“Just what the doctor ordered,” Rob assured me. “A river trip is nonprescription medicine with no side effects.”

In January 2012, I was standing next to Rocky Contos on the sodden banks of the raging Usumacinta River, the “Sacred Monkey River,” a name with some controversy and blending of Aztec and Mayan origins.

“Sacred Monkey,” Rocky said in his distinctively high-pitched, tightly nasal twang, “refers to the howler monkey’s sacred status in Mayan culture. And also to this jungle area, with the greatest density of primates in all of Central America.”

“Actually, there’s a howler in the tree right above your head!” Rob added.

I could hear its raspy growl and the thrashing branches and leaves as the monkey leaped around. I could also hear the rapid-fire motor drive on Rob’s camera zinging as he clicked away.

Rocky had agreed to take us down through the heart of the Mayan region, in the Mexican state of Chiapas, a densely jungled and vast rain forest wilderness featuring numerous Class II and III rapids, gigantic waves, sprawling beaches, and major archaeological sites. The Usumacinta, or Usu for short, had earned a reputation as one of the best river trips in the world but also one of the most dangerous. In the 1970s, the Usu had been a favorite winter destination for Grand Canyon guides and river aficionados, but then in the 1980s and ’90s, spurred by a simmering Guatemalan civil war and the Zapatista movement in southern Mexico, the banks of the river teemed with armed militants who routinely robbed boaters. Word of the dangers got out, and rather than be held up by machine gun or machete point, tourists stayed away. Rocky tried to explain that the eighty-eight-mile stretch we intended to run, paralleling and partly forming the Mexico-Guatemala border, wasn’t nearly as dangerous as it used to be.

“When I ran it solo two years ago, I was, to my knowledge, the first person to paddle it in a decade,” he said. “I did end up getting pulled over by armed men,” Rocky laughed, “but they were Guatemalan army, not banditos. Guatemalan soldiers actually police the region and help serve as caretakers of all the Mayan ruins along the river.”