Follow the northbound water and it runs into the Maranhão River, a tributary of the Tocantins, which crosses most of central Brazil before reaching the Atlantic beside the mouth of the Amazon — the Tocantins-Araguaia system is conventionally counted within the greater Amazon drainage, though it enters the sea through its own channel rather than the Amazon proper. Follow the southbound water and the trip is longer and stranger. The Brejinho joins the Fumal, which feeds the São Bartolomeu, which meets the Corumbá, which meets the Paranaíba, which becomes the Paraná — and the Paraná eventually delivers it to the Río de la Plata between Argentina and Uruguay. Two drops of rain landing a few hundred metres apart on the same flat sheet of cerrado end up separated by the width of a continent.

The divide is not always a divide. Between December and March the rains come hard, and because the terrain here is so flat, parts of the vereda simply flood. The two drainages briefly touch. Fish move across — species from the Tocantins basin and species from the Paraná basin mingling in the same shallow water before the wet season ends and the connection closes again. For biologists this is the whole point of the place. Very few spots on Earth let two of the world's great river systems exchange animals across a few centimetres of standing water, and fewer still are protected well enough to keep doing it.

The reserve was established in August 1968, when Brasília was eight years old and the northeast corner of the Federal District was still mostly empty. It began at 5,000 hectares and roughly doubled when the Mestre d'Armas lagoon — also known as the Lagoa Bonita — was folded in. In 1988 it was reclassified from biological reserve to ecological station, the strictest category Brazil applies, and in 1992 UNESCO named it one of the core areas of the Cerrado Biosphere Reserve on the strength of how intact its ecosystems remained. It now covers a little over 10,000 hectares in the administrative region of Planaltina. The public is not admitted: access is reserved for researchers and organised environmental education, with an information centre and auditorium handling the visitors it does receive.

What the fence protects is a nearly complete cross-section of a biome that has lost most of itself to soy and cattle. Open grassland gives way to scrub, to twisted low woodland, to dry gallery forest along the watercourses and flooded forest where the ground stays wet. Maned wolves work the open ground on their absurd long legs. Pampas deer, armadillos and anteaters share the same country. Toucans and parrots move through the gallery forest; caracaras patrol the fields; seriemas — long-legged, ground-running birds with a screaming call that carries for kilometres — stalk the grass. Almost none of this exists in the surrounding landscape any more.

Being an island has a price. Five roads cut across the station, and animals are killed on them regularly — a steady, unglamorous drain on populations that have nowhere else to go. Fire is the larger problem. The cerrado evolved with it, and the vegetation blackens in the dry months from May to October and comes back green when the rains return. But fires that used to be rare now arrive almost every year, driven by the growing population pressing in on all sides. A reserve created to sit quietly at the edge of a new capital has spent five decades watching that capital arrive at its fence line.