Wetlands are found under a wide range of hydrological conditions, but at least some of the time water saturates the soil. The result is a hydric soil, one characterized by an absence of free oxygen some or all of the time, and therefore called a "reducing environment." Plants (called hydrophytes or just wetland plants) specifically adapted to the reducing conditions presented by such soils can survive in wetlands, whereas species intolerant of the absence of soil oxygen (called "upland" plants) can not survive. Adaptations to low soil oxygen characterize many wetland species.

Intertidal wetlands are found in coastal areas of tropical regions where air temperature, wave action, salinity levels, and sediment movements are moderated by the locational features of the estuarine environment.