Wetlands are areas where water covers the soil, or is present either at or near the surface of the soil for at least part of the growing season. The occurrence and flow of water (hydrology) largely determine how the soil develops and the types of plant and animal communities living in and on the soil. Wetlands may support both aquatic and terrestrial species. The prolonged presence of water creates conditions that favor the growth of specially adapted plants (hydrophytes) and promote the development of characteristic wetland (hydric) soils.

Wetlands vary widely because of regional and local differences in soils, topography, climate, hydrology, water chemistry, vegetation, and other factors, including human disturbance. Indeed, wetlands are found from the tundra to the tropics and on every continent except Antarctica. Two broad categories of wetlands are recognized: tidally-influenced wetlands and non-tidal (or inland) wetlands.

Coastal Tidal Wetlands

Coastal wetlands in the United States, as their name suggests, are found along the Atlantic, Pacific, Alaskan, and Gulf coasts. They are closely linked to estuaries, where sea water mixes with fresh water to form an environment of varying salinities. The salt water and the fluctuating water levels (due to tidal action) combine to create a rather difficult environment for most plants. Consequently, many shallow coastal areas are unvegetated mud flats or sand flats. Some plants, however, have successfully adapted to this environment. Certain grasses and grasslike plants (or graminoids, including sedges and rushes) that adapt to the saline conditions form the tidal salt marshes that are found along the Atlantic, Gulf, and Pacific coasts. Mangrove swamps, with salt-loving shrubs or trees, are common in tropical climates, such as in southern Florida and Puerto Rico. Some tidal freshwater wetlands form beyond the upper edges.

Marine, or salt marsh, wetlands are located along the marine-influenced shoreline. They can be associated with these land forms:

• Barrier Beaches
• Barrier Islands
• Headlands
• Rivers/Estuaries

Brackish are generally located in areas which are influenced both by marine tidal waters and fresh waters. They are typically located at the upper reaches of estuaries but can also be found along the marine-influenced shoreline in areas where of significant fresh groundwater seeps. In addition, natural and manmade restrictions to tidal flow, such as shoals or roadway culverts, can result in the trasnsition from a salt marsh wetland to a brackish marsh wetland.

Nontidal Wetlands

Inland wetlands are most common on floodplains along rivers and streams (riparian wetlands), in isolated depressions surrounded by dry land (for example, playas, basins, and "potholes"), along the margins of lakes and ponds, and in other low-lying areas where the groundwater intercepts the soil surface or where precipitation sufficiently saturates the soil (vernal pools and bogs). Inland wetlands include marshes and wet meadows dominated by herbaceous plants, swamps dominated by shrubs, and wooded swamps dominated by trees. Certain types of inland wetlands are common to particular regions of the country: bogs and fens of the northeastern and north-central states and Alaska; inland saline and alkaline marshes of the arid and semiarid west; prairie potholes of Iowa, Minnesota and the Dakotas; playa lakes of the southwest and Great Plains; and bottomland hardwood swamps of the south.

Many of these wetlands are seasonal and, particularly in the arid and semiarid West, may be wet only periodically. The quantity of water present and the timing of its presence in part determine the functions of a wetland and its role in the environment. Even wetlands that appear dry at times for significant parts of the year-such as vernal pools-often provide critical habitat for wildlife adapted to breeding exclusively in these areas.

Depressional wetlands are surrounded or nearly so by uplands and lack a channelized stream--small order or internittent streams may enter or exit this type of wetland but they do not flow through as channels. Depressional wetlands can be:

• Isolated
• Inflow
• Outflow
• Thruflow
• Slope

Riverine wetlands are associated with flowing water systems (such as rivers, creeks, perenial streams, intermittent streams, and similar waterbodies) and contiguous wetlands. These wetland types can be clasified according to the river gradient:

• High (rapid water flow)
• Mid (fast to moderate)
• Low (slow)

Lacustrine wetlands are associated with large standing waterbodies (such as lakes and reservoirs) and contiguous wetlands formed in the lake basin; these types do not include large shallow waterbodies such as playa lakes (which are considered ponds). Lacustrine wetlands can be:

• Fringe
• Island

New England Wetlands: Ecology, Functions, and Degradation

New England Region Wetland Types

Wetland Ecology and Functions

Wetland Loss and Degradation

Projects

Wetland Assessment Projects