Lençóis Maranhenses National Park

Rivers carry sand to the coast. Strong winds blow it into dunes.

The dunes slowly move across the land.

NASA explains that rivers, including the Mearim and the Parnaíba, bring huge amounts of quartz sand to this very flat coast. Over the past few hundred thousand years, rising and falling seas helped pile it up.

Strong trade winds then blow the sand inland. UNESCO says small curved dunes called barchans form near the beach and grow up to 30 meters tall as they move inland, lined up in long, winding chains. They move 4 to 25 meters a year.

UNESCO: the dune field formed through the Quaternary from sediment of marine transgressions and regressions worked by wind, in the Barreirinhas Basin. Sand from the beach is eroded by mostly unidirectional wind into small barchans 50 cm to 1 m high near the shore, reaching up to 30 m as they migrate inland over older dunes, forming winding chains up to 75 km long and moving over 20 km inland at 4 to 25 m a year.

NASA: rivers including the Mearim and Parnaíba supply quartz sand; dry-season trade winds, often at least 50 km/h, push the dunes west; Landsat shows parts of the field extending about 0.5 km westward between 1986 and 2026.

When it rains a lot, water fills the low spaces between the dunes.

In the dry season, most of the lagoons dry up.

NASA says the park gets about 125 centimeters of rain a year. A layer of rock and clay under the sand stops much of the water from draining away, so lagoons fill the low ground between the dunes. They are fed only by rain, with no rivers flowing in or out.

As the rains ease between August and November, the water table falls, and by December most lagoons dry up. Because the dunes move, the lagoons come back in new places and shapes the next year.

NASA: about 125 cm of rain a year; a partly impermeable layer of bedrock and clay keeps lagoons from draining in the wet season; water levels typically peak around June; as rains subside (August to November) the water table falls and most lagoons dry by December. Shallow lagoons look turquoise; deeper ones darker blue, as deep water absorbs more red, yellow and green light; sediment, algae and dissolved organic matter add green and brown tones.

UNESCO: the lagoons, with no inlet or outlet, are fed exclusively by rainwater and controlled by the water table; with dunes migrating 4 to 25 m a year, they re-emerge in new places, their beds coated with brown or green algae and cyanobacteria.

Fish, birds, manatees and otters live in and around the park.

One fish hides in the damp mud when the lagoons dry up.

NASA says the park is home to more than 850 recorded species. Among them are endangered animals such as the West Indian manatee, the neotropical otter, the scarlet ibis and the oncilla, a small wild cat.

The trahira, a hunting fish with big, sharp teeth, lives in the lagoons. In the dry season it burrows under the sand to damp mud and rests until the rains return. The park lies where three Brazilian biomes meet: the Amazon, the Cerrado and the Caatinga.

NASA: more than 850 documented species, including four endangered species (neotropical otter, West Indian manatee, scarlet ibis, oncilla); the trahira (Hoplias malabaricus) burrows to damp mud in the dry season and becomes dormant, emerging after the rains to hunt insects and fish.

UNESCO: a transition zone between the Cerrado, Caatinga and Amazon biomes; restinga pioneer formations (more than 40,000 ha), mangroves and alluvial communities, with marine and freshwater environments, sustain species diversity; the moving dunes create new habitats for pioneer flora and fauna.

The white sand and water reflect the sun. Wear a hat and sunscreen.

Only swim with a grown-up watching.

The dunes have no shade, and sunlight bounces off sand and water. The CDC says UV rays reflect off water and sand, so wear sunscreen, sunglasses and a wide-brimmed hat.

The CDC also says adults should watch children closely around any water, and children should wear life jackets for activities in and around natural water.

CDC sun safety: UV rays reflect off surfaces such as water and sand; use broad-spectrum sunscreen, UV-blocking sunglasses and a wide-brimmed hat, and seek shade where possible.

CDC drowning prevention: close adult supervision of children around water, and life jackets for children in and around natural water.