The reserve sits astride the Serra do Cachimbo, a geological complex that is partly mountainous, partly plateau, with flat-bottomed valleys cut by rivers that begin right here. Eleven percent of the reserve drains south into the Tapajós basin, forming the Cristalino and São Bento rivers. The remaining 89 percent drains north into the Xingu basin. The Água Fria, Flecha, Nilana, Ipiranga, Xixé, Curuaés, Curuá, and Iriri all have their headwaters in these hills. Every one of those rivers eventually joins the Xingu, which joins the Amazon. The northern escarpments of the Serra do Cachimbo drop sharply into the peripheral depression of southern Pará, and where the rivers leave the plateau, they form rapids and waterfalls like the Salto do Curuá. The reserve covers parts of the Altamira and Novo Progresso municipalities and shares borders with the Panará and Menkragnoti indigenous territories.

Cachimbo sits on the transition between Amazon rainforest and cerrado savanna, and that meeting point is exactly what makes it extraordinary. In the north the climate is humid tropical year-round; in the south, a tropical pattern with a dry season takes over. The flora reflects this boundary. Sub-montane rainforest, seasonal and alluvial forest, and open campinarana scrubland all meet within the reserve, along with patches of rupestre and buritizais vegetation. Average rainfall is 2,000 millimeters annually, temperatures averaging 22 degrees Celsius. The range of habitats within small distances supports biodiversity that pure Amazon or pure cerrado cannot match. Endangered carnivores here include giant otter, ocelot, oncilla, margay, and jaguar. The regionally endemic white-cheeked spider monkey holds a protected status. Giant anteaters and giant armadillos still walk the forest floor.

The bird list reaches 409 species, and the varied relief - mountains, plateau, valleys - means many of them are specialists rather than generalists. The white-banded mockingbird passes through from June to August on migration. Threatened species include the hyacinth macaw, whose brilliant cobalt plumage makes it the largest flying parrot in the world, as well as the Chaco eagle, red-necked aracari, and blue-winged macaw. Twenty-two species are regionally endemic, including the red-throated piping guan, snow-capped manakin, and masked tanager. The rarities list includes the Tapajós hermit hummingbird, the bald parrot, and the chorozinho-do-cachimbo - a recently discovered Herpsilochmus antbird. Among amphibians, five regionally endemic frog species live here, including the Brazil-nut poison frog, which lays eggs in the empty capsules of Brazil-nut trees. Reptiles include emerald tree boa, false coral snake, the pit viper Bothriopsis taeniata, and seven lizards including the striped forest whiptail.

The reserve's creation on May 20, 2005 was slow. At the time the decree was signed, squatters from Mato Grosso had already been occupying roughly 94 locations in the area for fifteen years. By 2015, about 200 families were living inside a reserve that is classed as IUCN category Ia - a strict nature reserve meant to have no direct human interference. Roads tell the physical history. A 923-kilometer network has been cut through the forest, most of it to haul out mahogany. Once the valuable trees were gone, remaining lumber was dismissed as low-value, and trees were felled to clear pasture. As of 2013, 26,402 hectares had been deforested, placing Cachimbo among the ten most deforested federal conservation units in the Amazon Legal. More than 10,000 hectares have been burned by settlers, and about six percent of the reserve has been converted to pasture. The BR-163 highway runs adjacent to the reserve's western edge, and everything that makes deforestation possible - access, markets, buyers - travels that road.

The political pressure has not eased. In August 2015, Senator Flexa Ribeiro of the Brazilian Social Democracy Party proposed splitting the reserve into a 162,000-hectare national park open to tourism and a 178,000-hectare Environmental Protection Area where residents could legally breed livestock and grow rice, bananas, pineapples, and coffee. The management plan was approved on September 4, 2009, and an advisory council was created on November 8, 2011. The reserve contributes to a much larger conservation mosaic in this corner of the Amazon - twelve sustainable use areas and six fully protected areas together covering over 14 million hectares. Whether the mosaic holds is an open question. The rivers that start in the Serra do Cachimbo have to run hundreds of kilometers through landscapes far less protected before reaching either the Xingu or the Tapajós. What happens here matters far beyond here.