The benefits of restoration of degraded or destroyed wetlands and
creation of new wetlands has only recently been recognized. As the
population has expanded across the Nation during the past few
centuries, wetlands have been drained and altered to accommodate human
needs. These changes to wetlands have directly, or indirectly,
brought about changes in the migratory patterns of birds, local
climate, and the makeup of plant and animal populations. In the past,
people used wetland plants and animals for shelter and food. More
recently, people have become more aware of other benefits that
wetlands provide water-quality improvement, flood attenuation,
esthetics, and recreational opportunities. Now, it is recognized that
numerous losses are incurred when a wetland is damaged or destroyed.
Restoration and creation can help maintain the benefits of wetlands
and their surrounding ecosystems, and at the same time accommodate the
human need for development.
Wetland restoration rehabilitates a degraded wetland or reestablishes a wetland that has been destroyed. Restoration takes place on land that has been, or still is, a wetland. A term commonly associated with restoration is "enhanced." An enhanced wetland is an existing wetland that has been altered to improve a particular function, usually at the expense of other functions. For example, enhancing a site to increase its use by a particular species of bird commonly limits its use as habitat for other species. (For information on functions of wetlands see the articles "Wetland Hydrology, Water Quality, and Associated Functions" and "Wetland Functions, Values, and Assessment" in this volume.)
Wetland creation is the construction of a wetland on a site that never
was a wetland. This can be done only on a site where conditions exist
that can produce and sustain a wetland. Consequently, creation is
more difficult than restoration. A term commonly associated with
wetland creation is "constructed." A constructed wetland is a wetland
created specifically for the purpose of treating wastewater,
stormwater, acid mine drainage, or agricultural runoff (Hammer, 1989).
As used in this article, "project wetland" refers to restored or
created wetlands. (For a more complete discussion of the meaning of
these terms and others associated with restoration and creation, see
Wetland alterations have brought about changes in the migratory patterns of birds, local climate, and make up of plant and animal populations.
CHALLENGES OF RESTORATION
| Ecological issues and physical
limitations are important factors to consider when planning for
wetland restoration or creation. The relative merits of destroying
the function of an existing wetland, or other ecosystem, in exchange
for another wetland function involves the consideration of numerous
questions such as: (1) Which is more important, the existing or the
replacement function? (2) Will the proposed wetland increase wildlife
diversity? (3) Is the increased diversity worth the loss of habitat
of any endangered species? Questions of this type always arise during
planning for wetland restoration and creation.
A well-documented example of a physical limitation associated with restoring a wetland can be seen along the shoreline of the Salmon River Estuary, Oreg. (Frenkel and Morlan, 1990, 1991). In the past, many high marsh wetlands along the Pacific coast were diked to remove them from tidal action. After the area was diked, the wetlands dried up and the land was used for pasture. In 1978, in an effort to restore the Salmon River Estuary to its original condition, two dikes were removed to allow the original wetlands to reestablish themselves. However, after 10 years, the resulting wetlands (fig. 52) were not typical of other high marshes along the estuary. The land behind the dikes had subsided over time, and the restored wetlands were more typical of wetlands at lower elevations nearer the estuary (low marsh). Although the wetlands continue to evolve as sediments are trapped and deposited by the vegetation (thus raising the elevation), it might take another 50 years for the restored wetlands to become similar again to the original high marsh (Frenkel and Morlan, 1991). The time required and the ability to develop a fully functional soil system in project wetlands may be major determinants of the eventual acceptance or rejection of restoration and creation as management options.
| It is difficult to make a definitive
statement about the ability to replace wetland functions. Goals for
restoration and creation projects seldom are stated and information on
the existing functions of the wetlands seldom are documented. This is
due, in part, to the difficulty and expense of quantifying wetland
functions. Also, responsible monitoring during construction and after
completion of the project wetland is uncommon. Most information
available on project wetlands is in the form of qualitative case
Restoration and creation can help maintain the benefits of wetlands and accommodate the human
need for development.
DESIGNING FOR SUCCESS
| Much of the written material on
wetland restoration and creation deals with "project design." Project
design considers a large number of site-specific, interdependent
factors that determine the structure and function of a wetland.
Although there is no "cookbook" for restoring or creating wetlands,
documents describing general approaches to restoration and creation
and the conditions conducive to project success are available
(Garbisch, 1986; Marble, 1990; Pacific Estuarine Research Laboratory,
1990; Hammer, 1992; Maynord and others, 1992). Elements common to
wetland project design are site-selection criteria, hydrologic
analysis, water source and quality, substrate augmentation and
handling, plant material selection and handling, buffer zones
placement, and long-term management. A brief overview of each element
is presented here in a sequence similar to that followed in project
Site selection.- Sites for project wetlands often are selected on the basis of available land, or on policies that require wetlands to be restored or created to compensate for nearby wetland losses (mitigation). A wetland's structure, function, and ability to persist over time are greatly influenced by its location. Wetlands in settings with limited human influence can differ greatly in structure and function from wetlands in settings dominated by human activities. Therefore, the present and projected land uses of the surrounding area are a consideration when selecting the site. The characteristics of existing wetlands, in the same general area, or in an area with similar land uses, can be used as models for what might be expected of the project wetland. Benefits that extend beyond the wetland itself can be derived from the placement of a wetland if care is taken in site selection. For example, restoration of riverbank wetlands between agricultural land and a stream can improve downstream water quality (Olson, 1992).
Hydrologic analysis.- Hydrologic conditions probably are the
most important determinants of the type of wetland that can be
established and what wetland processes can be maintained (Mitsch and
Gosselink, 1993). Elements of site hydrology that are important to
maintaining a wetland are inflows and outflows of ground water and
surface water, the resulting water levels, and the timing and duration
of soil saturation or flooding.
One factor influencing hydrology is the configuration of the basin (depression) containing the wetland. The position of the basin surface relative to the water table influences the degree of soil saturation and flooding. To ensure that standing water is present year round, many project wetlands are excavated so that the deepest part of the basin is below the lowest anticipated water level. The slope of the basin banks determines how much of the site will be vegetated and by what kinds of plants (fig. 53). This is because the slope determines how far the substrate (soil or rock material that forms the surface of the basin) will be from water and how much of the substrate has the necessary conditions of wetness for specific plant species (Hollands, 1990). The ability to maintain the desired plant community, therefore, is ultimately dependent on the hydrology of the site. In a properly constructed freshwater marsh, the lowest point of the wetland will be inundated to a depth and for a period long enough that emergent vegetation can persist, but not so long as to destroy the plants.
Benefits can extend beyond the wetland if care is taken in site selection.
Water source and quality.- Although it is commonly
acknowledged that site hydrology is a major determinant of the success
or failure of wetland restoration or creation, the influence of water
quality often is ignored. Inputs of chemicals from the surrounding
landscape can overwhelm a wetland's ability to improve water quality
and can change the characteristics of the site. For example, deicing
salts are used extensively along highways and, if they enter a
wetland, can alter the productivity and composition of its plant
community, possibly favoring nuisance species such as purple
loosestrife (Niering, 1989).
Substrate augmentation and handling.- Wetlands are characterized by hydric soils, which develop as a result of an area being saturated, flooded, or ponded long enough during the growing season to develop anaerobic (oxygen-deficient) conditions (U.S. Soil Conservation Service, 1991) (fig. 54). Most of the chemical reactions in wetlands take place in the soils, where most chemicals are stored (Mitsch and Gosselink, 1993). The soils of project wetlands are receiving increased attention as studies link substrate characteristics to ecological function. Although a created wetland may be structurally similar to a natural wetland, its hydrology may differ greatly from that of the natural wetland if the permeability of the substrates differ (O'Brien, 1986). In addition to differences in permeability, soils in project wetlands commonly have a smaller amount of organic matter than soils in similar natural wetlands. Because organic matter in soils stores nutrients that are critical to plant growth (Pacific Estuarine Research Laboratory, 1990), the smaller amounts of organic matter in soils of project wetlands may limit plant growth (Langis and others, 1991). Augmenting, or mulching, the substrate of project wetlands with materials from a "donor" wetland can increase soil organic matter and provide a source of needed plant species, microbes, and invertebrates. Mulching makes the substrate more conducive to rapid revegetation by reducing the evaporation of pore water, runoff, soil loss and erosion, and surface compaction and crusting (Thornburg, 1977). Mulching also can cause problems such as the introduction of unwanted plant species.
| Plant material selection and
handling.- Vegetation is the most striking visual feature of a
wetland. Because of the unique and stressful conditions that develop
in wetlands, varying from long periods of flooding to periodic drying,
plants and animals found there have developed distinctive mechanisms
to deal with these stresses and conditions. It is important to
recognize the constraints of this unique environment when planning a
project wetland. Plant communities established in project wetlands
will fare better if they closely resemble communities in similar,
local wetlands. To increase the likelihood of successful
colonization, Garbisch (1986) suggests that project managers:
Hydrologic conditions probably are the most important determinants of wetland types and processes.
Buffer zone placement.- Protective measures are needed for many
restored and created wetlands, particularly in urbanized areas. This
protection can take the form of an undeveloped, vegetated band around the
wetland; a fence or barrier; or a lake or sediment basin. This buffer
between the wetland and surrounding land is desirable; however, the
characteristics of an appropriate vegetated buffer are not well defined.
Although composition is important, width is the most frequently cited
characteristic of an adequate buffer zone. Requirements for both
composition and width are dependent upon the adjacent land uses, their
potential effect on the functions of the wetland, and the requirements of
the animals that will use the wetland and buffer area. Buffers are used
|| Long-term management.- Careful
monitoring of newly established wetlands and the ability to make
mid-course corrections are critical to long-term success. However,
few project sponsors have been willing to assume long-term
responsibility for managing these new systems (Kusler and Kentula,
1990b). Because of this, project wetlands that are designed to be
self-sustaining or self-managing will have the best chance of
survival. The installation of control structures, such as tide gates
or pumps, that will require maintenance and are subject to vandalism
could be disadvantageous to the life of the project wetland.
Chemicals from the surrounding landscape can overwhelm a wetland's ability to improve water quality.
EVALUATION OF SUCCESS
One of the most vexing aspects of wetland restoration and creation projects
is defining success, primarily because there is no generally accepted
definition. This is true for many reasons-lack of clearly stated
objectives, lack of long-term monitoring (Kusler and Kentula, 1990b), and
the subjective point of view of the definer (Roberts, 1993). The vast
majority of project wetlands are ecologically young-10 years of age or
less. The lack of information on ecologically mature projects limits the
ability to predict whether or not the functions of project wetlands can
replace the functions of natural wetlands. Nevertheless, the results of
ongoing research and good professional judgment can be used to provide
insight into the selection of projects that have a high probability of
Various attempts have been made to define success criteria for wetland projects. The earliest criteria assumed that if conditions were correct for the establishment of wetland vegetation, then other ecological functions would either be present or develop over time. Now, it is known that a site "green" with vegetation does not necessarily mean success, and the standards by which projects are judged are more likely to be tied to wetland functions.
The Wetlands Research Program of the U.S. Environmental Protection
Agency (EPA) is developing an approach to establish quantitative
performance criteria for project wetlands. In this approach, groups
of natural wetlands serve as reference sites against which project
wetlands are judged. For example, Zedler (1993) uses reference data
from natural marshes being used by clapper rails (an indigenous bird
species) to define criteria that can be used to judge the suitability
of restored and created habitat for the birds. Older project wetlands
also are used as reference sites against which to judge newer project
wetlands, both to verify that development is as expected and to
identify developmental patterns that may have resulted from changes in
project design (Kentula and others, 1992). This approach is designed
to produce results that are regionally applicable to wetland
protection and management.
| One tool for comparing the
characteristics of project wetlands with similar, naturally occurring
wetlands is a performance curve (fig.
55). Functions in a group of restored wetlands can be expected to
increase gradually with time to a point of maturity at which time the
level of function has stabilized. The mean level of function in
mature project wetlands is generally less than that for natural
wetlands. Rate and time of maturation and functional level at
maturity will differ from project to project, depending on the type of
wetland being restored. The curve provides information on when to
monitor, how restored wetlands typically develop, and when project
goals have been met. Changes in the characteristics of project
wetlands can be expected in response to the maturation process, but
also in response to changes in the environment. Information on the
development of project wetlands and similar natural wetlands helps
managers determine whether an observed change is typical for a
particular year or stage of development.
Over time, successful project wetlands can be expected to become
similar to comparable natural wetlands. A comparison of plant
diversity on project wetlands and similar natural wetlands in Oregon
(Kentula and others, 1992), Connecticut (Confer and Niering, 1992),
and Florida (Brown, 1991) showed that, although the level of diversity
differs with each project, diversity tends to be higher on each
project wetland than on its natural counterpart. The type of wetland
studied was a pond with a fringe of freshwater marsh (fig. 56). If a project wetland develops as hoped and expected, after 2 to 5 years it probably will have a
plant diversity greater than or equal to that of similar natural
wetlands. As competition for space and resources increases and the
plants more completely cover the site, the diversity usually decreases
and the plant community tends to become more like that of a mature
Plants in project wetlands fare better if they closely resemble those in similar, local wetlands.
STATUS OF THE SCIENTIFIC KNOWLEDGE OF
| Current scientific knowledge about
successful wetland restoration and creation has been documented in
"Wetland Creation and Restoration: The Status of the Science" (Kusler
and Kentula, 1990a). Although the literature on wetland restoration
and creation has increased since the publication of that book, the
general assessment presented still applies. Key points from the
Executive Summary (Kusler and Kentula, 1990b) are discussed below.
(Additional information on restoration of aquatic systems, including
wetlands, can be found in a recent publication by the National
Research Council Committee on Restoration of Aquatic Ecosystems,
The status of scientific knowledge about wetland restoration and creation differs by wetland function, type, and location. It is still uncertain if the full suite of functions provided by a particular wetland type can be replaced. Full functional replacement has not yet been demonstrated. In the case of specific functions, the most is known about replacement of flood storage and waterfowl habitat, and the least is known about water-quality-improvement and ground-water-associated functions. The more complex the hydrology and ecology of a system, the more difficult it is to restore the system. Complete restoration might be impossible in some systems.
With respect to types and locations of wetlands, the most is known about
restoration and creation of intertidal salt marshes along the coasts of the
United States, in particular, the tall cordgrass marshes of the Atlantic
coast. However, these salt marshes comprise only about 5 percent of the
total wetland area of the Nation and are only a small part of the marine
and estuarine wetlands.
Much less is known about restoration and creation of inland freshwater wetlands, such as ponds, forested wetlands, or bogs and fens. Among these wetlands, most is known about restoration and creation of those dominated by open water, such as ponds, and the associated herbaceous vegetation. Much less is known about replacing forested wetlands because of the time needed for woody vegetation to mature. Experts agree, however, that the ecosystems that are least likely to be successfully replaced are bogs and fens. These are the wetlands with deep organic soils that have developed over thousands of years and that have hydrologic conditions that are difficult, if not impossible, to duplicate.
FEDERAL AGENCY RESEARCH ON WETLAND RESTORATION AND CREATION
Several Federal agencies have missions, and therefore conduct research
activities, that involve wetlands. This section presents a brief
overview of Federal research on wetland restoration and creation.
[For more information on wetland research by Federal agencies, see the
publications of the Wetlands Research Program of the U.S. Army Corps
of Engineers (Corps) and the article "Wetland Research by Federal
Agencies" in this volume.] The Corps has been leading an effort to
provide a reference source on current wetland research being conducted
by Federal agencies. The first edition (U.S. Army Corps of Engineers,
Wetlands Research Program, 1992) presents information provided by the
Corps, the EPA, the Soil Conservation Service (renamed Natural
Resources Conservation Service in October 1994), the Forest Service,
the National Marine Fisheries Service, the U.S. Fish and Wildlife
Service, the Bureau of Reclamation, and the U.S. Geological Survey.
The Corps surveyed over 25 agencies in 1993. To complement the Corps'
reference source, the U.S. Fish and Wildlife Service is maintaining
the Wetland Creation/Restoration data base to provide a current
compilation of the published literature. A hard copy of the
bibliographic material contained in the digital data base also has
been produced (Schneller-McDonald and others, 1989).
Federal agencies' research into wetland restoration and creation
generally falls into two categories-design implementation and
performance evaluation. Major contributions on project design have
been made by agencies involved in large-scale development, like the
Corps (Maynord and others, 1992) and the Federal Highway
Administration (Marble, 1990). The EPA has focused its research on
evaluation to support the agency responsibilities under Section 404 of
the Clean Water Act (Zedler and Kentula, 1986; Leibowitz and others,
1992). Agencies responsible for stewardship of living resources, such
as the National Marine Fisheries Service, have produced information
that will increase their effectiveness in management (Thayer,
The Natural Resources Conservation Service and the U.S. Fish and Wildlife Service probably will contribute the most information on practical, low-cost approaches to wetland restoration under the 1990 Farm Bill (Food, Agriculture Conservation and Trade Act of 1990-(P.L.101-624) and the Wetland Reserve Program. Under these programs, thousands of wetland acres previously converted to agriculture have been restored to wetlands. To support these efforts, both agencies have produced guidelines for their field personnel who are working with the farmers to restore wetlands (U.S. Soil Conservation Service, 1992; Wenzel, 1992). (For more information on legislation affecting wetlands, see the article "Wetland Protection Legislation" in this volume.)
Ecosystems that are least likely to be successfully replaced are bogs and fens.
|Wetland restoration and creation is more an art than a science, and functional replacement of wetlands has not been conclusively demonstrated. At the same time, the growing body of literature and experience is increasing the ability to discern which projects have a high probability of restoring or replacing damaged or lost ecosystems. Two factors that most limit the effective use of restoration and creation are: (1) lack of information on ecologically mature restored and created wetlands, and on the maturation process; and (2) the limited number of well designed and well constructed project wetlands that can be used as models.||In general, restoration is likely to be more successful than creation. Restoration of a damaged or destroyed wetland will have a greater chance of establishing the range of prior wetland functions, including critical habitat. Also, chances are greater for the long-term persistence of a restored wetland than for one created where none existed before.|
Restoration is likely to be more successful than creation.
For Additional Information:
Mary E. Kentula,
Wetlands Research Program,
U.S. Environmental Protection Agency,
Environmental Research Laboratory,
Corvallis, OR 97333
Maintainer:h2o Webserver Team Last modified: Tue Jan 29 08:55:12 EST 2002