The sand is carried to the park from the interior of the continent by the Parnaíba and Preguiças rivers, where it is then driven back inland up to by winds, creating a series of sand dunes rising as much as tall. During the rainy season, between the months of January and June, the rainstorms fill the spaces among the dunes with fresh water lagoons of up to in length and in depth, and together comprising as much as 41% of the area of the park. The water in the lagoons is prevented from draining by a layer of impermeable rock located beneath the sandy surface. The lagoons typically have a temperature between and , pH of between 4.9 and 6.2, and low levels of dissolved nutrients. When the dry season returns, the pools quickly evaporate, losing as much as of depth per month.