The village calendar runs on water. From August through January, the dry season, the Tapajos retreats from its flood banks and the beaches appear. Sand that was underwater six months earlier now stretches for hundreds of meters, fine and pale and bordered by clear water that glows blue-green over the pale bottom. From February through July, the wet season, the water returns and floods the forest around the town. What was beach becomes igapo, a flooded forest canopy where you paddle a canoe among tree trunks and look up at leaves that are partially submerged. Wildlife surfaces. Pink river dolphins and tucuxis move through what were recently streets. The village adapts to both regimes because it always has. A visitor arriving in mid-January can catch both seasons in one trip: receding water in some places, rising in others, a landscape that redraws itself by the week.

The Portuguese navigator Pedro Teixeira established a settlement here on 6 March 1626. He was already famous, or would be; in 1637 he led the first upstream navigation of the full Amazon by a European, rowing from Belem to Quito and back. Alter do Chao was one of his early staging posts. The town was formally elevated 132 years later, on 6 March 1758, by the colonial governor of Grao-Para and Maranhao, Francisco Xavier de Mendonca Furtado, brother of the Marques de Pombal. The name is a tribute to the Portuguese village of Alter do Chao in the Alentejo, an Old World echo in New World geography. Before the Portuguese, the Borari people lived here. Their presence is still on the ground, as pottery shards, stone pipes, and polished axes that turn up in the sand between villages. Jesuit and Franciscan missions arrived in the 17th and 18th centuries, and the cult of Our Lady of Remedies took root as the patron saint of the settlement.

Every September, Alter do Chao fills with its oldest and largest festival, the Saire. It is the most traditional popular celebration in the Amazon rainforest, an accumulated compound of Catholic procession, indigenous ceremony, and local invention. There are formal processions in honor of the saints. There are Caiibo groups performing traditional songs and dances. And since the 1970s, the festival has absorbed a competition called the Festival dos Botos, a rivalry between two dolphin teams, the pink river dolphin Cor de Rosa and the gray tucuxi, each represented by a samba-school-style performance crew with costumes, allegories, and choreographed numbers. The crowd sides with one or the other. The format descends from the better-known Festival dos Bois of Parintins, across the state line in Amazonas, which fills a stadium with competing bull teams each year. In Alter do Chao, the competing totems are aquatic, reflecting the river that shapes the town.

In the early 20th century, Alter do Chao briefly mattered to the Amazon rubber trade. Latex tapped in the plantations at Belterra and Fordlandia, the Ford Motor Company's catastrophic attempt to build a rubber empire of its own, passed through on its way downriver. The Ford venture collapsed by 1945. The wider Amazon rubber economy collapsed with it, undercut by Southeast Asian plantations that had been seeded with Brazilian tree stock decades earlier. Alter do Chao went quiet again. The 1950s through the 1980s were hard. The village survived on fishing and cassava and a trickle of travelers curious enough to take the bus 37 kilometers out from Santarem. Then, starting in the 1990s, foreign and Brazilian tourism rediscovered the Tapajos beaches, and the economic trajectory reversed. Today the village is a tourism hub in the high season, with inns, tour operators, restaurants, and a concentration of travelers that would be unrecognizable to anyone who knew it in 1980. The ruins of that hardship economy, the abandoned rubber warehouses downriver and the overgrown plantations, still stand.

The iconic image of Alter do Chao is a sandbar called the Ilha do Amor, the Island of Love. It appears in the dry season as a crescent of white sand stretching out from the village shore into a shallow bay of the Tapajos. During peak low water, you can wade across, or be ferried over on small canoes run by villagers. The water is warm, tannic-tinted, and deep enough for swimming but shallow enough for walking out hundreds of meters. Behind the beach, green hills rise. Beyond the hills, the forest continues for thousands of kilometers. The island is the kind of place that earns freshwater-beach accolades in British newspapers and the kind of place whose whole existence depends on the river's annual breath. Every flood season, the Tapajos swallows the Ilha do Amor whole. Every dry season, the island returns.