Barreirinhas sits on the right bank of the Rio Preguiças - the 'Sloths' River,' named for its languid current - about 40 kilometers from where the river meets the Atlantic at Atins. The riverside boardwalk is the town's social spine: stalls and bars spread along it in the evenings, families gather under streetlamps, and boats of every size are tied to railings and trees. Fishermen unload the day's catch at small docks. Teenagers jump from the walls into the brown, slow-moving water. The opposite bank is a rising dune face, bright white in midday sun, a physical preview of the national park that draws everyone here. Beyond the immediate downtown, the streets turn to sand again - a civic layout that acknowledges, from the ground up, that dune country begins right here.

Virtually every tour into Lençóis Maranhenses National Park leaves from Barreirinhas. The park itself, established in 1981, protects 155,000 hectares of white sand dunes - a landscape sometimes compared to the Sahara but radically different, because between the dunes, during and after the rainy season, rainwater collects in lagoons that glow turquoise, green, and cobalt blue. Tours to the two most popular lagoon complexes, Lagoa Bonita and Lagoa Azul, were running 25 to 30 pickup trucks a day at peak in recent years. The trucks climb sand roads, ford shallow streams, and drop visitors on top of dunes where they can walk down to the water, swim in rain-filled lagoons the size of small lakes, and then climb back up to photograph the landscape before the light goes flat. Getting around inside the park is only possible in off-road vehicles; even those struggle on steep sand.

Barreirinhas is remote, but less remote than it once was. The road from São Luís - four and a half hours by bus from the state capital's bus terminal, with four daily departures on Cisne Branco's schedule and shared vans filling the gaps - is the main artery. The airport in São Luís is the real entry point for most international travelers; shared vans in yellow polo shirts pick visitors up right off flights from São Paulo that land between 2 and 4 in the morning. Since late 2024, Azul has operated flights to Barreirinhas's small airstrip from Fortaleza, Jericoacoara, and Parnaíba on Thursdays, Saturdays, and Sundays. From Jericoacoara, the overland four-wheel-drive trip takes about seven hours along an improved road through Paulino Neves. From Tutóia in the east, buses make the two-hour run twice daily. The logistics are part of the adventure.

For those bound for Atins, Mandacaru, or the coast, the Rio Preguiças is faster and more beautiful than the land route. A daily boat leaves Barreirinhas around 9 a.m., stopping in Mandacaru - where a climb up the lighthouse at the river's mouth reveals an extraordinary view of dune and delta - and continuing to Atins or Caburé on request. The trip takes four hours and costs about R$5 for locals, though tourist-tour versions cost more. Monkeys, iguanas, and scarlet ibis are routinely spotted from the boat. Agencies can arrange elaborate combinations of water and overland transport onward to Parnaíba or Jericoacoara, multi-day journeys that piece together trucks, boats, and trekking across a region with few paved roads.

The region around Barreirinhas is a major producer of cashew nuts, though oddly they are rarely found at the central produce market. The shops near the church charge around R$50 per kilo. Stalls inside the national park sometimes sell them for less, R$40 per kilo. Most evenings there is something happening somewhere: a forró band playing in a bar along the river, a church service spilling out into the street, a kitesurf crowd back from Atins drinking beer under string lights. The town is not polished. Dust and sand accumulate on every surface. Internet is slow and expensive. Beds in the center of town command prices that surprise backpackers. But the view from the top of the marina-side dune at Pousada Lagoa Bonito - where R$40 buys a double room with fan and breakfast - is the kind of thing that makes the whole trip make sense.