed load, suspended sediment load, loss to floodplain channels, overbank de- position, and sediment lost to new bar or island formation. The summation of all of the input and output terms is presented in Figure 2C as the net storage of sediment per kilometre. In addition, as depicted in Figure 2A, the along-stream change in channel-floodplain geomorphology is not smooth, because the basinwide pattern is over- laid by features produced by intracratonic tec- tonics, local patterns of deformation, and major tributary inputs. General Along-Stream Geomorphic Patterns In the upstream reaches (Figs. 2A and 16A), from 863 km to 1528 km (Vargem Grande to Ju- tica), the water-surface gradient increases slightly (Fig. 2B), promoting erosion of sand (Fig. 2C). The sand erosion also corresponds to high rates of bank and island erosion (Figs. 8 and 9). These high rates probably promote or are the result of the high sinuosity of the river in the upstream reaches (Fig. 6). As a result of rapid channel mi- gration, both lakes and islands are small and streamlined. Lakes tend to be confined to swales between scroll bars as in “meander scroll lakes” (usage, Blake and Ollier, 1971). Islands tend to be shortlived, and therefore rarely attain a large size, due to the rapid change in the channel position. On the floodplain surface the upstream scroll bars appear fresh, and a large proportion of them are the same size as the bends of floodplain chan- nels rather than the larger main Amazon channel (Fig. 16B). Figure 17 depicts a schematic model for the development of the scroll-bar complexes that cover the upstream floodplain (Fig. 14).As the main channel migrates, bar deposits are left as a series of scroll bars on the scale of the main chan- nel (Fig. 17A). As time passes, the bend migrates farther, and floodplain channels also migrate, re- working the deposits of the large bend and leaving behind smaller-scale scroll bars (Fig. 17B). Even- tually the main channel bend is cut off by a more direct channel, and the old channel begins filling with sediment. Subsequent to the cut off, water continues to flow through floodplain channels in the bend area, and they rework the surface deposits (Fig. 17, C and D). If enough time passes, most of the surface features from the larger-scale bend can be erased, and the smaller-scale floodplain chan- nel scroll bars will dominate the surface morphol- ogy of the floodplain (Fig. 17E). The resulting scroll-bar complexes show arcs of two distinct sizes, outer arcs on the scale of the main channel and inner arcs on the scale of the floodplain chan- nels (Figs. 14 and 16A). Scroll topography on the Orinoco floodplain (Colonnello, 1990) also com- prises scroll bars at different scales. In the middle reaches of the river we observe a transition from upstream to downstream fea- tures (Fig. 16B). In the reaches from 1528 to 2228 km (Itapeua to São José do Amatari), the river tends to be straighter in a narrower valley, migration rates are slower, scroll bars cover pro- portionally less of the floodplain, and, on aver- age, islands are slightly larger. Scroll lakes are mixed with rounder, larger lakes. There is fairly consistent deposition of sand (Fig. 2C) through this middle section as the river gradient de- creases (Fig. 2B). In the downstream reaches from São José MERTES ET AL. 1100 Geological Society of America Bulletin, September 1996 Figure 13. (A) Distribution of widths of 105 floodplain channels measured from 1980 Brazilian Navy navigation charts. PA—Purús arch, MR—Madeira River confluence, SPO— São Paulo do Olivença, Ita—Itapeua, Man—Manacapurú, and Obi—Óbidos. (B) Distribu- tion of depths of 105 floodplain channels measured from 1980 Brazilian Navy navigation charts. PA—Purús arch, MR—Madeira River confluence, SPO—São Paulo do Olivença, Ita—Itapeua, Man—Manacapurú, and Obi—Óbidos. A B
    • CHANNEL-FLOODPLAIN GEOMORPHOLOGY, SOLIMÕES-AMAZON RIVER Geological Society of America Bulletin, September 1996 1101 do Amatari (2228 km) to the Tapajós River (2850 km) confluence, the Amazon River is fairly straight and migrates slowly compared to the upstream reaches (Fig. 16C). Scroll-bar sur- faces are blurred and hundreds of broad, shal- low, patchy lakes cover the floodplain. These features suggest that the construction of the floodplain is controlled by overbank deposition of fine material, with the main channel building levees that tend to confine the migration of the main channel and floodplain channels. Occa- sionally, floodplain channels migrate across lakes creating lake deltas (Fig. 16C) or “sedi- mentary jetties” (usage of Bird, 1964, as de- scribed by Blake and Ollier, 1971). Comparison of the less-well-defined lake boundaries in the downstream reaches to the meander scroll lakes of the upstream reaches indicates that the lake depressions may be due to local subsidence of the floodplain. Any bounding levees are gradu- ally being buried by overbank sedimentation. The blurred appearance of the scroll complexes in these downstream reaches also contrasts with their fresh appearance in upstream reaches. Blurring of the scroll bars may result from the combined effects of the main channel migrating slowly and floodplain channels being relatively confined, and therefore migrating slowly; hence, fresh new scroll complexes are slowly produced. Over time the old scroll surfaces are buried by sediment decanting from overbank flows. The delta plain of the river system extends downstream of the Tapajós River confluence to the estuary. The sinuosity is essentially 1.0, and the number and size of islands and lakes de- crease dramatically. In contrast to the reaches immediately upstream, in this reach the shallow lakes and floodplain channels have been filled (Klammer, 1984), perhaps by sedimentation from overbank flows, while the main channel has remained straight and locked in position. A sediment budget for this reach suggests that 300–400 Mt/yr of sediment deposition or 0.4 Mt/yr per km (Fig. 2C) could be responsible for the rapid filling of any lakes that may have formed (Mertes, 1985; Mertes and Dunne, 1988, Fig. 3; Dunne, Mertes, R. H. Meade, and J. E. Richey, unpub. data). Recent discovery of fluid muds on the continental shelf (Kineke and Sternberg, 1995) does not substantively alter this budget (Nittrouer et al., 1995) because the fluid muds are typically resuspended material and are not exported at high rates away from the shelf. Influence of Structural Arches The cratonic interior of the Amazon basin is interrupted approximately every 1000 km by structural arches (600–1300 km between arches; Figure 14. Scroll-bar geometry on the Amazon River floodplain between Vargem Grande and the Japurá River, traced from Radambrasil (1972) charts. The decreasing radius of cur- vature toward the center of each group (a–d) suggests that the outer scroll bars “a” were de- posited in main channel bends and the inner bars were deposited in smaller-scale bends of floodplain channels. Figure 15. (A) Frequency distribution of radii of curvature of floodplain and main channel bends in reaches of theAmazon River upstream of the Madeira River confluence. The means plus one standard deviation of the radii of the floodplain channel and main channel bends are shown above the respective distribution. (B) Frequency distribution of radii of curvature of inner and outer (marked “a” in Fig. 13) floodplain scroll bars. The means plus one standard deviation of the radii of curvature of floodplain channel and main channel bends are shown for com- parison. A B
    • Table 1 and Fig. 1) that traverse the main axis of the river system and form the boundaries of ap- parently subsiding basins. The river hydraulics in the form of the low-water surface slope appear to be responding to the presence of the three arches in Brazil; these are low water-surface gradients upstream of the arches, increasing water-surface gradients as the river crosses the apex of the arch, and then a rapidly decreasing water-surface gra- dient on the downstream limb of the arch. Simi- lar patterns have been described for other alluvial and bedrock channels, where high uplift rates have been estimated (Merritts andVincent, 1989; Bullard and Lettis, 1993) or more resistant lithol- ogy (Hack, 1973) occurs. Assuming that the water-surface slope is pro- portional to the boundary shear stress responsi- ble for transport of sediment, one can predict erosion in zones of increasing surface slope and deposition in zones of decreasing surface slope. In a downstream direction, the river erodes coarse material as the slope increases in the reaches between Vargem Grande and Jutica (863–1248 km). Between Jutica and Itapeua the slope decreases and both coarse and fine sedi- ment are deposited (see next section regarding local deformation). The slope increases again as the river crosses the Purús arch (2000 km), and there is a corresponding increase in coarse sedi- ment erosion. As the river crosses the arch, the slope decreases, and deposition of coarse and fine material occurs. As the river approaches the Gurupá arch in the delta plain, the surface slope again increases slightly. The estimated deposition rates of 0.4 Mt/yr per km (Fig. 2C) could result from de- creasing surface slopes on the downstream limb of the Gurupá arch. This reach of the river is ap- proximately at sea level. The effect of the Holo- cene rise in sea level could have been to en- hance the influence of the arch and graben, because the increased base level would initially further reduce water-surface slopes. In addition to influencing the patterns of hy- draulics and sediment transport, the arches are associated with narrowing of the alluvial valley. The valley width is <30 km near the Jutaí arch, <20 km near the Purús arch, and <40 km ap- proaching the Gurupá arch.As described herein, a correlation exists between the narrowest sec- tions of the alluvial valley and the lowest chan- nel sinuosity. Aside from its upstream reaches, the river is also unusually straight along most of its length, according to empirical relationships between width and bend amplitude for other al- luvial rivers as reported by Lewin and Brindle (1977) and Leopold and Wolman (1957). From these empirical relationships based on channel width, predicted Amazon bend amplitudes should range from 20 km upstream to 60 km downstream. Only two modern bends have am- plitudes >20 km. They are both upstream in the widest part of the valley. Hence, the river is ap- parently freely meandering only in the upstream reaches and elsewhere is confined. Whether the arches are restricting the migration of the chan- nel through influence on the valley width, or the valley is narrower because of reduced channel migration, is not clear. In either case, a combina- tion of reduced vertical incision or lateral migra- tion relative to activity upstream and down- stream could produce the geomorphic patterns spatially associated with the arches. Influence of Local Deformation The clearest example of the influence of local deformation not associated with the arches is in the reach between Jutica and Itapeua MERTES ET AL. 1102 Geological Society of America Bulletin, September 1996 01020km Figure 16. (A) Typical scroll-bar floodplain along upstream reaches of Amazon River near Xibeco (1051 km). (B) Typical transitional floodplain in middle reaches of Amazon River near Itapeua (1704 km). A B
    • (1528–1704 km, Fig. 2A), where the water-sur- face slope is high and then abruptly decreases (Fig. 2B). Nearly 100% of the floodplain is north of the main channel (Fig. 6), and the river runs nearly straight as it carves a path at the base of the terraces forming the south bank. Tricart (1977), Iriondo (1982), and Iriondo and Sugio (1981) described this as a southerly tilted basin extending at least as far north as the Japurá River. The impact of the tilted valley on channel behav- ior and geomorphology is significant: the river is straighter, deeper, and narrower, and the water- surface slope is high. These patterns are observed in part because flow is being forced to the south edge of the valley and the river cannot erode the terraces rapidly enough to form bends or even to widen the channel. As the river leaves this tilted section downstream of Itapeua, the water-surface slope decreases. As the river has carved its new position on the south bank, it also has cut across several blocked valley lakes (Fig. 16B) and has changed the position of the confluences of the southern tributaries. In this tilted reach there is also evidence that channel migration has been achieved partially through avulsions of the main channel. Fig- ure 17 shows the proposed development of typ- ical scroll complexes on theAmazon floodplain. As the scroll complexes develop through re- working of the surface deposits by floodplain channels, it is unlikely that oxbow lakes on the scale of the main channel would remain, in con- trast to many river systems where oxbow lakes are an important floodplain feature (e.g., the Mississippi River; Fisk, 1944). However, if the channel were to avulse to a distant location, then the floodplain channels would no longer receive flood waters, and it would be possible for an oxbow to remain on the floodplain sur- face. The only large-scale, oxbow-type features on the Brazilian Amazon floodplain are located north of Xibeco (1051 km) and north of Itapeua (1704 km). The oxbow lake north of Xibeco (Fig. 18A) lies along an old channel belt (Tri- cart, 1977; Iriondo, 1982; Iriondo and Sugio, 1981) and is the same size as the main channel bends. Between the northern channel belt con- taining the oxbow lake and the present channel, the floodplain surface is smooth, suggesting that recent channel migration was not continuous across its surface. Both the relatively smooth in- tervening floodplain surface and the presence of the oxbow lake suggest that the channel avulsed ≈40 km to its present location. Figure 18B shows that the river has cut across the blocked valley lake near Itapeua as it carved a tortuous bend south around older terrace deposits. North of the terrace deposits are signs of older channel deposits and some remnant oxbow-like lakes. All of this evidence suggests that the river may have avulsed to its southern position, perhaps captured by the former drainage of the blocked tributary. Without the south-tilting deformation, which may be the cause of these avulsions, the oxbow-type lakes may not have been preserved on this upstream part of the floodplain. This ev- idence also suggests that the tilted reach extends from Xibeco to Itapeua (1051–1704 km). Influence of Major Tributaries Local variability in channel behavior and floodplain geomorphology could also be the re- sult of input from the major tributaries, as has been suggested for much of the Orinoco River and floodplain (Hamilton and Lewis, 1990). However, unlike tributaries of the Orinoco, most of the tributaries of the Amazon have much smaller flows and sediment loads than the main channel. Therefore, their individual influence on the main channel behavior and floodplain mor- phology is difficult to assess, aside from the over- all effect of downstream dilution of the sediment load and the dramatic appearance of their large mouthbays. However, we can discern the inde- CHANNEL-FLOODPLAIN GEOMORPHOLOGY, SOLIMÕES-AMAZON RIVER Geological Society of America Bulletin, September 1996 1103 Figure 16. (Continued) (C) Typical downstream floodplain of Amazon River near Óbidos (2750 km). (D) Typical “filled” delta-plain floodplain of Amazon River between Tapajós and Xingu confluences (3000 km). C D
    • pendent influence of two of the major tributaries. The Japurá River confluence is in the middle of the tilted reach, and the location of the confluence may be controlled by the deformation. In turn, as the Japurá migrates to the south and builds its floodplain, it may be contributing to the overall tendency for the main Amazon to impinge along the southern edge of its alluvial plain. The Madeira River, with its confluence at 2300 km, can carry half of the sediment load of the main Amazon channel (Meade, 1985, 1994; Meade et al., 1985; Dunne, Mertes, R. H. Meade, and J. E. Richey, unpub. data). Downstream of the Madeira River confluence the banks of the Amazon tend to have a higher proportion of sand (Fig. 5), and the rate of channel migration (Fig. 8) and the number of islands (Fig. 7A) increase. These characteristics indicate that the Madeira River may be imparting an “upstream” character to the system as it provides a coarser sediment load from the Andes that is available for more frequent bar deposition and, therefore, more rapid rates of channel migration. Floodplain Aggradation, Erosion, and Recycling Time In considering the evolution of the Brazilian Amazon alluvial landscape it is instructive to esti- mate long-term rates of floodplain aggradation, erosion, and recycling based on the prevailing pat- terns of sediment transport along the main chan- nel. This discussion does not include the transport and deposition rates estimated for floodplain channels, which were shown (Figs. 13, 14, and 15) to represent a distinct population of chan- nels at least an order of magnitude smaller than the main channel. In developing the sediment budget for the Brazilian Amazon River, Dunne, Mertes, R. H. Meade, and J. E. Richey (unpub. data) found that the rates of main channel erosion essentially equaled the rates of overbank deposi- tion plus the rates of deposition of new bar or is- land formation along the main channel. Over the entire study reach, the annual rates of exchange (erosion or deposition) between the main channel and floodplain each summed to ≈1600 Mt, for a total of ≈3200 Mt of reworked floodplain sedi- ment. The exchange rates slightly exceed the rate of suspended sediment transport at Óbidos, which averages 1240 Mt/yr (Dunne, Mertes, R. H. Meade, and J. E. Richey, unpub. data). Account- ing for the local recycling of old sediments, we es- timate that a large proportion of the suspended sediment passing Óbidos has passed through floodplain storage and is not freshly eroded sedi- ment from the Andes or sub-Andean foreland. The likelihood that a high proportion of the sedi- ment spends time in floodplain storage and is in turn weathered was confirmed by Johnsson and Meade (1990), who reported that sediments in point-bar deposits near Xibeco (1051 km) show morphological and chemical characteristics that are likely the result of several thousands of years of in situ weathering. Hence, it is reasonable to es- timate that an individual particle may take thou- sands of years to travel the 3000 km length of the Brazilian Amazon. As the river exchanges material with its flood- plain, it is continuously recycling the sediments of the alluvial plain. Given the virtual balance be- tween the rate of destruction (erosion) and con- struction (overbank deposition and new bar for- mation) of the floodplain, it is reasonable to use the rates of bank erosion to calculate the order of magnitude of time required to recycle the sedi- ments of the floodplain and alluvial plain. Fig- ure 19 shows the area of erosion calculated from the overlay of radar images and navigation charts grouped into reaches that correspond to the gen- eral pattern of along-stream variation in channel migration rates. We divided the area of the allu- vial plain by the area of erosion per year to pre- dict the time period for recycling in years. The number of years required to recycle the alluvial plain generally increases from 1000 yr in the up- stream reaches to 2000 yr in the downstream reaches, with an increase to >4000 yr in the downstream reaches near Óbidos (2750 km). We do not have channel change data for the reaches downstream of Óbidos, but the geomorphic data suggest that the channel is relatively immobile, hence we predict that the recycling time in the delta plain is also >4000 yr. For this calculation we did not include sedi- ment transport associated with floodplain chan- nels. As suggested by Figure 17, the floodplain channels rework the surface deposits of the flood- plain and produce landforms at a smaller spatial scale than the main channel. The floodplain chan- nels do not penetrate deeply into the alluvium, and therefore reworking would account for only a small proportion of the total recycling of flood- plain sediments. In addition, for the calculation we included the entire area of the alluvial plain, and not just the modern, regularly inundated floodplain. By including the entire alluvial plain we are not accounting for the fact that in some MERTES ET AL. 1104 Geological Society of America Bulletin, September 1996 Figure 17. Schematic model for development of scroll-bar complexes. See text for a complete description of the pattern development over the five stages. Compare to Figures 14 and 16A.
    • reaches channel migration and river-water incur- sion (Mertes et al., 1995) are restricted to narrow bands of the floodplain. The more distal reaches of the alluvial plain may not have been reworked within the calculated time periods. For example, although we predict a turnover time for the allu- vial plain near Xibeco of ≈1000 yr, which matches radiocarbon dates of buried wood frag- ments showing ages of 1180 yr (L. Pessenda and P. Camargo, unpub. data), the channel belt on the northern part of the alluvial plain that contains the large oxbow lake (Fig. 18A) probably has not been recently reworked and may be older than 1000 yr. SUMMARY The Solimões-Amazon River annually carries 1240 Mt of suspended sediment as it crosses the cratonic landscape of Brazil, which has been punctuated by tectonic deformation in the form of structural arches and basin tilting.Variations in river slope and transport of sediment across this landscape result in upstream reaches character- ized by erosion of sand in the main channel and deposition of sediment in floodplain channels, producing an intricate scroll-bar topography forming the boundaries of hundreds of long, nar- row lakes. In contrast, downstream reaches are characterized by channels restricted by stabiliz- ing levee building, and floodplain construction dominated by overbank deposition that gradually buries scroll topography, resulting in a flat flood- plain characterized by a patchwork of large, more equant, shallow lakes. Imprinted on this along-stream geomorphic trend is the influence of basin structure and de- formation. Structural arches appear to exert a pri- mary influence on the channel-floodplain system by promoting entrenchment of the river as it passes through zones of deformation in Brazil, thus restricting channel movement. This pattern is similar to observations made by Dumont et al. (1990) for the western, Peruvian Amazon basin. Our schematic model for the eastern Amazon (Fig. 2) illustrates the spatial correlation between the narrowing of the alluvial valley as the river crosses the arches and the gradual widening of the valley on the downstream limb of the arches. This impact is clearest as the river crosses the Purús arch, where the valley narrows to <20 km, the wa- ter surface gradient decreases dramatically, large amounts of suspended sediment are deposited, and yet the rate of channel migration is negligible. Hence, the overall effect of intracratonic tectonic deformation on the continental-scale Solimões- Amazon River is to create a landscape where, on the spatial scale of hundreds of kilometres, the river is confined and entrenched in its valley, is straight, and is relatively immobile. On the basis of estimated rates of recycling of floodplain sediments due to activity by the main channel, the modern floodplain of the Brazilian Amazon could have been recycled in <5000 yr, and is recycled more rapidly in the upstream than the downstream reaches. These results sug- gest that during the Holocene, fluvial transport in the Amazon River in Brazil has reestablished a channel-floodplain system that disposes of its sediment load in a manner controlled by tectonic features and the modern hydrology and hy- draulics of the main channel, anabranches, and floodplain channels. By contrast, the more dis- tal alluvial plain appears to be a remnant feature CHANNEL-FLOODPLAIN GEOMORPHOLOGY, SOLIMÕES-AMAZON RIVER Geological Society of America Bulletin, September 1996 1105 Figure 18. (A) Oxbow lake on Amazon alluvial plain north of Xibeco (1051 km) that is the only oxbow lake of the same scale as main channel bend and has apparently not been re- worked since the channel avulsed to its present, more southern course. (B) Channel avulsion has caused theAmazon River to carve tortuous bend around older terrace deposits and across the blocked valley lake at Coari (Itapeua—1704 km). Open water areas northeast of terrace may be remnants of oxbow lakes left behind after channel avulsion. A B
    • that cannot be entirely explained by modern processes. In particular, the tremendous ex- panses of open water in the downstream flood- plain invite questions regarding the long-term processes active in the system. For example, it is not yet known whether this open alluvial plain is due to evacuation of sediment during lower sea level (e.g., Müller et al., 1995) or the result of continuing subsidence. ACKNOWLEDGMENTS This research was supported by National Sci- ence Foundation grant DEB-8107522 and Na- tional Aeronautics and Space Administration grant NAGW 2652. We thank R. H. Meade for his frequent involvement over many years in our analysis of sediment transport and the history of the Amazon floodplain and J. E. Richey for leading the data collection campaigns. H. O’R. Sternberg provided historical maps and insight into Amazonian processes. L. Pessenda and P. Camargo contributed radioisotope dates for floodplain sediments. B. Forsberg, A. Devol, J. Hedges, P. Quay, R. 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