Biomes do not stop at a line; they overlap, and the overlap is usually the richest ground. This park sits in the transition between the Amazon rainforest and the cerrado, Brazil's vast tropical savanna, and it collects species from both. Robust capuchins and howler monkeys work the forest canopy — Amazonian animals, at the southern edge of their range. Out on the open savanna the maned wolf hunts, long-legged and red as rust, and pampas deer graze. In the wetlands there are marsh deer and capybara. The rivers hold Amazon river dolphins, giant otters and turtles. Few places let you see a wolf of the dry grasslands and a dolphin of the flooded forest in the same landscape.

The park lies in the municipality of Novo Santo Antônio, hard against the borders of São Félix do Araguaia, Ribeirão Cascalheira and the state of Tocantins. It occupies the wedge between the Araguaia and its tributary the Rio das Mortes — the River of Deaths, a name given by colonists after the violence that played out along its banks between bandeirante expeditions, prospectors and the Indigenous peoples whose land it was. The Xavante know that river by their own name for it. Araguaia State Park forms the southernmost link in a chain of protected areas running up the right bank of the Araguaia, which means it functions less as an island than as a doorway: fauna can move north through connected habitat rather than dead-ending against fence line and soy.

Protection in Mato Grosso has never been evenly distributed, and the numbers say why. Since the 1970s, driven by federal and state tax incentives, enormous areas of Amazon forest and cerrado in this state have been converted to large-scale ranching and cropping. What survived was largely what nobody could farm easily — remote, inaccessible, or already flooded. The arithmetic is stark: roughly 83.77 percent of permanently flooded land in the region is protected, against just 3.87 percent of open treed savanna. Conservation here has often followed the path of least economic resistance. A park like this one, sitting on annually inundated ground, exists in part because the water made the argument for it.

Mato Grosso created the park by state law 7.517 on 28 September 2001, setting aside around 230,000 hectares to protect water resources, keep corridors open for native fauna, preserve a representative sample of the region's ecosystems, and allow controlled public use, education and research. It was classed as an IUCN category II protected area — a national park in all but name — and handed to the State Environment Foundation, FEMA, with a five-year deadline to produce a management plan. That deadline slipped. The park's boundaries were altered by law 8.458 in January 2006; the management plan appeared in 2007 and was finally approved in December 2008, seven years after the park was declared.

In June 2011 the state government published a law proposing an unusual swap. It would have moved the Xavante of the Marãiwatsédé Indigenous Territory into the state park, and in exchange regularised the holdings of the squatters occupying their land. The Xavante had been forced off Marãiwatsédé in 1966, when 233 people were flown hundreds of kilometres south to a mission; measles swept through them almost immediately, and within two weeks the group had been cut to around 160. They spent nearly half a century trying to get home. Funai, the federal Indigenous affairs agency, rejected the exchange outright — it had no interest in the trade, and Indigenous territory in Brazil is constitutionally inalienable and cannot be swapped away. In December 2012, following a Supreme Court ruling that October, courts ordered the illegal occupants removed and the Xavante returned to Marãiwatsédé. The park stayed a park, and the land stayed theirs.