Construction began in April 1982, in the final years of Brazil's military regime. Colonel Jorge Teixeira, governor of the federal territory of Rondonia, championed development at almost any cost. The dam was tied to POLONOROESTE, a World Bank-funded project that rebuilt and paved the BR-364 highway, opening Rondonia to a flood of migrants from Parana state. The road brought settlers, but it also brought devastation -- deforestation surged, and indigenous communities bore the brunt of the disruption. By 1987, World Bank President Barber Conable called the highway project "a sobering example of an environmentally sound effort which went wrong." Two hundred and thirty-eight farming families were relocated to make way for the reservoir. The first turbine spun on 24 July 1989; the last of five was installed on 2 August 1996. The final bill, excluding transmission lines, came to US$965 million.

The reservoir swallowed 560 square kilometers of land that was almost entirely tropical rainforest. Because the terrain around the Jamari River is relatively flat, engineers had to build 57 kilometers of dikes along both banks just to contain the water. The main lake stretches 25 kilometers long and up to 15 kilometers wide, with a southern arm extending another 15 kilometers. A well-publicized US$30 million animal rescue operation made headlines, but the effort saved only a fraction of the wildlife. The animals that were relocated to the lake's edge caused critical overpopulation in their new habitat. An exception to Brazilian law prohibiting log exports was carved out specifically for Samuel, and that loophole triggered a boom in illegal logging across the western Amazon basin. Satellite images comparing 1984 and 2011 reveal dramatic deforestation radiating outward from the dam in every direction.

The submerged forest decayed underwater, generating methane and carbon dioxide at rates that dwarfed the emissions of a comparable fossil fuel plant. Though the ratio has declined over time, the reservoir still emits roughly 2.6 times the greenhouse gases of its oil-burning equivalent. Meanwhile, the flooding triggered a chemical reaction that methylated mercury in the soil, contaminating fish throughout the reservoir. Fish diversity dropped after the dam was built, though certain species thrived in the new conditions -- tucunare, aracu comum, mapara, and the black piranha among them. The reservoir now supports intensive fishing that raises its own sustainability concerns, a far cry from the modest fishing that once took place near the river's mouth.

For all its environmental costs, the Samuel Dam did electrify Rondonia. By 1990 it provided 36 percent of the state's power; by 1996, 58 percent. Household connections tripled in seven years, from roughly 40,000 in 1983 to over 132,000 by 1990. In 2002, the grid extended to Rio Branco, the capital of neighboring Acre state. As of 2016, electricity reached 90 percent of Rondonia's municipalities. But the human cost lingered. In March 2016, protesters blockaded the BR-364 highway at Candeias do Jamari, demanding compensation that had still not been paid to families displaced decades earlier. The reservoir has found an unlikely second life as a recreational destination -- annual fishing tournaments draw competitors from three states -- but for the communities uprooted by its construction, the debt remains unsettled.

Engineers measure hydroelectric efficiency with a simple ratio: reservoir area divided by generating capacity. Lower is better. The Samuel Dam scores 2.59 square kilometers per megawatt -- among the worst in the Amazon, rivaled only by Curua-Una at 2.60 and the notorious Balbina Dam at 9.44. Its 216 megawatts of installed capacity required flooding an area larger than some European cities. To compensate for the lost forest, operator Eletronorte established the Samuel Ecological Station east of the reservoir, eventually expanding it to over 71,000 hectares. The station now covers a large swath of the reservoir itself and surrounding land. Whether this ecological offset balances the forest that was drowned, the mercury that was released, and the communities that were scattered is a question the numbers alone cannot answer.