Walk the crest of the dam and there is nothing mechanical to look at. Fifteen hundred metres of earth fill stretch away under your boots, roughly twelve million cubic metres of compacted material wrapped around a clay core, and the only hardware in sight is the spillway: five steel gates, each 15 metres wide and 20.4 metres tall, able to pass 15,000 cubic metres of water every second if the Tocantins ever demands it. The turbines are somewhere beneath the hill. Engineers hollowed a cavern out of the rock, 137 metres long and nearly 30 wide, with a ceiling reaching almost 70 metres at its highest point, and set three Francis units inside it. Together they are rated at 1,275 megawatts. It is a curious kind of monument, a power station that leaves almost no mark on the skyline, its noise absorbed entirely by the ground.

Eletrobras Furnas began studying the upper Tocantins in 1981, looking for places to put two large dams. Serra da Mesa, then called Sao Felix, had the geology the engineers wanted, and construction started in 1986. On 24 October 1996 the gates closed and the river began backing up into the valleys behind them. It took until 1998 for the reservoir to fill and for the generators to come fully online. What Furnas really built here was less a power plant than a savings account. Of the 54.4 billion cubic metres impounded, 43.25 billion is active storage, water that can be released on demand. That reserve does more work downstream than it does at Serra da Mesa itself, steadying the flow through Cana Brava just below and, more than a thousand kilometres north, through Tucurui in Para. One reservoir in Goias helps set the rhythm of the entire Tocantins.

Filling a reservoir 1,784 square kilometres across is not a quick operation, and for most of two years the water simply kept climbing. Farms, riverside settlements and rural districts near Niquelandia went under a metre at a time. Thousands of people had to move. Some of them were leaving houses their grandparents had built, land they had cleared themselves, small businesses whose customers lived along a stretch of river that no longer exists. The public record of who they were is thin, which is its own kind of loss. What replaced them is a landscape people now travel to on purpose. The lake turned out to be superb fishing water, full of tucunare, piau, corvina and traira, and a small industry of lodges, guides and boat ramps has grown along shorelines that used to be hillsides.

The nearest town has spent its whole modern life attached to whatever is being extracted nearby. Minacu, home to fewer than thirty thousand people, sits above the Cana Brava reservoir a short way downstream and takes its name and its payroll from the Cana Brava mine, one of the largest chrysotile asbestos operations in the world. Brazil's Supreme Court ended the domestic use of asbestos in 2017; Goias lawmakers responded in 2019 by allowing the mine to keep producing for export, and everything it digs now leaves the country. The town's population has been shrinking since 1996, when the dam works wound down. Then, in 2024, a rare earths operation opened at the Pela Ema deposit about 30 kilometres away, and Minacu found itself at the front of another commodity story. The lake stays. The reasons for being here keep changing.