Wetlands may be found on every continent with except of Antarctica and vary substantially based on local conditions such as topology, hydrology, climate and even the degree of human interference. Wetlands may take the form of mud flats that occur at low tide, hardwood swamps in the Southeast, prairie potholes in the Great Plains or bogs in the Northeast. Although there is a seemingly endless variation of wetland types, some of which are “wet” for only a small portion of the year such as vernal pools, they can generally be divided into two categories:

1. Coastal or Tidal Wetlands - As one might well guess, these are found in coastal regions and are related to the estuarial system and impacted by tidal flows. The degree of salination impacts the type of plant life that can survive and, in general, one will see barren mudflats give way to a variety of grasses and other vegetation as the salinity decreases.
2. Inland or Non-Tidal Wetlands - Riparian wetlands, those found within the floodplain of rivers and streams are the most common type of inland wetland, but wetlands can also form near lakes or ponds or in a variety of other low lying areas where the groundwater approaches the surface and/or the area acts as a collection point for surface water.
As varied as wetlands can be, there is a common theme that helps to define their nature. The hydrology of the wetland, which results in the extended presence of water, largely determines the nature of the soils (hydric) that develop and which plant (hydrophytes) and animal (both aquatic and terrestrial) varieties select it as their habitat.

The Emergency Wetlands Resources Act of 1986 (Public Law 99-645 (100 Stat. 3582)), requires that the Untied States Fish and Wildlife Service (FWS) map and provide information on the character, extent and status of wetland and deep water habitats. As part of that process, the Fish and Wildlife Service has divided them into five ecological categories:

1. Marine - open ocean and its shoreline
2. Estuarine - areas where the transition from salt to fresh water occur
3. Riverine - river channels and streams
4. Lacustrine - lakes, reservoirs and deep ponds
5. Palustrine - freshwater wetlands, shallow ponds and inland saline wetlands

Wetlands are further classified based on a variety of factors including, but not limited to, vegetation, hydrology, water chemistry and special modifiers that take into account not only human actions but those of beavers as well. (Yes, we are serious.)

The FWS has undertaken a significant mapping program and relies heavily on the use remote sensing tools such as infrared photography to monitor changes in wetland acreage. Additionally, the FWS has produced several “National Wetlands Status and Trend Reports” that provide a wealth of useful information. These reports are discussed in The Impact of Regulation section and links to the actual reports are provided there.