An increasing demand for electrical power can be observed in the Amazon region. Although hydropower is the main source of energy production, only a few reservoirs have been built. In view of the significant energy deficit, the construction of new reservoirs certainly merits serious consideration. However, reservoir construction has significant impacts on natural water systems. The environmental effects of reservoirs in the northern hemisphere have been well-documented but little is known about those in the Amazon area. Therefore, we studied Curuá-Una, a 23 year-old reservoir in the Amazon region, to evaluate developments in the catchment area and aquatic ecosystem, as well as dam use. The long-range environmental effects of the reservoir included deforestation in the watershed, erosion and the introduction of new land and water-borne diseases. Human migration and settlement was another significant consequence. New results of soil damage analysis are presented, illustrating a large export of iron and mercury from the system. The dam had several detrimental effects on the aquatic ecosystem, including enhanced eutrophication, inhibited decay of inundated vegetation and increased iron corrosion of electromechanical equipment related to the accumulation of iron in the water. The main downstream effect of the dam was the prevention of fish migration.