Reconnaissance of the river began in 1964, carried out by the U.S. Bureau of Reclamation and USAID. Brazilian studies followed through the late 1960s, and by 1973 the military government of President Emílio Médici was being asked to fund a dam on the Tocantins. Two sites were on the table; the Santo Antônio option was ruled out in 1974, and later that year the Tucuruí project was approved under President Ernesto Geisel. The World Bank declined to finance it. Brazil built it anyway, with Eletronorte and domestic institutions carrying most of the cost and a smaller share coming from Canadian, European and American lenders. Engevix and Themag drew up the designs in 1975; Camargo Corrêa won the construction contract in 1976. Ground broke that same year, concrete pouring began in 1977, and the first phase was finished in 1984. A second phase, begun in 1998, ran until 2010.

Filling the reservoir meant emptying the valley. Between 25,000 and 35,000 people were removed from the future flood zone in the early 1980s. Around 14,000 were relocated by the government, and roughly 3,750 of those ended up on islands the rising water had just created — new land with no roads, no clinics, and no reliable way to reach either. Some communities were not relocated so much as erased. For the Parakanã, Asurini and Parkatêjê peoples, whose lands lay in the path of the reservoir and its transmission corridors, the loss was not only of houses. A study done before construction concluded that of the 1,500 tonnes of fish these communities depended on, 900 tonnes would become unreachable. The finding did not change the plan. Without political standing in Brazilian society at the time, the affected peoples had to rely on others to argue their case, and those arguments went largely unheard.

The Tocantins had held some 280 species of fish. Around 178 remain. Manatees and river dolphins, the pink Amazon boto among them, once moved freely along this stretch; they now face spillways and turbines nothing in their evolution prepared them for. Behind the wall, nutrient-loaded still water bred dense mats of aquatic plants and clouds of mosquitoes, and malaria rates climbed as construction drew tens of thousands of migrants into the region. Downstream, industrial discharge, farm runoff and untreated sewage degraded a watershed that people and animals had drunk from for as long as anyone could say. The dam's electricity, meanwhile, went mostly where it had always been intended to go: about 60 percent to industry, especially aluminium smelting, an energy-hungry business that supports just under 2,000 jobs.

Opposition to Tucuruí has been continuous since the project was announced, and it has been organised largely by the people the reservoir moved. In 2007 members of the Movement of People Affected by Dams, the Landless Workers' Movement and Via Campesina occupied the plant for two days, ending the action only after officials agreed to meet their representatives. Dilma Ferreira Silva became the movement's best-known voice in the region, a regional coordinator who spent years pressing an unwelcome argument: that hydropower marketed as clean energy had, in this valley, cost communities their homes, their fisheries and their health. On 22 March 2019 she was murdered. Her husband and a friend were killed alongside her.

The engineering is genuinely enormous. The main concrete gravity structure runs 6.9 kilometres; add the Mojú and Caraipé earth-fill dikes and the barrier stretches 12,515 metres across the valley. Its Creager-type service spillway, controlled by 20 floodgates, can pass 110,000 cubic metres per second — the second-largest spillway capacity in the world. The reservoir holds 45 cubic kilometres of water. Twelve 350-megawatt Francis turbines filled the first powerhouse, and navigation locks were built to lift river traffic between the upper and lower Tocantins. In 1985 the site turned up on cinema screens worldwide as a location in John Boorman's film The Emerald Forest, a story, as it happens, about a dam and the people the forest loses to it.