The Tapajos does more than cut the park in half. It functions as a biogeographic barrier. Some species on the western bank never cross to the east, and some on the east never move west. Biologists have documented this pattern across Amazon rivers, where genetic isolation between banks produces close cousins that have diverged over millions of years into distinct species of birds, primates and frogs. The upper Tapajos is unusual in the Amazon basin for its areas of white-sand grassland beside the river, patches of nutrient-poor soil where a specialized flora adapted to the extreme conditions. Elsewhere the park is dense lowland rainforest, the classic multi-tiered cathedral of green with the largest emergents reaching 50 meters into the canopy.

Federal Decree 73.683 established the park on February 19, 1974, at the height of the Brazilian military government's push to open and occupy the Amazon. Its boundaries were altered in 1985, 2006 and 2011. In 2012, Law 12678 redrew the limits of the Amazonia, Campos Amazonicos and Mapinguari national parks along with several national forests and a protected area, reducing almost all of them to accommodate the Belo Monte and Teles Pires hydroelectric complexes. Amazonia lost area and ended up at its current 1,070,737 hectares. The management plan was drafted in 1988 but never made official. A consultative council was finally created in 2004, three decades after the park itself. The halting institutional history reflects the larger story of Brazilian Amazon conservation: ambitious creation, grudging enforcement, piecemeal reduction.

About 98 percent of the park is dense humid tropical forest, its uniform canopy letting enough filtered light through to sustain a rich understory of vines, lichens, mosses and orchids. Many of the mammals are nocturnal, padding unseen through the gloom, including jaguar and ocelot. Several of the park's most charismatic species are endangered: the giant otter, which hunts in noisy family groups along blackwater lakes; the Amazonian manatee, a ponderous gray vegetarian drifting through flooded forest; and the giant anteater, its huge front claws shredding termite mounds. The rivers themselves teem with fish, reptiles and aquatic invertebrates. The Pau-Rosa National Forest adjoins the park to the west, and both are proposed components of the South Amazon Ecological Corridor, a still-theoretical green seam meant to link protected areas across thousands of kilometers.

Itaituba is the gateway. The first park base lies about 53 kilometers out. The Transamazonian highway runs east to west through the area, paved as far as Itaituba and then reverting to unpaved earth roads topped with gravel. Rainy season, November through April, turns sections to mud that needs repair every May and June. Many visitors arrive instead from Santarem, the city where the Tapajos meets the Amazon. Santarem has an airport and regular flights from Manaus and Belem. Once in the region, travel usually means a mix of four-wheel drive vehicles and boat, because the river remains the truest road here. The park receives few tourists compared to its size, and facilities are minimal. This is not a manicured national park experience. It is rainforest that has barely been curated.