Midway between Jericoacoara and the city of São Luís, the Parnaíba Delta is an unspoiled ecosystem of rivers, mangrove swamps, islands, beaches, and sand dunes. It has the size and majesty of the Mississippi Delta without the urban infrastructure to foul it up. Senhor Ramos dropped us at its eastern edge. He would meet us later on the other side.

We picked up the boat—the equivalent of a small Boston Whaler, with an outboard motor and a canopy to shield us from the furnace sun—in a small riverside village called Araioses. The pilot steered us in and out among the infinite labyrinth of emerald channels. Two men in a dugout canoe were foraging for açaí, an Amazonian palm prized for its antioxidant fruit. Jungle vines closed in, trailing vegetation in the water lilies along the shore. The mud was watery like gruel, and the water was muddy like pea soup. Fish rose in lazy circles. The channel became narrower and narrower, until the boat was whispering among the shallow reeds. I waited for the anaconda to drop on my neck. Then we ran aground.

The river was a river no longer. The motor was switched off. The pilot handed us a line; we stripped to our shorts, hopped overboard, and began to drag our vessel downstream. Some people might find wading in tropical slop, with who knows what sluicing between bare toes, unnerving. The human male will do anything if someone else does it first. After a few hundred yards of wading and hauling, the stream grew deeper once again, and we hopped back in the boat.

Toward noon, halfway through our delta traverse, we rounded a wide sweeping bend. Suddenly, rising overhead behind the deep jungle canopy on the bank was a golden sand formation much larger than a dune. The sand glacier reached the shore, sloping into the water as if it, too, were thirsty and hot. Between the blue water and the long smooth golden wall was a one-hut hamlet, with a thin smoke column rising from a cooking fire. A man and a woman squatted near it, intently focused on mending their fishing nets. One child romped along the shore with his dog. With the ominous power of the marching motionless sand behind it, the scene looked like a poster designed to warn of global warming. It was a visible parable, concise and precise: Those who have eyes, let them see.

Still shaken by this close encounter, we pulled ashore for lunch at the delta version of a voyagers' rest stop: A thatched roof shaded a few benches and tables. There was a spigot for drinking water and a canteen. As we stretched our legs, I noticed a monkey chained to a stake in the yard. One of his relatives, still enjoying his freedom, was crouched on a low bough directly overhead, deconstructing a coconut with his slender fingers and sharp teeth. I sat with the two of them: three primates eating lunch.

I shared my sandwich with the prisoner, all the while entranced by the dexterity and focus of the one who was free. He worked with a quiet frenzy at his task, deliberate as he peeled the tough bark to get at the pure white meat inside. The drive, intensity, and even the malice in his energy reminded me of jungle behavior back home. He was only a few feet away, and the frankness of his gaze was unnerving. I was looking in a fur mirror, where intelligence and malevolence intertwined. When we finished our meal, I bowed to the monkey in the tree and climbed back into the boat.