Most large hydroelectric dams create massive reservoirs -- drowned valleys that displace communities and obliterate ecosystems. Santo Antonio was designed differently. As a run-of-the-river facility, the dam stands only 13.9 meters tall and stretches 3,100 meters across the Madeira, creating a reservoir of 271 square kilometers, of which 164 square kilometers was already the existing river channel. The distinction matters: instead of storing months of water behind a towering wall, Santo Antonio uses the river's natural flow, channeling it through 50 Kaplan-bulb turbines rated at 71.6 megawatts each. The original design called for 44 turbines, but the project was expanded in 2013 to 50. The first unit began commercial production in March 2012, and by December 2016 all turbines were online. Two spillways can handle a combined maximum discharge of 84,000 cubic meters per second -- a volume difficult to visualize until you stand beside the Madeira during flood season and watch cafe-au-lait water stretching to the horizon.

Brazil's 2001-2002 energy crisis -- a year of nationwide rationing caused by drought and underinvestment -- created political urgency for new generating capacity. Construction of both Santo Antonio and the Jirau Dam upstream was accelerated in 2009, with a combined estimated cost of $15.6 billion. Santo Antonio alone accounted for $7 billion. Critics argued that the approval process was rushed, that environmental and social criteria were rubber-stamped before all concerns from affected communities had been addressed. The Worldwatch Institute warned that fast-tracking the Madeira dams set a dangerous precedent, allowing projects to circumvent Brazil's environmental laws under the guise of eco-friendly run-of-the-river design. FUNAI, Brazil's indigenous protection agency, noted that uncontacted indigenous populations may exist in the region. Traditional fishermen and small-scale farmers along the Madeira reported increased flooding they attributed to the dam. The Serra dos Tres Irmaos Ecological Station was reduced from 89,847 hectares to 87,412 hectares to accommodate the reservoir.

Santo Antonio is one piece of a four-dam complex envisioned for the Madeira River. Santo Antonio and the 3,750-megawatt Jirau Dam, roughly 100 kilometers upstream, are both operational. Two additional dams -- one at Guayaramerin on the Brazil-Bolivia border, and another at Cachuela Esperanza inside Bolivia -- remain in planning stages. The complex is part of IIRSA, the Initiative for the Integration of the Regional Infrastructure of South America, a continent-wide effort to connect economies through new investments in transportation, energy, and communication. Every dam in the complex would include ship locks capable of raising and lowering oceangoing vessels, and if the full project is completed, the rapids that have blocked navigation on the Madeira for centuries would become navigable lakes. The vision is not new -- in 1851, the U.S. government commissioned Lieutenant Lardner Gibbon to study a rail link along these same rapids for access to Bolivian rubber markets. What Gibbon imagined as a railroad, and what the disastrous Madeira-Mamore Railway briefly achieved, the dams could accomplish permanently.

Santo Antonio features two fish ladders and a shipping lock, engineering concessions to the ecological and economic realities of damming one of the Amazon's largest tributaries. The environmental stakes are specific: if the fish ladders fail, several valuable migratory fish species could face near-extinction. Both Madeira dams include environmental remediation programs, and proponents point out that the run-of-the-river design avoids the massive reservoir flooding that made earlier Amazon dam projects so destructive. But the Madeira is not a laboratory experiment -- it is a living river system that supports fisheries, agriculture, and indigenous communities across multiple countries. Whether 50 turbines, two fish ladders, and a set of ship locks can coexist with that system is a question the river itself will answer over the coming decades. Most of the generated power flows southeast through the Rio Madeira HVDC system, the world's longest high-voltage direct current transmission line, linking the Amazon to the cities of Sao Paulo state 2,375 kilometers away.