San Martín divides itself into four distinct zones that together cover just about every kind of terrain the Andes-Amazon transition produces. In the west, the eastern slope of the Andean plateau falls steeply in ravines and rough topography. Next, wide valleys open with stepped terraces along the Huallaga and its tributaries - the agricultural heart of the region, where cattle and farms have replaced forest. To the southwest, the Cordillera Azul descends with lower elevation, creating a dramatic canyon called Cajón de Sión and ending in the Cayumba rapids. Finally, a small patch of low-lying jungle floods easily when the rivers rise. Aircraft crossing this region see every transition visible at once: snow-capped Andean peaks to the west, green valleys in the middle distance, and unbroken forest to the east where the Huallaga becomes part of the Amazon system.

The conventional picture of the Inca Empire is mountain civilization. But the Inca Tupac Yupanqui pushed east into the montaña and absorbed Moyobamba - then known by its indigenous name, Muyupampa - into his dominion. In 1539, the Spaniard Alonso de Alvarado reached the Mayo River and founded Santiago de los Valles de Moyobamba, which eventually became the capital of Maynas province covering enormous stretches of what is now northeastern Peru. Between 1787 and 1790, a priest named Manuel Sobreviela accomplished something no Spaniard had managed: a full traverse of the Huallaga, documented in a map titled Planning for the course of the Huallaga and Ucayali Rivers and for the Pampa del Sacramento. Before modern roads, the rivers were the only way in or out. Missions, traders, and rubber tappers all followed the water.

San Martín is home to the San Martín titi monkey, known locally as mono tocón, scientific name Callicebus oenanthe. The animal is endemic: it exists nowhere else on Earth. It is also, according to the IUCN, critically endangered - one of the most threatened primate species in Peru. Conservation International Peru maintains an office in Rioja and leads the Alto Mayo Protected Forest Conservation Initiative. The Alto Mayo forest is one of the last places where the titi hangs on, alongside the spectacled bear and several dozen endemic bird species. The pressures are familiar: deforestation for coffee, cattle, and coca over the last half-century has fragmented the small ranges these animals need. Whether the mono tocón survives depends on choices being made right now, in towns most travelers have never heard of.

On the border with La Libertad, in the cloud forests above the Marañón and Huallaga, sits Gran Pajatén - a pre-Inca complex of circular slate buildings decorated with carved condors and human figures. Twenty-six circular structures cluster on terraces cut from a ridge. Access requires a multi-day trek through Río Abiseo National Park, and since the 1960s - when a government helicopter landing destroyed much of the protective vegetation - Peru's government has prohibited tourism to protect the fragile ruins. Most people learn about Gran Pajatén through photographs they will never duplicate, or descriptions by the handful of archaeologists who have permits to work there. The site is attributed to the Chachapoyas culture, whose territory extended into what is now Amazonas department. They built in stone high above the jungle, a society that chose elevation over rivers, mist over lowland heat. Gran Pajatén sits as they left it, slowly being reclaimed by the clouds.

The department is divided into ten provinces - Bellavista, El Dorado, Huallaga, Lamas, Mariscal Cáceres, Moyobamba, Picota, Rioja, San Martín, and Tocache - covering 78 districts in total. Lamas, a short drive from Tarapoto, preserves a Quechua Lamista cultural enclave descended from Chanka refugees who fled the Inca. Juanjuí, capital of Mariscal Cáceres, provides the jumping-off point for expeditions into Río Abiseo. Tarapoto itself has grown into a noisy, humid commercial hub - three-wheeled motokars darting between waterfalls and ayahuasca retreats, tourism pushing outward into places like the Lamas highlands and Laguna Azul. The highway that connects San Martín to the coast transformed the economy in recent decades. Before it arrived, the rivers were everything, and the Huallaga still remembers being the only road.