A watershed is an invisible ridge that determines which ocean a raindrop reaches. In Altamira National Forest, the ridge divides the Tapajos basin from the Xingu basin - two of the Amazon's largest tributaries, running parallel for hundreds of kilometers before both empty into the main river near Santarem. The forest holds part of the Curuaes River, which flows into the Xingu system. The municipalities it crosses - Altamira, Itaituba, and Trairao - are sprawling Amazon units the size of small European countries, and Altamira National Forest occupies a chunk of all three. You can walk for days in any direction without encountering a road. The land is almost flat, the canopy is continuous, and the water tables are high enough to turn parts of the forest into wetland during the rainy season.

The forest was created by presidential decree on February 2, 1998, under Brazil's then-new category of protected areas that permit economic use. It is administered by the Chico Mendes Institute for Biodiversity Conservation, the federal agency named after the rubber tapper whose 1988 murder by ranchers turned him into a global symbol of the fight over the Amazon. The 2009 annual plan allocated 380,000 hectares - more than half the forest - to two private companies under forty-year contracts. The deal promised roughly 900 formal jobs, 80 percent of them local, in exchange for selective logging done according to management plans that specify which trees come down and which stay. Whether any of that works over the long term depends on whether the monitoring holds.

Rapid ecological surveys found 212 species of flora across 145 genera - not the most species-dense forest in the Amazon, but a cross-section of what the upper Xingu-Tapajos ecotone looks like. The primates of interest to researchers are two endemics: the white-cheeked spider monkey, Ateles marginatus, which moves in small groups through the canopy, and the white marmoset, Mico leucippe, one of the smallest monkeys in the Amazon. The rivers hold endemic fish found nowhere else - Aspidoras poecilus, Microschemobrycon elongatus, Harttia dissidens, Hopliancistrus tricornis - species whose names sound like taxonomy and whose absence from the forest would mean extinction, because they live here and nowhere else.

Three Amazonian icons patrol the protected species list. Jaguars, Panthera onca, the largest cat in the Americas, move through the forest floor mostly at night, hunting capybaras, peccaries, and caimans. Giant otters, Pteronura brasiliensis, live in river family groups of six or seven - vocal, noisy, social animals that can grow to almost two meters long and whose presence usually indicates a healthy fish population. Amazonian manatees, Trichechus inunguis, are the freshwater cousins of their coastal relatives, slow grazers in the slow water of oxbow lakes. All three are what biologists call indicator species: when they thrive, the forest around them is working. When they vanish, you know the forest is hollowing out before you see it.

Altamira National Forest does not stand alone. It sits inside a mosaic of twelve sustainable-use areas and six fully protected ones, covering in total more than fourteen million hectares of adjacent forest. The fully protected neighbors include Amazonia, Jamanxim, Rio Novo, and Serra do Pardo national parks, plus the Nascentes da Serra do Cachimbo Biological Reserve and the Terra do Meio Ecological Station. The sustainable-use zone adds the Tapajos environmental protection area and the Altamira, Amana, Jamanxim, Trairao, Itaituba I, Itaituba II, and Tapajos national forests. From altitude, it looks like one continuous canopy - a green expanse interrupted only by rivers, occasional clearings, and the pale scar of BR-163 where it cuts north-south through the mosaic, bringing soybean trucks within reach of forest that was meant to be just out of range.