A megariver under threat : the construction and environmental impacts of dams on the Madeira River in Brazil, in context of the fluvial geomorphology

The Madeira River in Brazil is the fifth largest river in the world in terms of water discharge and is the largest contributor of sediment to the Amazon River. The Madeira River Hydroelectric Complex (MRHC), currently under construction, poses a threat to this fluvial system and to knowledge of the way in which this unique megariver functions in context of the fluvial geomorphology. Following a two-pronged approach, this research presents an assessment of the impacts of the Santo Antonio and Jirau Dams, which are part of the MRHC, and an overview of the fluvial geomorphologic environment of the Madeira River. The perceived environmental impacts of the dams differ between the dam proponents, which include the construction companies and the Brazilian federal government, and scientific critics and environmental non-governmental organizations. These opposing viewpoints are analyzed with the objective of synthesizing these findings into a comparative evaluation of prioritized impacts. The implementation of these dams is controversial and wrought with political and scientific conflict over the licensing procedure for the dams' construction and the anticipated magnitude and nature of the resulting environmental impacts. The geomorphologic overview contributes to knowledge of this understudied large fluvial system and contextualizes the anticipated environmental impacts in terms of the geomorphologic environment. Channel patterns downstream of the Santo Antonio Dam are a primary focus of this geomorphologic overview, with special attention given to the channel and hydraulic characteristics that generate anabranching channel structures, which are unique to megarivers. This reach of the river has not been evaluated for environmental impacts in official reports, and this study provides a foundation for further analysis of changes in channel morphology due to dam operations downstream of the MRHC.