There are tidal marshes, mud flats, freshwater swamps, rivers, lakes, bogs and wet meadows just to name a few. However, all wetlands share three characteristics which are used to define them. The first is hydrology , the presence of water. Wetlands form wherever water is present for extended periods during the growing season. Water may be present on the surface or, the soils in an area may saturated for a sufficient period of time to develop the second characteristic of a wetland, hydric soils.
All soils are porous, meaning that there are small spaces between the grains of the soil.
Typically soils with larger grain sizes have larger spaces through which air and water can move more freely. For example a sandy soil drains much more quickly than soils with small particles like clay. In an upland soil the spaces within the soil are usually filled by air. This allows oxygen to react with the elements within the soil. In a wetland soil, the spaces within the soil are filled by water for a long enough period of time to reverse the chemical reactions that can occur between oxygen and the soil's elements giving the soil a darker color such as black, bluish-gray or dark brown.