The low-lying barrier island (latitude 26.436394, longitude -82.155589) has a surface area of around 4,900 hectares and was formed by sediment accretion, resulting in linear dune systems that vary in height (between 0 and 3 m above sea level). Natural sand dune ridges encircle the lower elevation of the island's core, forming a freshwater wetland. Osmotic pressure caused by seasonal rains and a thin clay layer protects Sanibel Island's freshwater wetlands from subterranean saltwater intrusion from salty aquifers beneath the island and the Gulf of Mexico. These wetlands flood during the summer and fall wet seasons, when Sanibel receives 85% of its yearly precipitation, and the water levels decrease throughout the winter and early spring dry seasons.
