In the São Marcos bay, the watershed of the Mearim river and its tributaries flows, while in the São José/Arraial bay the watersheds of the Itapecuru and Munim rivers break. In this region, the amplitude of the tides can exceed seven meters. The region presents numerous streams and tidal channels. Several agents have modeled relief such as those of climatic, hydrological and oceanographic origin, as well as intense wind, marine and fluvial activity, with vegetation characterized by remnants of the Amazonian Forest, Mangroves and Campo de Perizes, an extensive fluvial plain with predominantly herbaceous, located on the mainland.