The 2001 U.S. Supreme Court's decision in Solid Waste Agency of Northern Cook County (SWANCC) held that isolated intrastate non-navigable waters could not be protected under the Clean Water Act (CWA) based on the presence of migratory birds. SWANCC represented a major reinterpretation of the CWA by re-emphasizing the importance of navigability in the definition of “waters of the United States” protected by the statute. The decision also implied that isolated waters might be “waters of the United States” where they had a “significant nexus” to navigable waters. Understanding the significance of SWANCC requires a historical look at the geographic scope of federal laws and regulations protecting surface waters. The concept of navigability had been prominent in the Rivers and Harbors Act of 1899, but the principal implementation focus for the CWA after its enactment in 1972 and prior to SWANCC had been on the hydrologic cycle and the relevance of links to interstate commerce for determining what waters were protected under the CWA. In upcoming years and months, the geographic jurisdiction of the CWA will continue to be debated in the courts, within Federal agencies, and by the public. Aquatic resource science will play a key role in helping ensure that the CWA is implemented in a scientifically defensible manner, consistent with SWANCC. One area in need of particular emphasis is additional research on the ways in which isolated waters help ensure the physical, chemical, and biological integrity of navigable waters and their tributaries. It is this question—the “significant nexus” between an intrastate non-navigable isolated water and the rest of the aquatic ecosystem—that will likely determine whether the water will be protected by the CWA.

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 recent U.S. Supreme Court case of Solid Waste Agency of Northern Cook County v. U.S. Army Corps of Engineers (SWANCC) has had profound implications on the legal status of isolated wetlands. As a result, policymakers need ecological information on the definition and functions of isolated wetlands to respond to this decision. The term “isolated wetlands” is of fairly recent usage and has been poorly defined. In response, I recommend Tiner's (2003b) definition as wetlands “that are completely surrounded by upland.” Isolation needs to be considered with respect to specific processes and functions. I suggest that isolation not be viewed discretely but be considered within an isolation-connectivity continuum. Isolation has a fundamental influence on the way water enters and leaves a wetland. This consequently affects any wetland function that depends on water as a vector (e.g., pollutant transport and certain types of dispersal). These wetlands can also have a high level of endemism, extensive plant zonation, and high biodiversity. Isolated wetlands, however, do not represent ecologically isolated habitat for many organisms. I conclude that the effect of isolation may not be as significant as the term “isolated wetlands” suggests: many of the biological features of isolated wetlands may result from environmental conditions that also occur in non-isolated wetlands. As a result of SWANCC, assessment methods are needed that can help regulators distinguish between jurisdictional and non-jurisdictional isolated wetlands. I propose that the merger of simple, source-sink-transport vector concepts with landscape-level assessment methods could be useful in this regard. I point to the need for documented examples of organisms that spend most of their lives in waters of the U.S. but also require isolated wetlands. I conclude that wetland science would benefit from the development of a comprehensive view of isolation as a formative process across different regional wetland types.

Wetlands that are not connected by streams to other surface-water bodies are considered to be isolated. Although the definition is based on surface-water connections to other water bodies, isolated wetlands commonly are integral parts of extensive ground-water flow systems, and isolated wetlands can spill over their surface divides into adjacent surface-water bodies during periods of abundant precipitation and high water levels. Thus, characteristics of ground-water flow and atmospheric-water flow affect the isolation of wetlands. In general, the degree that isolated wetlands are connected through the ground-water system to other surface-water bodies depends to a large extent on the rate that ground water moves and the rate that hydrologic stresses can be transmitted through the ground-water system. Water that seeps from an isolated wetland into a gravel aquifer can travel many kilometers through the ground-water system in one year. In contrast, water that seeps from an isolated wetland into a clayey or silty substrate may travel less than one meter in one year. For wetlands that can spill over their surface watersheds during periods of wet climate conditions, their isolation is related to the height to a spill elevation above normal wetland water level and the recurrence interval of various magnitudes of precipitation. The concepts presented in this paper indicate that the entire hydrologic system needs to be considered in establishing a definition of hydrologic isolation.

Isolated wetlands occur in many hydrogeomorphic settings, and while they appear to be physically isolated from other water bodies, they are almost never completely decoupled from surface-water or ground-water systems. In this paper, we examine water-quality data for isolated wetlands in three hydrogeomorphic classes (depressions, slopes, flats). Some isolated wetlands are dominated by atmospheric exchanges and have little ground-water or surface-water connections with adjacent systems. Other isolated wetlands are dominated by ground-water inputs and have intermittent or continuous hydrologic connections to adjacent systems. Water-quality characteristics of isolated wetlands are highly variable and depend primarily on the sources of water, substrate characteristics, and land uses associated with the wetland watershed. We were unable to identify any general pattern of water-quality characteristics within or between isolated wetlands in the three hydrogeomorphic classes. Alteration of hydrologic conditions (e.g., ditching, filling), however, usually results in increased nutrient export to downstream systems. From a water-quality perspective, we conclude that so-called isolated wetlands are rarely isolated, and isolation is a term that is not very useful from an ecosystem perspective. Isolated wetlands are nutrient sinks and, because most are hydrologically connected to other waters and wetlands, the loss of isolated wetlands would potentially have negative impacts on the water quality of downstream systems.

Carolina bays, depression wetlands of the southeastern United States Coastal Plain, are “islands” of high species richness within the upland landscape and are the major breeding habitat for numerous amphibians. The 2001 Supreme Court decision that removes isolated wetlands from protection under the Clean Water Act has potential for great losses of these wetland ecosystems. Most Carolina bays are not naturally connected with stream drainages or other water bodies, and their hydrology is driven primarily by rainfall and evapotranspiration. Their potential interaction with shallow ground water is not well-understood. Water levels in these wetlands may vary seasonally and across years from inundated to dry, and organisms inhabiting Carolina bays must be adapted to fluctuating and often unpredictable hydrologic conditions. The ecological importance of these wetlands as habitats for species that require an aquatic environment for a part of their life cycle has been well-documented. Many Carolina bays have been drained and converted to agriculture or other uses, and many of the smaller bays have been poorly inventoried and mapped. If these wetlands are not protected in the future, a major source of biological diversity in the southeastern United States will be lost.

Surveys have shown that pocosins (swamp-on-a-hill) occur on the southeastern Coastal Plain of the U.S. from Virginia to north Florida and once covered more than one million hectares in North Carolina. A broad definition of pocosins (sensu lato) would include all shrub and forested bogs, as well as Atlantic white cedar stands and some loblolly pine stands on flooded soils on the Coastal Plain. A stricter definition (sensu stricto) of pocosins would only include the classic shrub-scrub (short pocosin) and pond-pine-dominated tall pocosin. Common synonyms for pocosins, including bay, bayland, bayhead, xeric shrub bog, and evergreen shrub bog, further confuse what is and is not classified as a pocosin. Over 51% of the forested palustrine wetlands in North Carolina have been disturbed, with approximately 33% of pocosins having been destroyed. Pocosins are rainfall driven and lack a well-defined stream surface-flow connection to major rivers on the landscape. However, they are often found adjacent to estuaries and have surface hydrologic connections that are linked to the regional water quality and salinity gradients found in estuarine areas along the southeastern coast. This hydrologic connection, combined with the vast continuous expanses of pocosins on the landscape, suggests that they are connected to regulated tributary waters of the United States. In addition, a survey of U.S. Army Corps of Engineers personnel in North Carolina indicates that most pocosins are considered hydrologically connected to regional water supplies since they are the source of water flow on the landscape where they dominate. However, the potential impact of the U.S. Supreme Court's 2001 decision in the case of Solid Waste Agency of Northern Cook County v. U.S. Army Corps of Engineers (SWANCC) on the future development of pocosins is still unknown in most states. Moreover, the Bush administration's recent (January 2003) review and redefinition of the Clean Water Act's (CWA) jurisdiction over isolated wetlands may remove federal oversight on 20% of the nation's wetlands, including pocosins not immediately adjacent to estuaries. The high rate of past wetland loss, especially for pocosin wetlands, suggests that stricter wetland laws are needed at the state and local level if we are to support the concept of “no net loss” of wetlands.

Playa wetlands provide functions critical to the existence of life on the High Plains portion of the Great Plains, including surface drainage, aquifer recharge, and wildlife habitat. These small, circular, isolated depressional wetlands with closed watersheds have a dynamic, unpredictable hydroperiod, which is essential to the maintenance of biodiversity. Most numerous in the Southern High Plains of northwestern Texas and eastern New Mexico, playas have been impacted by sedimentation, pit excavation, road construction, industrial and municipal wastewater, feedlot runoff, urban development, overgrazing, and deliberate filling. Despite being declared, as a wetland class, jurisdictional “waters of the United States” since 1977, regulations and laws for conservation of wetland functions have seldom been applied to playas. The January 2001 Supreme Court decision, Solid Waste Agency of Northern Cook County (SWANCC) v. United States Army of Corps of Engineers, likely eliminated federal regulation of impacts covered by the Clean Water Act in all but a few playas. Although still subject to the Federal “Swampbuster” provision enacted by the 1985 Food Security Act, extended natural dry periods allows for frequent cultivation and other activities in playas without incurring violation, contributing to the continued degradation of playa functions. None of the states with significant numbers of playas have regulations for the conservation of playa functions. Suggestions for the successful future conservation of playas and their associated functions include (1) increased promotion and implementation of existing federal and state conservation programs specifically for playas; (2) proposed state regulations for playa conservation; (3) recognition of agricultural impacts on wetland determinations; (4) creation of Wetland Management Districts to preserve intact, functioning playas; and (5) increased public education on the value of playas.

Collectively, the millions of wetlands in the prairie pothole region create one of the most important waterfowl breeding areas in North America. Their use by migratory waterfowl provided a legal rationale, the Migratory Bird Rule, for extending Clean Water Act Section 404 protection to them even though prairie potholes are generally not associated with navigable waters. The U. S. Supreme Court's decision in the case of the Solid Waste Agency of Northern Cook County vs. U. S. Army Corps of Engineers, the SWANCC decision, seems to have invalidated the use of the Migratory Bird Rule. We have evaluated the adjacency of potholes to navigable waters and the implications of the SWANCC decision. For most groups of wetland organisms, with the important exception of fish, potholes are not ecologically isolated habitats. Their flora and fauna and functions (primary production, food webs, mineral cycling, water storage capacity, etc.) are largely determined by water levels, not degree of isolation. Annual changes in precipitation can significantly change their water levels, and this can result in major shifts in their flora and fauna. Hydrologically, prairie potholes are usually connected by ground-water flows and, during wet years, even by surface-water flows. According to landscape analyses done by Ducks Unlimited on selected areas in North and South Dakota, most prairie potholes are not adjacent to (>95%) or even within 1 km (ca. 50%) of navigable waters. Consequently, because of SWANCC, most prairie potholes may no longer be protected under section 404 of the Clean Water Act. Most prairie potholes, however, are located on farms and are thus protected by the Swampbuster provision of the 1985 Food Security Act. Even with Swampbuster, there will be minor losses of potholes without any mitigation around cities and towns, and various exemptions under Swampbuster will result in some loss or degradation of prairie potholes. The SWANCC decision, however, will significantly weaken the protection of prairie potholes because it lends strength to existing pressures to weaken or eliminate Swampbuster.

Vernal pools, broadly defined as ephemeral wetlands that predictably form in permanent basins during the cooler part of the year but which dry during the summer months, are distributed throughout the world. In the U. S., they are particularly abundant on the Pacific Coast and in various forms in the glaciated landscapes of the north and northeast. Vernal pools are ecosystems that have evolved in a balance between isolation and connectedness. Because of isolation at several scales, the vernal pools biota includes many regionally endemic species. Because of connectedness, vernal pools also share many taxa with continent-spanning distributions at the generic and species level. Vernal pools serve an important local biodiversity function because of their connection to surrounding terrestrial habitats. Along with other ephemeral wetlands, they are the primary habitat for animal species that require relatively predator-free pools for feeding or breeding, including many amphibians. The recent U. S. Supreme Court decision (SWANCC), which deemed “isolated” wetlands to be outside the class of “waters of the United States,” places some significant but unknown proportion of vernal pools at risk. In the worst case, the consequences could be immediate reductions in biodiversity at a local level, and regional reductions over longer periods of time. Ideally, federal law should be rewritten to establish unambiguously the value of ephemeral wetlands. It will also be necessary for conservationists to educate the public and to bring the issue of vernal pool protection to the notice of their local and state governments.

The term fen has been variously used by peatland ecologists, ground-water hydrologists, and vegetation scientists. The common denominator among all types of fens is recognition of the importance of ground-water discharge, especially mineral-rich ground water, in determining fen hydrology, chemistry, and vegetation, in contrast to wetlands whose characteristics are determined primarily by precipitation or surface-water inputs. Thus, fens tend to occur where climate and hydrogeologic setting sustain flows to the plant-rooting zone of mineral-rich ground water. In the United States, these areas include the glaciated Midwest and Northeast, as well as portions of the Appalachian Mountains and mountainous West. Individually and collectively, fens are among the most floristically diverse of all wetland types, supporting a large number of rare and uncommon bryophytes and vascular plant species, as well as uncommon animals including mammals, reptiles, land snails, butterflies, skippers, and dragonflies. Several species listed under the federal Endangered Species Act inhabit or use fens. Fens also help maintain stream water quality through denitrification and phosphorus sorption. Few estimates of loss and current extent exist, but where estimates are available, they indicate extensive loss, fragmentation, and degradation. Cultural eutrophication threatens the biological and functional integrity of remaining fens because, along with mineral-rich water, low availability of nitrogen and phosphorus controls many of their distinctive characteristics. Because they occur where ground water discharges to the surface, fens are isolated from neither ground water nor surface water. However, the majority of fens develop in headwater areas and could be defined as “isolated” for jurisdictional purposes because of their distance from navigable-in-fact waters. If so defined, the critical roles that fens play in maintaining biological diversity and stream water quality are at risk regarding federal jurisdiction over “isolated waters” because of the 2001 U.S. Supreme Court ruling in the case of Solid Waste Agency of Northern Cook County v. U.S. Army Corps of Engineers.

A wetland ecosystem, no matter how small or isolated, includes biotic and abiotic features that interact to promote biodiversity at larger landscape scales. Isolated wetlands, in particular, harbor a significant portion of regional fauna and are often critical habitats for maintaining herpetofaunal biodiversity in southern wetlands. Long-term research on isolated wetlands reveals that two terrestrial habitats contiguous with the wetland—the terrestrial periphery and terrestrial corridors that connect isolated wetlands—are vital for much of the animal community. The U.S. Supreme Court's ruling on the SWANCC decision has severely threatened the continued existence of such wetlands and their associated animals. Recognition that terrestrial habitats associated with isolated wetlands are essential elements for enhancing biodiversity could garner support from regulators, resource managers, and the general public in strengthening wetlands protection.

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.

On January 9, 2001, the U. S. Supreme Court decision in Solid Waste Agency of Northern Cook County v. U. S. Army Corps of Engineers (SWANCC) limited the scope of the Clean Water Act's jurisdiction by limiting the definition of Waters of the U.S. The Court invalidated the “Migratory Bird Rule” as the sole basis for federal regulation of non-navigable, isolated, and intrastate waters (“isolated wetlands”) under the Clean Water Act (CWA). While specifically invalidating the long-standing policy that waters used by migratory birds were included in CWA jurisdiction (termed the migratory bird rule), the decision does not make clear which of the waters and wetlands covered by 33 CFR 328(a)(3) remain under CWA jurisdiction. In January of 2003, the U.S. Environmental Protection Agency (USEPA) and the U.S. Army Corps of Engineers (Corps) published an Advanced Notice for Proposed Rulemaking on CWA Definition of Waters of the United States in the Federal Register. This document requested input from the public as a prelude to rulemaking on behalf of the federal agencies in response to SWANCC. In the absence of clear guidance that can be applied consistently throughout the country, jurisdictional determinations have been left to the individual field offices of the Corps and USEPA and vary widely. Thus, the extent of wetlands and other waters that are impacted as a result of SWANCC still cannot be determined accurately. The potential changes in jurisdiction could alter dramatically the framework for federal-state partnerships in wetlands protection and will require changes in either state or federal programs if protection of aquatic ecosystems is to be sustained at pre-SWANCC levels. The uncertainty over the extent of the change restricts the states' ability to respond. Despite this uncertainty, some states have taken action. Wisconsin passed legislation in the months following the SWANCC decision. Several other states have attempted to make changes through legislation, regulations, and/or guidance with limited success. In half of the states in the U.S., there are no state programs in place or planned to address the reduction in federal jurisdiction. In these states, a significant change in federal regulation could mean the loss of important wetlands.

In Solid Waste Agency of Northern Cook County v. U.S. Army Corps of Engineers (SWANCC), the U.S. Supreme Court held that isolated, intrastate, non-navigable waters could not be protected under the Clean Water Act based solely on their use by migratory birds. The SWANCC decision has created a need to compile and make available scientific information for post-SWANCC policy development. In response, this article reviews the state of our scientific understanding of isolated wetlands, based on the major findings of papers contributed to this special issue of Wetlands. Because the term “isolated wetland” has not been used consistently in the scientific literature, we recommend that geographically isolated wetlands be defined as “wetlands that are completely surrounded by upland,” as proposed by Tiner, for the purposes of scientific studies. Geographically isolated wetlands are not homogeneous but have a broad range of functional response, partly due to their occurrence over a wide range of climatic and geologic settings. One major question addressed through this special issue is the role that isolation plays in the function of geographically isolated wetlands. It appears that isolation is not a primary factor and that many of the functions performed by isolated wetlands are also performed by non-isolated wetlands and non-wetland ecosystems. Variability in moisture conditions plays an important role in the function of many geographically isolated wetlands. However, hydrologic isolation may affect moisture conditions, and biotic isolation could be important for certain populations. Depending on the factor being considered, geographically isolated wetlands are not entirely isolated but are better viewed as occurring within an isolation-connectivity continuum that has both hydrologic and biotic expressions. The juxtaposition of isolation and connectivity occurring in geographically isolated wetlands may represent a semi-isolated state that uniquely shapes these wetlands and their functions. Comprehensive data, designating the number, total area, and functional classification of isolated wetlands, would provide the foundation for monitoring impacts to isolated wetlands. Studies are needed to examine and quantify how isolated wetlands, wetland complexes, and other potentially impacted waters contribute hydrologically, chemically, and biologically to waters of the U.S. Methods to assess and map the degree of connectivity between geographically isolated wetlands and waters of the U.S., based on ground-water travel time, recurrence frequency of intermittent surface-water connections, and home ranges of species that require both types of waters, could be useful for regulators. Whatever policies are developed, scientific input and technical information will continue to play a crucial role in the policy and regulatory arena. Maintaining and enhancing the dialogue among wetland scientists, policy-makers, and regulators will ensure that critical information is developed and communicated and also continue to invigorate wetland science.