Is Lencois Maranhenses a desert? How the lagoons form

Is Lencois Maranhenses a desert? It certainly looks like one. The state tourism office even calls it technically a desert full of white dunes, some up to 40 metres high.⁶ But by the measure scientists use, rain, it is nothing of the kind. NASA's Earth Observatory counts roughly 125 centimetres of rain in an average year and says plainly that the park looks like a desert but is far from it.^1,7

An older NASA caption, written for an astronaut photograph, gave 1500 mm a year and made the same point: the wind and the supply of sand keep the dunes bare despite that much rain.⁸ The two figures differ, but both describe a wet place. ICMBio's management plan lists among the park's goals the protection of coastal dunes with many lagoons in a region of high rainfall.⁹ In other words, the agency that runs the park treats the rain as part of what it protects.

So the paradox is the whole story. A desert is defined by too little water; this one has plenty, but it all arrives in one half of the year. In the management plan's words, half the year brings heavy rain and the other half none at all.¹⁰ What you see on any visit depends on where in that cycle you arrive, which is why a visit in July and a visit in December can feel like visits to different countries.

Wind gives the sand its shape. NASA's older caption describes the basic form repeated across the field as a crescent-shaped dune, and notes that where sand is plentiful the crescents merge into chains many miles long.²⁷ The UNESCO description in Brazil's register says the same in technical terms: along an 80 km coast the prevailing winds shape the dunes into long chains of barchans.²²

The wind also keeps them moving. In the dry season the easterly trade winds often blow at 50 km per hour or more, strong enough to pile up the dunes and shove them west by roughly 4 to 25 metres a year.³ Satellite images show the field creeping westward by about 0.5 km in some areas between 1986 and 2026.²⁸ That constant motion is also why so little grows on the open sand.²⁹ Plants that take root are soon buried or stripped bare by the next season's wind.

Look at the dunes from a high point such as Lagoa Bonita and you can read the wind. The long ridges line up across the field, and NASA notes that the lines of dunes were created by winds blowing from the north-east.³⁰ On a windy afternoon you can watch fine sand streaming off the crests, which is the whole process happening in miniature in front of you. Shield your camera and your eyes when it does.

A lagoon that dries every year would seem a poor home for fish, yet fish are there. Some small species, says the state tourism office, outlast the dry months as eggs hidden in the sand.¹⁴ The trahira, a sharp-toothed hunter, goes further still: NASA explains that it digs down through the sand to damp mud, sleeps out the dry season there and resurfaces to hunt once the rains return.³⁶ It is one of the strangest survival tricks in Brazil.

NASA counts more than 850 documented species in the park.²³ Among them is the pininga turtle, Trachemys adiutrix, which the public use plan describes as endemic, threatened and the park's emblem.³⁷ The management plan adds that the park lies on the migration route of birds from the Northern Hemisphere during their wintering season.³¹ Binoculars are worth the space in your bag.

This hidden life is a good reason for the rules you meet on any tour. No motorboat may run on a dune lagoon, and the whole park is closed to glass and to fires.^38,33,32 A lagoon looks empty for half the year, but under the sand and in the surrounding scrub, a whole community is waiting for the next rains. Treat the sand as a living place and you will see far more of it.

The dunes were protected long before they became famous. The national park was created by decree on 2 June 1981.⁴¹ In 2024 it joined UNESCO's World Heritage roll, honoured for its geology and its wildlife, NASA notes.⁴² ICMBio, the federal agency for protected areas, manages it today, with 156,608.16 hectares under its care.⁴³ Accredited guides and drivers work under its rules.

Protecting a moving landscape means protecting the process, not just the view. The management plan lists the natural dynamics of the free and fixed dune fields among the park's aims, and keeps motorboats off the lagoons.^44,38 ICMBio bans private cars, motorbikes, buggies and quad bikes inside the park, allowing only accredited vehicles to carry visitors.^45,46 Every visitor who books through them shares in that responsibility.

For you as a visitor, that translates into a few simple habits. Follow the tracks your guide uses, keep off fragile vegetation at the edges of the dunes, carry out everything you carry in, and leave the fish and turtles in peace. The lagoons will form again next year whatever you do, but the life that depends on them is more fragile than the sand. A careful visit is the best way to say thank you for the swim.