Aldose, amino acid, and elemental compositions were determined for flux‐weighted samples of coarse (>63 µm) and fine (1,000 Daltons) dissolved organic matter collected at three sites along the Brazilian Amazon River and six of its major tributaries. Concentrations of total organic C (TOC) were relatively uniform (550±100 µM) at all sites, with DOC comprising the major (50–100%) component. An average of 77% of the total DOC was isolated by ultrafiltration.The greatest compositional differences observed in the Amazon River system were among the coarse, fine, and dissolved organic fractions. All coarse particulate fractions were nitrogen‐poor (atomic C : N = 21) and exhibited stable carbon isotope, aldose, and amino acid compositions similar to those of angiosperm tree leaves. Coarse particulate organic materials, although the least degraded of the three fractions, had lost appreciable carbohydrate and had immobilized excess nitrogen of apparent bacterial origin. Fine particulate materials were more nitrogen‐rich (C : N = 9) than coarse counterparts and had lower total aldose yields and glucose percentages. Fine particles gave greater total yields of amino acids, characterized by high ratios of basic vs. acidic components. Coexisting dissolved organic materials recovered by ultrafiltration were nitrogen‐poor (C : N = 27–52) and yielded the lowest amounts of aldoses, among which deoxy sugars were concentrated. Dissolved fractions gave extremely low yields of amino acids in mixtures that were enriched in nonprotein components and in acidic vs. basic molecules. These yield and composition patterns are consistent with a “regional chromatography” model in which highly degraded leaf material is solubilized and then partitioned between soil minerals and water during transport to the river, resulting in suspended fine particulate organic materials of soil origin that are nitrogen‐rich and coexisting dissolved organic substances that are nitrogen‐poor.

Depth‐integrated, discharge‐weighted water samples were collected over 1,800 km of the Amazon River on eight cruises at different stages of the hydrograph, 1982–1984. Fine (FPOC, 63 µm) particulate organic carbon as weight percentage of suspended sediment varied between 0.9–1.5% for FPOC and 0.5–3.4% for CPOC. Concentrations of FPOC ranged from 5 mg liter⁻¹ upriver to 2 mg liter⁻¹ downriver in the mainstem and from 6 mg liter⁻¹ in the Rio Madeira to <1 in the Rio Negro. CPOC had similar distribution patterns, but with concentrations 15–30% those of FPOC. Dissolved organic carbon (DOC) averaged 4–6 mg liter⁻¹ in the mainstem and up to 12 mg liter⁻¹ in the Rio Negro. Upriver dissolved inorganic carbon (DIC) concentrations of about 1,200 µM were diluted by tributaries and floodplain drainage to 600 µM at the most downriver site. Evasion of CO₂, invasion of O₂, and in situ oxidation were of comparable magnitude, 3–8 µmol m⁻² s⁻¹.The average export of total organic carbon (TOC) was 36.1 Tg yr⁻¹ (8.5 g m⁻² yr⁻¹), of which 62% was DOC, 34% was FPOC, and 4% was CPOC. TOC inputs were insufficient to support in situ oxidation by a factor of at least two. A relatively small, rapidly cycling pool of labile organic matter may coexist with a much larger pool of more refractory material.

Increased energy demand has led to plans for building many new dams in the western Amazon, mostly in the Andean region. Historical data and mechanistic scenarios are used to examine potential impacts above and below six of the largest dams planned for the region, including reductions in downstream sediment and nutrient supplies, changes in downstream flood pulse, changes in upstream and downstream fish yields, reservoir siltation, greenhouse gas emissions and mercury contamination. Together, these six dams are predicted to reduce the supply of sediments, phosphorus and nitrogen from the Andean region by 69, 67 and 57% and to the entire Amazon basin by 64, 51 and 23%, respectively. These large reductions in sediment and nutrient supplies will have major impacts on channel geomorphology, floodplain fertility and aquatic productivity. These effects will be greatest near the dams and extend to the lowland floodplains. Attenuation of the downstream flood pulse is expected to alter the survival, phenology and growth of floodplain vegetation and reduce fish yields below the dams. Reservoir filling times due to siltation are predicted to vary from 106–6240 years, affecting the storage performance of some dams. Total CO₂ equivalent carbon emission from 4 Andean dams was expected to average 10 Tg y^-1 during the first 30 years of operation, resulting in a MegaWatt weighted Carbon Emission Factor of 0.139 tons C MWhr^-1. Mercury contamination in fish and local human populations is expected to increase both above and below the dams creating significant health risks. Reservoir fish yields will compensate some downstream losses, but increased mercury contamination could offset these benefits.

This paper presents the first application and validation of a 2D hydrodynamic model of the Amazon at a large spatial scale. The simulation results suggest that a significantly higher proportion of total flow is routed through the floodplain than previously thought. We use the hydrodynamic model LISFLOOD‐FP with topographic data from the Shuttle Radar Topography Mission to predict floodplain inundation for a 240 × 125 km section of the central Amazon floodplain in Brazil and compare our results to satellite‐derived estimates of inundation extent, existing gauged data and satellite altimetry. We find that model accuracy is good at high water (72% spatial fit; 0.99 m root mean square error in water stage heights), while accuracy drops at low water (23%; 3.17 m) due to incomplete drainage of the floodplain resulting from errors in topographic data and omission of floodplain hydrologic processes from this initial model.