Status and trends of wetlands in the conterminous United States 2004 to 2009

              U.S. Fish & Wildlife Service
Report to Congress
Status and Trends of Wetlands
in the Conterminous United States
2004 to 2009
Status and Trends of Wetlands in the
Conterminous United States
2004 to 2009
T. E. Dahl
U.S. Fish and Wildlife Service
Fisheries and Habitat Conservation
Washington, D.C.
3
Acknowledgments
Many agencies, organizations, and
individuals have contributed to
the completion of this study. The
author would like to specifically
recognize the following individuals
for their contributions: From the
Fish and Wildlife Service, Bryan
Arroyo, Assistant Director, Fisheries
and Habitat Conservation; Jeff
Underwood, Deputy Assistant
Director, Fisheries and Habitat
Conservation; David J. Stout, Chief,
Division of Habitat and Resource
Conservation; Robin NimsElliott,
Deputy Chief, Division of Habitat and
Resource Conservation; and Martin
Kodis, Chief, Branch of Resource
and Mapping Support1. Editorial,
administrative and outreach
assistance was provided by Cheryl
Amrani and Jo Ann Mills, U.S. Fish
and Wildlife Service, Arlington, VA.
A Fish and Wildlife Service Technical
Review Team was responsible for
ensuring the validity of standard
operating procedures, appropriate
implementation of technological
advances and adaptations, review
of source materials, project
documentation and quality assurance
plans. This Technical Team was
composed of the following individuals:
Jim Dick, Regional Wetland
Coordinator, Albuquerque, NM;
Jerry Tande, Regional Wetland
Coordinator, Anchorage, AK;
Bill Kirchner, Regional Wetland
Coordinator, Portland, OR.
Key personnel from the U.S. Fish and
Wildlife Service, National Standards
and Support Team, Madison, WI,
contributed greatly to this effort.
Special acknowledgement goes to
Mitchell T. Bergeson, Geographic
Information Systems Specialist;
Andrew Cruz, Information
Technology Specialist; and Jane
Harner, Geographic Information
Analyst.
Additional support and assistance
for field operations and analysis was
provided by John Swords, Regional
Wetland Coordinator, Atlanta, GA;
Bill Pearson and Drew Rollman of the
Alabama Ecological Services Field
Office, Daphne, AL; Audrey Wilson,
U.S. Fish and Wildlife Service,
Albuquerque, NM.
Close cooperation with the U.S.
Environmental Protection Agency,
Office of Wetlands, Oceans and
Watersheds, Wetlands Division has
been instrumental. David Evans,
Lynda Hall, Michael E. Scozzafava,
Myra Price, Gregg Serenbetz,
Elizabeth Riley and Chris Faulkner
have generously contributed their
time and expertise to this study.
Assistance from the U.S. Geological
Survey has been provided by James
M. (Mike) Duncan and the staff
of the Commercial Partnerships
Team, National Geospatial Technical
Operations Center, Rolla, MO;
Gary Latzke, Interagency Liaison,
Wisconsin Water Science Center,
Middleton, WI; and Michelle
Greenwood, Reports Specialist,
USGS Wisconsin Water Science
Center, Middleton, WI.
Review and assistance also was
provided by Lauren B. McNamara,
Office of Environment and Energy,
U.S. Department of Housing and
Urban Development, Washington,
D.C.
1 Currently Deputy Chief, Division of Congressional and
Legislative Affairs, U.S. Fish and Wildlife Service.
4
Statistical oversight and
programming was done by
Dr. Kenneth Burnham, Statistician,
Colorado Cooperative Fish and
Wildlife Research Unit, Department
of Statistics, Colorado State
University, Fort Collins, CO.
Peer review of the manuscript was
provided by the following subject
matter experts: Dr. Mary Kentula,
U.S. Environmental Protection
Agency, National Health and
Environmental Effects Research
Laboratory, Western Ecology
Division, Corvallis, OR; Dr. Daniel
Hubbard, Department of Wildlife
and Fisheries Sciences, South
Dakota State University, Brookings,
SD; Dr. Ralph Morgenweck, Senior
Science Advisor, U.S. Fish and
Wildlife Service2; Susan-Marie
Stedman, National Oceanic and
Atmospheric Administration,
National Marine Fisheries Service-
Office of Habitat Conservation,
Silver Spring, MD; Dr. N. Scott
Urquhart, Research Scientist,
Department of Statistics, Colorado
State University3, Fort Collins,
CO; Dr. Bill O. Wilen, U.S. Fish
and Wildlife Service, Arlington,
VA; Josh Collins, Lead Scientist,
San Francisco Estuary Institute,
Oakland, CA; and Cherie L. Hagen,
Wetland Team Leader & Policy
Coordinator, Wisconsin Department
of Natural Resources, Spooner, WI.
This report is the culmination
of technical collaboration and
partnerships. A more complete
listing of some of the cooperators
appears in Appendix A.
This report should be cited as
follows:
Dahl, T.E. 2011. Status and trends
of wetlands in the conterminous
United States 2004 to 2009.
U.S. Department of the Interior;
Fish and Wildlife Service,
Washington, D.C. 108 pp.
2 Currently Scientific Integrity Officer, Department of the Interior.
3 Retired.
Funding or technical assistance
for this study was provided by
the following agencies:
Environmental Protection Agency
Department of the Army
Army Corps of Engineers -‑
Department of Agriculture
Natural Resources Conservation Service
Department of Commerce
National Oceanic and Atmospheric Administration
 National Marine Fisheries Services
Department of the Interior
Fish and Wildlife Service
Photograph by A. Cruz, USFWS
7
Preface
Members of Congress:
I am pleased to provide the U.S.
Fish and Wildlife Service’s (Service)
Status and Trends of Wetlands in the
Conterminous United States 2004
to 2009 (Report) to Congress on the
status and trends of our Nation’s
wetland resources. The Service
prepared the Report after a two
year study period and a rigorous
statistical analysis and peer review.
The Service is the principal Federal
agency that provides information to
the public on the extent and status
of the Nation’s wetlands and it
works with partner organizations to
maintain an active Federal role in
monitoring wetland habitats of the
Nation. This Report is the latest
in a continuous series spanning
50 years of wetland data. It
represents the most comprehensive
and contemporary effort to track
wetlands resources on a national
scale.
While I am heartened to note that
the Nation is making important
progress in the conservation of our
wetland resources, there is also
reason for concern and continued
diligence. Findings from this study
indicate that between 2004 and 2009,
wetland losses outdistanced wetland
gains. The reasons for these
changes are complex but they serve
as a warning signal that additional
work is needed to protect wetland
resources. In 2009, I cosigned a
letter emphasizing the importance
of the Clean Water Act and its
ramifications to the waters of the
United States including wetlands.
While we have made tremendous
strides, it is apparent that we
continue to face challenges and
wetlands continue to face pressure(s)
from the effects of sea level rise,
changes in climate, competing
demands for natural resources,
and the cumulative effects of an
array of environmental stressors.
The oil spill in the Gulf of Mexico
has reminded us of the importance
that our wetland resources play in
maintaining environmental quality,
habitat for fish, and wildlife species,
as well as supporting social and
economic pillars for the American
people.
This report does not draw
conclusions regarding trends in the
quality of the Nation’s wetlands. The
Status and Trends Study collects
data on wetland acreage gains and
losses, as it has for the past 50
years. However, the information
contained in this and previous
reports have provided a context
for the examination of wetland
condition. The process for such an
examination is already underway
and the information contained in
this report should be viewed as the
initial step in Federal partnerships.
The Administration is committed
to working with governmental,
corporate, and private partnerships
to secure and conserve our treasured
landscapes.
Ken Salazar,
Secretary, Department of the Interior
8
General Disclaimer
The use of trade, product, industry or firm names or products in this report is for informative
purposes only and does not constitute an endorsement by the U.S. Government or the Fish and
Wildlife Service.
U.S. Customary to Metric
inches (in.) × 25.40 = millimeters (mm)
inches (in.) × 2.54 = centimeters (cm)
feet (ft) × 0.30 = meters (m)
miles (mi) × 1.61 = kilometers (km)
square feet (ft2) × 0.09 = square meters (m2)
square miles (mi2) × 2.59 = square kilometers (km2)
acres (A) × 0.40 = hectares (ha)
Fahrenheit degrees (°F)  0.556 (°F – 32) = Celsius degrees (°C)
Metric to U.S. Customary
millimeters (mm) × 0.04 = inches (in.)
centimeters (cm) × 0.39 = feet (ft)
meters (m) × 3.28 = feet (ft)
kilometers (km) × 0.62 = miles (mi)
square meters (m2) × 10.76 = square feet (ft2)
square kilometers (km2) × 0.39 = square miles (mi2)
hectares (ha) × 2.47 = acres (A)
Celsius degrees (°C)  1.8 (°C) + 32) = Fahrenheit degrees (°F)
Conversion Table
9
Acknowledgments......................................................................................................................................................3
Preface......................................................................................................................................................................... 7
Executive Summary ................................................................................................................................................15
Introduction.............................................................................................................................................................. 17
Study Design and Procedures................................................................................................................................19
Study Objectives...............................................................................................................................................20
Sampling Design ..............................................................................................................................................24
Types and Dates of Imagery...........................................................................................................................27
Methods of Data Collection and Image Analysis...........................................................................................30
Wetland Change Detection ..............................................................................................................................30
Field Verification...............................................................................................................................................31
Data Quality Control........................................................................................................................................31
Statistical Analysis............................................................................................................................................32
Limitations.........................................................................................................................................................33
Attribution of Wetland Losses .........................................................................................................................34
Results....................................................................................................................................................................... 37
Status of the Nation’s Wetlands.......................................................................................................................37
National Trends, 2004 to 2009..........................................................................................................................40
Attribution of Wetland Gain and Loss, 2004 to 2009.....................................................................................41
Discussion and Analysis..........................................................................................................................................45
Marine and Estuarine Wetlands......................................................................................................................45
Changes in Sea Level and Coastal Processes Affecting Marine and Estuarine Wetlands......................54
Freshwater Wetlands........................................................................................................................................59
Additional Analysis of Recent Changes .........................................................................................................68
Wetland Restoration, Reestablishment, and Creation..................................................................................71
Potential Vulnerability of Selected Wetland Types to Climatic Changes...................................................86
Summary................................................................................................................................................................... 89
References Cited ..................................................................................................................................................... 91
Appendix A. Acknowledgment of Cooperators.....................................................................................................99
Appendix B. Definitions of Habitat Categories Used by Status and Trends..................................................101
Appendix C. Physiographic Regions of the Conterminous United States as Used in This Study...............105
Appendix D. Estimates of Acreage by Classification and Change between 2004 and 2009..........................106
Contents
10
Figure 1. Freshwater wetlands of Bon Secour National Wildlife Refuge, southern Alabama, 2010.............19
Figure 2. Permanently flooded lakes are examples of deepwater components of the study..........................22
Figure 3. Borrow pits found in association with a highway interchange have filled with water....................23
Figure 4. Numerous ponds and small residential lakes, including golf course ponds have been
created in this rapidly developing area..................................................................................................................23
Figure 5. An aerial image of artificially created ponds........................................................................................23
Figure 6. A small sized farmed wetland about 0.1 acre (0.04 ha)........................................................................24
Figure 7. Near-shore coastal wetland included salt marsh (A), shoals (B), tidal flats
(not pictured) and bars (C)......................................................................................................................................25
Figure 8. Physiographic subdivisions of South Carolina and an example of sample plot
distribution allocated in proportion to the amount of wetland area as used in this study...............................26
Figure 9. Color infrared satellite imagery (GeoEye) was used to identify and classify wetlands ................27
Figure 10. Spring flood waters cover both wetland and upland along the Lemonweir River, WI.................28
Figure 11. Early spring, leaf off imagery helped identify small wet forested pockets as shown
in this GeoEye satellite image from eastern Michigan in March 2009..............................................................29
Figure 12. Ground level view of a small wetland swale under heavy tree canopy............................................29
Figure 13. Drainage ditches visible on aerial imagery provided indicators of change....................................30
Figure14. States with field verification work (green) conducted between 2009 and 2010...............................31
Figure 15. Earthen berms divide a farm field used in rotation with other crops for commercial
rice production, Arkansas, 2010.............................................................................................................................33
Figure 16. Planted pine forest as an example of upland forested plantation, South Carolina, 2010..............35
Figure 17. Status of estuarine wetland area by type, 2009..................................................................................39
Figure 18. Status of freshwater wetland area by type, 2009...............................................................................39
Figure 19. Average annual net loss and gain estimates for the conterminous United States,
1954 to 2009...............................................................................................................................................................40
Figure 20. Estimated average annual loss of vegetated freshwater wetland area, 1974 to 2009...................41
Figure 21. Percent occurrence of freshwater pond types, 2009..........................................................................41
Figure 22. Estimated net gains and losses of wetland acres attributed to the various upland land
use categories and deepwater, 2004 to 2009...........................................................................................................42
Figure 23. Loss of freshwater forested wetland as attributed to upland and deepwater categories,
2004 to 2009...............................................................................................................................................................42
Figure 24. Wetland losses attributed to “other” landuse indicated the land may be in transition
from one land use to another and the final land use type cannot be determined.............................................43
Figure 25. This temporarily flooded wetland has reestablished naturally on lands that were part
of an agricultural program set-aside......................................................................................................................43
Figure 26. Estuarine salt marsh wetland, Florida, 2010......................................................................................46
Figure 27. The attribution of estuarine emergent losses between 2004 and 2009............................................47
Figure 28. Oil and gas field development located in estuarine wetlands of southern Louisiana....................47
Figure 29. Comparison of aerial images from 2004 and 2009 showing areas of estuarine marsh
along the northern Texas coast..............................................................................................................................48
Figure 30. An example of shoreline protection measures along the coast of southeastern Louisiana..........49
List of Figures
11
Figure 31. Man-made structures in areas of former estuarine marsh in southern Louisiana.......................50
Figure 32. Mangrove shrub wetlands along the west coast of Florida..............................................................51
Figure 33. Estimated percent area of intertidal non-vegetated wetland along the Pacific
coastline of Washington, Oregon and California compared to the coastline of the Atlantic and Gulf of
Mexico, 2009..............................................................................................................................................................52
Figure 34. The fishing pier on Dauphin Island, Alabama no longer reaches the water line as
coastal sediments have been deposited along this shore (2010)..........................................................................53
Figures 35 A and B. Sea birds rest and feed on intertidal habitats such as beaches and
tidal flats .................................................................................................................................................................53
Figure 36. Beached oil from the Deepwater Horizon oil spill, 2010...................................................................54
Figure 37. Cliffs and rocky shorelines along California’s Pacific coastline restrict any possible
migration (retreat) of coastal wetlands as sea levels rise....................................................................................55
Figure 38. Shoreline armoring and stabilization along this beach in North Carolina was designed
to protect coastal dunes and development.............................................................................................................55
Figure 39. Eroding shoreline along the Atlantic coast in Georgia......................................................................56
Figure 40. Estuarine shoreline along the northwestern Florida coast illustrated the effects of
erosion and confinement of coastal plants to a narrow beach-line.....................................................................57
Figure 41. Acreage immigration and emigration of freshwater emergent wetland, 2004 to 2009.................61
Figure 42. Gains and losses of selected wetland, upland and deepwater categories that influenced
a net gain of freshwater shrub wetland 2004 to 2009...........................................................................................62
Figure 43. A freshwater shrub wetland composed of true shrub species, Tennessee.....................................63
Figure 44. Long-term trends in freshwater shrub net changes, 1974 to 2009..................................................63
Figure 45. Long-term trends in forested wetland area as measured since the 1950s.....................................64
Figure 46. Minor drainage and the installation of ditches have been considered a normal
silviculture activity in wetlands designed to “temporarily dewater” a wetland...............................................65
Figure 47. Both long-leaf (Pinus palustris) and slash pine (Pinus elliottii) occur naturally
in southern wetlands................................................................................................................................................66
Figure 48. A former forested wetland in South Carolina one year following clear-cut...................................67
Figure 49. This study found particular regions of the conterminous United States experienced
different rates of wetland loss depending on many factors.................................................................................69
Figures 50 A. Originally, approximately 93 percent of the land area pictured was vegetated
wetland with level, poorly drained or very poorly drained hydric soils (NRCS 2010) typical
of the sloughs and wet flatwoods of south Florida (Liudahl et al. 1989)............................................................70
Figure 50 B. Updated loss information showing cumulative wetland losses 1998 to 2004
and 2004 to 2009........................................................................................................................................................70
Figure 51. Remnant cypress (Taxodium sp.) remain as part of a former forested wetland
complex in south Florida.........................................................................................................................................71
Figure 52. This series of image maps illustrate the end result of a 121 acre (49.0 ha) wetland
reestablishment project in southern Wisconsin....................................................................................................75
Figure 53. Former aquaculture ponds in west-central Mississippi supported wetland emergent
plant growth in 2009.................................................................................................................................................77
Figure 54. A created pond in an urban subdivision has been used to drain an adjacent vegetated
wetland and serves as a retention basin to compensate for the increase in impervious surface
from the development..............................................................................................................................................77
Figure 55. Distribution of created ponds in the conterminous United States..................................................78
Figure 56. Many created wetlands share common characteristics of a deeper open-water basin
ringed by a band of emergent vegetation..............................................................................................................79
12
Table 1. Wetland, deepwater, and upland categories used to conduct the wetland status and
trends study..............................................................................................................................................................21
Table 2. Change in wetland area for selected wetland and deepwater categories, 2004 to 2009....................38
Table 3. Status and changes to intertidal marine and estuarine wetlands, 2004 to 2009.................................46
Table 4. Status and changes in freshwater wetland types between 2004 to 2009.............................................59
Table 5. Wetland types identified in this study exhibiting change in extent or distribution
from climatic conditions...........................................................................................................................................87
List of Tables
15
Executive
Summary
This study examined recent trends
in wetland extent and habitat type
throughout the conterminous United
States between 2004 and 2009.
Wetland trends were measured
by the examination of remotely
sensed imagery for 5,042 randomly
selected sample plots. This imagery
in combination with field verification
provided a scientific basis for
analysis of the extent of wetlands
and changes that had occurred over
the four and half year time span in
this study.
This information provides a
quantitative measure of the areal
extent of all wetlands, regardless
of ownership, in the conterminous
United States. Wetlands were
defined using biological criteria
and standardized nomenclature
for the classification of wetland
types. Recently acquired remotely
sensed imagery was used as
the principle means to assess
wetland change with a number
of geoprocessing and quality
control measures implemented
to ensure data completeness and
accuracy. The spatial sample design
involved randomized sampling of
geospatial information on 4.0 mi2
(10.4 km2) plots. This was a well-established,
time-tested procedure
that provided a practical, scientific
approach for measuring wetland
area extent (status) and change
rates (trends) in the conterminous
United States. Statistical estimates
provided national status and change
information as well as estimates
by major wetland type. Field
verification was completed for 898
(18 percent) of the sample plots
during 2009 to 2010. Field sites were
dispersed in portions of 42 States.
Enhancements to this study
included augmentation to the
number of sample plots along the
Pacific coast of Washington, Oregon
and California. This augmentation
was done to provide estimates of
estuarine and marine wetlands
not included in the original sample
design and provide a more complete
estimate for these wetland types
nationally.
Because of the increased area
of created freshwater ponds in
recent years, additional descriptive
categorization for freshwater ponds
was developed and implemented
as part of this study. Further
categorization of the physical
and ecological characteristics of
freshwater ponds was intended to
provide information about what
types of ponds have been created
over time.
This report did not draw conclusions
regarding trends in the quality
or condition of the Nation’s
wetlands, but rather it provided
data regarding trends in wetland
extent and type and provided
baseline information to facilitate
ongoing collaborative efforts to
assess wetland condition. Further
examination of wetland condition on
the national level has been initiated
by the Environmental Protection
Agency in conjunction with the
Fish and Wildlife Service and other
Federal, State and Tribal partners.
16
The study indicated that there were
an estimated 110.1 million acres
(44.6 million ha) of wetlands in the
conterminous United States in 20094
(the coefficient of variation of the
national estimate was 2.7 percent).
An estimated 95 percent of all
wetlands were freshwater and
5 percent were in the marine or
estuarine (saltwater) systems. With
the exception of minor statistical
adjustments to the area estimates,
the overall percentage of wetland
area and representation by saltwater
and freshwater components
remained unchanged.
Estuarine emergent (salt marsh)
wetland was the most prevalent
type of all estuarine and marine
intertidal wetland. Salt marsh made
up an estimated 66.7 percent of
all estuarine and marine wetland
area. Forested wetlands made
up the single largest category
(49.5 percent) of wetland in the
freshwater system. Freshwater
emergents made up an estimated
26.3 percent, shrub wetlands
17.8 percent and freshwater ponds
6.4 percent by area.
The difference in the national
estimates of wetland acreage
between 2004 and 2009 was not
statistically significant. Wetland
area declined by an estimated
62,300 acres (25,200 ha) between
2004 and 2009. The reasons for this
are complex and potentially reflect
economic conditions, land use trends,
changing wetland regulation and
enforcement measures and climatic
changes. Certain types of wetland
exhibited declines while others
increased in area. The result of these
gains and losses yielded the net
change and it was possible to have
losses or gains of particular wetland
types that exceed the overall net
change for all wetlands.
Collectively, marine and estuarine
intertidal wetlands declined by an
estimated 84,100 acres (34,050 ha)
or an estimated 1.4 percent between
2004 and 2009. The majority of
these losses (73 percent) were to
deepwater bay bottoms or open-ocean.
Losses of estuarine emergent
(salt marsh) and changes in marine
and estuarine non-vegetated
wetlands reflected the impacts of
coastal storms and relative sea
level rise along the coastlines of the
Atlantic and Gulf of Mexico. The
majority (99 percent) of all estuarine
emergent losses were associated
with processes related to the marine
environment such as saltwater
inundation and/or coastal storm
events. The effects of sea level on
wetlands are subject to considerable
uncertainties; however, recent
changes in non-vegetated intertidal
wetlands (beaches, bars and shoals)
along the South Atlantic and Gulf
of Mexico indicated considerable
instability and change. Coastal
environments continue to face a
variety of stressors that can interact
with climate-related processes and
potentially increase the vulnerability
of coastal wetlands.
Overall, freshwater wetlands
realized a slight increase in
area between 2004 and 2009.
Freshwater ponds have continued
to increase although the rate of
pond development had slowed
from previous reporting periods.
Freshwater vegetated wetlands
continued to decline albeit at a
reduced rate. This most recent
annual rate of loss represented a
reduction in the loss rate of roughly
50 percent since 2004. Declines
in freshwater forested wetland
area (633,100 acres or 256,300 ha)
negated area gains in freshwater
emergent and shrub categories.
Forested wetlands sustained their
largest losses since the 1974 to 1985
time period. Freshwater wetland
losses continued in regions of the
country where there has been
potential for wetlands to come into
conflict with competing land and
resource development interests.
Between 2004 and 2009,
489,600 acres (198,230 ha) of former
upland were re-classified as wetland.
These increases were attributed
to wetland reestablishment and
creation on agricultural lands and
other uplands with undetermined
land use including undeveloped land,
lands in conservation programs
or idle lands. The rate of wetland
reestablishment increased by an
estimated 17 percent from the
previous study period (1998 and
2004). Conversely, the estimated
wetland loss rate increased
140 percent during the same time
period and, as a consequence,
national wetland losses have
outdistanced gains.
The cumulative effects of losses in
the freshwater system have had
consequences for hydrologic and
ecosystem connectivity. In certain
regions, profound reductions in
wetland extent have resulted
in habitat loss, fragmentation,
and limited opportunities for
reestablishment and watershed
rehabilitation.
4 This estimate has been revised to reflect 2010
wetland status as well as the addition of wetland
area in the coastal zone of the Pacific coast for
WA, OR, and CA as described in the Sample
Design section of this report.
17
Introduction
The mission of the U.S. Fish and
Wildlife Service (Service) is to
conserve, protect, and enhance fish,
wildlife, plants, and their habitats
for the continuing benefit of the
American people. The Service
has been entrusted with legal
authorities and responsibilities
for fish and wildlife conservation
including the management of fish
and wildlife populations; conserving
endangered and threatened
species, inter-jurisdictional fish,
and migratory birds; managing
an extensive conservation land
base; and collaborating in carrying
out conservation activities under
international conventions, treaties,
and agreements. The Service
communicates information
essential for public awareness and
understanding of the importance
of fish and wildlife resources and
changes reflecting environmental
conditions that ultimately will affect
the welfare of people.
Wetlands are transitional from
true aquatic habitats to dry land
(upland) and as a result, their
abundance, type, and condition
are directly reflected in the health
and abundance of many species. In
1986, the United States Congress
enacted the Emergency Wetlands
Resources Act (Public Law 99-645)
recognizing that wetlands are
nationally important resources
and that these resources have
been affected by human activities.
Under the provisions of this Act,
the Service is required to update
wetland status and trends studies
of the Nation’s wetlands at 10 year
intervals. To date, there have been
five national reports on wetland
status with this study being the
latest. Recently, Congress has
considered a number of policy issues
that involve wetlands. Some of these
reflect long-standing interests of the
Federal government and influence a
number of incentive and disincentive
measures to conserve wetlands and
if possible increase both the extent
and improve the environmental
quality aspects wetlands provide
(Copeland 2010). This study tracks
and quantifies wetland losses,
reestablishment (restoration) or
creation and provides a measureable
element to gauge Federal policy
success and provide information
crucial to understanding this
important resource type.
There has been keen interest in
wetland trends since the Supreme
Court decisions in 2001 and 2006
that narrowed the interpretation of
the scope of waters and wetlands
protected by the Clean Water Act5.
Previous information on wetland
trends pre-dated the 2006 Rapanos
and Carabell decisions (Rapanos v.
United States and Carabell v. United
States) and changes in the wetland
regulatory process. The Supreme
Court decisions narrowed the prior
interpretation of the scope of waters
protected under the Clean Water Act
and agencies have faced challenges
implementing those decisions
(Council on Environmental Quality
2009). The effects of those decisions
are reflected in the data collected
between 2004 and 2009 and reported
here.
5 The 1977 amendments, the Clean Water Act
of 1977 [P.L. 95-217].
18
Since 2004, several severe
hurricanes have struck the coastline
along the Gulf of Mexico and these
data afford an indication of wetland
area changes sustained as a result of
those storms.
In addition, the wetland extent
information presented in this report
has important uses by resource
managers as they interpret the
role of wetlands on the national
landscape. This study was designed
to provide scientific information to
resource specialists and decision
makers about wetlands resource
trends. These data help guide
decisions on wetland-related
issues, such as reestablishment and
enhancement, endangered species
habitat availability, possible changes
resulting from climatic change,
strategic habitat conservation, and
ecosystem management planning.
Wetland status and trends data
continue to be used extensively
by Federal, State, local and Tribal
governments to develop wetland
conservation strategies, strategic
management actions, and validate
performance toward halting loss and
reestablishing wetlands.
The goals of this study were to:
•• Describe the resource type,
extent, trends and reporting the
results for the Nation through
time;
•• Maintain survey integrity and
avoid bias;
•• Provide relevant, contemporary
data to aid in assessment or
formulation of policy;
•• Establish high standards
for data quality; and update
procedures to incorporate new
and proven technologies and
enhancements.
In 2004, the Service’s Wetlands
Status and Trends data indicated
that for the first time there had
been a net increase in wetland area
(estimated gain of 32,000 acres
or 12,900 ha) between 1998 and
2004; however, qualitative aspects
of wetlands remained unknown.
Since 2000, observed changes in
wetland type(s) and the continued
loss of freshwater vegetated
wetlands coupled with increases
in freshwater ponds have raised
questions regarding the ecological
integrity of the existing wetlands.
As more comprehensive assessment
of wetland condition has become a
higher priority for Federal agencies,
this study has contributed relevant
data on wetland type, location, and
extent to be used as part of the
first national wetland condition
assessment currently being
conducted by the Environmental
Protection Agency (EPA). The
Service has worked closely with
EPA in preparation for the National
Wetland Condition Assessment
Study scheduled to be released
in 2013. The two agencies have
been collaborating on a number
of technical monitoring and data
collection efforts. The potential
outcome of these studies on wetland
quantity and quality will assist
in further assessment of wetland
status and efficacy of programs and
policies.
The Service has continued to work
closely with other key partner
organizations and this multi-agency
involvement has enhanced the
wetlands status and trends study
design, data collection, verification,
peer review and data applications
to address challenges of resource
management, research and policy
formulation. In 2009, collaboration
with the National Oceanic and
Atmospheric Administration
(NOAA–Fisheries), produced a
report based on further analysis
of the 1998 to 2004 national status
and trends information for the
coastal watersheds of the Atlantic,
Gulf of Mexico, and Great Lakes.
The results of that effort indicated
that coastal watersheds were losing
wetlands despite the national trend
of net gains, and pointed to the
need for an expanded effort on
conservation of wetlands in those
coastal watersheds. These findings
have stimulated subsequent actions
from agencies addressing the need
for further policy considerations and
focused conservation measures in
those coastal areas.
Continued monitoring of wetland
resources has been widely
considered essential for identifying
changes in the wetland community
type, spatial extent, and guiding
additional research or management
actions. This information combined
with historical perspectives increase
our understanding of landscape
patterns and processes.
19
Study Design
and Procedures
Figure 1. Freshwater wetlands of
Bon Secour National Wildlife Refuge,
southern Alabama, 2010.
20
Study Objectives
This study was designed to
provide the Nation with current,
scientifically valid information on
the status and extent of wetland
resources and to measure change
in those resources over time. It is
a quantitative measure of the areal
extent of all wetlands, regardless
of ownership, in the conterminous
United States and provides no
indication of wetland quality outside
of the changes in wetland area, by
category.
Wetland Definition and Classification
During the mid-1970s, the Fish and
Wildlife Service began work on a
biological definition of wetland and
standardized nomenclature for the
classification of wetland types. This
system described by Cowardin et al.
(1979) was adopted as a standard
by the Service and subsequently
became a Federal Geographic Data
Committee (FGDC) Standard for
mapping, monitoring, and reporting
on wetlands (FGDC 1996). This
institutionalization of a biological
definition and classification system
has facilitated its use in each of the
national wetland status and trends
studies and has provided consistency
and continuity by defining the
biological extent of wetlands and
common descriptors for wetland
types.
This study continued the use of the
Cowardin et al. (1979) definition of
wetland. It is a two-part definition
as indicated below:
Wetlands are lands transitional
between terrestrial and aquatic
systems where the water table
is usually at or near the surface
or the land is covered by shallow
water.
For purposes of this classification,
wetlands must have one or more
of the following three attributes:
(1) at least periodically, the
land supports predominantly
hydrophytes, (2) the substrate
is predominantly undrained
hydric soil, and (3) the substrate
is nonsoil and is saturated with
water or covered by shallow water
at some time during the growing
season of each year.
Cowardin et al. (1979) and other
researchers (Gosselink and Turner
1978; Mitsch and Gosselink 1993)
recognized that hydrology was
universally regarded as the most
basic feature of wetlands and
that hydrology, not the presence
of vegetation, determines the
existence of wetland (Cowardin
and Golet 1995). For this reason,
in areas that lack vegetation or
soils (e.g., mud flats, sand or gravel
bars, and shorelines), hydrology
determines that these areas are
wetlands.
21
Ephemeral waters6, which are not
recognized as a wetland type, and
certain types of “farmed wetlands”
as defined by the Food Security
Act were not included in this study
because they do not meet the
Cowardin et al. definition. Habitat
category definitions including the
latest categorization of freshwater
ponds developed for this study are
given in synoptic form in Table 1.
Complete definitions of wetland
types and land use categories
used in this study are provided in
Appendix B.
Deepwater Habitats
Wetlands and deepwater habitats
are defined separately by Cowardin
et al. (1979) because the term
wetland does not include deep,
permanent water bodies. Deepwater
habitats are permanently flooded
land lying below the deepwater
boundary of wetlands (Figure 2).
Deepwater habitats include
environments where surface water
is permanent and often deep, so
that water, rather than air, is the
principal medium in which the
dominant organisms live, whether
or not they are attached to the
substrate. For the purposes of
conducting status and trends work,
all lacustrine (lake) and riverine
(river) waters were considered
deepwater habitats.
Upland Categories
Upland included lands not meeting
the definition of either wetland or
deepwater habitats. An abbreviated
upland classification system
patterned after the U. S. Geological
Survey land classification scheme
described by Anderson et al. (1976),
with five generalized categories,
was used to describe uplands in
this study. These upland categories
as well as all other wetland and
deepwater categories are listed in
Table 1.
Table 1. Wetland, deepwater, and upland categories used to conduct the
wetland status and trends study. The definitions for each category appear in
Appendix B.
Salt Water Habitats Common Description
Marine Subtidal* Open Ocean
Marine Intertidal Near shore
Estuarine Subtidal* Open-water/bay bottoms
Estuarine Intertidal Emergents Salt marsh
Estuarine Intertidal Forested/Shrub Mangroves or other estuarine shrubs
Estuarine Intertidal Unconsolidated Shore Beaches/bars
Riverine* (may be tidal or non-tidal) River systems
Freshwater Habitats
Palustrine Forested Forested swamps
Palustrine Shrub Shrub wetlands
Palustrine Emergents Inland marshes/wet meadows
Palustrine Farmed Farmed wetlands
Palustrine Unconsolidated Bottom (ponds) Open-water ponds/aquatic bed
Pond – Natural characteristics Small bog lakes, vernal pools, kettles, beaver
ponds, alligator holes
Pond – Industrial Flooded mine or excavation sites (including
highway borrow sites), in-ground treatment
ponds or lagoons, holding ponds
Pond – Urban use Aesthetic or recreational ponds, golf course
ponds, residential lakes, ornamental ponds,
water retention ponds
Pond – Agriculture use Ponds in proximity to agricultural, farming
or silviculture operations such as farm ponds,
dug outs for livestock, agricultural waste
ponds, irrigation or drainage water retention
ponds
Pond - Aquaculture Ponds singly or in series used for aquaculture
including cranberries, fish rearing
Lacustrine* Lakes and reservoirs
Uplands
Agriculture Cropland, pasture, managed rangeland
Urban Cities and incorporated developments
Forested Plantations Planted or intensively managed forests;
silviculture
Rural Development Non-urban developed areas and
infrastructure
Other Uplands Rural uplands not in any other category;
barren lands
*Constitutes deepwater habitat
6 This refers to temporary surface water
and should not be confused with ephemeral
(temporary) wetlands.
22
Addition of Descriptive Categories
for Freshwater Ponds
This study was designed as a
scientific approach to monitor the
Nation’s wetlands using a consistent,
biological definition. Cowardin
et al. (1979) recognized ponds as an
important component of the aquatic
ecosystem and included them within
a larger system of freshwater
wetlands. This classification system
for wetlands became a Service
Standard (USFWS 1980) as well as
the FGDC standard for monitoring
and reporting on wetlands (FGDC
1996). Open water ponds have been
included in every wetland status
and trends report conducted by the
Service using the Cowardin et al.
classification system. These past
studies have provided a quantitative
measure of the areal extent of all
wetlands in the conterminous United
States. Qualitative assessment of
wetland function was beyond the
scope of the status and trends study
objectives.
Because of the proliferation of
created open water ponds in recent
years, there have been questions
regarding the ecological implications
of increasing the number and area
of open water wetlands identified
during the 2005 wetlands status
and trends analysis. In 2006, EPA
and the Service began working
together to design a method for
further categorizing the physical
characteristics and ecological
contributions of freshwater ponds
on the landscape. As a result of
that effort, additional descriptive
categories for freshwater ponds have
been added as part of this study.
This information was intended to
provide users with additional insight
about what types and how many
ponds were created over time.
Water features that have been
excluded from this study as non-wetland
include stock watering
tanks, swimming pools, industrial
waste pits, stormwater drains (non-retention
features), garden ponds
or fountains (coy or koi ponds),
water treatment facilities, municipal
or industrial water storage tanks,
sewage treatment facilities (other
than wetlands designed to filter
effluent), water cooling towers or
tanks, road culverts or ditches, and
other “ephemeral” waters.
Further subdivision of freshwater
ponds (palustrine unconsolidated
bottom wetlands) was carefully
considered to allow the
re-aggregation of the data to
the original classification unit
(all ponds). Another important
consideration was the ability
to accurately determine the
appropriate descriptive pond
category by the use of remotely
sensed imagery. Pond descriptive
categories were field tested to
ensure that a consistent scientific
approach was implemented
and the descriptive terms
used would provide users with
additional information about pond
characteristics and numbers.
Five descriptive categories of
freshwater ponds were used as part
of this study. These are listed below
together with a brief description of
characteristics and remote sensing
indicators used to identify and
classify these areas.
Figure 2. Permanently flooded lakes are
examples of deepwater components of the
study (Jackson Lake, Wyoming, 2010).
tac11-0632_fig 03
23
Freshwater Pond Categories:
Descriptive Types
(1) Ponds with natural features or
characteristics as indicated by lack of
human modification or development.
These include naturally occurring ponds,
bog lakes, vernal pools, potholes, kettles,
beaver ponds, alligator holes, etc.
(2) Ponds used for industrial purposes
such as mine reclamation sites, excavated
pits or mine drainage ponds, highway
borrow pits (Figure 3), sewage lagoons,
and other wetlands designed to filter
effluent, and industrial holding ponds.
(3) Urban ponds built and used for
aesthetics or recreational purposes such
as golf course ponds, small (<20 acres)
residential lakes, ornamental water
bodies, water retention basins (Figure 4).
(4) Ponds found in conjunction to
agriculture, farming, or silvicultural
operations such as farm ponds, dug outs
for livestock, agricultural waste ponds,
irrigation or sediment retention ponds.
(5) Aquaculture ponds that occur singly
or in series (Figure 5) and are used for
some form of aquaculture including
fish or shellfish rearing. Commercial
cranberry growing operations also are
placed in this category.
Figure 3. (Top) Borrow pits (indicated
by the blue arrows) found in association
with a highway interchange have filled
with water (color infrared aerial image).
The shape and proximity of these ponds
provided good indicators for further
descriptive categorization.
Figure 5. (Bottom) An aerial image of
artificially created ponds (blue and green
geometric shapes). Ponds in series provided
indicators of aquaculture operations
such as the catfish farm shown here
(Mississippi, 2009).
Figure 4. (Middle) Numerous ponds and
small residential lakes (indicated by the
red arrows), including golf course ponds
(blue arrows) have been created in this
rapidly developing area. These types of
ponds were classified as “urban ponds” in
this study.
Figure 6. A small sized farmed wetland about 0.1 acre (0.04 ha). Findings from
this study indicated that wetlands smaller than 1 acre were routinely detected
as part of the survey, however, there was no assurance that all wetlands less than
the minimum target size were identified.
24
Sampling Design
Sample-based surveys and
monitoring methods such as
those used in this study have
been an effective means to gather
information regarding various
resource types. Because continued
pressures on wetland resources
require effective monitoring at
temporal and spatial scales that are
useful for contributing to wetland
conservation efforts, resource
managers, researchers, and policy
makers have come to rely on
recent wetlands status and trends
information.
This study used a practical,
scientific approach for measuring
wetland area extent (status)
and change rates (trends) in the
conterminous United States. The
development of the target population
for wetlands, sample frame,
probabilistic sampling procedures
and the recent improvements used
have been described in previous
reporting (Dahl 2000; 2006) and
further reviewed in detail (Dahl in
manuscript). The study measured
wetland extent and change using
a statistically stratified, simple
random sampling design. The
foundations and scientific principles
underlying such surveys are well
developed and have been applied
for several iterations of national
reporting. These techniques have
been used to monitor conversions
between ecologically different
wetland types, as well as measure
wetland gains and losses in area.
The essentials of survey design
provide the basis for (a) selecting a
subset of sampling units from which
to collect data, and (b) choosing
methods for analyzing the data.
Olsen et al. (1999) have described
the conceptual relationships among
the key elements in a probabilistic
sampling survey design. These
same elements were incorporated
in the design of this study as
initially developed and implemented
by interagency statisticians.
Sample plots were examined
with the use of remotely sensed
imagery in combination with field
reconnaissance work to determine
wetland change.
Monitoring All Wetlands
To monitor changes in wetland
area, the 48 conterminous States
were stratified or divided by State
boundaries and 35 physiographical
subdivisions described by Hammond
(1970) and shown in Appendix C.
Habitats were identified primarily
by the analysis of imagery, and
wetlands were identified based on
vegetation, visible hydrology, and
geography. There was a margin
of error inherent in the use of
imagery, thus detailed on-the-ground
inspection of any particular
site may result in revision of the
wetland boundaries or classification
established through image analysis
(Dahl and Bergeson 2009). The
accuracy of image interpretation
depended on the quality of the
imagery, the experience of the image
analysts, the amount and quality of
the collateral data, and the amount
of ground truth verification work
conducted. The minimum targeted
delineation size for wetlands was
1 acre (0.40 ha). Results from this
and past status and trends studies
indicated the minimum feature
routinely delineated was about
0.1 acre (0.04 ha), but there was no
assurance that all wetlands this size
were detected (Figure 6).
A
B
C
25
Some natural resource assessments
stop at county boundaries or at a
point coinciding with the census
line for inhabitable land area. Doing
so may exclude offshore wetlands,
shallow water embayments or
sounds, shoals, sand bars, tidal flats,
and reefs (Figure 7). These were
important resources to quantify
and monitor especially in light of
climatic change(s) that may result in
sea level rise7. This study included
wetlands in coastal areas by adding
a supplemental sampling stratum
along the coastal fringes of the
conterminous United States. This
stratum included the near shore
areas of the coast with its barrier
islands, coastal marshes, exposed
tidal flats and other offshore
features not a part of the landward
physiographic zones.
The coastal zone stratum of the
Atlantic and Gulf of Mexico included
28.2 million acres (11.4 million ha).
At its widest point in southern
Louisiana, this zone extended
about 92.6 mi (149 km) from Lake
Pontchartrain to the farthest extent
of estuarine wetland resources.
In this area, saltwater was the
overriding influence on biological
systems. The coastal zone as
described in this study was not
synonymous with any State or
Federal jurisdictional coastal zone
definitions. The legal definition of
“coastal zone” has been developed
for use in coastal demarcations,
planning, regulatory and
management activities undertaken
by other Federal or State agencies.
A substantial enhancement to
this study included the addition
of 290 supplemental sample plots
to form a coastal stratum along
the Pacific coast of Washington,
Oregon, and California. These
plots were randomly distributed
within an additional stratum that
approximated the extent of coastal
watersheds. Sampling included
all types of wetlands (fresh and
saltwater) that were physically
located within the 8-digit Hydrologic
Unit Code areas (watersheds)
that drained directly to the Pacific
Ocean. The number of sample
plots was determined by the total
area within the stratum. Working
in cooperation with the EPA and
NOAA, this sampling stratum was
incorporated as part of the national
sampling effort. In the past, Pacific
coast estuarine wetlands, such
as those in San Francisco Bay,
California; Coos Bay, Oregon; or
Puget Sound, Washington, were not
extensively sampled because they
occurred in discontinuous patches
that precluded establishment of a
coastal stratum similar to that of the
Gulf and Atlantic coast (Dahl 2006).
Improved geographic information
systems and increased knowledge
of wetland distribution allowed
the Pacific coastal wetlands to be
incorporated as part of this update.
Augmentation was done to provide
estimates of estuarine and marine
wetlands not included in the original
sample design and provide a more
complete estimate for these wetland
types nationally.
Figure 7. Near-shore coastal wetland included salt marsh (A), shoals (B),
tidal flats (not pictured), and bars (C).
7 Including other catastrophic events such as
hurricanes and tropical storms..
Coastal
Zone
Appalachian
Highlands
Dry
Wet
Sample Plot Location
Gulf-Atlantic Rolling Plain
Gulf-Atlantic
Coastal Flats
Figure 8. Physiographic subdivisions of South Carolina and an
example of sample plot distribution allocated in proportion to the
amount of wetland area as used in this study.
26
To permit even spatial coverage
of the sample plots, the 36
physiographic regions formed by
the Hammond subdivisions and
the coastal zone stratum were
intersected with State boundaries to
form multiple subdivisions or strata.
An example of this stratification
approach and how it relates to
sampling intensity is shown for
South Carolina (Figure 8).
Weighted, stratified sample
plots were randomly allocated in
proportion to the amount of wetland
acreage expected to occur in each
physiographic strata described
above. Each sample area was a
surface plot 2.0 mi (3.2 km) on a
side or 4.0 mi2 of area equaling
2,560 acres (1,036 ha). Plots
were examined at two different
time periods (2004 and 2009) to
determine wetland type, extent, and
change between the two periods.
Stratification of the Nation based on
differences in wetland density made
this study an effective measure
of wetland resources as it offered
ecological, statistical, and practical
advantages for determining wetland
acreage trends and monitoring
conversions between ecologically
different wetland types. These
plots formed a geospatially fixed,
permanent sampling network. Such
monitoring networks provide the
advantage of measuring cumulative
impacts accurately over time (Smith
2004).
Because declining wetland loss
rates require finite measurement
techniques to ensure a high
degree of statistical reliability, the
sample size of this study has been
systematically augmented with
additional sample plots since the
late 1990s. The area analyzed in this
study was comprised of 5,042 sample
plots (total area equal to 20,192 mi2
or 51,893 km2).
27
Types and Dates
of Imagery
Remotely sensed imagery has
become an invaluable source for
ecological characterization, land
cover survey, and change detection
(Miller and Rogan 2007). Various
commercial satellite platforms
with improved spatial resolution
and sensors have made detailed
imagery more readily available and
applicable to wetlands identification,
classification, and monitoring
work. The comparison of historical
and recent imagery to determine
change increases our understanding
of natural and human-induced
processes at work on the landscape
(Jenson 2007).
In this study, image analysts relied
primarily on observable physical or
spectral characteristics evident on
high altitude imagery, in conjunction
with collateral data, to make
decisions regarding wetland extent
and classification8. Remote sensing
techniques to detect and monitor
wetlands in the United States and
Canada have been used successfully
by a number academic researchers
and governmental agencies (Frohn
et al. 2009; Jenson 2007; Dechka
et al. 2002; Watmough et al. 2002;
McCoy 2005; National Research
Council 1995; Patience and Klemas
1993; Lillesand and Kiefer 1987).
The use of remotely sensed imagery,
either from aircraft or satellite,
has been a cost effective way to
conduct surveys over expansive
areas (Dahl and Watmough 2007)
and the frequency and repeatability
of remotely sensed information
is invaluable for detecting and
monitoring changes on the landscape
(Rogan et al. 2002). The Fish and
Wildlife Service has successfully
used remote sensing techniques to
determine the biological extent of
wetlands for the past 35 years.
Recent imagery from multiple
platforms and direct on-the-ground
observations were used to determine
wetland changes. Only high quality
imagery was used and in some
instances multiple dates of imagery
were acquired to better determine
wetland extent and change. To
recognize and classify wetland
vegetation, color infrared imagery
was preferred (Figure 9).
8 Analysis of imagery was supplemented
with substantial field work and ground
observations.
Figure 9. Color infrared satellite imagery (GeoEye) was used to identify and classify wetlands. Several
wetland basins and cover types (indicated by arrows) were evident in this example from Florida, 2008.
28
Past studies found that leaf-off
(early spring or late fall) imagery
worked well to detect some types
of wetlands under forested canopy;
however, changes in cyclical climatic
conditions are increasingly forcing
reassessment of the timing of
image capture in some regions.
Imagery obtained when vegetation
was dormant allowed for better
identification of wetland boundaries
as long as this timing did not
coincide with seasonal flood events,
drought, or wildfires that prevented
accurate landscape characterization
(Figure 10). For some habitat types
such as forested wetlands, there
have been distinct advantages to
using leaf-off imagery to detect the
extent of early season inundation.
Under most circumstances, leaf-off
imagery enhanced the visual
evidence of hydrologic conditions
such as saturation, flooding, or
ponding in closed canopy habitats
(Figures 11 and 12). However, for
other wetland types, mid-growing
season may offer advantages for
wetland detection. Jensen (2007)
points out that the best time of
imagery acquisition for detecting
smooth cordgrass (Spartina
alterniflora) in South Carolina’s
salt marshes was from July through
October. Thus, the optimum time
to obtain imagery depended
on many factors including the
resource extent, habitat type, and
seasonal conditions. The use of
additional sources of information
to complement remotely sensed
imagery has always been important
for accurate analysis. Imagery
combined with collateral data
sources such as soil surveys,
topographic maps, and wetland
or vegetation maps were used to
identify and delineate the areal
extent of wetlands in this study.
Multiple sources of satellite imagery
in combination with recently
acquired digital photography were
used to complete this study. Satellite
imagery made up about 40 percent
of the source imagery and offered
the advantage of higher resolution
digital imagery that had been
acquired close to the target date.
Satellite imagery was supplemented
with National Agriculture Imagery
Program (NAIP) imagery
acquired during the agricultural
growing season. NAIP and other
sources of aerial imagery made
up about 60 percent of the source
imagery analyzed. (For technical
specifications of NAIP imagery
see: http://www.fsa.usda.gov/FSA/.)
The mean date of the imagery used
to complete this study was 2009,
thus there was a 4.5 year mean
differential between target dates
(2004 to 2009).
Figure 10. Spring flood waters cover both wetland and upland along the Lemonweir River, WI. Extreme climatic conditions
can negate the value of early spring (leaf-off) imagery intended to aid in the identification of wetland habitats.
tac11-0632_fig11
Figure 11. (Top) Early spring, leaf off imagery helped identify small wet forested pockets (green arrows
indicate some example areas) as shown in this GeoEye satellite image from eastern Michigan in March 2009.
Figure 12. (Bottom) Ground level view of a small wetland swale under heavy tree canopy.
29
30
Methods of Data
Collection and
Image Analysis
The identification of wetlands
through image analysis forms
the foundation for deriving all
subsequent products and results.
Consequently, a great deal of
emphasis has been placed on the
quality of the image interpretation9.
Information on the elements of
image interpretation techniques
have been discussed by a number
of authors (Jensen 2007; Philipson
1996; Lillesand and Kiefer 1987).
Specific protocols used for image
interpretation of wetlands in this
study have been documented by
Dahl and Bergeson (2009). Wetlands
were identified based on vegetation,
visible hydrology, and physical
geography. Delineations on the
sample plots reflected ecological
change or changes in land use that
influenced the size, distribution, or
classification of wetland habitats.
Wetland Change
Detection
Technological advances in the
acquisition of remotely sensed
imagery and computerized mapping
techniques often provide the ability
to capture more detailed information
about Earth objects. The integration
of Geographic Information
Systems (GIS) and remote sensing
for ecological monitoring has
become even more important as
technologies have improved and
ecological assessments address
more challenging issues (Miller
and Rogan 2007). The use of such
technologies as part of this study
provided tremendous advantages
for producing higher quality natural
resource information including
wetland location, extent and type.
In this study, change detection
and analysis involved identifying
wetland gains and/or losses, cover
type changes as well as upland
land use changes. To determine
changes between eras required
the comparison of the existing
sample plot information from
the past era (circa 2004) to more
recent imagery for the same area
(circa 2009). Changes in wetland
area represented realistic and
logical analysis, avoiding any false
or unlikely changes10. All change
information was carefully scrutinized
and verified. Examination of sites
in the field or the use of collateral
data assisted in this process. To
ensure accuracy, the temporal
dynamics of wetlands and the
subtleness of many of the wetland
alterations required substantial
reliance on the analysis of imagery
and proper implementation of the
prescribed protocols and techniques
in combination with field verification.
False changes were avoided by
observing positive visual evidence
of a change in land use. Examples
included the presence of new
drainage ditches (Figure 13),
canals or other man-made water
courses, evidence of dredging, spoil
deposition or fills, impoundments,
excavations, structures, pavement
or hardened surfaces, in addition to
the lack of any hydrology, vegetation
or soil indicators indicative of
wetland. Difficulties in determining
wetland change have been related
to availability, timing or quality
of the imagery (Watmough et al.
2002; Dahl 2004), and correctly
interpreting wetland change has
been especially challenging at times
when hydrologic conditions were
not optimal (i.e. drought or flooded
conditions).
Figure 13. Drainage ditches visible on aerial imagery provided indicators of change.
9 The Service makes no attempt to adapt or
apply the products of these techniques to
regulatory or legal authorities regarding
wetland boundary determinations or to
jurisdiction or land ownership.
10 An example of an unlikely change might
involve upland-urban development converted
to palustrine forested wetland in a short
period of time (less than 5 years).
Texas
Utah
Montana
California
Arizona
Idaho
Nevada
Oregon
Iowa
Colorado
Kansas
Wyoming
New Mexico
Illinois
Ohio
Missouri
Minnesota
Florida
Nebraska
Georgia
Oklahoma
Alabama
Washington
South Dakota
Arkansas
Wisconsin
North Dakota
Virginia
Maine
New York
Indiana
Louisiana
Michigan
Mississippi
Kentucky
Tennessee
Pennsylvania
North Carolina
South
Carolina
West Virginia
Vermont
Maryland
New Jersey
New Hampshire
Massachusetts
Connecticut
Delaware
Rhode Island
States Field Verified
tac11-0632_fig14
Figure 14. States with field verification work (green) conducted between 2009 and 2010.
31
The goal of updating wetland status
and trends plots was to produce
data that match existing wetland
and deepwater conditions (on-the-ground)
as closely as possible.
These data derived from the plot
information reflected ecological
change(s) that influenced the size,
distribution, or classification of
wetland habitats.
Field Verification
Field verification was completed for
898 (18 percent) of the sample plots
distributed in 42 States (Figure 14).
Field work was done primarily as
a quality control measure to verify
that plot delineations were correct.
Verification involved field visits to
a cross section of wetland types,
geographic settings, and to plots
with different image types, scales
and dates. Field work was not
conducted in some Western States
because of the remote location
(limited access) and logistical
problems associated with these
areas. Of the 898 sample plots
reviewed in the field, 28 percent
used satellite imagery as the source
data and 72 percent used high
altitude digital photography. All field
verification work took place between
May 2009 and September 201011.
Approximately 39 percent of the
total population of sample plots have
had some field reconnaissance work
completed within the past 10 years.
Data Quality
Control
Advances in information technology
and geographic information systems
have influenced public expectations
for greater utility and functionality
from Government data sources
and there has been a growing
importance and sensitivity placed
on data quality and integrity. To
ensure the reliability of wetland
status and trends data, procedural
guidelines and various quality
assurance and quality control
measures were followed. The goal
of these guidelines was to ensure
that the data collection, analysis,
verification and reporting methods
used supported decisions for which
the data were intended. Some of the
major quality control steps included:
11 Results of field verification work indicated
no discernible differences in the size or
classification of wetlands delineated using
either satellite imagery or the high altitude
photography. Errors of wetland omission were
2 percent based on occurrence but less than
1 percent based on area (omitted wetlands
generally were small < 1.0 acre or 0.4 ha).
Errors of inclusion of upland were less than
1 percent in both occurrence and area. There
was no difference regionally, between States
or data analysts in the number of errors found
based on field inspections, although not all
plots were included in the field analysis.
32
Plot Location and Positional
Accuracy
Sample plots were permanently
fixed georeferenced areas used
to monitor land use and cover
type changes. The same plot
population has been re-analyzed
for each status and trends report
cycle. The plot coordinates were
positioned precisely using a
system of redundant locators in a
geographic information system.
Topographic maps, other maps used
for collateral information and the
aerial imagery were used during the
study to reaffirm sample locations.
All plots were also verified for the
correct spatial coordinates, size and
geographic projection.
Quality Control of Interpreted Images
This study used well established,
time-tested, fully documented data
collection and analysis procedures.
To facilitate training and consistent
application of data collection and
quality control measures, a relatively
small cadre of highly skilled and
experienced personnel was used for
image analysis. Image analysis was
reviewed by technical expert(s) with
the review consisting of adherence
to geospatial data standards,
ecological logic and other quality
requirements.
Data Verification
All digital data files were subjected
to rigorous quality control
inspections. Digital data verification
included quality control checks that
addressed the geospatial topology,
data completeness and integrity as
well as some geoprocessing aspects
of the data. These steps took place
following the review and qualitative
acceptance of the updated change
information. Implementation of
quality checks ensured that the data
conformed to the specified criteria,
thus achieving the project objectives.
Quality Assurance of Digital Data
Files
There were tremendous advantages
in using advanced technologies to
store and analyze the geographic
data. The geospatial analysis
capabilities built into this study
provided a complete digital
database to better assist analysis
of wetland change information. All
digital data files were subjected
to rigorous quality control
inspections. Automated checking
modules incorporated in the
geographic information system
(Arc/GIS) were used to correct
digital artifacts including polygon
topology. Additional customized data
inspections were made to ensure
that the changes indicated at the
image analysis stage were properly
executed. Digital file quality control
reviews also provided confirmation
of plot location, stratum assignment,
and total land or water area
sampled.
Customized digital data verification
tools designed specifically for use
with this sample plot work were
used to check for improbable
changes that may represent errors
in the image interpretation. The
software considered the length of
time between update cycles and
identified certain unrealistic cover-type
changes and other types of
potential errors in the data.
Statistical
Analysis
The wetland status and trends study
was based on a scientific probability
sample of the surface area of the
48 conterminous States. The area
sampled was about 1.93 billion acres
(0.8 billion ha), and the sampling
did not discriminate based on
land ownership. The study used a
stratified, simple random sampling
design. Given the total possible plot
population, the sampling design was
stratified by use of the 36 physical
subdivisions described in the “Study
Design” section. Once stratified,
the land subdivisions represented
large areas where the samples were
distributed to obtain an even spatial
representation of plots. The final
stratification, based on intersecting
physiographic land types with State
boundaries, guaranteed an improved
spatial random sample of plots.
Geographic information system
software organized the information
for the 5,042 random sample plots.
All sample plots in a stratum were
given equal selection probabilities.
In the data analysis phase, the
adjustments were made for varying
plot sizes (some lots were split by
study boundaries) by use of ratio
estimation theory. For any wetland
type, the proportion of its area in
the sample of plots in a stratum
was an unbiased estimator of the
unknown proportion of that type in
that stratum. Inference about total
wetland acreage by wetland type
or for all wetlands in any stratum
began with the ratio (r) of the
relevant total acreage observed in
the sample (Ty), for that stratum
divided by the total area of the
sample (Tx). Thus, y was measured
in each sample plot; r = Ty/Tx,
and the estimated total acreage of
the relevant wetland type in the
stratum was A x r. The sum of these
estimated totals over all strata
provided the national estimate
for the wetland type in question.
Uncertainty, which was measured
as sampling variance of an estimate,
was estimated based on the variation
among the sample proportions in a
stratum (the estimation of sample
variation is highly technical and
not presented here). The sampling
variation of the national total was
the sum of the sampling variance
over all strata. These methods have
been a standard for ratio estimation
in association with a stratified
random sampling design (Sarndal
et al. 1992; Thompson 1992).
33
By use of this statistical procedure,
the sample plot data were expanded
to specific physiographic regions,
by wetland type, and statistical
estimates were generated for the 48
conterminous States. The reliability
of each estimate generated is
expressed as the percent coefficient
of variation (% C.V.) associated with
that estimate. Percent coefficient of
variation was expressed as (standard
deviation/mean) × (100).
Procedural Error
Procedural or measurement errors
occur in the data collection phase of
any study and must be considered.
Procedural error was related to
the ability to accurately recognize
and classify wetlands both from
multiple sources of imagery and
on-the-ground evaluations. Types of
procedural errors may have included
missed wetlands, inclusion of
upland as wetland, misclassification
of wetlands or misinterpretation
of data collection protocols. The
amount of introduced procedural
error is usually a function of the
quality of the data collection
conventions; the number, variability,
training and experience of data
collection personnel; and the rigor
of any quality control or quality
assurance measures (Dahl and
Bergeson 2009).
Rigorous quality control reviews
and redundant inspections were
incorporated into the data collection
and data entry processes to help
reduce the level of procedural error
and have been described in more
detail by Dahl and Bergeson (2009).
Estimated procedural error ranged
from 3 to 5 percent of the true values
when all quality assurance measures
had been completed. This error rate
has remained steady since 2000.
Limitations
The identification and delineation
of wetland habitats through image
analysis forms the foundation for
deriving the wetland status and
trends data results reported here.
Because of the limitations of aerial
imagery as the primary data source
to detect some wetlands, certain
wetland types were excluded
from this monitoring effort. These
limitations included the inability
to detect small wetland areas
(see Sampling Design Section);
inability to accurately detect or
monitor certain types of wetlands
such as seagrasses that may
require hyperspectral or other
specialized imagery or analysis
techniques (Dierssen et al. 2003;
Peneva et al. 2008), submerged
aquatic vegetation, or submerged
reefs (Dahl 2005); and inability to
consistently identify certain forested
wetlands either because of their
small size, canopy closure, or lack of
visible hydrology.
Figure 15. Earthen berms divide a farm field used in rotation with other crops for commercial rice production, Arkansas, 2010.
34
Other habitats intentionally
excluded from data summary results
in this study include:
Commercial Rice—Throughout
the southeastern United States and
in California, rice (Oryza sativa) is
planted on drained hydric soils and
on upland soils. When rice was being
grown, the land was flooded and
the area functioned as wetland. In
years when rice was not grown, the
same fields were used to grow other
crops (e.g., corn, soybeans or cotton)
as shown in Figure 15. Commercial
rice lands were identified primarily
in California, Arkansas, Louisiana,
Mississippi and Texas. These
cultivated rice fields were not able
to support hydrophytic vegetation
in the absence of artificial pumps.
Consequently, these lands were not
included in the base wetland acreage
estimates.
Attribution of
Wetland Losses
The process of identifying or
attributing cause for wetland losses
or gains has been investigated by
both the Fish and Wildlife Service
and Natural Resources Conservation
Service (NRCS). In past studies,
specialists from both agencies
made a concerted effort to develop
a uniform approach to attribute
wetland losses and gains as to their
causes (Dahl 2000). Interagency
field evaluations were conducted to
test these definitions on the wetland
status and trends plot data. This was
done by conducting field visits where
interagency field teams evaluated
a number of sites with different
wetland types and changes in a
variety of geographical locations.
Field evaluations compared land use
descriptors, wetland classification,
and attribution of the losses or
gains observed. Ultimately, this
process resulted in no disagreement
among agency representatives
with how wetland losses or gains
were attributed as to cause. These
descriptors have been used in
subsequent reporting on wetland
status and trends (Dahl 2000; 2006).
The Fish and Wildlife Service and
NRCS continue to coordinate on
issues related to wetland change and
attribution of those changes.
The USDA’s Natural Resource
Inventory (NRI) categorization of
wetlands is slightly different than
that used by the Fish and Wildlife
Service’s Wetlands Status and
Trends study. The NRI and the Fish
and Wildlife Service have different
legislative mandates; sampling
methodology, inventory protocols,
data handling, and analysis routines
have evolved independently, even
though both survey programs use
the hierarchical Cowardin et al.
(1979) wetland classification system.
Recent collaborative efforts have
resulted in enhancements for
both programs, but wetlands data
collected by the two agencies are
currently neither comparable nor
interchangeable.
The categories used to determine
the causes of wetland losses and
gains are described below. Draining,
filling or otherwise altering a
wetland to conform to these land
use descriptions constituted a
loss in wetland area. Wetlands
reestablished or created from these
land use(s) constituted a gain in
wetland area.
Agriculture
The definition of agriculture followed
Anderson et al. (1976) and included
land used primarily for production
of food and fiber. Agricultural
activity was shown by distinctive
geometric field and road patterns
on the landscape and/or by tracks
produced by livestock or mechanized
equipment. Agricultural land uses
included horticultural crops, row
and close grown crops, hayland,
pastureland, native pastures and
range land and farm infrastructures.
Examples of agricultural activities in
each land use include:
Horticultural crops consisted of
orchard fruits (limes, grapefruit,
oranges, other citrus, apples,
peaches, and like species). Also
included were nuts such as
almonds, pecans and walnuts;
vineyards including grapes
and hops; bush-fruit such as
blueberries; berries such as
strawberries or raspberries;
and commercial flower and fern
growing operations.
Row and Close Grown Crops
included field corn, sugar cane,
sweet corn, sorghum, soybeans,
cotton, peanuts, tobacco, sugar
beets, potatoes, and truck
crops such as melons, beets,
cauliflower, pumpkins, tomatoes,
sunflower and watermelon. Close
grown crops also included wheat,
oats, barley, sod, ryegrass, and
similar graminoids.
Hayland and pastureland
included grass, legumes,
summer fallow and grazed native
grassland.
Other farmland included
farmsteads and ranch
headquarters, commercial
feedlots, greenhouses, hog
facilities, nurseries and poultry
facilities.
Figure 16. Planted pine forest as an example of upland forested plantation, South Carolina, 2010. (Photograph
by M. Bergeson, USFWS.)
35
Forested Plantations (Silviculture)
Forested plantations were uplands
that consisted of planted and
managed forests including planted
pines, Christmas tree farms, clear
cuts, and other managed forest
stands. These were identified by the
following remote sensing indicators:
(1) trees planted in rows or blocks;
(2) forested blocks growing with
uniform crown heights; or (3) logging
activity and use patterns (Figure 16).
Rural Development
Rural developments occurred in
rural and suburban settings outside
distinct cities and towns. This type
of land use was disjunctive areas
of development not within a well
defined urbanized outgrowth or
corridor. This classification shares
only some of spatial characteristics
of sprawl as found in the literature
and summarized by Hasse (2007).
Rural development was not based
on number of dwelling units but may
have included isolated infrastructure
or development characterized
by non-intensive land use and
sparse building density. Scattered
suburban communities located
outside of major urban centers,
described as “sprawl” (Wolman
et al. 2005) also were included in this
category as were some industrial
and commercial complexes;
isolated transportation, power,
and communication facilities; strip
mines; quarries; and recreational
areas.
Urban Development
Urban land consisted of areas of
intensive use in which much of the
land was covered by structures
(high building density). Urbanized
areas were cities and towns that
provided goods and services through
a central business district. Services
such as banking, medical and legal
office buildings, supermarkets
and department stores made
up the business center of a city.
Commercial strip developments
along main transportation routes,
shopping centers, dense residential
areas, industrial and commercial
complexes, transportation, power
and communication facilities, city
parks, ball fields and golf courses
were included in the urban category.
Other Land Uses
Other Land Use was composed
of uplands not characterized by
the previous categories. Typically
these lands included native prairie,
unmanaged or non-patterned upland
forests, conservation lands, scrub
lands, and barren land.
Lands in transition between
different uses also were in this
category. These were lands in
transition from one land use to
another and generally occurred in
large acreage blocks of 40 acres
(16 ha) or more. They were
characterized by the lack of any
remote sensor information that
would enable the interpreter to
reliably predict future use. The
transitional phase occurred when
wetlands were drained, ditched,
filled or when the vegetation had
been removed and the area was
temporarily bare.
Results
37
This study examined the status
and recent trends of wetlands to
monitor the changes in aerial extent
from 2004 to 2009. Updated data on
wetland area by type(s) and change
information have been provided as
well as new information derived
from enhancing the study to include
the estuarine wetlands along the
Pacific coast of Washington, Oregon,
and California. Because portions of
the Pacific coastal region had not
been sampled in previous wetland
status and trends studies, there
has been an adjustment to the
total wetland area estimate for
the conterminous United States.
There also has been a statistical
adjustment to the estimate of total
wetland area for the United States12.
The data presented here do not
provide qualitative assessment nor
do they address functional condition
of the Nation’s wetlands beyond
changes in extent by type.
Status of
the Nation’s
Wetlands
There were an estimated 110.1
million acres (44.6 million ha) of
wetlands in the conterminous United
States in 200913 (the coefficient of
variation of the national estimate
was 2.7 percent). The percent
of surface area and distribution
by major wetland type had not
changed since the previous era as
wetlands composed 5.5 percent of
the surface area of the conterminous
U.S. An estimated 95 percent of
all wetlands were freshwater and
5 percent were in the marine or
estuarine (saltwater) systems.
With the exception of minor
statistical adjustments to the area
estimates, the overall percentage
of wetland area and representation
by saltwater and freshwater
components remained unchanged.
In 2009, there were an estimated
104.3 million acres (42.2 million ha)
of freshwater wetland and
5.8 million acres (2.4 million ha) of
intertidal (saltwater) wetlands in
the conterminous United States.
Data for the 2004 to 2009 study
period are presented in a change
matrix and shown in Appendix D.
The distribution of wetlands by
type, estimated area and change has
been summarized and presented in
Table 2.
Within the marine and estuarine
systems, estuarine emergent (salt
marsh) made up an estimated
66.7 percent of all estuarine and
marine intertidal wetland area
(Figure 17). The mean size of salt
marsh included in the sample was
34.6 acres (14.0 ha). Estuarine shrub
wetlands made up an estimated
11.8 percent of the total intertidal
wetland area in 2009. The mean
size of estuarine shrub wetland
sampled was 15.8 acres (6.4 ha).
Non-vegetated intertidal wetlands
represented 21.5 percent of all
intertidal wetland area with a mean
size of 11.8 acres (4.8 ha).
12 The current estimate reflects a 2.0 percent
adjustment to the national wetland acreage
base. This was within the 2.7 percent
coefficient of variation associated with the
statistical estimate.
13 This estimate has been revised to reflect
2010 wetland status as well as the addition of
wetland area in the coastal zone of the Pacific
coast for WA, OR, and CA as described in the
Sample Design section of this report.
38
Table 2. Summary of study findings. Change in wetland area for selected wetland and deepwater
categories, 2004 to 2009. The coefficient of variation (CV) for each entry (expressed as a percentage) is
given in parentheses.
Wetland/Deepwater Category
Area, In Thousands of Acres
Estimated Area,
2004
Estimated Area,
2009
Change,
2004–2009
Change,
(In Percent)
Marine Intertidal 219.2 227.8 8.5 3.9%
(15.2) (14.8) (48.4)
Estuarine Intertidal Non-Vegetated 999.4 1,017.7 18.3 1.8%
(13.5) (13.3) (48.2)
Estuarine Intertidal Vegetated 1 4,650.7 4,539.7 -110.9 -2.4%
(4.4) (4.4) (16.6)
All Intertidal Wetlands 5,869.3 5,785.2 -84.1 -1.4%
(4.6) (4.6) (20.2)
Freshwater Ponds 6,502.1 6,709.3 207.2 3.2%
(4.6) (4.5) (29.6)
Freshwater Vegetated 2 97,750.6 97,565.3 -185.3 -0.2%
(2.9) (2.9) (*)
Freshwater Emergent 27,162.7 27,430.5 267.8 1.0%
(7.7) (7.6) (85.8)
Freshwater Shrub 18,331.4 18,511.5 180.1 1.0%
(4.2) (4.2) (*)
Freshwater Forested 52,256.5 51,623.3 -633.1 -1.2%
(2.7) (2.7) (30.7)
All Freshwater Wetlands 104,252.7 104,274.6 21.9 0.0%
(2.8) (2.8) (*)
All Wetlands 110,122.1 110,059.8 -62.3 -0.1%
(2.7) (2.7) (*)
Lacustrine 3 16,786.0 16,859.6 73.6 0.4%
(10.1) (10.1) (60.0)
Riverine 7,517.9 7,510.5 -7.4 -0.1%
(8.7) (8.7) (*)
Estuarine Subtidal 18,695.4 18,776.5 81.1 0.4%
(2.5) (2.5) (25.4)
  All Deepwater Habitats 42,999.4 43,146.6 147.2 0.3%
(4.3) (4.3) (33.8)
  All Wetlands and Deepwater Habitats 153,121.4 153,206.4 85.0 0.1%
(2.4) (2.4) (*)
* Statistically unreliable.
1Includes the categories: Estuarine Intertidal Emergent and Estuarine Intertidal Forested/Shrub.
2Includes the categories: Palustrine Emergent, Palustrine Shrub, and Palustrine Forested.
3Does not include the open-water area of the Great Lakes.
Percent coefficient of variation was expressed as (standard deviation/mean) × 100.
tac11-practice_fig17
Estuarine
Emergent
66.7%
Estuarine Shrub
11.8%
Marine and
Estuarine non-vegetated
21.5%
Forested
49.5%
Emergent
26.3%
Ponds
6.4%
Shrubs
17.8%
39
Among the freshwater types,
forested wetlands made up the single
largest category (49.5 percent).
Forested wetland area represented
less than 50 percent of the total
wetland acreage in the conterminous
United States for the first time.
The mean size of forested wetland
was 20.3 acres (8.2 ha). Freshwater
emergent wetland made up an
estimated 26.3 percent of the
total freshwater wetland area,
shrub wetlands 17.8 percent and
freshwater ponds 6.4 percent
(Figure 18). The mean size of
freshwater emergent, shrub and
open water pond wetlands sampled
in this study was 6.1 acres (2.5 ha),
7.6 acres (3.1 ha), and 1.3 acres
(0.5 ha) respectively.
Wetlands were found in all 48 States
and in every physiographic region
of the country as part of this study.
Spatial associations with land use
types varied. Of the freshwater
wetland population contained in the
national sample, ponds were the
most prevalent wetland type found
in urban areas, whereas freshwater
emergent wetlands were the least
common type. On agricultural lands,
there was a fairly even distribution
of wetland types with forested,
emergent and ponds represented.
Land predominantly in silviculture
had the highest percentage of
forested and shrub wetland. Rural
areas exhibiting growth had a mix of
all freshwater wetland types, as they
represented the interface of new
development activities.
Figure 17. Status of estuarine wetland area by type, 2009.
Figure 18. Status of freshwater wetland area by type, 2009.
-458,000
-290,000
-58,550
32,000
-13,800
0
-450,000
-500,000
-400,000
-350,000
-300,000
-250,000
-200,000
-150,000
100,000
50,000
-50,000
1950s–1970s 1970s–1980s 1980s–1990s 1998–2004 2004–2009
Acres
Figure 19. Average annual net loss and gain estimates for the conterminous United States, 1954 to 2009. Estimates of error are
not graphically represented. Sources: Frayer et al. 1983; Dahl and Johnson 1991; Dahl 2000; 2006; and this study.
40
National Trends,
2004 to 2009
The difference in the national
estimates of wetland acreage
between 2004 and 2009 was not
statistically significant. Wetland
area declined by an estimated
62,300 acres (25,200 ha) between
2004 and 2009. This equated to an
average annual loss of 13,800 acres
(5,590 ha) during the 4.5 year time
interval of this study (Figure 19) 14
as there were notable losses that
occurred to intertidal estuarine
emergent wetlands (salt marsh) and
freshwater forested wetlands.
Collectively, marine and estuarine
intertidal wetlands declined by an
estimated 84,100 acres (34,050 ha).
The loss rate of intertidal emergent
wetland increased to three times the
previous loss rate between 1998 and
2004. The majority of these losses
(83 percent) were to deepwater
bay bottoms or open ocean. There
were area gains in marine intertidal
wetlands (beaches/shores) and
estuarine non-vegetated wetlands
including near shore shoals and sand
bars. Over the period of this study, non-vegetated
intertidal wetlands increased
in area by an estimated 2.2 percent.
Freshwater vegetated wetlands
continued to decline albeit at a
reduced rate. The annual rate of loss
for freshwater vegetated wetlands
had been reduced by roughly
50 percent since 2004 (Figure 20).
Declines in freshwater forested
wetland area (633,100 acres or
256,300 ha) negated area gains in
freshwater emergent and shrub
categories. Forested wetlands
sustained their largest losses since
the 1974 to 1985 time period. An
estimated 392,600 acres (158,950 ha)
of forested wetland area was lost to
upland land use types or deepwater
between 2004 and 2009.
Gains in freshwater ponds offset
losses of vegetated wetland area15
although the 3.2 percent increase
in pond area was four times less
than reported in prior studies. The
distribution of freshwater ponds
by descriptive categories is shown
in Figure 2116. Farm ponds and
ponds in urban (developed) areas
increased, whereas ponds described
as having natural characteristics and
aquaculture ponds declined during
the same time period. The overall
estimated net gain in all freshwater
wetland area (vegetated and non-vegetated
types) between 2004 and
2009 was 21,900 acres (8,870 ha).
This estimate had declined
substantially from a net increase in
freshwater wetland of 220,200 acres
(89,140 ha) reported for the period
between 1998 and 2004.
The estimated area of lacustrine
and riverine deepwater habitats17
increased slightly (<0.3 percent)
between 2004 and 2009.
14 There are statistical uncertainties associated with this estimate.
15 This report did not draw any conclusions regarding trends in quality or condition of the any wetland type.
16 Ponds were open-water bodies (freshwater) less than 20 acres (8.1 ha).
17 Because of the sample design, these estimates do not represent total area of all freshwater lakes and rivers.
Figure 20. Estimated average annual
loss of vegetated freshwater wetland
area,18 1974 to 2009. Sources: Dahl and
Johnson 1991; Dahl 2000; 2006; and this
study.
tac110632_fig 20
334,400
117,900
82,500
41,200
0
50,000
100,000
150,000
200,000
250,000
300,000
350,000
400,000
1974–1984 1986–1997 1998–2004 2004–2009
Acres
Industrial
6%
Farm Ponds
44%
Aquaculture
4%
Urban
15%
Natural
31%
41
Attribution of
Wetland Gain
and Loss,
2004 to 2009
Figure 22 illustrates the net gains
and losses of wetlands that occurred
between 2004 and 2009 relative to
the various land use categories.
In the saltwater systems, there has
been a trend toward an increase
in non-vegetated tidal wetland as
salt marsh areas have diminished.
In combination, intertidal marine
shorelines as well as estuarine
flats, bars, and shoals increased
in area and made up 21.5 percent
of all intertidal wetlands in 2009.
Figure 21. Percent occurrence of
freshwater pond types, 2009.
18 Includes palustrine forested, palustrine
shrub and palustrine emergent wetlands.
This increase in tidal non-vegetated
area came primarily from former
salt marsh wetlands as estuarine
emergent area declined by an
estimated 111,500 acres (45,140 ha)
or 2.8 percent between 2004 and
2009. One percent of the losses of
salt marsh habitats were the result
of conversion to upland land use.
Eighty-three percent of the estuarine
emergent losses were attributed
to saltwater intrusion or other
forms of inundation and the vast
majority (99 percent) of all estuarine
emergent losses were affected by
open ocean generated processes (i.e.,
saltwater inundation, coastal storms,
etc.). There was very little gain in
estuarine vegetated wetland (either
shrubs or emergent) as a result of
reestablishment or creation during
the time covered by this study.
Between 2004 and 2009, 489,600 acres
(198,130 ha) of former upland were
re-classified as wetland. These
increases were attributed to wetland
reestablishment and creation on
agricultural lands and other uplands
with undetermined land use (i.e.,
undeveloped land, lands in conservation
programs or left idle). Further
explanation of “other” uplands with
undetermined land use has been
provided in the inset (page 43). When
these wetland gains were balanced
with losses, freshwater wetlands
realized a net increase of an estimated
21,900 acres (8,870 ha).
Silviculture
38%
Development
26%
Agriculture
13%
Deepwater
4%
Upland Other
Land Uses
19%
tac110632_fig 22
-115,960
-307,340
-61,630 -66,940
100,020
389,600
500
400
300
200
100
-100
-200
-300
-400
0
Deep Water Urban Rural
Development
Silviculture Agriculture Other
Acres (in thousands)
Land Use Category
42
Freshwater wetland losses were
primarily attributed to urban and
rural development and silviculture
operations. Urban and rural
development combined accounted
for 23 percent of the wetland losses
and were estimated to have been
128,570 acres (52,050 ha). This was
an 8.0 percent decline in wetland
area lost and attributed to urban
or rural development as compared
to the period between 1998 and
2004. Wetland losses to silviculture
increased considerably since 2004.
Silviculture accounted for 56 percent
of all wetland losses from 2004 to 2009.
All freshwater wetland types
increased in area with the exception
of forested wetlands. Forested
wetlands declined by 1.2 percent in
area (633,100 acres or 256,200 ha).
Attribution of the loss of freshwater
forested wetland to uplands and
deepwater from 2004 to 2009 is
shown in Figure 23.
Freshwater ponds increased in
area by 3.2 percent. An estimated
207,200 acres (83,890 ha) of
freshwater ponds were created
between 2004 and 2009. These
wetlands ameliorated some of the
Figure 23. Loss of freshwater forested wetland as attributed to upland and
deepwater categories, 2004 to 2009.
Figure 22. Estimated net gains and losses of wetland acres (saltwater and freshwater) attributed to the various upland land
use categories and deepwater, 2004 to 2009.
losses in area of other freshwater
wetland types, but the functional
characteristic of these water bodies
continues to be debated.
43
Wetland Gains and Loss Examples on “Other” Lands
(Undetermined Land Use)
This study found that an estimated 389,600 acres (157,730 ha) net increase in wetland came from
uplands classified as “other” lands or lands with undetermined land use. What are these “other” lands?
Other lands have included areas such as native prairie, unmanaged or non-patterned upland forests,
scrub lands, barren and abandoned land, lands enrolled in set-aside programs, conservation easement
or other lands designated as wildlife management areas. Lands in transition also may fit into this
category when land has been cleared but not yet developed to the point of a distinguishable land use (i.e.,
silviculture or agriculture) as seen in Figure 24.
Wetland changes attributed to “other” lands have become more prominent. This has been due to the
success of conservation programs that have developed streamside buffers, soil conservation measures,
crop retirement programs, easements and land set-aside programs. As some of these areas have been
enlisted into conservation programs, wetlands have been reestablished either by design or through
natural processes (Figure 25). Natural changes on “other” lands such as buffers along stream corridors
or in riparian areas were not uncommon. Riparian dynamics have the ability to create and destroy
wetlands along stream corridors or in floodplains (Kudray and Schemm 2008).
Figure 24. Wetland losses attributed to “other” land use
indicated the land may be in transition from one land
use to another and the final land use type can not be
determined. This example of a wetland area in the process
of being drained and filled provided no indication of
the final land characterization (South Carolina, 2010,
photograph by M. Bergeson, USFWS).
Figure 25. This temporarily flooded wetland has reestablished
naturally on lands that were part of an agricultural program set-aside.
The surrounding upland was no longer in active agriculture
and was classified as “other” upland (Minnesota, 2009).
Crystal River, FL.
Photograph courtesy of USFWS45
Discussion and
Analysis
This study, as a long-term
monitoring effort, has helped
document the historical trends in
wetland gains and losses and traced
policy and land use practices that
have had consequences for these
resources. At the time the study
was originated (1970s), the average
annual wetland loss rate was
458,000 acres (185,400 ha). During
the period between the mid-1970s to
mid-1980s, the loss rate had declined
to 290,000 acres (117,400 ha)
annually. In 1998, the wetland
loss rate was about 59,000 acres
(23,900 ha) annually and in 2005
wetland area gains had exceeded
losses by an estimated 32,000 acres
(13,000 ha) per year.
Wetland losses increased between
2004 and 2009 reversing this long-standing
trend in wetland loss
reduction. The reasons for this were
complex and subject to many factors
including economic conditions
(such as crop prices or property
values), land use trends, changes to
wetland regulation and enforcement
measures and possible climatic
changes.
Data indicate that the rate of
wetland reestablishment or creation
between 2004 and 2009 increased
by 17 percent from the previous
study period (1998 and 2004). Yet,
the overall estimated net gain in all
freshwater wetland area (vegetated
and non-vegetated types) between
2004 and 2009 was 21,900 acres
(8,870 ha), a substantially lower
net increase than the 220,200 acres
(89,140 ha) reported for the
period between 1998 and 2004. A
comparable analysis of the wetland
loss rate showed an increase of
140 percent from 2004 to 2009 from
the previous era. As a consequence,
national wetland losses have
outdistanced gains.
Marine and
Estuarine
Wetlands
Table 3 shows the current status
and change for the marine and
estuarine intertidal (saltwater)
wetlands between 2004 and 2009.
Cowardin et al. (1979) defined
“estuarine” and “marine” wetlands
as saltwater systems. Marine and
estuarine wetlands have been
grouped into three types: estuarine
intertidal emergent wetlands (salt
and brackish water marshes),
estuarine shrub wetlands (mangrove
swamps and other salt-tolerant
woody species), and estuarine and
marine intertidal non-vegetated
wetlands. This latter category
included exposed coastal beaches
subject to tidal flooding, as well
as sand bars, tidal sand or mud
flats, shoals, and sand spits. These
tidal wetlands are subjected to a
multitude of anthropogenic stressors
originating from the landward side,
natural forces affecting change from
the sea (Stedman and Dahl 2008),
as well as increasing sea levels and
climatic change. There is growing
awareness of the threats posed by
climate related changes on fresh
and saltwater systems in coastal
areas. Recently, the Army Corps
of Engineers and NOAA published
frameworks to guide how to consider
the impacts of factors such as
sea level rise in coastal wetlands
(USEPA 2010a).
Saltwater intertidal wetlands are
dynamic areas of tremendous
ecological, economic and social
importance. The ecological value
of tidal wetlands has been well
documented by a number of
researchers (Mitsch and Gosselink
2007; Costanza et al. 2008;
Harrington 2008; USEPA 2008)
as these wetlands provide crucial
migratory habitat for the majority
of shorebirds that breed in the
United States (Withers 2002);
support adult stocks of commercially
harvested shrimp, blue crabs,
oysters, and other species of fish
and shellfish (Stedman and Hanson
2000); and provide protection from
storms (Costanza et al. 2008). In
the Pacific Northwest, coastal
fishes and particularly anadromous
species such as the salmonids,
utilize coastal marshes as areas to
transition from freshwater to open
ocean environments (Adamus 2005;
Simenstad et al. 2002).
Figure 26. Estuarine salt marsh wetland, Florida, 2010.
46
Trends in Estuarine Emergent
(Salt Marsh) Wetland
The largest acreage change in the
saltwater system was an estimated
loss of more than 111,500 acres
(45,140 ha) of estuarine emergent
wetland (salt marsh as shown in
Figure 26). This rate of loss was
three times greater than estuarine
emergent losses from 1998 to 2004
and continued a long-term trend in
the decline of estuarine emergent
wetland area. In this study, there
were very few (< 1 percent)
estuarine emergent losses attributed
to discrete anthropogenic actions19
that fill or otherwise convert salt
marsh areas to uplands.
Table 3. Status and changes to intertidal marine and estuarine wetlands, 2004 to 2009. The coefficient of variation
(CV) for each entry (expressed as a percentage) is given in parentheses.
Wetland/Deepwater Category
Area, In Thousands of Acres Area (as
percent) of
all Intertidal
Wetlands,
2009
Estimated
Area,
2004
Estimated
Area, 2009
Change,
2004–2009
Change,
(In Percent)
Marine Intertidal 219.2 227.8 8.5 3.9% 3.9%
(15.2) (14.8) (48.4)
Estuarine Intertidal Non-Vegetated 999.4 1,017.7 18.3 1.8% 17.6%
(13.5) (13.3) (48.2)
Marine and Estuarine Intertidal 1,218.6 1,245.5 26.8 2.2% 21.5%
Non-Vegetated (11.5) (11.2) (35.3)
Estuarine Emergent 3,971.4 3,859.8 -111.5 -2.8% 66.7%
(4.6) (4.7) (16.6)
Estuarine Forested/Shrub 679.3 679.9 0.6 0.1% 11.8%
(12.4) (12.4) (*)
Estuarine Intertidal Vegetated 1 4,607.7 4,539.7 -110.9 -2.4% 78.5%
(4.4) (4.4) (16.6)
Changes in Coastal Deepwater area, 2004–2009
All Estuarine and Marine Intertidal 5,869.3 5,785.2 -84.1 -1.4% ��
(4.6) (4.6) (20.2)
* Statistically unreliable.
1 Includes the categories: Estuarine Intertidal Emergent and Estuarine Intertidal Forested/Shrub.
Percent coefficient of variation was expressed as (standard deviation/mean) × 100.
19 Land subsidence and sea level rise may be
attributed to human actions but could not
be traced to a specific event or geospatial
change such as filling, draining, or otherwise
mechanically altering wetland area.
tac11-practice_fig27
Tidal Non-vegetated
16%
Deepwater
83%
Upland
1%
47
This suggests that marine and
estuarine vegetated wetlands (tidal
salt marsh and shrubs) have been
afforded protection by various State
and Federal coastal regulatory
measures including Federal
protection under the Section 404
of the Clean Water Act as waters
of the United States (Dahl 2000).
These wetlands, however, have been
susceptible to oceanic influences
including sea level rise and storm
events. An estimated 99 percent of
the losses of estuarine emergent
wetlands between 2004 and 2009
were attributed to effects from
coastal storms, land subsidence, sea
level rise, or other ocean processes
(Figure 27) and the vast majority
of these losses were in the northern
Gulf of Mexico along the coastline of
Louisiana and Texas.
Factors responsible for the loss
of estuarine emergent wetland in
the northern Gulf included land
subsidence (sinking of the land),
compaction of sediments and
extraction of subsurface fluids, such
as oil, gas, and water. In portions
of coastal Louisiana and Texas, oil,
gas, and groundwater extractions
have been recognized as factors
that contributed to subsidence and
relative sea level rise (Galloway et al.
1999; Morton et al. 2003; Dokka
2006; Lavoie 2009). Throughout
the northern Gulf coastal region,
marine and estuarine wetlands
have been adversely impacted by
the cumulative effects of energy
development (Figure 28), coastal
storms and development in the
upper portions of the watershed.
Figure 27. The attribution of estuarine emergent (salt marsh) losses between
2004 and 2009. An estimated 99 percent of these losses were attributed to
deepwater and tidal non-vegetated areas and were the result of coastal storms
or ocean derived processes.
Figure 28. Oil and gas field development located in estuarine (salt-marsh)
wetlands of southern Louisiana. Such modifications have increased the
vulnerability of these wetlands to climate related change (Twilley 2007) and
the cumulative impacts have contributed to relative sea level rise, marsh
fragmentation, and subsidence.
48
Figure 29. Comparison of aerial images from 2004 (top) and 2009 (bottom) showing
areas of estuarine marsh along the northern Texas coast. At site A, the open water
(dark blue) in this color infrared (CIR) image has been restored to emergent marsh
seen as gray or brown in the true-color image in 2009. Wetland mitigation was
completed in 2008 using approximately 500,000 cubic yards (381,680 cubic meters)
of dredge material to restore 240 acres (97 ha) of open water to emergent marsh.
Site B seen as emergent salt marsh (reddish color) in the 2004 CIR image, has been
impacted by a series of tropical storms including Hurricane Rita (2005), Hurricane
Humberto (2007) and Hurricane Ike (2008). The 2009 true-color image shows this
wetland area has been physically scoured removing the marsh vegetation and
inundated by high salinity sea water (olive-green color). Marsh losses also have
been accentuated by regional drought conditions.
The construction of levees and
canals, such as the hundreds of
miles of Mississippi River levees
constructed to control flooding,
also weaken the sustainability of
the landscape and have contributed
to coastal wetlands loss (GAO
2007). These actions have reduced
freshwater and sediment that has
been crucial to maintain estuarine
wetland elevation as a mechanism
to overcome rising sea levels. In
these areas and elsewhere, wetlands
have been vulnerable to salt water
intrusion and marsh disintegration
as development has interfered with
natural hydrological processes that
transport sediment and freshwater
necessary to sustain the structure,
function, and extent of wetland
ecosystems (Kling and Sanchirico
2009). The interconnection between
fresh and saltwater systems has
become more apparent as impacts
to freshwater wetlands have
compounded the effects of sea level
rise and the ability of wetlands in
coastal watersheds to adapt.
Since the mid-1980s, there has
been recognition that the majority
of losses to these tidal wetlands
have resulted from coastal erosion
and inundation by salt water. This
situation has been exacerbated by
a series of hurricanes in the Gulf
of Mexico that damaged property
and natural resources in proximity
to coastal areas. Attempts to
re-nourish tidal wetlands have been
implemented following several
hurricane events from 2005 to 2008
(Figure 29). There also has been
considerable work in the northern
Gulf of Mexico to armor near-shore
areas that were damaged as a result
of hurricanes or relative rise in
sea level.
Rock outcrops
49
The data from this study provided
little evidence of increased
estuarine wetland area resulting
from reestablishment. Wetland
reestablishment (restoration) or
creation has been more challenging
in tidal systems and potentially more
costly where land values fueled by
development were high. Additionally,
successful reestablishment of
many tidal wetlands has hinged on
consideration of physical processes
including flow, circulation, and
transport of nutrients, salinity and
sediments (Sanders and Arega
2002). Because of the recent storm
events along the Gulf coast, local,
State and Federal agencies have
renewed their emphasis on coastal
wetland reestablishment (Working
Group for Post-Hurricane Planning
for the Louisiana Coast, 2006;
Twilley 2007; Day et al. 2008).
Under the auspices of the Coastal
Wetlands Planning, Protection
and Restoration Act (CWPPRA),
Federal agencies and the State of
Louisiana have designed and/or
constructed 147 projects intended
to restore and protect more than
120,000 acres of coastal wetlands
(Government Accountability
Office [GAO] 2007). Some of these
projects included wetland and land
protection efforts, salinity control
and water diversion. Some projects
have yet to be implemented and as
a consequence, the results have not
been recognized as wetland acreage
gains. A review conducted by GAO
indicated that of the 147 projects,
22 were demonstration projects and
17 projects had been delayed due to
problems such as land rights, oyster
leases, and uncertain benefits of the
project design. Shoreline protection
projects (building barriers from
Figure 30. An example of shoreline protection measures along the coast of
southeastern Louisiana. Rock outcrops have been systematically placed in
shallow water parallel to the shoreline.
rock or plants, see Figure 30) and
hydrologic restoration projects
(returning areas to their natural
drainage patterns) made up more
than one-half of the 90 projects
that were completed or under
construction. An example of
a large scale project designed
to trap sediment and restore
estuarine marsh is shown in
Figure 31. The CWPPRA program
also has faced several challenges,
such as increasing project costs,
limited capability to monitor
project effectiveness, uncertain
project performance, issues with
private landowner rights, and
damage from hurricanes and
storms (GAO 2007). Additionally,
the GAO found that many of these
projects were expected to erode
and subside over time as a result
of naturally occurring hydrologic
and geologic processes.
“In addition to the storms,
sea level rise, and land
subsidence (sinking)
that have contributed to
and continue to cause
coastal wetlands loss, the
construction of levees
and canals, such as the
hundreds of miles of
Mississippi River levees
constructed to control
flooding, also weaken
the sustainability of the
landscape and contribute
to coastal wetlands loss.”
GAO 2007
tac11-0632_fig 31B
50
Estuarine Shrub Wetlands
Estuarine shrub wetlands were
comprised of halophytic trees and
shrubs growing in brackish or saline
tidal waters. This category was
dominated by species of mangroves
(Rhizophora mangle, Avicennia
germinans, and Laguncularia
racemosa) but also may have
included other salt tolerant woody
species, such as buttonwood
(Conocarpus erectus), saltbush
(Baccharis halimifolia), bay cedar
(Suriana maritina), and false willow
(Baccharis angustifolia). Mangrove
dominated wetlands (Figure 32)
serve as valuable nurseries for
a variety of recreationally and
commercially important marine
species (National Park Service
2010).
Overall, estuarine shrubs
had a small net gain in area
(0.1 percent) as losses to upland
were outdistanced by gains. Area
gains in estuarine shrubs came
from both palustrine wetlands
(1,789 acres or 724 ha), presumably
from salt water inundation of low
lying freshwater wetland20; and from
agricultural lands and unspecified
other uplands (2,314 acres or
937 ha collectively). There were
an estimated 1,370 acres (555 ha)
of estuarine shrub wetlands lost
to upland between 2004 and 2009.
Eighty-three percent of those losses
were attributed to urbanization
and related development. Human
induced impacts to mangrove
wetlands included proliferation of
invasive species, cutting/removal,
coastal development resulting
in drainage, filling or changes to
shoreline structure.
Figure 31. Man-made structures (identified by red arrows) in areas of former estuarine marsh in southern Louisiana. Projects
such as this were designed to trap sediment and hopefully reestablish vegetation.
20 Saltwater inundation of other woody
species also was possible.
Long-term trends in area of
estuarine shrub wetland has
remained fairly constant since the
1980s despite long-term stressors
including invasion by exotic
species such as Brazilian pepper
(Schinus terebinthifolius) and a
high vulnerability to change due
to natural causes such as coastal
storms, drought, frost, fire, sea
level changes and stress due to
increased salinity. Climax stands of
mangrove forest are uncommon in
the conterminous United States as
they survive within a very limited
geographic range and have been
vulnerable to physical damage from
high winds that accompany coastal
storms.
51
Figure 32. Mangrove shrub wetlands along the west coast of Florida.
tac11-practice_fig33
Atlantic and
Gulf of Mexico
60%
Pacific Coast
Washington,
Oregon, and California
40%
52
Marine and Estuarine Non-Vegetated
Wetlands
Non-vegetated coastal wetland
habitats included tidal flats, shoals,
sandbars, sandy beaches and small
barrier islands. Study findings
provided new information about
the extent of tidal non-vegetated
wetland along the Pacific coast of
the conterminous United States.
An estimated 40 percent of all non-vegetated
tidal wetlands were found
along the near-shore areas of the
Pacific coast (Figure 33). Most of
these non-vegetated tidal wetlands
were located around Puget Sound,
Willapa Bay and Grays Harbor in
Washington; Tillamook Bay and Coos
Bay in Oregon; and San Francisco
Bay, California. The extent of these
wetlands remained stable when
compared to the same type of areas
of the Atlantic and Gulf of Mexico.
The Pacific coast of the conterminous
United States experienced no change
in the estimated area of tidal non-vegetated
wetland between 2004 and
2009, and insignificant (<100 acres or
41 ha) change in estuarine vegetated
wetland area.
In contrast, intertidal non-vegetated
wetlands along the Atlantic and the
Gulf of Mexico sustained considerable
change. Over the time-span of this
study the area of intertidal non-vegetated
wetland increased by an
estimated 2.2 percent (26,800 acres
or 10,850 ha). All of these changes
occurred along the south Atlantic and
Gulf coastlines and were attributed
to storm events that transported
sediments, over-washed barrier
islands, or scoured shorelines and
other near-shore features along
the coast. Intertidal non-vegetated
wetlands (shores and flats) have
Figure 33. Estimated percent area of intertidal non-vegetated wetland along
the Pacific coastline of Washington, Oregon, and California compared to the
coastline of the Atlantic and Gulf of Mexico, 2009.
exhibited marked change and
instability and, despite an increase
in acreage, are most likely to sustain
additional changes from ongoing and
future coastal processes (Figure 34).
Seaward events such as storms,
tidal-surge causing erosion and
deposition, saltwater intrusion and
inundation have contributed to the
modification of these coastal wetland
types and extent (Steadman and
Dahl 2008).
The effects on non-vegetated
wetland types has often been
overshadowed by losses to
vegetated wetland areas, but
these wetlands provide crucial
habitats for a variety of coastal
bird species, including pelicans,
cormorants, gulls, terns, and
roughly 50 species of sandpipers,
plovers, and their allies known as
shorebirds. (Harrington and Corven
[no date]) have described shorebird
guilds, enumerating species and
habitat types.) Some of these bird
populations are at risk because
of their dependence on narrow
ribbons of marine and estuarine
tidal habitats that are subjected to
rapid and unpredictable changes
resulting from coastal storms,
habitat alteration by man, and other
changes in marine ecosystems that
can affect the availability of marine
invertebrates (a food resource),
water temperature, nutrients, and
phytoplankton. Rising sea levels are
expected to continue to inundate or
fragment low-lying coastal areas
including sandy beaches, barrier
islands, and mudflats that support
sea and shorebirds dependent on
marine waters (North American
Bird Conservation Initiative
[NABCI] 2010) (Figure 35A and
35B).
Figures 35 A and 35B. Sea birds (A) including these Royal Terns and Black Skimmers rest and feed on intertidal habitats such
as beaches and tidal flats (Photograph by J. Dahl). At lower tides, shorebirds (B) prefer foraging on invertebrates characteristic
of sandy, intertidal habitats, such as sandbars or barrier beaches (Harrington 2008). Pictured are Short-billed Dowitcher
(Limnodromus griseus) and Willet (Tringa semipalmata). (Photograph by A. Cruz, USFWS).
53
Figure 34. The fishing pier on Dauphin Island, Alabama, no longer reaches the water line as coastal sediments have been
deposited along this shore (2010).
Figure 36. Beached oil from the Deepwater Horizon oil spill, 2010. (Photograph
courtesy of Denise Rowell, Alabama Ecological Services Field Office, USFWS).
54
Most recently, tidal beaches,
shoals, bars, and barrier islands
along the northern Gulf of Mexico
were exposed to the impacts
from the Deepwater Horizon oil
spill (Figure 36). Although data
on any wetland losses resulting
from that event are not included
in these results21, the incident
served to highlight the ecological
and economic importance of these
marine and estuarine resources.
Changes in
Sea Level
and Coastal
Processes
Affecting Marine
and Estuarine
Wetlands
There is strong scientific consensus
that climate change is accelerating
sea level rise and affecting coastal
regions, however, many researchers
point to the uncertainties associated
with predicting the response
that increased sea level will have
given other coastal processes and
interactions (National Academy of
Sciences 2008; Lavoie 2009). Sea
level rise directly threatens coastal
infrastructure through inundation,
increased erosion, more frequent
storm-surge flooding, and loss of
habitat through drowned wetlands
(NOAA Congressional Budget
Hearing 2009). Coastal habitats
will likely be increasingly stressed
by climate change impacts that
have resulted from sea level rise
and coastal storms of increasing
frequency and intensity (Field
et al. 2007). The difficulty in linking
sea level rise to coastal change
stems from shoreline changes not
solely the result of sea level rise
21 The period covered by this study was 2004
to 2009.
(Lavoie 2009). Natural and physical
processes that act on the coast
(e.g., storms, waves, currents,
sand sources, sinks, relative sea
level), as well as human actions that
affect coastal processes in both the
saltwater and freshwater systems,
(e.g., development, dredging,
dams, coastal engineering and
modification), all have contributed
to coastal changes.
In the conterminous United States,
the Gulf of Mexico and mid-Atlantic
coasts have experienced the highest
rates of relative sea level rise and
recent wetland loss (NABCI 2010).
Stedman and Dahl (2008) found that
in addition to the wetland losses
already recognized, climate change
models project additional wetland
degradation in coastal areas as sea
level continues to rise throughout
this century. This trend has
presented long-term challenges to
managing and monitoring wetlands
that abut the coast in coming
decades.
55
Inundation of coastal wetlands by
rising sea levels threatens wetland
plants particularly those not able
to adjust to higher salinities or
increased wave or tidal energy. For
many of these systems to persist,
a continued input of suspended
sediment from inflowing streams
and rivers is required for soil
accretion (Poff et al. 2002). Migration
or movement of coastal wetlands
may offset some losses; however,
this possibility is limited in areas
with cliffs and steeper topography,
such as areas on the Pacific Coast
(Figure 37) and parts of the north
Atlantic or, where shorelines are
extensively developed (e.g., around
Mobile Bay, Pensacola Bay, Tampa
Bay, Biscayne Bay, portions of
Chesapeake Bay, and San Francisco
Bay). The construction of levees
and flood protection infrastructure
may put some wetlands at additional
risk by restricting water flow,
sediment, and nutrient inputs.
Corbett et al. (2008) estimated that
about 30 percent of the shoreline
along the Neuse River Estuary in
North Carolina had been modified
with stabilization structures. Coastal
development, urbanization, and
infrastructure to support tourism
throughout the coastal watersheds
have an increased cumulative effect
on the loss and modification of
freshwater and estuarine wetland
habitats. With continued growth
and development, more shorelines
have been cleared and stabilized
(Figure 38), shallow waters