Inscribed as a World Heritage site in 1999. The nominated World Heritage Site includes a total area of 111,930ha. The site is surrounded by a buffer zone, the area of which has not yet been specified. River sediments upstream of marine intrusions are made up of stilt gravel and sand. At the river mouths and old lagoons there are deposits of stilt and clay, which are rich in organic matter.
Marine sediments penetrate into the riverbeds up to a long distance from the coast.
They all run westwards and flow into the Atlantic Ocean. Wetlands and seasonally flooded areas exist in large U-shaped river valleys. In general, rivers tend to swell in April and May, due to intense rains. Some streams are saturated with humic acids and show black coloration. Dry seasons are more conspicuous inland and away from the oceanic influence.