SummaryMore than 100 hydropower dams have already been built in the Amazon basin and numerous proposals for further dam constructions are under consideration. The accumulated negative environmental effects of built and proposed dams, if constructed, will trigger massive hydrophysical and biotic disturbances that will impact the Amazon basin's floodplains, estuary, and sediment plume. By introducing a Dam Environmental Vulnerability Index (DEVI) we quantify the current and potential impacts of dams in the basin. The scale of foreseeable environmental degradation indicates the need for collective action among nations and states to avoid cumulative, far-field impacts. We suggest institutional innovations to assess and avoid the likely impoverishment of Amazon rivers.

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This paper presents an overview of tropical river systems around the world and identifies major knowledge gaps. We focus particularly on the rivers draining the wet and wet-dry tropics with annual rainfall of more than 700 mm/year. The size of the analyzed river basins varies from 10 4 to 6 Â 10 6 km 2 . The tropical rivers across the globe drain a variety of geologicgeomorphologic settings: (a) orogenic mountains belts, (b) sedimentary and basaltic plateau/platforms, (c) cratonic areas, (d) lowland plains in sedimentary basins and (e) mixed terrain. All of them show clearly high but variable peak discharges during the rainy season and a period of low flow when rainfall decreases. Some tropical rivers show two flood peaks, a principal and a secondary one, during the year. We computed the intensity of floods and discharge variability in tropical rivers. The relationship between sediment yield and average water discharge for orogenic continental rivers of South America and Asia was also plotted. Insular Asian rivers show lower values of sediment yield related to mean annual discharge than continental orogenic rivers of Asia and South America. Rivers draining platforms or cratonic areas in savanna and wet tropical climates are characterized by low sediment yields. Tropical rivers exhibit a large variety of channel form. In most cases, and particularly in large basins, rivers exhibit a transition from one form to another so that traditional definitions of straight, meandering and braided may be difficult to apply. In general, it is more useful to apply the terminology of single and multi-channel systems or complex anabranching systems at least for selected regional segments.Present-day knowledge of tropical systems and its potential application to improve interpretation of older alluvial sequences and facies models are briefly discussed. Human impact and river management issues including land use changes, mining, dams, interbasin water transference as well as flood hazards are some of the daunting problems in tropical river basins today. D

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Deforestation changes the hydrological, geomorphological, and biochemical states of streams by decreasing evapotranspiration on the land surface and increasing runoff, river discharge, erosion and sediment fluxes from the land surface. Deforestation has removed about 55% of the native vegetation and significantly altered the hydrological and morphological characteristics of an 82,632 km 2 watershed of the Araguaia River in east-central Brazil. Observed discharge increased by 25% from the 1970s to the 1990s and computer simulations suggest that about 2/3 of the increase is from deforestation, the remaining 1/3 from climate variability. Changes of this scale are likely occurring throughout the 2,000,000 km 2 savannah region of central Brazil.

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