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.
There are areas covered with secondary forests in varying degrees of regeneration. They are rich in epiphytes and parasitic plants and have a dense shrub layer. On sand coastal soils, there is restinga vegetation which, depending on the distance to the shore, includes from humid prairies and shrubs to low forests. Bare sand areas are colonised by very specialised vegetation. Restinga forests contain lianas, orchids and epiphytes.