The Madeira and Tapajos do something that mountains do elsewhere: they isolate populations. The white-fronted capuchin and hairy saki live west of the Tapajos but not east of it. The white-nosed saki exists only on the eastern side. This pattern repeats across dozens of species. Because the rivers are wide, fast, and ancient, the genetic separation has had millions of years to deepen, producing an unusual concentration of range-limited species. The Santarem marmoset is endemic to the northern part of the ecoregion. Roosmalens' dwarf marmoset -- one of the smallest primates on Earth -- clings to a narrow strip along the eastern Madeira and the lower Aripuana. In the waterways themselves, Amazon river dolphins and tucuxi share channels with black caiman and Amazonian manatees.

At its northern edge, where the ecoregion meets the Amazon River, the land sits just 20 meters above sea level. Travel south and the terrain rises gradually toward the Parecis plateau, where table mountains reach 1,126 meters and the vegetation shifts from dense rainforest to rocky meadows and open savanna. Between these extremes, the ecoregion contains flooded varzea forest along the Madeira, dark-water igapo forest along the Tapajos and Aripuana, and broad stretches of campina -- white-sand grasslands unlike anything most people picture when they think of the Amazon. The canopy in the dense lowland areas stands around 30 meters tall, with emergent trees punching up to 45 meters. One of those trees is Theobroma cacao, the wild ancestor of cultivated chocolate, which reaches the eastern edge of its natural range here in the Tapajos basin.

The numbers are staggering: 183 mammal species including 90 species of bats, 621 species of birds, six species of macaws, and a roster of dangerous reptiles that includes fer-de-lance vipers, bushmasters, and boa constrictors. Endangered species include the white-cheeked spider monkey, the white-nosed saki, the giant otter, and the red-necked aracari. Yet the World Wildlife Fund classifies the entire ecoregion as Vulnerable. Between 2004 and 2011, habitat disappeared at an annual rate of 0.61 percent -- a figure that understates the damage in hotspots. Central Rondonia has already been stripped, its forests replaced by degraded pastureland. The BR-364 highway from Cuiaba to Porto Velho and the Trans-Amazonian Highway from Humaita to the middle Tapajos have opened corridors of destruction.

Along the banks of both the Tapajos and the Madeira, large-scale illegal mining has left visible scars -- muddy pits, mercury-contaminated waterways, and stripped riverbanks that look from the air like wounds in the forest. Conservation units exist on paper: the Amazonia National Park, the Pacaas Novos National Park, and dozens of smaller reserves dot the region. But many are underfunded and poorly enforced, their boundaries challenged by loggers, miners, and settlers. The ecoregion's southwestern edge adjoins the Beni savanna in Bolivia, adding a cross-border dimension to conservation challenges. What remains is still extraordinary -- a place where the sheer scale of the forest can still swallow the horizon. But that scale shrinks a little more each year.