Ralph W. Tiner

GEOGRAPHICALLY ISOLATED WETLANDS OF THE UNITED STATES

While many wetlands form along floodplains of rivers, streams, lakes, and estuaries, others have developed in depressions far removed from such waters. Depressional wetlands completely surrounded by upland have traditionally been called “isolated wetlands.” Isolated wetlands are not confined to basins, as some occur on broad flats and others form on slopes. The term “geographically isolated wetlands” better describes these wetlands, since many are hydrologically connected to other wetlands and waterbodies through ground-water flows or by intermittent overflows (spillovers). Numerous types of geographically isolated wetlands occur throughout the United States. They may be naturally formed or the result of human action. Naturally formed types include prairie pothole wetlands, playas, Nebraska’s Rainwater Basin and Sandhills wetlands, West Coast vernal pools, sinkhole wetlands, Carolina bays, interdunal and intradunal wetlands, desert springs, terminal basins in the Great Basin, and kettle-hole bogs in glaciated regions. Human-caused isolated types may be intentionally built, such as ponds designed for various purposes and wetlands built on mined lands, or they may be accidentially created (e.g., natural wetlands that were once connected to rivers and streams but are now isolated by roads, railroads, and other development or isolated by altered river hydrology). Many of the functions and benefits attributed to non-isolated wetlands are present in isolated wetlands.

THE CONCEPT OF A HYDROPHYTE FOR WETLAND IDENTIFICATION

deepwater aquatic systems and terrestrial systems. Although often found along rivers, lakes, ponds, and estuaries, wetlands also exist on gentle slopes or in isolated depressions surrounded by uplands. Wetlands can be considered to occur along a natural soil-moisture gradient between permanently flooded deepwater areas and dryland (Figure 1). Wetland hydrologic conditions, therefore, range from permanent inundation by shallow water or permanent soil saturation to periodic inundation or soil saturation.

Wetlands of the United States

This paper discusses the general types of wetlands found in the United States (including Alaska and Hawaii) as well as their classification, ecology, status and trends, and regional problem areas. It is based upon the work performed by the U.S. Fish and Wildlife Service’s National Wetlands Inventory and also upon a review of existing information about the wetlands of the United States.

ESTIMATED EXTENT OF GEOGRAPHICALLY ISOLATED WETLANDS IN SELECTED AREAS OF THE UNITED STATES

In preparing a major report on geographically isolated wetlands, the U.S. Fish and Wildlife Service (FWS) initiated a study of the extent of these wetlands across the country. The FWS used geographic information system (GIS) technology to analyze existing digital data (e.g., National Wetlands Inventory data and U.S. Geological Survey hydrologic data) to predict the extent of isolated wetlands in 72 study areas. Study sites included areas where specific types of “isolated” wetlands (e.g., Prairie Pothole marshes, playas, Rainwater Basin marshes and meadows, terminal basins, sinkhole wetlands, Carolina bays, and West Coast vernal pools) were known to occur, as well as areas from other physiographic regions. In total, these sites represented a broad cross-section of America’s landscape. Although intended to show examples of the extent of isolated wetlands across the country, the study was not designed to generate statistically significant estimates of isolated wetlands for the nation. As expected, the extent of isolated wetlands was quite variable. The study found that isolated wetlands constituted a significant proportion of the wetland resource in arid and semi-arid to subhumid regions and in karst topography. Eight study areas had more than half of their wetland area designated as isolated, while 24 other areas had 20–50 percent of their wetland area in this category. For most sites, isolated wetlands represented a greater percent of the total number of wetlands than the percent of wetland area. This was largely attributed to difference in wetland size, with most non-isolated wetlands being larger than the isolated wetlands. Forty-three sites had more than 50 percent of their total number of wetlands designated as isolated. The estimates of isolated wetlands presented in this study cannot be readily translated to wetlands that have lost Clean Water Act “protection” based on a recent U.S. Supreme Court ruling for several reasons, including the lack of written guidance on interpreting the Court’s decision for identifying jurisdictional wetlands. The results of this GIS analysis present one perspective on the extent of geographically isolated wetlands in the country and represent a starting point for more detailed assessments.

ASSESSING CUMULATIVE LOSS OF WETLAND FUNCTIONS IN THE NANTICOKE RIVER WATERSHED USING ENHANCED NATIONAL WETLANDS INVENTORY DATA

The coterminous U.S. has lost more than 50% of its wetlands since colonial times. Today, wetlands are highly valued for many functions including temporary storage of surface water, streamflow maintenance, nutrient transformation, sediment retention, shoreline stabilization, and provision of fish and wildlife habitat. Government agencies and other organizations are actively developing plans to help protect, conserve, and restore wetlands in watersheds. The U.S. Fish and Wildlife Services National Wetlands Inventory Program (NWI) has produced wetland maps, digital geospatial data, and wetland trends data to aid these and other conservation efforts. Most recently, the NWI has developed procedures to expand the amount of information contained within its digital databases to characterize wetlands better. It has also developed techniques to use these data to predict wetland functions at the watershed level. Working with the states of Delaware and Maryland, the NWI applied these techniques to the Nanticoke River watershed to aid those states in developing a watershed-wide wetland conservation strategy. Wetland databases for pre-settlement and contemporary conditions were prepared. An assessment of wetland functions was conducted for both time periods and comparisons made. Before European settlement, the Nanticoke watershed had an estimated 93,000 ha of wetlands covering 45% of the watershed. By 1998, the wetland area had been reduced to 62% of its original extent. Sea-level rise and wetland conversion to farmland were the principal causes of wetland loss. From the functional standpoint, the watershed lost over 60% of its original capacity for streamflow maintenance and over 35% for four other functions (surface-water detention, nutrient transformation, sediment and particulate retention, and provision of other wildlife habitat). This study demonstrated the value of enhanced NWI data and its use for providing watershed-level information on wetland functions and for assessing the cumulative impacts to wetlands. It provides natural resource managers and planners with a tool that can be applied consistently to watersheds and large geographic areas to show the extent of wetland change and its projected effect on wetland functions.

Use of high-altitude aerial photography for inventorying forested wetlands in the United States

Abstract The U.S. Fish and Wildlife Service is conducting an inventory of the wetlands of the United States through its National Wetlands Inventory Project (NWI). After considering alternative remote-sensed data sources, the NWI selected high-altitude aerial photography (1:40 000 to 1:130 000 scale) as its primary data source. Stereoscopic interpretation of this photography is an efficient and cost-effective method for identifying, classifying, and inventorying wetlands on a national, regional, and statewide basis. This paper outlines the NWI procedures and summarizes NWIs experience in inventorying forested wetlands, emphasizing problems encountered and their resolution.

The primary indicators method—A practical approach to wetland recognition and delineation in the United States

Over the past 30 years, various methods have been developed to identify and delineate wetlands for regulatory purposes in the United States. This paper discusses major limitations of existing methods and offers an alternative method called the “primary indicators method.” This new method is based on using features (national and regional plant and soil characteristics) unique to wetlands for identifying wetlands and their boundaries. These primary indicators permit accurate wetland determinations and delineations in the absence of significant hydrologic modification because these features only develop in wetlands. Wetlands subject to significant drainage require an assessment of the current hydrology.

Remote sensing of wetlands : applications and advances

Submerged aquatic vegetation (SAV) refers to all underwater flowering plants, of which seagrass is the most important from a marine ecosystem perspective. Being submerged underwater, the remote sensing of SAV, as well as coral reefs, is considerably more challenging than for terrestrial targets. The air-water interface and overlying water column form a dynamic medium that strongly influences the transfer of electromagnetic radiation (e.g., visible sunlight) that is used in a remote sensing situation to communicate information about submerged targets. As a result, the spectral differentiation of SAV and coral habitats using optical remote sensing demands specialized strategies, even if the depth of submergence is only a few meters. When submerged features are visible within remote sensing imagery, a situation termed “optically shallow water,” SAV, and coral habitats can be mapped. Conversely, if the lake or seabed is invisible due to excessive turbidity or water depth, termed “optically deep water,” benthic features cannot be mapped using satellite or airborne optical remote sensing. Therefore, optically deep and optically shallow waters require different application of technologies and/or also integration of field and image-based datasets. This chapter will provide insight into the use of remote sensing to map submerged features found in coral reef and SAV environments.

advances in remotely sensed data and techniques for wetland mapping and monitoring

Submerged aquatic vegetation (SAV) refers to all underwater flowering plants, of which seagrass is the most important from a marine ecosystem perspective. Being submerged underwater, the remote sensing of SAV, as well as coral reefs, is considerably more challenging than for terrestrial targets. The air-water interface and overlying water column form a dynamic medium that strongly influences the transfer of electromagnetic radiation (e.g., visible sunlight) that is used in a remote sensing situation to communicate information about submerged targets. As a result, the spectral differentiation of SAV and coral habitats using optical remote sensing demands specialized strategies, even if the depth of submergence is only a few meters. When submerged features are visible within remote sensing imagery, a situation termed “optically shallow water,” SAV, and coral habitats can be mapped. Conversely, if the lake or seabed is invisible due to excessive turbidity or water depth, termed “optically deep water,” benthic features cannot be mapped using satellite or airborne optical remote sensing. Therefore, optically deep and optically shallow waters require different application of technologies and/or also integration of field and image-based datasets. This chapter will provide insight into the use of remote sensing to map submerged features found in coral reef and SAV environments.

LISTS OF POTENTIAL HYDROPHYTES FOR THE UNITED STATES: A REGIONAL REVIEW AND THEIR USE IN WETLAND IDENTIFICATION

The U.S. federal government has developed lists of plant species that occur in wetlands. The initial purpose of these lists was to enumerate plants that grow in wetlands and that could be used to identify wetlands according to the U.S. Fish and Wildlife Service’s wetland classification system. The first list was generated in 1976 by the Service, and since that time, the list has undergone several iterations as more information was reviewed or became available through field investigations and scientific research. Two lists are currently published and available for use: a 1988 list and a 1996 draft list. The latter list represents an improvement based on nearly 10 years of field work by the four signatory agencies plus comments from other agencies, organizations, wetland scientists, and others. The national list was generated from 13 regional lists. These data have not been summarized previously; this note provides an interregional summary of vital statistics. The 1988 list included 6,728 species, while the 1996 list has nearly 1,000 additions for a total of 7,662 species (a 14% increase). Roughly one-third of the nation’s vascular plants have some potential for being hydrophytes—plants growing in water or on a substrate that is at least periodically deficient in oxygen due to excessive wetness. Each species on the list is assigned an indicator status reflecting its frequency of occurrence in wetlands: 1) obligate (OBL; > 99% of time in wetlands), 2) facultative wetland (FACW; 67–99% in wetlands), 3) facultative (FAC; 34–66%), 4) facultative upland (FACU; 1–33%), and 5) upland (UPL; < 1%). From 1988 to 1996, the regional lists of potentially hydrophytic species increased by more than 39 percent in three regions: Caribbean, North Plains, and Central Plains. The percent of OBL, FACW, and FAC species on the lists decreased in the Northeast and Hawaii. The percent of OBL and FACW species also decreased in the Southeast and Northwest. The number of OBL species declined in all but three regions, whereas the number of FACU species added to the lists increased in all regions except Hawaii. The regional “wetland plant” lists have been used to help identify plant communities that possess a predominance of wetland indicator plants (i.e., a positive indicator of hydrophytic vegetation) and to identify wetlands that can be recognized solely based on their vegetation.