Amazon River System Research Articles

Seasonality affects the parasitism levels in two fish species in the eastern Amazon region

Summary Fish parasite communities have strong interactions with the environment, the host fish, and the aquatic invertebrate communities. Thus, factors directly involving their different life cycles, such as different host populations and environments, as well as seasonal fluctuations in water levels can cause different responses. The main factors structuring parasite communities may be the hydrodynamic variations, together with seasonal variations in the availability of infectious stages of parasites in the environment. The purpose of this study was therefore to investigate the influence of the rainy and dry seasons on parasite infracommunities in fish species in the Amazon River System of Brazil. Between October 2009 and April 2011, specimens of Colossoma macropomum and Colossoma macropomum × Piaractus brachypomus hybrids (tambatinga) were examined for the presence of parasites. The parasite communities were similar in these two species that both showed aggregate dispersion, although the hybrid specimens were less parasitized. For both hosts, the Brillouin diversity, species richness, evenness and Berger-Parker dominance indices were similar in the rainy season and dry season, except that the Berger-Parker dominance index for the C. macropomum × P. brachypomus hybrid was higher during the dry season. Ichthyophthirius multifiliis was the dominant parasite, followed by Piscinoodinium pillulare, in both hosts. However, C. macropomum had a higher prevalence and abundance in the dry season, while in the C. macropomum × P. brachypomus hybrid, only the I. multifiliis prevalence increased. The abundance of monogenean species (Anacanthorus spathulatus, Linguadactyloides brinkmanni, Mymarothecium and Notozothecium janauachensis) was higher during the dry season in the gills of C. macropomum. Despite overdispersion of the lernaeid Perulernaea gamitanae, a seasonal pattern was seen only in the C. macropomum × P. brachypomus hybrid, with higher prevalence in the rainy season. The low prevalence and abundance of the ectoparasites Tetrahymena sp., Trichodina sp. and Braga patagonica, as well as of the endoparasites Procamallalus (Spirocamallanus) inopinatus and Neoechinorhynchus buttnerae, were not affected by seasonality. This information is applicable in farming practice for these economically important fish, as it indicates the best time for prophylactic management and treatment against parasites, in order to prevent economic losses in fish farms.

A checklist for the zooplankton of the Middle Xingu - an Amazon River system.

A zooplankton checklist is presented for the Middle Xingu River, based on surveys conducted at four sites in the main channel and two fluvial lakes. A total of 175 taxa are listed, including 141 rotifers, 20 cladocerans, and five copepods. Rapids presented the greatest species richness, with up to 124 taxa, while Ilha Grande lake had 70 taxa, the lowest number. Non-planktonic benthic larvae were recorded frequently in the samples.

Structure of the parasites communities in two Erythrinidae fish from Amazon river system (Brazil).

This study compared the parasite communities of Hoplias malabaricus and Hoplerythrinus unitaeniatus from Amazon river system. Hoplias malabaricus were infected by Ichthyophthirius multifiliis, Piscinoodinium pillulare, Tetrahymena sp., Urocleidoides eremitus, Braga patagonica, metacercariae of Clinostomum marginatum, Procamallanus (Spirocamallanus) inopinatus, larvae of Contracaecum sp. and larvae of Nomimoscolex matogrossensis. Hoplerythrinus unitaeniatus were also infected by these same species of protozoans, nematodes, digeneans and cestodes, except for Tetrahymena sp. and B. patagonica, which were replaced by Argulus pestifer, Urocleidoides sp., Whittingtonocotyle caetei, Whittingtonocotyle jeju and Gorytocephalus spectabilis. For both hosts, I. multifiliis and P. pillulare were the predominant parasites. Most of the parasites presented an overdispersion. Parasite species richness, Brillouin diversity, evenness and Berger-Parker dominance were similar for the two hosts. The length and weight of H. malabaricus showed a positive correlation with the abundance of U. eremitus and Contracaecum sp., while the weight of H. unitaeniatus showed a positive correlation with the abundance of I. multifiliis. The diversity of ectoparasites seemed to be influenced by the behavior of these two hosts. This was shown by the similar parasite communities and was characterized by low species diversity, low evenness and low richness, and by a high prevalence of ectoparasites.

Length-weight relationship of 11 fish species from a tributary of the Amazon River system in northern Brazil

Summary The length–weight relationships (LWRs) of 11 freshwater fish for eight families (Leporinus friderici, Pterophyllum scalare, Geophagus camopiensis, Curimata incompta, Astyanax bimaculatus, Tetragonopterus chalceus, Moenkhausia lepidura, Metynnis lippincottianus, Plagioscion squamosissimus, Hemiodus unimaculatus and Colomesus asellus) captured in tributary of the Amazon River system (Brazil) were investigated. These results represent the first reference on the LWRs for all 11 species in the eastern Amazon.

Diversity of parasites in Curimata incompta (Curimatidae), a host from Amazon river system in Brazil.

This paper is the first study on host-parasite relationship in wild Curimata incompta Vari, 1984 (Curimatidae) from Amazon river system, Northern Brazil. In 40 specimens examined from December 2012 to November 2013, 615,818 parasites were collected, such as Ichthyophthirius multifiliis, Piscinoodinium pilullare, Urocleidoides sp., Posthodiplostomum sp., Gorytocephalus elongorchis and Braga patagonica. The parasites' component community had a low Brillouin diversity (0.16±0.15), a low species richness (3.1±0.7), a low evenness (0.09±0.09) and a high dominance of Berger-Parker (0.96±0.06). I. multifiliis was the dominant parasite species and it showed the highest prevalence and intensity in the host population. There was an aggregate dispersion of parasites, but the low parasitism did not affect the body condition of the host. The occurrence of parasites in C. incompta was due to their life habits and food behavior. This study, besides expanding the geographical distribution of G. elongorchis in Brazil, records the first occurrence of these six parasites in C. incompta.

Are the mixing zones between aquatic ecosystems hot spots of bacterial production in the Amazon River system?

Mixing zones between aquatic ecosystems are known diversity hot spots, but so far no study evaluated the effect of mixing waters on bacterial metabolism. Samplings were performed in four of the major Amazonian rivers, and aquatic bacterial production (BP) was measured both before the confluences and in the mixing zones downstream of the confluences. BP in mixing zones was higher than before the confluence in 75% of cases, representing an increase ranging from 31% to more than 1,000% of BP. In most of these cases, the confluence zone was formed by aquatic ecosystems that differed in the water type (environmental conditions and limiting nutrients to BP). The proportional contribution of the water from each source in the mixture had no impact on the outcome of BP in the mixtures, while BP in the mixtures peaked after 12 h of mixing, indicating that the increase in BP can continue downstream from the confluence zone. Overall, the results presented here do not contradict the low aquatic bacterial metabolism suggested for most Amazonian waters, but show that this general pattern may be sprinkled with hot spot areas of high bacterial metabolism in the confluence of aquatic ecosystems.

Five Cryptic Species in the Amazonian Catfish Centromochlus existimatus Identified Based on Biogeographic Predictions and Genetic Data

Accurately quantifying biodiversity is fundamental for both evolutionary theory and conservation strategies. DNA-based studies are exposing high cryptic diversity irrespective of taxonomic group or environmental setting, and increasing the ever-growing estimates of global biodiversity. This has severe implications for under-sampled and species-rich tropical regions, such as the Amazon Basin. We used biogeographic predictions derived from geomorphological history and contemporary hydrochemical and genetic data to examine cryptic diversity in the Amazonian driftwood catfish Centromochlus existimatus. Using both nuclear and mitochondrial DNA markers, five deeply divergent cryptic lineages are reported, for which at least three are sympatric in distribution. These lineages appear relatively old, with divergence times dating back to middle Miocene. Diversification events appear to be chronologically associated with the formation of the modern Amazon River system, and perhaps influenced by hydrochemical gradients between tributaries. The cause of apparent morphological stasis in the C. existimatus species complex is speculated within the context of hydrochemistry and non-visual mating cues and a full taxonomic revision is recommended. Our findings suggest that the diversity of Amazonian ichthyofauna is vastly underestimated and highlight the relevance of biogeographic predictions to guide sampling efforts in ecologically complex and under-studied ecosystems.

The Amazon River system as an ecological barrier driving genetic differentiation of the pink dolphin (Inia geoffrensis)

2 . Previous morphological and genetic studies have proposed the existence of at least two evolutionary significant units: one distributed across the Orinoco and Amazon basins and another confined to the Bolivian Amazon. The presence of barriers in the riverine environment has been suggested to play a significant role in shaping present-day patterns of ecological and genetic structure for this species. In the present study, we examined the phylogeographic structure, lineage divergence time and historical demography using mitochondrial (mt)DNA sequences in different pink dolphin populations distributed in large and small spatial scales, including two neighbouring Brazilian Amazon populations. mtDNA control region (CR) analysis revealed that the Brazilian haplotypes occupy an intermediate position compared to three previously studied geographic locations: the Colombian Amazon, the Colombian Orinoco, and the Bolivian Amazon. On a local scale, we have identified a pattern of maternal isolation between two neighbouring populations from Brazil. Six mtDNA CR haplotypes were identified in Brazil with no sharing between the two populations, as well as specific cytochrome b (cyt b) haplotypes identified in each locality. In addition, we analyzed autosomal microsatellites to investigate male-mediated gene flow and demographic changes within the study area in Brazil. Data analysis of 14 microsatellite loci failed to detect significant population subdivision, suggesting that male-mediated gene flow may maintain homogeneity between these two locations. Moreover, both mtDNA and microsatellite data indicate a major demographic collapse within Brazil in the late Pleistocene. Bayesian skyline plots (BSP) of mtDNA data revealed a stable population for Colombian and Brazilian Amazon lineages through time, whereas a population decline was demonstrated in the Colombian Orinoco lineage. Moreover, BSP and Tajima's D and Fu's Fs tests revealed a recent population expansion exclusively in the Bolivian sample. Finally, we estimated that the diversification of the Inia sp. lineage began in the Late Pliocene (approximately 3.1 Mya) and continued throughout the Pleistocene. Published 2011. This article is a US Government work and is in the public domain in the USA. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102, 812-827.

Effects of DOM photochemistry on bacterial metabolism and CO2 evasion during falling water in a humic and a whitewater river in the Brazilian Amazon

In the Amazon river system, the source of the large quantity of CO2 evading from river surfaces remains unidentified. Photochemical transformation of dissolved organic matter (DOM) into dissolved inorganic carbon (DIC) and low molecular weight organic acids (LMWOAs) is a promising candidate. Few studies in the Amazon river system, and river systems in general, have attempted to quantify the contribution of these specific photoproducts to CO2 evasion. We conducted photochemical degradation and 14C addition experiments to measure the rate of production and the impact on bacterial metabolism, respectively, in the black water Rio Negro and in the white water Rio Solimoes during low water. We found statistically significant production of both photoproducts in the Rio Negro and none in Rio Solimoes. We also found that two photochemically produced LMWOAs—acetic and formic acid—may play a significant role in bacterial metabolism in both rivers. Based on our experimental results, we estimate that photochemically produced CO2, acetic acid and formic acid alone contribute to only 0.5% of the CO2 evading from the Rio Negro. Due to our experimental set-up, analytical methods and time of sampling, we caution that our estimate is very conservative. More extensive research is needed before drawing conclusions on the contribution of photochemistry to CO2 evasion from river surfaces of the Amazon basin.

Endemism in Neotropical Podostemaceae1

Abstract Podostemaceae, a family of plants restricted to river rapids and waterfalls, are widely reported to have a high degree of local species endemism. We tested this idea for Neotropical members of the family using historical records, herbarium holdings, personal field collections, and geographic information systems analyses. In contrast to estimates of endemism based on the landmark studies of P. van Royen (66%), we report 15%–37% based on current taxonomy. Examples of regional endemism, based on extent of occurence and longest geographic axis measures, are discussed for a subset of species. Major hydrographic regions (Amazon River System, Parana River System) and major areas (eastern Brazil) are shown to possess largely unique podostemad floras. We propose rivers and river systems as the most appropriate units to assess endemism for Podostemaceae, and consider one-river and two-river endemics as narrowly distributed. Limitations in the current taxonomy are discussed relative to establishment of mean...

Description of Dicrossus foirni sp. n. and Dicrossus warzeli sp. n. (Teleostei: Perciformes: Cichlidae), two new cichlid species from the Rio Negro and the Rio Tapajós, Amazon drainage, Brazil

Dicrossus foirni sp. n. and Dicrossus warzeli sp. n. are described from the Brazilian Amazon River system. Both species are differentiated from all other Neotropical cichlids by exhibiting three rows of longitudinal lateral spots. Dicrossus foirni sp. n., which is distributed in the northern Rio Negro drainage, exhibits a pattern of lateral spots formed by the clustering of individual dots. By contrast, Dicrossus warzeli sp. n., which is endemic to the Rio Tapajós, exhibits three longitudinal rows of horizontal dash-like spots on the sides of the body. Furthermore, the two species are distinguished by having different relationships between body height and body length.

Descrição de nove espécies novas de Phenacogaster (Ostariophysi: Characiformes: Characidae) e comentários sobre as demais espécies do gênero

Nine new species of Phenacogaster Eigenmann, 1907 are described: Phenacogaster capitulatus sp. nov. from the Neshuya River system, Ucayali River basin; Phenacogaster maculoblongus sp. nov. from the upper Cuyuni River, and tributaries of the lower Orinoco River; Phenacogaster napoatilis sp. nov. from the Napo River system, upper Amazon River basin; Phenacogaster ojitatus sp. nov. from the Xingu River system; Phenacogaster prolatus sp. nov. from the Orinoco River, canal Casiquiare and upper and middle Negro River; Phenacogaster retropinnus sp. nov. from the Negro, Madeira and Xingu river systems; Phenacogaster simulatus sp. nov. from the Potaro River, Essequibo drainage; Phenacogaster wayampi sp. nov. from the Oiapoque River, and Phenacogaster wayana sp. nov. from the Corantijn (Suriname), Maroni, Mana, Sinnamary and Approuague rivers (French Guiana). The geographic distribution of P. megalostictus, previously recorded for the Negro River, Amazon River system, is restricted to the Essequibo River system, Guiana. The distribution of P. carteri, previously known only from the type-locality (Cuyuni River, Guyana), is extended to the lower Orinoco River. The reduction of the latero-sensory canal, mentioned in the literature as an autapomorphy for P. tegatus was observed in three additional species of Phenacogaster. The posterior humeral spot, found only in P. tegatus, is herein considered autapomorphic for the species. Diagnoses are given for previously described species based on the analysis of type material and additional specimens, and a key to the species of the genus is provided.

Dissolved rare earth elements in a seasonally snow-covered, alpine/subalpine watershed, Loch Vale, Colorado

Dissolved rare earth elements (REEs) were determined in a four-year time series at the outlet of Loch Vale. The Loch Vale watershed is a seasonally snow-covered alpine/subalpine basin in Rocky Mountain National Park, USA. The time series was mainly distinguished by an annual early spring peak in the concentrations of all REEs. REE concentrations at this time were as much as 8-fold greater than at other times of the year. This annual peak was coincident with an early spring peak in dissolved organic carbon (DOC) which results from flushing of soils at the beginning of spring snow melting. The REE/DOC peak occurs as discharge starts to increase from wintertime lows but well before the spring peak in discharge. Speciation considerations suggest complexation of the REEs by DOC. The Ce anomaly also increases (i.e., is less fractionated) during the spring flush indicating that the most reducing (or least oxidizing) REE sources in the system are comparatively more important at that time. Mn data and the La/Yb ratio also support this. The behavior of REEs in the Loch Vale system has additionally been compared with metal and DOC behavior in other systems. Hydrologic and climatic differences can be important especially with regard to timing and duration of the spring flush peak. Damping of hydrologic events in the lower floodplain of major rivers may also partially result in the differences observed between Loch Vale and the lower Mississippi River. However, comparison with the Amazon River system additionally suggests that seasonal flooding of wetlands may be an important regulator of REE concentrations. Chemical differences are also important for these systems. This includes pH and suspended matter concentrations which affect the balance between adsorption and complexation. Additionally, the relative complexing ability of DOC in different systems is a factor needing further consideration.

Organic matter diagenesis and bacterial contributions to detrital carbon and nitrogen in the Amazon River system

Amino sugars and amino acids, including the bacterial biomarkers muramic acid and D‐amino acids, were measured to investigate the diagenetic alterations of and microbial contributions to detrital organic matter in the Amazon River system. Three size fractions of detrital material were analyzed: coarse and fine particulate organic matter (CPOM and FPOM), and ultrafiltered dissolved organic matter (UDOM). CPOM was depleted in amino sugars and had high percentages of total N as amino acids (32‐37%), consistent with relatively fresh plant debris being a major source of CPOM. FPOM had the highest percentages of total C as amino acids (5‐8%), and its composition appeared to be influenced by the preferential sorption of N‐containing molecules. UDOM had the highest percentages of total N as amino sugars (up to 2.3%) indicating the relatively important contributions from microorganisms. A consistent trend of increasing proportions of D‐amino acids among detrital size fractions, CPOM < FPOM < UDOM, suggests this parameter is a useful diagenetic indicator. About 4‐17% of the C and 17‐37% of the N in FPOM and CPOM were of bacterial origin based on D‐alanine and D‐glutamic acid yields. Similar bacterial contributions to UDOM were evident from the biomarker data, but they were not quantified due to insufficient information about representative yields of source biomarkers. Relationships between biomarker N and total N suggest that microbial activity influences the N content of detrital material. Bacterial contributions to detrital material were primarily as cellular remnants rather than living cells.

The influence of historical and potential future deforestation on the stream flow of the Amazon River – Land surface processes and atmospheric feedbacks

In this study, results from two sets of numerical simulations are evaluated and presented; one with the land surface model IBIS forced with prescribed climate and another with the fully coupled atmospheric general circulation and land surface model CCM3-IBIS. The results illustrate the influence of historical and potential future deforestation on local evapotranspiration and discharge of the Amazon River system with and without atmospheric feedbacks and clarify a few important points about the impact of deforestation on the Amazon River. In the absence of a continental scale precipitation change, large-scale deforestation can have a significant impact on large river systems and appears to have already done so in the Tocantins and Araguaia Rivers, where discharge has increased 25% with little change in precipitation. However, with extensive deforestation (e.g. >30% of the Amazon basin) atmospheric feedbacks, brought about by differences in the physical structure of the crops and pasture replacing natural vegetation, cause water balance changes of the same order of magnitude as the changes due to local land surface processes, but of opposite sign. Additionally, changes in the water balance caused by atmospheric feedbacks are not limited to those basins where deforestation has occurred but are spread unevenly throughout the entire Amazon by atmospheric circulation. As a result, changes to discharge and aquatic environments with future deforestation of the Amazon will likely be significant and a complex function of how much vegetation has been removed from that particular watershed and how much has been removed from the entire Amazon Basin.

Simulating the surface waters of the Amazon River basin: impacts of new river geomorphic and flow parameterizations

AbstractThis paper describes the impacts of new river geomorphic and flow parameterizations on the simulated surface waters dynamics of the Amazon River basin. Three major improvements to a hydrologic model are presented: (1) the river flow velocity equation is expanded to be dependent on river sinuosity and friction in addition to gradient forces; (2) equations defining the morphological characteristics of the river, such as river height, width and bankfull volume, are derived from 31 622 measurements of river morphology and applied within the model; (3) 1 km resolution topographic data from the Shuttle Radar Topography Mission (SRTM) are used to provide physically based fractional flooding of grid cells from a statistical representation of sub‐grid‐scale floodplain morphology. The discharge and floodplain inundation of the Amazon River is simulated for the period 1968–1998, validated against observations, and compared with results from a previous version of the model. These modifications result in considerable improvement in the simulations of the hydrological features of the Amazon River system. The major impact is that the average wet‐season flooded area on the Amazon mainstem for the period 1983–1988 is now within 5% of satellite‐derived estimates of flooded area, whereas the previous model overestimates the flooded area by about 80%. The improvements are a consequence of the new empirical river geomorphologic functions and the SRTM topography. The new formulation of the flow velocity equation results in increased river velocity on the mainstem and major tributaries and a better correlation between the mean monthly simulated and observed discharge. Copyright © 2007 John Wiley & Sons, Ltd.

The riverscape of Western Amazonia – a quantitative approach to the fluvial biogeography of the region

AbstractAim To provide a quantitative spatial analysis of the riverscape (open‐water bodies and their surrounding areas) of the Western Amazonian lowlands using a consistent surface of remotely sensed imagery. Taking into account the essential significance of fluvial environments for the Amazonian biota, we propose that an enhanced understanding of the Amazonian riverscape will provide new insight for biogeographical studies in the region and contribute to the understanding of these megadiverse tropical lowlands.Location An area of 2.2 million km2 covering the Western Amazonian lowlands of the Andean foreland region, i.e. the upper reaches of the Amazon river system. Areas in Colombia, Venezuela, Ecuador, Peru, Brazil and Bolivia between longitudes 83 °W and 65 °W and latitudes 5 °N and 12 °S are included.Methods A mosaic of 120 Landsat TM satellite images was created with 100‐m resolution, and water areas of over 1 ha in size or c. 60 m in width were extracted using a simple ratio threshold applicable to a large set of data. With this method, 99.1% of the water areas present in 30‐m imagery were mapped with images with 100‐m resolution. Water pixels of distinct river segments were assigned to river classes on the basis of their channel properties, and islands and lakes were distinguished separately and classified. Measures of water patterns such as structure, composition, richness and remoteness were provided for various spatial units. Riverine corridors were computed from the open‐water mask by outer limits of active channels and floodplain lakes. Analytical results are shown as both thematic maps and statistics.Results A total of 1.1% of Western Amazonia is covered by open‐water bodies over 1 ha in size or 60 m in width. River‐bound waters comprise 98% of the total water surface. Whilst isolated lakes are scarce, river‐bound oxbow and backchannel lakes are plentiful, comprising 17.5% of all waters. They are particularly frequent along meandering channels, which dominate both in area and length. The riverine corridors including active channels and floodplain lakes cover 17% of the land area. The average distance from any point of land to the nearest water is 12 km. Geographically speaking, the distribution of waters is uneven across the region, and the detailed characteristics of the riverscape are geographically highly variable. Three major, fluvially distinct regions can be identified: central Western Amazonia, the south, and the north‐east. The proportional surface areas of the riverine corridors, numbers of lakes, sizes of islands and their distributions depend largely on the types and sizes of the rivers.Main conclusions Our results support the notion of Western Amazonia as a dynamic, highly fluvial environment, highlighting and quantifying considerable internal variation within the region in terms of fluvial patterns and the processes that they reflect and control. Biogeographically, the variety of types of fluvial environments and their characteristics are important constituents of what influences the distribution of species and dynamics of terrestrial habitats. Spatially consistent riverscape data can serve as a consistent and scalable source of relevant information for other biogeographical approaches in the region.

Floodplain lake fish assemblages in the Amazon River: directions in conservation biology

The Neotropical region is renowned for its high biodiversity, and the Amazon River basin contains the highest number of fish species of any river system in the world. In recent years, habitat fragmentation and exploitation of biotic resources have threatened biological integrity and provoked to need for sustainable management and conservation of the Amazon River system. We studied 36 floodplain lakes along 2000 km of the Amazon River. The fish assemblages associated with flood forests are moderately diverse, with low species dominance and reduced populations. To detect nestedness of fish assemblage composition in floodplain lakes, a nested subset analysis was performed on species presence–absence. The incidence matrix (species × lakes) was maximally packed using the Nestedness Temperature Calculator software. The results of ranking lakes and species allow us to establish targets for conservation. Such strategy for sustainable management should be focused on maintaining the Amazonian biodiversity.

Iron isotopes in the Amazon River system: Weathering and transport signatures

Trace metal clean plankton tows, river samples, aerosol leachates, and porewater samples from the Amazon River system were measured for their iron isotopic composition. The overall isotopic range observed was 1.5‰ with Fe isotopic variability observed between different types of tributaries. Dissolved Fe and suspended load Fe from two main channel sites were isotopically similar (≈ − 0.1 to − 0.3‰). In contrast, the dissolved Fe and suspended load Fe from an organic rich tributary, the Negro River, were isotopically distinct with the dissolved slightly isotopically heavy (+ 0.3‰) and the suspended load isotopically light (− 0.9‰). The isotopically lightest sample from the Amazon region was shelf porewater (− 1.3‰). In river water–seawater mixing experiments, the Fe isotopic signal of dissolved Fe of river water was modified by flocculation of isotopically heavy Fe. The variability of the Fe isotopic composition of natural samples including biological and aqueous samples demonstrates that significant fractionation is associated with Fe biogeochemistry in the environment.

The Pan-Amazonian Ucayali Peneplain, late Neogene sedimentation in Amazonia, and the birth of the modern Amazon River system

We review the Neogene geologic history of lowland Amazonia in an attempt to focus attention on areas of agreement, as well as areas in dispute, in this research arena. We reinterpret pre-existing hypotheses, present new data, and discuss new insights intended to support a unified synthesis of the Amazon Basin as a single sedimentary basin, albeit on a vast scale, during the late Miocene to middle late Pliocene. We document the Ucayali Peneplain as an isochronous, Pan-Amazonian geologic feature that formed following the early to mid-Miocene Quechua I orogenic phase of Andean tectonism. Peneplanation began possibly as early as ∼ 15 Ma and terminated abruptly near the beginning of the late Miocene Quechua II orogenic event at ∼ 9.5–9.0 Ma. Subsequently, a thin cover of sediments comprising the Madre de Dios Formation began blanketing most of lowland Amazonia, excepting only the eastern Subandean Fold-and-Thrust Belt and isolated highlands within the basin. The buried peneplain is readily observed in river cutbanks throughout Amazonia as the marked, often angular Ucayali Unconformity that separates eroded, older, often folded, faulted, and weathered, moderately to well consolidated Tertiary formations from unconsolidated, near horizontal, upper Neogene deposits. The dates of formation of major unconformities and subsequent depositional events at widely separated areas within the Andes of Bolivia, Ecuador, and Peru are coincident with that of the Ucayali Unconformity and deposition of the Madre de Dios Formation and suggest that the events are linked to a common cause, which is interpreted to be the still on-going collision between the South American and Nazca tectonic plates. The Madre de Dios Formation has three members, the oldest of which documents a short-lived, high energy depositional environment followed by a moderate-energy depositional environment, both occurring at a time when drainage from the basin was unobstructed. The upper two members record fluctuations between moderate and low energy continental depositional environments during a period when drainage from the basin was obstructed, disorganized, and took place over long distances with extremely low gradients. The sedimentology of the Madre de Dios Formation, particularly the thick, massive beds of clay, and the widespread presence of paleodeltas and associated geomorphic features on the Amazonian planato are consistent with the hypothesis that much of the upper two members formed as lacustrine and deltaic deposits within a gigantic lake, Lago Amazonas, or, more probably, within a complex series of interconnected mega-lakes that occasionally united to cover most or all of lowland Amazonia to a shallow depth from the latest Miocene until ∼ 2.5 Ma. The presence of the Ucayali Unconformity and the relatively uniform lithostratigraphy basin-wide of the fluvial, fluviolacustrine, and lacustrine sediments of the upper Neogene Madre de Dios Formation are consistent with the hypothesis that the Amazon Basin acted as a single, undivided sedimentary basin in the late Neogene. The biostratigraphic correlation across important modern drainage divides of both micro- and macro-sized, late Miocene fossil vertebrates recovered from basal conglomerates of the Madre de Dios Formation, and the absence therefrom of fossil vertebrates of any other age, is also consistent with this hypothesis. Two 40Ar/ 39Ar dates on ash deposits within the Madre de Dios Formation corroborate the upper Miocene age of the basal horizons of that formation indicated by fossil vertebrates and support an upper Pliocene age for the youngest sediments of the formation. The modern Amazon River drainage system was established in the late Pliocene, at ∼ 2.5 Ma, by the breaching of the eastern rim of the sedimentary basin as a result of the basin being overfilled, or by headward erosion of the lower Amazon River, or both. Data on Cenozoic mass accumulation rates and the chemistry of terrigenous sediments in the Atlantic Ocean obtained by Ocean Drilling Project Leg 154, Ceara Rise support the postulated timing of the establishment of the modern Amazon River drainage system at ∼ 2.5 Ma, rather than the long-held view that this event occurred in the late Miocene. We discuss the important role of ocean currents and sea level fluctuations on terrigenous mass accumulation rates on the Ceara Rise. The postulated time of formation of the modern Amazon River is nearly coincident with the onset of the Plio-Pleistocene glacial climatic regime and the lowest sea level stands since the latest middle Miocene. This analysis indicates that modern Amazonia is a product of terrain development within an erosional regime since ∼ 2.5 Ma.