Although there is controversy over the precise, legal definition of a wetland, wetlands are scientifically defined by their hydrology, vegetation, and soils. The many different types of wetlands, found in many different geographic settings, have different functions. Wetlands can be grouped according to these differences using a nationally consistent terminology (Cowardin and others, 1979) to identify mapping units for Federal and State wetland inventories and to determine wetland status and trends that can aid in planning and management of the resource. The different types of wetlands and the classification systems describing them are presented in "Wetland Definitions and Classifications in the United States."
An understanding of the basic hydrologic processes that control the formation, persistence, size, and functions of wetlands is necessary for determining appropriate protective measures for particular wetlands and for determining the success of those measures. The source and distribution of water is a major factor in the differences in wetland types and distribution across the country. Both a favorable geologic setting and an adequate and persistent supply of water are necessary for the existence of a wetland. Different wetlands receive water from different sources; ground water, streams, lakes, tides, snow, and rain. The source of water largely determines its quality, which in turn is largely responsible for wetland vegetation. The wetland vegetation affects the value of the wetland to animals and people. Wetlands provide many beneficial water-related functions. Some wetlands provide flood control, some provide water for aquifers, others feed streams, some modify climate, others improve water quality, some help maintain the salt balance necessary for estuarine life, and still others control erosion. "Wetland Hydrology, Water Quality, and Associated Functions" describes the different water-related factors that determine what types of wetlands will be established and what functions each will perform.
One of the best known functions of wetlands is as habitat for birds. About one-third of the North American bird species use wetlands for water, food, shelter, or breeding. About 138 of the 1,900 bird species in the conterminous United States are wetland dependent. For wetland-dependent birds, habitat loss or degradation usually translates to population loss. Some international treaties-The Migratory Bird Treaty and the Ramsar Convention-are partly responsible for much of the formal wetland protection in this country. "Wetlands as Bird Habitat" discusses the relation of birds and wetlands and the effects of wetland losses on birds, and describes some efforts to reduce wetland loss.
The recent understanding of wetland values and the benefits that they provide has been broadened by the research efforts. In 1992, wetland research was being done by 18 Federal agencies-12 of which had expenditures of $1 million or more-as part of their mission or responsibilities defined by Congress. In 1992, Federal wetland research expenditures totaled about $63 million. Ecological processes and functions differ with wetland type; therefore, research needs and techniques also differ. Types of Federal wetland research fall into one of the following broad categories: wetland processes, wetland functions, human-induced stresses, delineation and identification, and management. Research needs also differ among agencies; nevertheless, efforts are coordinated to share information and to avoid duplication. Disappearing coastal and bottom-land hardwood wetlands are among the major areas of research. These and other areas of research are discussed in "Wetland Research by Federal Agencies."
Wetland mapping is a prerequisite for wetland inventory, regulation, management, protection, and restoration. Maps are used to analyze wetland trends and the effects of projects, policies, and activities on wetlands. The U.S. Fish and Wildlife Service has a major responsibility for the mapping and inventory of the Nation's wetlands as mandated by legislation enacted in the past 40 years. This responsibility is satisfied through the agency's National Wetlands Inventory program by producing maps, establishing a wetland data base, publishing and distributing reports on the status and trends of wetlands in this country, and by providing other products related to the identification, mapping, and inventory of wetlands. To date, the National Wetlands Inventory has produced more than 43,300 maps, covering more than 83 percent of the conterminous United States, 28 percent of Alaska, and all of Hawaii and the U.S. Territories. Other Federal agencies with wetland mapping and inventory activities, specific to their missions, are the Natural Resources Conservation Service (formerly known as the Soil Conservation Service)-freshwater wetlands with the potential for agricultural conversion; the National Oceanic and Atmospheric Administration-coastal wetlands associated with marine resources; and the U.S. Geological Survey-geographically significant wetlands. More information can be found in "Wetland Mapping and Inventory."
Placing a value on wetlands facilitates decisions on which sites should be developed to ensure that the most valuable wetlands are preserved. The value of a wetland lies in the benefits that its habitat, water-quality, and hydrologic functions provide to the environment or to people. Economic value can be placed on some wetland products, but true value goes beyond money. Some wetland values extend beyond the perimeter of the wetland and provide benefits on a local, regional, or global scale. Several systems of wetland evaluation have been or are being developed to assign numerical values to wetland functions in order to allow for the comparison of the worth of one wetland to another. The article "Wetland Functions, Values, and Assessment" discusses three different wetland evaluation methods-the Federal Highway Administration's "Wetland Evaluation Technique," the U.S. Environmental Protection Agency's "Environmental Monitoring Assessment Program-Wetlands," and the U.S. Army Corps of Engineers' "Hydrogeomorphic Approach."
In August 1992, Hurricane Andrew caused massive destruction in southern Florida and in Louisiana-two States with some of the largest wetland acreages in the country. The storm passed directly over the Florida Everglades-the largest wetland complex in the United States-and the Atchafalaya River Basin, La., which contains the largest hardwood swamp in the United States. Although there were some immediate detrimental effects on plants and animals, the long-term effects seem to have been minimal in Florida. In Louisiana, the hurricane may have hastened the coastal erosion and wetland deterioration processes that were already at work. "Effects of Hurricane Andrew (1992) on Wetlands in Southern Florida And Louisiana" describes the effects of this major hurricane on these wetlands.
The Great Midwest Flood of 1993, in the Mississippi and Missouri River Basins, was the most devastating flood in United States' history. The areal extent, intensity, and long duration makes this flood unique in the 20th century. Effects of the flood were both detrimental and beneficial to wetlands. Trees were uprooted, islands were eroded, many wetland plants were destroyed, and several bird species fledged few young. Massive sedimentation buried mussels; mammals displaced from the flood plain suffered higher than normal mortalities on highways and railroads; the floodwaters transported large amounts of contaminants and nutrients into and down streams; nuisance plants replaced native vegetation; and turbidity made it difficult for some fish to feed. Nevertheless, some fish spawn and feed on inundated flood plains when temperature rise accompanies flooding-which was the case in this flooding. Also, some fish habitat was improved by the creation of deep scour holes and massive underwater debris piles that provide cover. Effects of the flooding are discussed in "Effects of the Great Midwest Flood of 1993 on Wetlands."
The "Types and Distribution" section of each State summary also contains a brief discussion of wetland distribution in the State and a map that shows the general distribution of major wetlands. The State maps were derived from a national map that was compiled by the U.S. Fish and Wildlife Service . Because the data used to compile the map differ in reliability from State to State, the distribution of wetlands shown should be considered approximate. Also, because small areas physically cannot be represented at the scale at which the map was compiled, only relatively large wetlands are shown.
The factors that determine where and when wetlands form include precipitation amount and timing, evaporation and transpiration rates, topography, and geologic characteristics (see "Wetland Hydrology, Water Quality, and Associated Functions" in this volume for a discussion of wetland hydrology). The "Hydrologic Setting" section of the State summaries provides an overview of the factors that determine wetland hydrology in each State.
Each State summary contains a table (such as the accompanying table for Maryland and the District of Columbia) that lists selected wetland-related activities of Federal, State, and local government agencies and private organizations in the State. The information contained in the table and in the "Conservation" section was compiled in 1993; because of the often dynamic nature of government bureaucracies and agency responsibilities, the names of agencies and the activities listed for them can be considered reliable as of that date and no later.
Example of table 1 used in each State summary (in this case Maryland and the District of Columbia) showing selected wetland-related activities of government agencies and private organizations within the State.
[Source: Classification of activities is generalized from information provided by agencies and organizations. , agency or organization participates in wetland-related activity; ... , agency or organization does not participate in wetland-related activity. MAN, management; REG, regulation; R&C, restoration and creation; LAN, land acquisition; R&D, research and data collection; D&I, delineation and inventory]
|Agency or organization||MAN||REG||R&C||LAN||R&D||R&I|
|Department of Agriculture|
|Consolidated Farm Service Agency||...||...||...||...||...|
|Natural Resources Conservation Service||...||...|
|Department of Commerce|
|National Oceanic and|
|Department of Defense|
|Army Corps of Engineers|
|Department of the Interior|
|Fish and Wildlife Service||...|
|National Park Service||...|
|Environmental Protection Agency||...||...||...|
|Department of the Environment|
|Water Management Administration|
|Department of Natural Resources|
|Chesapeake Bay and Watershed Programs||...||...|
|Natural Heritage Program||...||...||...|
|Program Open Space||...||...||...||...||...|
|Office of State Planning||...||...||...||...||...|
|State Highway Administration||...||...||...||...||...|
|University of Maryland||...||...||...||...||...|
|DISTRICT OF COLUMBIA|
|Department of Consumer and|
|Department of Public Works||...||...||...||...|
|Metropolitan Council of Governments||...||...||...||...||...|
|Soil and Water Conservation District||...||...||...|
|SOME COUNTY AND LOCAL
|Chesapeake Bay Foundation||...||...||...||...||...|
|Environmental Concern, Inc.||...||...||...||...|
|Maryland Land Trust Alliance||...||...||...||...|
|The Nature Conservancy||...||...||...||...|
Dahl, T.E., 1990, Wetlands-Losses in the United States, 1780's to 1980's: Washington, D.C., U.S. Fish and Wildlife Service, 13 p.