Lençois Maranhenses National Park, a coastal dune field (measuring some 155,000 hectares) that is flooded with freshwater lakes during the January to May rainy season. Strong prevailing winds from the East mobilize the barchan dunes during the dry season and, combined with the equatorial sun, evaporate the lakes and prevent permanent vegetation from taking root. The dunes are composed of quartz grains that are eroded from the granite and washed down from the Parnaiba River and then carried north by ocean currents before being deposited on the coast of Lençois Maranhenses. The name means "bed sheets of Maranhão Province", as from above they resemble undulating sheets of pure white.
Photos taken along a branch of the Rio Negro as it winds its way towards Baixa Grande and Quemada dos Britos, the two oases in the middle of Lençois Maranhenses. The waters are stained dark brown by tannic acids leached from plants in the forest before the river reaches the sands. Other lakes here are blue and green, as they are filled with rainwater from the heavy wet season rains.