NATIONAL WETLAND CONDITION 

ASSESSMENT 2011

A Collaborative Survey of the Nation’s Wetlands

Draft for Public Review and Comment
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NATIONAL WETLAND CONDITION ASSESSMENT 2011:

A Collaborative Survey of the Nation’s Wetlands

Draft for Public Review and Comment

U.S. Environmental Protection Agency

Office of Wetlands, Oceans and Watersheds

Office of Research and Development

Washington, DC 20460

EPA-843-R-15-005

October 2015
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National Wetland Condition Assessment 2011

Acknowledgements

The U.S. Environmental Protection Agency (EPA) Office of Water (OW) and Office of Research and 

Development (ORD) would like to thank the many people who contributed to this project. Without the 

collaborative efforts and support by state and tribal environmental agencies, federal agencies, 

universities, and other organizations, this groundbreaking assessment of wetlands would not have been 

possible. In addition, the survey could not have been done without the innumerable field biologists, soil 

scientists, taxonomists, statisticians, and data analysts, as well as program administrators, EPA regional 

coordinators, project managers, quality control officers, and reviewers. To the many hundreds of 

participants, EPA expresses its profound thanks and gratitude.

State and Tribal Agency Partners

Alaska Department of Environmental Conservation

Alabama Department of Environmental 

Management

Arizona Department of Environmental Quality

California State Water Resources Control Board

Colorado Natural Heritage Program

Delaware Department of Natural Resources and 

Environmental Control

Fort Peck Tribe

Georgia Department of Natural Resources

Georgia Environmental Protection Division

Florida Department of Environmental Protection

Idaho Department of Environmental Quality

Illinois Natural History Survey

Iowa Department of Natural Resources

Kansas Alliance for Wetlands and Streams

Kansas Department of Health and the Environment

Kansas Water Office

Kentucky Division of Water 

Leech Lake Band of Ojibwe, Division of Resource 

Management

Maine Department of Environmental Protection

Maryland Department of the Environment

Massachusetts Bays National Estuary Program

Massachusetts Department of Environmental 

Protection

Michigan Department of Environmental Quality

Minnesota Pollution Control Agency

Missouri Department of Natural Resources

Montana Natural Heritage Program

Navajo Environmental Protection Agency

Nebraska Game and Parks Commission

New Hampshire Department of Environmental

Services

New Jersey Natural Heritage Program

New Mexico Environment Department

New Mexico Natural Heritage Program

New York Natural Heritage Program

North Carolina Department of Environment and 

Natural Resources

North Dakota Department of Health

Ohio Environmental Protection Agency

Oklahoma Conservation Commission

Oregon Department of Environmental Quality

Oregon Division of State Lands

South Carolina Department of Health and 

Environment Control

Utah Division of Water Quality

Utah Geological Survey

Vermont Department of Environmental 

Conservation

Virginia Department of Environmental Quality

Washington State Department of Ecology

West Virginia Department of Environment 

Protection

Wind River Environmental Quality Commission

Wisconsin Department of Natural Resource

Federal Partners

U.S. Army Corps of Engineers

U.S. Department of Agriculture, Forest Service

U.S. Department of Agriculture, Natural Resource 

Conservation Service

U.S. Department of Interior, Fish and Wildlife Service

U.S. Department of Interior, National Park Service

U.S. EPA Office of Research and Development

U.S. EPA Office of Water

U.S. EPA Regions 1-10

U.S. Geological Survey
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National Wetland Condition Assessment 2011

Other Partners and Collaborators

Battelle Memorial Institute

BioLogics

California State University System - Moss Landing 

Marine Laboratories

Center for Plains Conservation and Biodiversity

Dynamac

East Carolina University

EcoAnalysts, Inc

Great Lakes Environmental Center

Kenyon College

Michigan State University

Midwestern Biodiversity Institute

Nature Serve

New England Interstate Water Pollution Control 

Commission

North Carolina State University

North Dakota State University

Oregon State University

Partnership for the Delaware Estuary

Pennsylvania State University

San Francisco Estuarine Institute

Southern California Coastal Water Research Project

Tetra Tech, Inc.

University of Central Missouri

University of Florida

University of Houston-Clear Lake

University of Nebraska-Lincoln

Virginia Institute of Marine Sciences

Virginia Tech University

Wells National Estuarine Reserve

Tom Dahl, Bill Wilen, Mitch Bergeson, and Rusty Griffin from the U.S. Fish and Wildlife Service’s National 

Wetland Inventory Program provided invaluable technical assistance on national wetland mapping and 

extent data and generated wetland maps of the NWCA sites. 

Lenore Vasilas, Phil King, Steve Monteith, Chris Smith, Larry West, Leander Brown, Rich Ferguson, and 

Edward Griffin from the USDA Natural Resource Conservation Service Soil Science Division provided 

invaluable technical support and advice developing field protocols, training field crews, reviewing field 

data and processing soil samples for NWCA. 

The following people played a pivotal role and lent their expertise to the data analysis in this project:

Mary Kentula, Teresa Magee, Karen Blocksom, Tony Olsen, Steve Paulsen, Tom Kinkaid, Marc Weber, 

Dave Peck, Anett Trebitz, Brian Hill, and Janet Nestlerode from EPA Office of Research and 

Development; Amanda Nahlik and Siobhan Fennessy from Kenyon College; Alan Herlihy from Oregon 

State University; Josh Collins from San Francisco Estuary Institute; Gregg Lomnicky from Dynamac 

Corporation; Marlys Cappaert from SRA International, Inc; and Keith Loftin from U.S. Geological Survey.

The report was written by a team of contributors that included Mary Kentula, Teresa Magee, and Steve 

Paulsen from EPA Office of Research and Development, Amanda Nahlik from Kenyon College; Alan 

Herlihy from Oregon State University; Gregg Lomnicky from Dynamac; Gregg Serenbetz, Laura Nollen, 

Sue Bartow, Sarah Lehmann, Regina Poeske, Chris Faulkner, Mike McDavit, and Susan Holdsworth from 

EPA Office of Water; and Annie Rossi, an ORISE Research Participant on the National Wetland Condition 

Assessment (NWCA).

We would also like to acknowledge the contributions of several EPA interns and managers who are no 

longer on the NWCA project team but whose support and leadership were instrumental in the planning 

and field implementation of NWCA. Mike Scozzafava and Lynda Hall from EPA Office of Water led NWCA 

efforts through these stages and Elizabeth (Riley) Donovan, Leah Medley, Natalie Allen, and Rachel 

Sullivan provided outstanding technical and project research as ORISE participants during their time on 

NWCA.
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Table of Contents

Acknowledgements............................................................................................................................ i

Table of Contents..............................................................................................................................iii

Tables and Figures .............................................................................................................................v

State and Partner Highlights.............................................................................................................vii

Executive Summary.........................................................................................................................viii

Key Findings ........................................................................................................................................... viii

Biological Condition ........................................................................................................................... viii

Indicators of Stress............................................................................................................................... ix

Implications.............................................................................................................................................. ix

Chapter 1: Introduction......................................................................................................................1

What are wetlands?..................................................................................................................................1

Why are wetlands important? ..................................................................................................................2

Why do we need a national assessment of wetland condition? ..............................................................6

How was the national assessment developed?........................................................................................7

Chapter 2: Design of the National Wetland Condition Assessment ....................................................11

Which wetlands are included in the NWCA?..........................................................................................11

How are the NWCA results presented? ..................................................................................................13

How were the sampling sites chosen and what do they represent?......................................................14

How were wetlands sampled?................................................................................................................17

What data were collected and why? ......................................................................................................21

How were the NWCA core data used to report on ecological condition and stress? ............................23

Evaluating Wetland Biological Condition............................................................................................27

Evaluating Indicators of Stress............................................................................................................31

Estimating the extent of wetland area for condition classes or stress-level classes..............................37

Other data collected as part of NWCA 2011 but not reported in national and regional results............37

Water Chemistry .................................................................................................................................37

Algae Species Data..............................................................................................................................38

Chapter 3: National Results..............................................................................................................42

Biological Condition Based on Vegetation MMI .....................................................................................43

Indicators of Stress..................................................................................................................................45

Physical ...............................................................................................................................................45

Chemical..............................................................................................................................................47
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Biological.............................................................................................................................................49

Ranking of Stressors................................................................................................................................50

Relative Extent and Relative Risk ........................................................................................................50

Attributable Risk..................................................................................................................................51

Microcystin Presence and Risk................................................................................................................52

Chapter 4: Ecoregion and Wetland Type Results ...............................................................................57

Coastal Plains Ecoregion .........................................................................................................................59

Landscape setting of the ecoregion ....................................................................................................59

Summary of findings...........................................................................................................................60

Eastern Mountains and Upper Midwest.................................................................................................65

Landscape setting of the ecoregion ....................................................................................................65

Summary of findings...........................................................................................................................66

Interior Plains..........................................................................................................................................75

Landscape setting of the ecoregion ....................................................................................................75

Summary of findings...........................................................................................................................76

West Ecoregion.......................................................................................................................................79

Landscape setting of the ecoregion ....................................................................................................79

Summary of findings...........................................................................................................................80

Estuarine Wetlands.................................................................................................................................83

Landscape setting for estuarine wetlands..........................................................................................83

Summary of findings...........................................................................................................................83

Chapter 5: Summary and Next Steps.................................................................................................86

Summary of Major Findings and Implications........................................................................................86

Advancing Wetland Science....................................................................................................................87

Next Steps: Preparing for the 2016 Assessment.....................................................................................88

Glossary of Terms ............................................................................................................................94

Sources and References ...................................................................................................................99

List of Abbreviations and Acronyms................................................................................................103
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Tables and Figures

Table 2-1. FWS S&T Wetland Categories that comprise the NWCA Target Wetland Types. .....................12

Table 2-2. Number of probability sites sampled nationally and within each of the NWCA Aggregated 

Ecoregions and acres of wetland area these sites represent...................................................16

Table 2-3. Heavy metals included in the Heavy Metal Index as a chemical indicator of stress. ................33

Table 2-4. Soil phosphorus concentration thresholds as chemical indicators of stress.............................34

Table 2-5. World Health Organization thresholds of risk associated with exposure to microcystin..........34

Table 2-6. Summary of NWCA 2011 indicators of condition and stress and information on how 

thresholds were set. .................................................................................................................36

Figure 1-1. Examples of wetlands commonly found across the U.S.............................................................3

Figure 2-1. Nine Aggregated Ecoregions used in other NARS (map on left) further combined into the four 

NWCA Aggregated Ecoregions (map on right). ........................................................................13

Figure 2-2. NWCA Sample Sites. .................................................................................................................15

Figure 2-3. Standard NWCA assessment area and buffer sampling layout................................................18

Figure 2-4. Core data types and relationships to how they are used to develop indicators of condition 

and stress..................................................................................................................................24

Figure 2-5. Criteria for setting VMMI thresholds for good, fair, and poor condition classes based on 

VMMI values observed for Least Disturbed (Reference) sites. ................................................28

Figure 3-1. Estimated extent of wetland biological condition by condition classes (good, fair, poor) based 

on the VMMI.............................................................................................................................44

Figure 3-2. Estimated extent of vegetation alteration in wetlands by stressor levels as indicated by 

vegetation removal and vegetation replacement....................................................................46

Figure 3-3. Estimated extent of hydrologic alteration in wetlands by stressor levels as indicated by 

damming, ditching, hardening, and filling/erosion. .................................................................47

Figure 3-4. Estimated extent of chemical indicators of stress in wetlands by stressor levels as indicated 

by a Heavy Metal Index and soil phosphorus concentrations..................................................48

Figure 3-5. Estimated extent of biological stress in wetlands by stressor levels as indicated by the 

Nonnative Plant Stressor Indicator...........................................................................................49
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Figure 3-6. National level estimates for relative extent of stressor indicators when stressor level is high, 

relative risk associated with each stressor indicator, and attributable risk for each stressor 

indicator relative to wetland biological condition....................................................................51

Figure 3-7. Estimated occurrence of microcystin in wetlands....................................................................53

Figure 3-8. Estimated extent of recreational health risk from exposure to microcystin by risk category 

(low, moderate, and high) based on WHO guidelines..............................................................54

Figure 4-1. The NWCA Aggregated Ecoregions...........................................................................................58

Figure 4-2. NWCA 2011 survey results for the wetlands (i.e., all target wetland types) across the Coastal 

Plains.........................................................................................................................................61

Figure 4-3. NWCA survey results for the inland herbaceous wetland type across the Coastal Plains. ......61

Figure 4-4. NWCA 2011 survey results for the inland woody wetland type across the Coastal Plains. .....62

Figure 4-5. NWCA 2011 survey results for the wetlands (i.e., all target wetland types) across the Eastern 

Mountains and Upper Midwest................................................................................................67

Figure 4-6. NWCA 2011 survey results for the inland herbaceous wetland type across the Eastern 

Mountains and Upper Midwest................................................................................................68

Figure 4-7. NWCA 2011 survey results for the inland woody wetland type across the Eastern Mountains 

and Upper Midwest..................................................................................................................68

Figure 4-8. NWCA 2011 survey results for the wetlands (i.e., all target wetland types) across the Interior 

Plains.........................................................................................................................................77

Figure 4-9. NWCA 2011 survey results for the inland herbaceous wetland type across the Interior Plains.

..................................................................................................................................................78

Figure 4-10. NWCA 2011 survey results for the inland woody wetland type across the Interior Plains....78

Figure 4-11. NWCA 2011 survey results for the wetlands (i.e., all target wetland types) across the West.

..................................................................................................................................................81

Figure 4-12. NWCA 2011 survey results for the inland herbaceous wetland type across the West..........82

Figure 4-13. NWCA 2011 survey results for the inland woody wetland type across the West..................82

Figure 4-14. NWCA 2011 survey results for estuarine herbaceous wetland types across all coastal areas 

of the conterminous U.S...........................................................................................................84

Figure 4-15. NWCA 2011 survey results for estuarine woody wetland types across all coastal areas of the 

conterminous U.S. ....................................................................................................................85
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National Wetland Condition Assessment 2011

State and Partner Highlights

Gulf of Mexico Coastal Wetlands Pilot Project: Setting the Stage for the NWCA ........................................9

California’s Intensification Project: Learning More about California’s Wetlands and Refining Monitoring 

Techniques................................................................................................................................19

Wisconsin’s Intensification Study: A Floristic Characterization of Wetlands in Eastern Wisconsin...........29

Nebraska’s Intensification Project: Impacts of Land Use and Management on Wetland Condition .........39

Alaska’s Arctic Wetlands Assessment.........................................................................................................55

Southeast Wetlands Monitoring and Assessment Intensification Study: North Carolina, South Carolina, 

Alabama, and Georgia ..............................................................................................................63

Establishing a Baseline for Ohio’s Valuable Wetland Resources: A National Wetland Condition 

Assessment Intensification.......................................................................................................69

Minnesota’s Intensification Project: NWCA Intensification Survey Helps Reveal Important Regional 

Variation in Minnesota .............................................................................................................72

The U.S. Fish and Wildlife Service’s Wetlands Status and Trends Program ...............................................90
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Executive Summary

The National Wetland Condition Assessment (NWCA) 2011: A Collaborative Survey of the Nation’s 

Wetlands presents the results of the first national evaluation of the ecological condition of the nation’s 

wetlands. The NWCA is part of a broader effort by EPA and state, tribal, and federal partners to conduct 

national scale assessments characterizing the ecological condition of the nation’s waters. Under the 

National Aquatic Resource Survey (NARS) program, studies have been completed for wadeable streams 

(2004), lakes (2007), rivers and streams (2008-2009), and coastal waters (2010). The issuance of the 

NWCA 2011 report marks the completion of the first full-cycle of assessments by EPA and its partners 

under the NARS program.

Wetlands are a vital component of our nation’s waters, providing a wide array of benefits that 

contribute to the overall health and integrity of aquatic ecosystems and people’s well-being. Wetlands 

help improve water quality by filtering pollutants, protecting downstream or coastal areas from floods 

and erosion, serving as homes or sources of food for a diverse and abundant range of species including 

humans, and offering places for recreation and scientific and cultural exploration. Though we are aware 

of the important benefits wetlands provide, we know very little about the overall ecological condition of 

these systems nationally. The NWCA begins to address some of the gaps in our understanding of 

wetland health by providing information about the ecological condition of the nation’s wetlands and 

stressors most commonly associated with poor condition.

During the spring and summer of 2011, more than 50 field crews sampled 1,179 wetland sites across the 

country. Each crew used standardized field protocols to sample vegetation, soils, hydrology, algae, water 

chemistry, and potential stressors at each site. Most sites were selected using a random sampling 

technique that ensures that the results of the survey reflect the range of wetlands in the target 

population across the U.S. Data collected at these randomly selected sites are used to produce national 

and regional estimates of wetland condition.

Key Findings

Biological Condition

Vegetation is a major component of the biodiversity and structure found in wetlands, provides 

important habitat and food sources for birds, fish, and other wildlife, and both responds to and 

influences other physical features (e.g., soils, hydrology) and chemical processes (e.g., nutrient cycling) 

in wetland systems. Thus, vegetation can reflect and integrate different components of wetland 

ecosystem integrity and serve as an effective indicator of wetland condition. NWCA 2011 uses 

vegetation to assess the condition of wetlands across the conterminous U.S. and in four major 

ecoregions. Vegetation data collected at each sampling site was used to develop a national Vegetation 

Multi-Metric Index (VMMI), which indicates “good,” “fair,” or “poor” condition based on properties of 

vegetation that vary in relation to human-mediated disturbance. For NWCA 2011, “good” condition 

generally reflects diversity and abundance levels for species and plant traits (e.g., native species,

tolerance for disturbance) appropriate to ecoregion and wetland type.

NWCA 2011 found that nationally, 48% of the wetland area is in good condition, 20% is in fair condition

and the remaining 32% of the area is in poor condition. Of the four major ecoregion-based units 

reported on by NWCA, the West has the lowest percentage of wetland area, 21%, in good condition. The 
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Coastal Plains, Eastern Mountains and Upper Midwest, and Interior Plains have a range of 44% to 52% 

wetland area in good condition.

Indicators of Stress

Wetland condition can be influenced by physical, chemical, and biological factors that impact (i.e., cause 

stress to) a wetland’s physical structure or ecological processes. NWCA 2011 developed and measured a 

number of physical, chemical, and biological indicators of stress that reflect potential negative impact to 

wetland condition. At each site, all of these indicators were evaluated in a core assessment area and 

within a surrounding 100 meter radius buffer. Indicators of stress at each wetland site are assigned to 

“low,” “moderate,” or “high” stressor levels depending on criteria established for each indicator.

Physical

Six physical indicators of stress are assessed for NWCA using field-based observational data collected at 

each site. These indicators represent physical alterations to vegetation (removal and replacement) or 

hydrologic alterations (damming, ditching, surface hardening, and filling/erosion) observed at the 

sampling site. Vegetation removal, surface hardening (e.g., pavement, soil compaction), and ditching are 

found to be the most widespread stressors nationally. Vegetation removal and hardening stressors are 

high for 27% of wetland area, while the ditching stressor is high for 23% of wetland area.

Chemical

Two chemical indicators of stress are assessed for NWCA using soil data collected at each site: a Heavy 

Metal Index and soil phosphorus concentration. Stressor levels for both of these indicators are low for 

the majority of wetland area nationally. However, stressor levels for the Heavy Metal Index are 

moderate for 47% of wetland area in the West and 31% of wetland area in the Eastern Mountains and 

Upper Midwest. Stressor levels for soil phosphorus are high for 13% of wetland area in the Eastern 

Mountains and Upper Midwest.

NWCA conducted the first national study of algal toxins in wetlands. Microcystin, a chemical toxin that 

can harm people, pets, and wildlife, was detected in 12% of wetland area nationally. However, based on 

recreational exposure risk levels established by the World Health Organization, very little wetland area 

(<1%) poses either moderate or high risk levels.

Biological

A Nonnative Plant Stressor Indicator developed for NWCA is used to assess the level of biological stress 

in wetlands. Nationally, 61% of wetland area has low stressor levels for nonnative plants. These results 

are not uniform across the country, however. The Eastern Mountains and Upper Midwest and the 

Coastal Plains have similar percentages of low stressor levels, 74% and 66% of wetland area, 

respectively, for the nonnative plant stressor indicator. In contrast, the West and Interior Plains have

only 14% and 27% of wetland area, respectively, with low stressor levels.

Implications

For resource managers and other decision-makers, the NWCA provides important information about the 

condition of wetlands and several wide-spread stressors influencing their biological condition. 

Additionally, the results point to potential improvement in condition that might be seen nationally by 

reducing these stressors. The NWCA found that wetlands with high levels of vegetation removal and 

surface hardening stress are about twice as likely to have poor biological condition as those with low or 
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moderate levels of these stressors. Further analysis that looks at how condition might improve if these

two stressors are reduced, called attributable risk, suggests a possible 20% reduction in wetland area 

with poor biological condition if the stressor level changed from high to moderate or low.

The NWCA developed a robust VMMI that was successfully used to evaluate the condition of wetlands 

across major ecoregions and wetland types. In addition, NWCA developed several indicators of stress 

based on readily collected field data and used these to evaluate the relationship between common 

stressors and biological condition. NWCA scientists also conducted research into other potential 

indicators of wetland condition, and while not highlighted in this public report, findings from this 

research will help inform future scientific studies.

The NWCA marks a beginning in our endeavors to assess wetland condition nationally. Work conducted 

under the NWCA has advanced the state of science into indicators of wetland condition and improved 

our ability to evaluate wetland condition at national and regional scales. Subsequent studies and 

research by EPA, states, and other partners will continue to build upon the knowledge gained through

the NWCA and allow us to further explore and evaluate the condition of wetlands at multiple scales. We 

will be better able to answer important policy and management questions about the overall health of 

this critical resource, and design effective strategies to fulfill the objectives of the Clean Water Act—to 

restore and maintain the chemical, physical, and biological integrity of the nation’s waters.
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Chapter 1: Introduction

The National Wetland Condition Assessment (NWCA) 2011 is the first national evaluation of the 

ecological condition of the nation’s wetlands. The survey encompassed both tidal and nontidal wetlands 

ranging from the expansive marshes of our coasts to the forested swamps, meadows, and waterfowlrich prairie potholes of the interior plains.

The NWCA is part of a broader effort by EPA, state, tribal, and federal partners to conduct national-scale 

assessments characterizing the ecological condition of the nation’s waters. Under the National Aquatic 

Resource Survey (NARS) program, studies have been completed for wadeable streams (2004), lakes 

(2007), rivers and streams (2008-2009), and coastal waters (2010). NWCA 2011 marks the completion of 

the first full cycle of NARS assessments. EPA and its partners plan to continue to conduct the surveys on 

a five-year basis, rotating through each of the aquatic resources beginning with lakes and ending each 

cycle with wetlands.

Similar to the other NARS studies, the NWCA identified the following key goals for the project:

 Produce a national report describing the ecological condition of the nation’s wetlands and 

stressors commonly associated with poor condition;

 Collaborate with states and tribes in developing complementary monitoring tools, analytical 

approaches, and data management technology to aid wetland protection and restoration 

programs; and

 Advance the science of wetland monitoring and assessment to support wetland 

management needs.

The NWCA builds not only upon the groundbreaking work of its sister NARS studies, but also that of the 

U.S. Fish and Wildlife Service’s (FWS) Wetland Status and Trends (S&T) program, which has been 

documenting changes to the extent of wetland area in the U.S. for more than 30 years. Taken together, 

these two efforts provide government agencies, wetland scientists, and the public with comparable, 

scientifically-defensible information documenting the current status and, ultimately, trends in both 

wetland quantity (i.e., area) and quality (i.e., ecological condition). These studies help us to better 

understand the overall condition and health of all of our nation’s waters and provide information to 

more effectively manage and protect this valuable resource.

This report presents the major findings of NWCA 2011. It begins with an introduction on wetlands and 

the importance of these aquatic systems. Subsequent chapters provide information on the design and 

implementation of the study, national and regional results, and the study’s implications and next steps. 

Key concepts and study components are emphasized in special text boxes throughout the report. In 

addition, there are a number of highlights provided by states and other partners detailing studies and 

work associated with NWCA.

What are wetlands?

“Wetlands” is the collective term given to areas of the landscape that are transitional between land and 

water. Some wetlands occur alongside streams, rivers, lakes, and coastal waters, while others occur in 

depressions and other low-lying areas apart from surface waters, and still others are associated with 

springs high in the mountains. Wetlands can be saturated with water at varying intervals throughout the 
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day and year, and have plant and animal communities adapted to live in conditions ranging from 

permanently wet to fluctuating wet and dry. Wetlands are defined by three important attributes:

 plants that have adapted to survive and thrive in wet conditions (known as hydrophytes);

 soils that exhibit features of prolonged saturation and changing wet and dry cycles; and

 the presence of water at or near the surface of the ground for a time sufficient to produce soils 

and plant communities characteristic of wetlands.

Scientific and regulatory definitions for wetlands can differ in the criteria used to incorporate these 

three attributes. The NWCA uses a scientific definition for wetlands described by Cowardin et al. (1979)

that is broader than the regulatory definition of wetlands used in the federal Clean Water Act (CWA). 

Thus, NWCA includes wetlands that may not be considered jurisdictional for purposes of the CWA.

Wetlands take on a variety of different forms 

and are known by many different names 

depending on their principal characteristics 

and location in particular regions of the 

country. Some examples include marsh, wet 

meadow, swamp, bog, and prairie potholes. 

See Figure 1-1 for pictures and descriptions of 

several common wetland types.

A number of classification systems have been 

developed based on distinctive wetland 

characteristics to organize the many kinds of 

wetlands into groups that share similar 

attributes. The two most common systems 

used nationally are the Cowardin and the 

Hydrogeomorphic (HGM) classification 

systems. The Cowardin system considers water regime, the underlying substrate and vegetation 

communities to catalog wetland and deep water habitats (Cowardin et al. 1979). The HGM system 

considers the wetland’s location in the landscape (i.e., proximity to a lake, stream, or topographic 

setting), along with its morphology, primary water sources, and hydrodynamics (Brinson 1993).

Why are wetlands important?

Wetlands are found on every continent and make up an estimated 5 to 8% of the Earth’s land surface 

(Mitsch and Gosselink 2007; Zhu et al. 2014). In the U.S., wetlands are found in every state, from the 

tundra of Alaska to the playas of the Great Plains and the swamps of the Florida Everglades. Wetlands 

even occur in the driest areas of the American West. Wetlands often occur on the edges of lakes, rivers, 

streams, coastal seas, and other surface water bodies, but also occur independent of these waters 

where precipitation or groundwater is abundant enough to sustain plants, soils, and animals that are 

characteristic of wetlands.

COWARDIN DEFINITION OF WETLANDS

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.
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National Wetland Condition Assessment 2011

Figure 1-1. Examples of wetlands commonly found across the U.S.
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Believed to be an obstacle to development for much of our country’s history, wetlands were often 

drained and filled to make way for other uses. The FWS estimates that more than 220 million acres of 

wetlands existed in the conterminous U.S. prior to European settlement. Since then, extensive losses 

have occurred and over half of those wetlands have been drained and converted to other uses (Dahl 

1990). Beginning in the 1970s, the rate of wetland losses slowed substantially as a result of changes in 

national and state wetland policies and heightened awareness of the important benefits aquatic 

systems, including wetlands, bring to society.

We now know that wetlands are a vital component of the nation’s waters and provide many crucial 

benefits including water quality improvement, flood abatement and carbon storage, biodiversity 

support, plus aesthetic, recreational, educational, and scientific opportunities. Today these benefits are 

part of the decision process when permits are issued for activities that result in the loss or degradation 

of wetlands.

Wetlands play a critical role in sustaining healthy surface and ground waters. The physical structure of 

wetlands can allow them to intercept the flow of surface water, resulting in the retention and filtration 

of nutrients, sediment, and pollutants. Microbes living in wetlands can break down or transform 

potentially toxic compounds. Plants and microbes also consume and transform excess nutrients,

improving water quality and slowing eutrophication of downstream waters. Together these wetland 

processes often reduce the amount of undesirable substances entering other surface water bodies (e.g., 

rivers, streams, lakes, coastal waters) where they can degrade water quality and pose environmental 

and human health risks.

Wetlands can act as natural sponges, capturing and slowly releasing surface water resulting from heavy 

rains, snow melt, and other floodwaters. Trees, grasses, and other wetland plants help slow the speed of 

floodwaters and disperse the excess water across floodplains. These processes lower flood heights, 

reduce erosion, and decrease the otherwise destructive effects of swiftly moving floodwaters. In 

addition, wetlands at the edges of lakes, rivers, bays, and the ocean buffer shorelines from the 

damaging effects of storm surges caused by hurricanes, tropical storms, and other powerful weather 

events.

The position of wetlands at the interface of land and water make them some of the most biologically 

diverse and productive ecosystems in the world. Many unique plant communities provide diverse plant 

species, physical structure, and distinct aesthetics to wetlands. A great variety of life from the tiniest 

microbes to plants, birds, fish, and mammals inhabit and depend upon wetlands for part or all of their 

life-cycles. Chemical and biological processes occurring within wetlands provide abundant supplies of 

food for a diverse range of species. Microbes and small aquatic insects break down dead plant material, 

forming small particles of organic material that feed larger aquatic insects and shellfish. These, in turn, 

feed fish and wildlife. Wetland dependent fish species make up 71% of the commercial and recreational 

fisheries in the U.S. (Fisheries and Water Resources Policy Committee 2004). Ducks and other waterfowl 

depend on wetlands for food and shelter. For example, prairie potholes and marshes dotting the 

Midwest are rich in plant and aquatic life vital for breeding waterfowl. Bogs and fens are important 

habitat for rare plants and animals. About 50% of threatened or endangered animal species in the U.S. 

depend on wetlands for their survival and 28% of threatened or endangered plant species are associated 

with wetland ecosystems (Mitsch and Gosselink 2007).

Wetlands provide plentiful opportunities for recreation and enjoyment by people. Bird and other 

wildlife watchers are drawn to wetlands to view or photograph the diverse species that call wetlands 

home. Hunters wade or boat through wetlands to hunt waterfowl. The abundance of fish in some types 
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National Wetland Condition Assessment 2011

of wetlands attracts fishermen to these habitats. Marsh ecosystems line our coasts and provide food 

supply, shelter, and nursery areas for both marine and freshwater species, fueling a commercial and 

recreational fishery economy worth billions of dollars (Fisheries and Water Resources Policy Committee 

2004). The 2011 National Survey of Fishing, Hunting, and Wildlife-Associated Recreation revealed that 

over 90 million U.S. residents participated in wildlife-related recreation such as fishing, hunting, or 

wildlife watching. This recreation is entirely dependent on having clean and healthy waters, including 

wetlands, to support the fish and wildlife at the center of these activities.

Wetlands are important settings for scientific research. Tens of thousands of research papers have been 

published about wetlands by scientists since the 1950s, and today, wetlands continue to be the subject 

of much scientific research, including studies on nutrient cycling, global climate change, bird migration, 

unique plant communities, and countless others. Despite all of the historic and current wetlands 

research, the NWCA is the first assessment to characterize the ecological condition of wetlands on a 

national scale. The NWCA also has inspired dozens of researchers to conduct novel scientific research 

that will add to our knowledge of wetlands and their importance in our national, global, and human 

environments.

WATER CLEANSING PROCESSES OF NATURAL WETLANDS AND THEIR USE IN ENGINEERED WATER 

TREATMENT SYSTEMS

Wetlands are vital to the health of waterways and downstream communities. Wetlands can naturally remove 

excess nutrients, sediment and other pollutants, keeping them from reaching lakes, streams and the ocean. 

The result is cleaner water resources and a healthier aquatic ecosystem.

After being slowed by the vegetation in a wetland, incoming water moves around plants, allowing suspended 

sediment to drop out and settle to the wetland bottom. Nutrients dissolved in water—which reach wetlands 

from various sources including anthropogenic ones such as fertilizer application, manure, leaking septic tanks, 

and municipal sewage—are often absorbed by plant roots and microbes in the soil while other pollutants 

adhere to soil particles. In many cases, this filtration process removes much of the water’s nutrient and 

pollutant load by the time it leaves a wetland.

Engineers have designed water treatment facilities to use similar processes to remove pollutants through 

coagulation, settling (in sedimentation tanks), filtration, and disinfection. Just as with the natural processes in 

wetlands, stormwater and sewage go through physical, chemical and biological processes at treatment 

facilities which clean the wastewater. Constructed wetlands are treatment systems that use natural processes 

involving wetland vegetation, soils, and their associated microbial assemblages to improve water quality. 

Around the world, thousands of constructed wetlands have been created to replicate the water cleaning 

abilities of natural wetlands.

 Picture from USEPA 2004
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National Wetland Condition Assessment 2011

Why do we need a national assessment of wetland condition?

Efforts by the FWS S&T program have provided national scale information on the location and extent of 

wetlands (i.e., the areal coverage on the landscape) since the 1950s. Their most recent report, published 

in 2011, found a slight decrease in wetland area between 2004 and 2009. While the report noted gains 

for some wetland types, such as freshwater ponds, it found continued declines in area of forested 

wetlands and salt marshes. Companion reports focused specifically on wetlands in coastal watersheds 

(2013) and the prairie pothole region (2014) also found that wetland area is decreasing in these areas.

See “The U.S. Fish and Wildlife Service’s Wetland Status and Trends Program” highlight at the end of 

Chapter 5 for more information on this program.

While the S&T reports provide invaluable information on wetland extent and change among wetland 

types, they do not provide data on wetland condition. Compared to other aquatic systems, such as 

lakes, streams, rivers, and coastal waters, wetlands have not been comprehensively assessed to 

determine their condition or ability to meet water quality goals under the CWA. In a 2002 Water Quality 

Report to Congress, information provided by states addressed only 1% of the estimated area of 

wetlands, compared to approximately 20% of streams and rivers, 40% of lakes, and 35% of coastal 

waters (USEPA 2007). The lack of data makes it difficult to answer basic questions about the condition of 

our wetlands nationwide and to support key management decisions for most effectively protecting and 

restoring this valuable resource.

The NWCA is designed to address the gaps that exist in our understanding of wetland condition. EPA and 

its state, tribal, academic, and federal agency partners, are collaborating to provide improved 

environmental information about wetlands at national and regional scales. The NWCA, like all NARS 

assessments, is a statistical survey that provides a cost-effective and scientifically-valid way for 

informing the public and decision-makers about wetland quality because it:

 Is nationally consistent,

 Produces data representative of the resource being sampled,

 Uses standardized field and laboratory protocols, and

 Follows rigorous quality assurance protocols.

The findings of the NARS are not water quality reports prepared by the states under Section 305(b) of 

the CWA, nor are they impaired water determinations under Section 303(d) of the CWA. Such 

determinations are made by states on specific water body segments using applicable state water quality 

standards. State CWA monitoring and assessment approaches may vary significantly from those used in 

NARS and may yield different results (see text box “How Does NARS Reporting Differ from State Water 

Quality Reports Required by the CWA?”).

Rather, the NARS are designed to answer such questions as:

 What percent of waters support healthy biological and recreational condition?

 How widespread are major stressors that impact water quality?

 How is condition changing over time?

 Are our national investments in the protection and restoration of aquatic systems working?

The focus of the surveys is on water bodies as groups or populations, rather than as individual waters. 

Accordingly, this report does not provide wetland managers with information on the condition of a 

specific wetland. Instead, the NWCA allows us to assess the percent of wetland area within particular 
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National Wetland Condition Assessment 2011

condition classes (e.g., good, fair, poor) at the national scale and in four major ecoregions across the U.S. 

As additional surveys are implemented, we will be able to track changes in condition over time as well.

How was the national assessment developed?

EPA began planning for the NWCA in 2006. Throughout the planning process, EPA engaged with a broad 

group of stakeholders from state environmental and natural resource agencies, tribes, federal agencies, 

academia, and other organizations to help inform different aspects of the assessment. Planning the firstever national survey of wetland condition entailed a number of steps, each with its own set of 

challenges to overcome, including, but not limited to, creating a survey design, selecting efficient, 

scientifically valid indicators, developing new field protocols, and piloting protocols. The development 

and selection of the NWCA field methods were also influenced by logistic considerations, such as the 

need to complete sampling for each site in one day.

In addition to reporting on the condition of wetlands nationally, another objective of the NWCA is to 

provide support to states and tribes that are interested in pursuing research to develop assessment 

methodologies and undertake statistically valid surveys of wetlands at finer geographic scales (i.e., within 

state or tribal boundaries). To accomplish this, EPA encouraged and provided support for states to do 

more intensive sampling in conjunction with the NWCA at state or regional scales. These intensifications 

involved sampling additional sites, parameters, or both within a state or region. States worked closely 

with EPA to develop intensification survey designs. This resulted in projects throughout the country, with 

state-driven focuses such as:

 Designing state-scale assessments that inform state-level management and policy needs;

HOW DOES NARS REPORTING DIFFER FROM STATE WATER-QUALITY REPORTS REQUIRED BY THE CWA?

Under Section 305(b) of the CWA, states are expected to monitor, and are required to assess and report on the 

condition of their waters, including the extent of waters that support the goals of the Act. Under Section 303(d), states 

are to identify waters that are impaired, do not meet state water quality standards, and require additional pollution 

controls. States use a variety of monitoring and assessment approaches to meet these requirements of the CWA and to 

address state-specific information needs for managing state waters. 

States usually collect information for many parameters at locations of importance to the state, such as swimming 

beaches, near dischargers, or at drinking water sources using methods developed for the state’s specific purposes. 

However, state methods of collecting and assessing data can change over time and vary widely between states; so too 

do the state water quality standards used to determine impairment. This makes it difficult to aggregate this information 

for the nation as a whole, or over time. State monitoring programs are not designed to answer national-level questions 

such as whether or not U.S. water quality is improving.

One of the long-term goals of the NARS is to determine trends for the nation as a whole. To do this, the surveys use a set 

of standardized methods to monitor for a discrete set of stressors. Field crews collect the same data at each of the 

randomly selected, representative sites across the country; results are compared to conditions at least-disturbed sites in 

the ecoregions (and not to state water quality standards). Survey results are then aggregated into an overall assessment 

of water quality. It is the intent of EPA that these surveys complement state-specific CWA information and provide 

national and regional context to decision-makers.

To learn more about state CWA reporting, visit www.epa.gov/waters/ir/.
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National Wetland Condition Assessment 2011

 Capturing wetlands of interest that may have been missed in the national survey due to being 

rare (e.g., vernal pools); and

 Testing additional indicators or assessment procedures.

These intensifications serve to augment the national and ecoregion results of the NWCA 2011. Some of 

these projects, as well as their findings, are highlighted throughout this report.
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National Wetland Condition Assessment 2011

Highlight

Gulf of Mexico Coastal Wetlands Pilot Project: Setting the Stage for the NWCA

Janet Nestlerode, U.S. EPA Office of Research and Development, Gulf Ecology Division

Objectives: The EPA, in 

collaboration with the 

United States Geological 

Service (USGS), 

implemented a pilot project 

along the northern coast of 

the Gulf of Mexico (GOM) in 

order to evaluate the 

effectiveness of a three-tier 

survey framework for 

regional wetland 

assessment. The results of 

the pilot study provided 

insight into the usefulness 

of the chosen indicators in 

determining condition and 

provided several “lessons 

learned” that were 

beneficial to the 

development of the NWCA

2011.

Overview: One hundred 

wetland points along the GOM Coastal Region, which includes five states and 2,500 km of coastal 

lands, were selected for the pilot study and included five wetland classes (Figure 1). Five hundred

oversample sites were also generated to replace original points that had to be dropped due to 

inaccessibility, or for other reasons. A 3-tier assessment framework was implemented during the 

2007 and 2008 field seasons. Landscape-level (tier 1) assessments were conducted off-site and (tier 

2 and 3) sampling was undertaken by crew members for each site, typically within one field day.

Crews utilized a tier 2 rapid assessment, called the Gulf Rapid Assessment Method (GRAM), based 

on the California Rapid Assessment Method (CRAM). Tier 3 (intensive site assessments) included the 

collection of physical, chemical, and biological data at each site. This included soil and water 

chemistry, vegetation and macrophyte samples, as well as other measures. Between the first and 

second field seasons for the pilot, minor modifications were made to the field protocols.

Planning, logistics, and field conditions presented several challenges for the team before and during 

the field season. These ranged from identifying landowners, obtaining permission to sample a site, 

and determining how best to reach remote sites. Identifying landowners and gaining access to sites 

Figure 1. GOM coastal wetland survey sites shown with in the boundary of GOM 

coastal watersheds as defined by all USGS 8-digit hydrologic unit code (HUC) 

system segments that abut the coast and modified by NOAA’s Estuarine Drainage 

Areas where needed.
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National Wetland Condition Assessment 2011

often proved difficult, as the team had to take into consideration both land ownership at the sample 

site and ownership of lands that had to be crossed to access the site. Landowner records and access 

to those records differed by localities, and it was a learning curve to find contact information and

coordinate access permissions.

During GOM pilot implementation, crews sampled sites that were often remote and/or difficult to 

access. In South Florida, for example, getting to some sites required hiking for miles through waistdeep waters and mucky soils, transportation by boat, 4-wheel drive sport utility vehicles, or swamp 

buggies with balloon tires, or some combination of these modes of transportation. The team 

coordinated with local experts to determine the best ways to access remote sites, but in some cases 

sites were inaccessible and had to be dropped altogether.

Through data collection and analysis, 

the pilot project identified gear and 

equipment needed by survey crews, as 

well as an appropriate division of 

responsibilities among crew members. 

Implementation of the pilot study also 

provided an opportunity to determine 

the time needed for various data 

collection protocols, and helped gauge 

what could realistically be 

accomplished by crews within a single 

field sampling day, once factoring in 

travel times. This information was used 

in the planning and protocol 

development for the NWCA.

Overall, the GOM coastal wetlands 

pilot project highlighted the great 

cooperation and effort necessary to conduct a regional condition assessment, and it provided critical 

information and lessons learned that informed planning efforts and development of the NWCA 

2011.

To learn more, contact Janet Nestlerode (Nestlerode.Janet@epa.gov; 850-934-2492), EPA Office of 

Research and Development, Gulf Ecology Division.

Swamp buggy used to access a remote wetland site in Big 

Cypress National Preserve.
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National Wetland Condition Assessment 2011

Chapter 2: Design of the National Wetland Condition Assessment

The NWCA is designed to answer basic questions about the condition of our nation’s wetlands and the 

prevalence of key stressors at national and regional scales. As noted, it is intended to complement and 

build upon the achievements of the FWS S&T Program, which characterizes changes in wetland acreage 

across the conterminous U.S.

Which wetlands are included in the NWCA?

This report covers the conterminous U.S., which currently contain an estimated 110 million acres of 

wetlands (Dahl 2011). Neither Alaska nor Hawaii are included in the national results presented in this 

report. Wetlands in Hawaii were not sampled, but the State of Alaska conducted sampling on the North 

Slope of the Alaskan coastal plain, using protocols similar to those used in the NWCA. A summary of the 

results of the North Slope assessment are presented as a special highlight later in the report (see 

“Alaska’s Arctic Wetlands Assessment”).

The specific wetlands targeted for sampling in the NWCA include tidal and nontidal wetlands within the 

conterminous U.S. with rooted vegetation and, when present, shallow open water less than one meter 

deep, that are not currently being used in the production of crops1. EPA used the same digital map of 

wetland locations that FWS uses in their Wetlands S&T Program to select sites for sampling. The S&T

Program defines and classifies habitats into wetland, deep water, and upland categories and groups 

wetlands into S&T categories based on hydrology, geomorphology, vegetation, and water chemistry 

(Dahl 2011, Dahl and Bergeson 2009, Cowardin et al. 1979). EPA uses a subset of the S&T wetland 

categories for the NWCA. Table 2-1 provides detailed descriptions of the seven S&T wetland categories 

that are included in the target wetland population for NWCA.

Two major S&T wetland categories, Marine Intertidal (near shore coastal waters) and Estuarine 

Intertidal Unconsolidated Shore (beaches, bars, and mudflats), are not included in the NWCA because 

they are outside of the NWCA target population. They typically occur in deeper water (> 1 meter), or are 

unlikely to contain rooted wetland vegetation. Unique wetland types with more limited extents across

the conterminous U.S. may also not be included, or may be underrepresented, in the NWCA 2011 if they 

are not included in the S&T Program or due to inherent constraints associated with the number and 

locations of the sites randomly selected for sampling.

 

1 Wetlands that have been mechanically or physically altered for the production of crops, but where wetland 

plants would become reestablished if farming is discontinued, are identified in the FWS S&T Program as “farmed 

wetlands.” NWCA included these wetlands in its target population only if they were not currently being used for 

the production of crops.
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Table 2-1. FWS S&T Wetland Categories that comprise the NWCA Target Wetland Types. Descriptions adapted from Dahl 

(2006) and Dahl and Bergeson (2009). See NWCA 2011 Technical Report (USEPA 2015), Chapter 1 for more details.

S&T Categories included in 

NWCA (NWCA Aggregated 

Wetland Type) Common Name Technical Description

Estuarine Intertidal Emergent

(Estuarine Herbaceous)

Salt marsh Emergent wetlands in estuarine system characterized by erect, 

rooted, herbaceous hydrophytes, excluding mosses and lichens. 

This vegetation is present for most of the growing season in 

most years. These wetlands are usually dominated by perennial 

plants.

Estuarine Intertidal Scrub 

Shrub/Forested

(Estuarine Woody)

Mangroves

Other estuarine 

shrub or forest 

wetlands

Shrub or forested wetlands in estuarine system dominated by 

woody vegetation less than 20 feet (6 meters) tall. The species 

include true shrubs, young trees, and trees or shrubs that are 

small or stunted because of environmental conditions. Forested 

wetlands are characterized by woody vegetation that is 6 

meters or taller.

Palustrine Emergent

(Inland Herbaceous)

Inland marshes

Wet meadows

Emergent wetlands in the palustrine* system and characterized 

by erect, rooted, herbaceous hydrophytes, excluding mosses 

and lichens. This vegetation is present for most of the growing 

season in most years. These wetlands are usually dominated by 

perennial plants.

Palustrine Scrub Shrub

(Inland Woody) 

Shrub wetlands Shrub wetlands in the palustrine* system and dominated by 

woody vegetation less than 20 feet (6 meters) tall. The species 

include true shrubs, young trees, and trees or shrubs that are 

small or stunted because of environmental conditions.

Palustrine Forested

(Inland Woody)

Forested swamps Forested wetlands in the palustrine* system and characterized 

by woody vegetation that is 6 meters tall or taller.

Palustrine Farmed

(Inland Herbaceous)

Farmed wetland Farmed wetlands in the palustrine* system and having the soil 

surface mechanically or physically altered for production of 

crops, but where hydrophytes will become reestablished if 

farming is discontinued.

Palustrine Unconsolidated 

Bottom/Aquatic Bed

(Inland Herbaceous)

Ponds

Bog lakes

Vernal pools

Kettle ponds

Beaver ponds

Alligator holes

Farm ponds

Recreation ponds

Golf course ponds

Residential lakes

Water retention 

ponds

Aquatic beds

Pondweeds

Aquatic beds in the palustrine* system dominated by plants that 

grow principally on or below the surface of the water for most 

of the growing season in most years. Examples include 

pondweeds (Potamogeton spp.), wild celery (Vallisneria 

americana), waterweed (Elodea spp.), and duckweed (Lemna 

spp.).

Unconsolidated bottom wetlands in the palustrine* system and 

with at least 25% cover of particles smaller than stones, and a 

vegetative cover less than 30%. Examples of unconsolidated 

substrates are: sand, mud, organic material, and cobble gravel.

Aquatic bed and unconsolidated bottom wetlands must also 

have the following four characteristics: (1) area less than 20 

acres (8 ha); (2) an active wave formed or bedrock shoreline 

features are lacking; (3) water depth in the deepest part of a 

basin less than 6.6 feet (2 meters) at low water; and (4) salinity 

due to ocean derived salts less than 0.5 parts per thousand.

* Due to differences in classifying and mapping wetlands under the Cowardin system, these S&T categories may 

include wetlands in shallow riverine and lacustrine systems.
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National Wetland Condition Assessment 2011

How are the NWCA results presented?

The NWCA reports on wetlands at two scales. The broadest scale is nationwide. Results are also 

reported for ecoregions that correspond to major climate and landform patterns, because the patterns 

of response to stress, and the stressors themselves, are often better understood in this context.

Ecoregions developed and described by Omernik (1987, USEPA 2011a) are used in this report, as well as 

in other NARS studies. Omernik Level III ecoregions across the conterminous US were combined into 

nine Aggregated Ecoregions to analyze data and report results in previous NARS studies of wadeable 

streams (2004), lakes (2007), and rivers and streams (2008-2009) (see left map in Figure 2-1). Ideally,

NWCA would have used this same set of nine ecoregions to analyze and report results. However, 

attempting to evaluate each of the seven NWCA Target Wetland Types within each of the nine 

Aggregated Ecoregions would have required sampling nearly three times as many sites to achieve 

statistically valid results, which was beyond the logistical capacity of the NWCA. To allow assessment of 

condition for distinct wetland types across ecoregions with an acceptable degree of statistical certainty

(i.e., a sufficient number of sampled sites by wetland type), NWCA further combined the nine 

Aggregated Ecoregions into four NWCA Aggregated Ecoregions (see right map in Figure 2-1):

 Coastal Plains

 Eastern Mountains and Upper Midwest

 Interior Plains

 West

Figure 2-1. Nine Aggregated Ecoregions used in other NARS (map on left) further combined into the four 

NWCA Aggregated Ecoregions (map on right).

The seven S&T wetland categories included in the NWCA Target Wetland Types (see Table 2-1) sampled 

also had to be combined to allow a sufficient number of sites in each reporting group. This was done by 

maintaining the distinction between estuarine and inland wetland types, and within these two groups 
Chapter 2 Design of the NWCA

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National Wetland Condition Assessment 2011

distinguishing wetlands by herbaceous-dominated or woody-dominated vegetation, resulting in four 

NWCA Aggregated Wetland Types:

 Estuarine Herbaceous – emergent wetlands

 Estuarine Woody – scrub-shrub and forested wetlands

 Inland Herbaceous – emergent, unconsolidated bottom/aquatic bed, and farmed wetlands not 

in crop production

 Inland Woody – forested and scrub-shrub wetlands

Chapter 3 presents the results at the national level and in comparison to the four NWCA Aggregated 

Ecoregions. Chapter 4 presents the condition and stressor results for each NWCA Aggregated Ecoregion

and, within each ecoregion, for inland herbaceous and inland woody wetland types. Results for 

estuarine herbaceous and woody wetlands are presented nationally. While aggregating wetland types 

allows for the reporting of statistically valid national and ecoregional results, differences among unique 

wetland types across the conterminous U.S. may be obscured by combining the various wetland types 

into the four wetland types used for this NWCA 2011 report.

How were the sampling sites chosen and what do they represent?

NWCA sampling locations were randomly selected using a survey design commonly applied in a 

variety of research fields (e.g., ecological assessments, health surveys, election polls, monthly labor 

estimates) to determine the status of populations using a representative sample of relatively few 

members of the group. This approach is especially cost-effective if the population is so numerous 

that all members cannot be sampled, or if it is not necessary to sample the entire population to 

reach a desired level of statistical precision.

To identify a group of wetland sites to be sampled in the NWCA, also known as the target 

population (i.e., potential sample points), it was necessary to know the location of the NWCA Target 

Wetland Types. The NWCA design team used the same digital map of wetland locations as the FWS 

S&T Program in their 2005 survey to select wetland sample points and to facilitate comparisons of 

the findings from both programs. The S&T Program updates wetland mapping for a fixed set of 

statistically selected locations across the conterminous U.S. every five to ten years. While not a 

comprehensive map of all wetlands throughout the U.S., these mapped locations are used to

statistically represent the extent of wetlands nationally and, at the time of the survey, was the most 

consistent and up‐to‐date source of digitally mapped wetlands available on a national scale for the 

NWCA 2011. Sample points for the NWCA were distributed based on the prevalence of wetlands

across the U.S. and the seven NWCA Target Wetland Types (see Table 2-1). For example, more 

sample points were located in regions with greater wetland area.

The 967 sites sampled based on the NWCA design were identified using a technique called 

Generalized Random Tessellation Stratified (GRTS) survey design (see NWCA 2011 Technical Report, 

Chapter 1 (USEPA 2015)). In such a design, every element in the population has a known probability 

of being selected for sampling. This important feature ensures that the results of the survey reflect 

the full range of wetlands in the target population across the U.S. Site selection rules were 

implemented to provide balance in the number of wetlands from each class. Site selection was also 

controlled for spatial distribution to ensure each state received a minimum number of sites, which 

also improved the national spatial balance of the sites (see Figure 2-2).
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National Wetland Condition Assessment 2011

Figure 2-2. NWCA Sample Sites. The inset shows the sample sites for the study in the North Slope of Alaska described in 

a highlight later in this report.

Research teams from EPA and the states screened the points identified for sampling by the survey 

design using aerial photo interpretations and Geographic Information System (GIS) analyses to eliminate 

locations not suitable for NWCA sampling (e.g., wetlands converted to dry land or upland due to 

development). Next, field reconnaissance was conducted to determine if the sites met established 

criteria for inclusion in the survey. If a site was eliminated by the screening process or determined in the 

field to be a non-target wetland type or inaccessible (e.g., the landowner denied access to the site, the 

site was unsafe to access, the site was too remote to access under the logistical constraints of the 

survey), it was removed from the sampling effort and systematically replaced with another site from a 

pool of replacement sites within the random design.

The treatment of sites eliminated from sampling, as either non-target or inaccessible, affects how 

the final population results for the NWCA are estimated and reported. Taking into account the sites 

identified, during screening and field reconnaissance, as non-target (e.g., wetlands in active crop 

production, deeper water ponds, mudflats, uplands), the NWCA estimated there are 95 million 

acres of wetlands in the NWCA target population. The area represented by sites that are part of the 

target population, but not sampled because of accessibility issues, is not included for reporting on 

assessment of condition and stress. As a result, the final acreage represented by the probability 

sites sampled and reported on in the NWCA is 62 million acres. In addition, not all wetland types 

included in the FWS S&T studies (estimated wetland area 110 million acres) are included as wetland 
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National Wetland Condition Assessment 2011

types in the NWCA target population. Table 2-2 provides the distribution of sampled probability 

sites and the acres and percent of wetland area they represent within the NWCA ecoregions and 

aggregated wetland types.

Table 2-2. Number of probability sites sampled nationally and within each of the NWCA Aggregated Ecoregions and acres of 

wetland area these sites represent. Number of sites sampled and the acres represented is also shown for the aggregated 

wetland types used in NWCA.

Region

Sampled 

Sites in 

Population

Acres 

Represented 

by Sampled 

Sites, (% of 

Population) NWCA Wetland Type

Sampled 

Sites in 

Population

Acres 

Represented 

by Sampled 

Sites, (% of 

Population)

National

967 62,156,199

(100%)

Estuarine Herbaceous 258 4,987,824

(8%)

Estuarine Woody 69 497,821

(1%)

Inland Herbaceous 302 13,599,611

(22%)

Inland Woody 338 43,070,943

(69%)

Coastal Plain

513 30,893,305

(50%)

Estuarine Wetlands 288 5,283,489

(9%)

Inland Herbaceous 62 3,750,551

(6%)

Inland Woody 163 21,859,265

(35%)

Eastern Mtn & Upper Midwest

152 19,956,668

(32%)

Estuarine Wetlands 14 29,173

(0.04%)

Inland Herbaceous 55 3,762,089

(6%)

Inland Woody 83 16,165,406

(26%)

Interior Plains

156 7,659,166

(12%)

Estuarine Wetlands 0 0

Inland Herbaceous 115 4,598,831

(7%)

Inland Woody 41 3,060,335

(5%)

West

146 3,647,060

(6%)

Estuarine Wetlands 25 172,985

(0.3%)

Inland Herbaceous 70 1,488,139

(2%)

Inland Woody 51 1,985,936

(3%)
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National Wetland Condition Assessment 2011

How were wetlands sampled?

NWCA field work was conducted during the spring and summer of 2011 by more than 50 crews 

composed of four or more trained personnel from state and tribal environmental agencies, EPA, 

universities, and contract staff. Wetland sites were sampled using standardized field protocols (see 

NWCA Field Operations Manual, USEPA 2011a; NWCA Laboratory Operations Manual, USEPA 2011b; 

and the NWCA Quality Assurance Project Plan, USEPA 2011c) to collect data relevant to describing the 

ecological condition of wetlands and quantifying indicators of stress to condition. Protocols were 

designed to allow sampling at each site to typically be completed in one day.

During each site visit, field crews collected ecological data in a standard 0.5-hectare assessment area, 

representing the sample point from the survey design, and in the immediately adjacent 100 meters 

extending from the assessment area edge, designated as the buffer (Figure 2-3). The crews collected 

data on vegetation, soils, hydrology, algae, and water chemistry from the assessment area. In the buffer

crews collected data on habitat and the presence of stressors that could impact the assessment area.

Some sites were not conducive to use of the standard layout shown in Figure 2-3 because of the size and 

shape of the wetland (e.g., long and narrow) or because parts of the assessment area contained deep 

water, non-target wetland types, or upland. In such cases, the NWCA protocols provided specifications 

for alternate assessment area layouts.

The use of standardized field and laboratory protocols is a key feature of the NWCA and all NARS 

studies, and allows the data to be combined to produce a nationally consistent assessment. As part of 

the quality assurance procedures, each field crew was trained and evaluated on applying the NWCA 

protocols by wetland experts. Field checks were conducted at the beginning of the sampling season to 

ensure that the protocols were being correctly implemented by the crews, thereby minimizing human 

error in data collection. In addition, roughly 10% of the sites were resampled two to four weeks after the 

initial visit to a site to evaluate sampling variability.
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Figure 2-3. Standard NWCA assessment area and buffer sampling layout
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Highlight

California’s Intensification Project: Learning More about California’s Wetlands and 

Refining Monitoring Techniques

Cara Clark, Moss Landing Marine Laboratories

California has a great diversity of wetlands. Different wetland 

types provide different services and support diverse wildlife, 

such as water birds, birds of prey, otters, bears, deer, and a 

wealth of fish species. California has lost more than 90% of its 

historical wetlands and today, many remaining wetlands are 

threatened. Wetlands continue to be drained for agriculture, 

filled for development, or disturbed by modifications to the 

watershed such as dams or water diversions. Climate change 

poses a significant threat, as many wetlands today are 

dependent on artificial water delivery systems or high 

groundwater levels, and may be impacted by changing climatic 

conditions. Further, wetlands along the coast face flooding from 

potential sea level rise.

Project Objectives: California’s intensification project augmented the 2011 NWCA survey to support two 

primary goals. First, California increased the number of sites sampled to produce a statistically valid state-level 

assessment of California wetland quality. Second, the data collected for NWCA were used to further validate 

the California Rapid Assessment Method (CRAM) - an important tool in California’s toolbox for assessing 

wetland health - and examine additional associations between CRAM and potential stressors to wetland 

quality.

Sampling and Protocols: The 2011 

NWCA included 23 sites in California 

and California added 22 additional 

sites bringing the total to 45. The 

additional California intensification 

project sites were selected from the 

same framework as the NWCA, so that 

the two sets of sites could be 

combined for analysis and reporting. 

NWCA data and protocols were used 

at all 45 sites and the state also 

conducted the CRAM (Figure 1).

The CRAM is a standardized tool for 

wetland monitoring in California, 

developed with support from EPA. It is 

based on the concept that the 

structure of a wetland is indicative of 

its capacity to provide important 

functions. Four general attributes, 

Estuarine Wetland Site sampled in China 

Camp State Park, San Rafael, CA.

Figure 1. Sampled NWCA sites in California.
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National Wetland Condition Assessment 2011

Buffer and Landscape Context, Hydrology, Physical Structure, and Biotic Structure, are assessed visually in the 

field. The scores for each attribute are compiled into an overall score, with higher scores relating to better 

condition. This project served as a valuable opportunity to further validate CRAM by comparing CRAM scores 

to the more intensive parameters in the NWCA including water nutrient concentrations.

What They Have Learned: Although assessment of the information from the California intensification is still 

ongoing, staff have begun looking at the site-specific information from the CRAM, including a comparison of 

estuarine and depressional wetlands. California’s estuarine wetlands are often large, connected salt marsh 

systems, in contrast to depressional wetlands which are usually smaller or more fragmented. As a result, 

estuarine wetlands have fewer direct landscape stressors putting pressure on them, and so tend to be in 

better condition. This is borne out by CRAM results from 2011.

The sites sampled in 2011 indicate that estuarine wetlands tended to have higher CRAM scores (indicating 

better condition) than depressional wetlands. The median (50th percentile) CRAM score of depressional 

wetlands was 62, whereas the median score for estuarine wetlands was 79 (Figure 2). Additionally, 

depressional wetlands tended to have lower scores than estuarine wetlands for all of the attributes included 

in CRAM, indicating they are subject to more 

stressors.

Other stressors: Excess nutrients in wetlands 

often have anthropogenic sources, as 

nutrients drain to wetlands from high intensity 

land uses, such as agriculture or urban 

development. Excess nutrients entering the 

wetland can have direct impacts, causing 

eutrophication and excess growth of algae, 

which leads to hypoxia. When algae die and 

decompose, oxygen is depleted from the 

water, and the lack of oxygen can kill fish and 

other organisms. Researchers found weak, but 

statistically significant, correlations between 

water nutrient concentrations and CRAM 

scores in California’s wetlands. Wetlands with 

higher nitrate and nitrite concentrations in 

surface water samples tended to have lower CRAM scores. The correlation observed between CRAM scores 

and nitrate/nitrite concentrations validates that the CRAM score reflects environment factors that can cause 

wetland degradation. Higher total phosphorous in surface water was also associated with wetlands having 

lower Buffer and Landscape Context scores (one of the CRAM components). Like nitrogen, excess 

phosphorous in wetlands is often due to intensive anthropogenic activities in the surrounding area, which is 

suggested by the lower Buffer and Landscape Context scores.

What’s Next: The California intensification project provided valuable information for state-level wetland 

condition, as well as validation for the CRAM tool that is utilized throughout the state. In addition to continued 

analysis of this dataset, implementation of the NWCA encouraged the state to develop its own wetland status 

and trends program with the goal of augmenting the FWS Status and Trends wetland area mapping 

information with consistent and complete state-level wetland maps. While the sites selected for the NWCA 

survey were spread across the state, there were clusters of sites in the San Francisco Bay area and the Central 

Valley. At the time of NWCA 2011 site selection, California did not have comprehensive or consistently-scaled 

wetland maps or a state level wetland mapping program. While this program is still under development, it is 

hoped that wetland monitoring and assessment goals will be better met with improved wetland maps for the 

state providing a more robust characterization of the entire population of wetlands in California.

To learn more, contact Cara Clark (cclark@mlml.calstate.edu; 831-771-4428), Central Coast Wetlands Group at 

Moss Landing Marine Laboratories.

Figure 2. Cumulative Frequency of CRAM scores for depressional and 

estuarine wetlands, based upon unweighted data.

6

2 7

9
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What data were collected and why?

The NWCA collected data to characterize biological, chemical, and physical features of each site. 

Vegetation, soil, hydrology, water chemistry, algae, and buffer characteristics were chosen for 

evaluation based on their utility in reflecting ecological condition of wetlands or key indicators of stress 

that may influence condition across broad national and regional scales (see NWCA 2011 Technical 

Report (USEPA 2015)). Data for each of these indicator groups were obtained from field observations

and laboratory analyses of samples collected in the field. Vegetation characteristics were used in the 

development of a biological indicator of condition. Data collected on vegetation, soils, hydrology, buffer 

characteristics, and algae were used to develop core indicators of stress for the NWCA.

Brief descriptions of key NWCA data that were collected and why each data type is important to the 

determination of wetland condition or stress class are provided in the remaining paragraphs of this 

section. Additional data, not discussed here, were collected in the NWCA 2011 primarily for research 

purposes. For information on these other data, see the NWCA 2011 Field Operations Manual (USEPA 

2011a) and the NWCA 2011 Technical Report (USEPA 2015).

Vegetation: The status of natural vegetation has been increasingly and effectively used to evaluate 

ecological integrity in wetlands. In wetland ecosystems, vegetation provides biodiversity, primary 

productivity, habitat for other organisms, and responds to and influences hydrology, water chemistry, 

and physical and chemical properties of soils. Because plants respond directly to physical, chemical, and 

biological factors at multiple temporal and spatial scales, they can be excellent indicators of ecological

condition or stress. For example, wetland plant species represent diverse adaptations, ecological 

tolerances, and life history strategies, and integrate environmental conditions, species interactions, and 

human-caused disturbance. As a result, many human-mediated disturbances are reflected in shifts in 

the presence or abundance of particular plant species or in the types of plants occurring at a particular 

location. Data describing plant species identity, presence, and abundance were collected in the field for 

the NWCA 2011. Information on plant species traits was compiled from a variety of sources including 

the PLANTS database (USDA-NRCS 2013), National Wetland Plant List (USACE 2014), regional and state 

floristic databases and floras, state and regional lists of coefficients of conservatism (describing species 

sensitivity to disturbance) and other published literature (see NWCA 2011 Technical Report (USEPA 

2015) for details). Field and trait data describing vegetation characteristics are powerful and robust, and 

can be summarized into myriad candidate metrics or indices of ecological condition. Some plant species 

or plant groups also can be indicators of stress to wetlands. Nonnative plant species, in particular, are 

recognized as indicators of stress or declining ecological condition, or as direct stressors to condition.

Soils: Wetland soils cycle nutrients, store pollutants, mediate groundwater, are a growth medium for 

plants, and provide habitat for microbes and macroinvertebrates (e.g., insects, worms, crayfish, crabs). 

Wetland soils develop distinct characteristics as a result of the hydrology and biota (e.g., microbes, 

vegetation) associated with wetlands, as well as other factors that affect soil development across all 

environments (e.g., climate, geology). These characteristics can be altered by chemical, physical, and 

biological stressors, which impacts the ability of the soil to perform functions necessary to healthy 

wetlands. Field crews described soil morphology (characteristics such as color, texture, and evidence of 

saturation) and collected soil samples to be analyzed for chemical and physical properties to identify 

disturbance and indicators of stress. The Natural Resources Conservation Service (NRCS), the federal 

agency that provides technical assistance in the management and protection of natural resources 
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including soils, worked in partnership with the EPA to provide laboratory analysis for soil samples 

collected.

Hydrology: Wetland hydrology is a primary driver of wetland formation and persistence. Hydrology 

describes the movement, distribution, and physical and chemical characteristics of surface and 

subsurface water. How water moves in or out of a wetland, how long water remains in a wetland, and 

how much water is in a wetland impacts many wetland characteristics, including the plant community 

composition and productivity, nutrient cycling, and the loss and retention of sediment. Water levels and 

patterns of water movement within a wetland can be very dynamic, changing over a period of hours, 

days, months, and/or years, making it difficult to assess hydrology with a single site visit (such as in the 

NWCA). Field crews collected information about the hydrology of each site by observing the presence or 

absence of specific water sources and evidence of alterations to water flow and retention (e.g., drainage 

ditches, damming features, evidence of sedimentation or erosion, impervious surfaces).

Water Chemistry: Characterizing water chemistry is an integral part of the assessment of aquatic 

resources, because the physical and chemical properties of water directly reflect the surrounding 

environment, including anthropogenic influences. However, wetlands differ from lakes, streams, and 

coastal waters in that standing water is not necessarily present (Mitsch and Gosselink 2007). At sites 

with standing water greater than 15 centimeters deep, water samples were collected and analyzed for a 

number of water quality parameters. Field crews also made qualitative assessments of water clarity. 

Data were analyzed to investigate relationships between water measurements and natural conditions 

and anthropogenic stressors.

Algae: Algae respond quickly to ecological change in wetlands and have been widely used as indicators 

of recent changes in wetland condition because of their rapid reproduction rates, short life cycles, broad 

distribution, and sensitivity to changes in nutrient levels (McCormick and Cairns Jr. 1994). In addition, 

diatom species can provide insights into past hydrology such as recent flooding, standing water, or 

droughts (McCormick and Cairns Jr. 1994, USEPA 2002, Lane and Brown 2007). Algae samples from the 

water column, sediment, and vegetation were collected from sites with standing water and those 

without standing water that had evidence of recent inundation. Samples were analyzed to characterize 

the algal communities present at sites and to investigate relationships between certain communities 

and ecological condition of wetlands.

Blue-Green Algae: Toxins produced by some blue-green algae species can pose potential human health 

risks or limit human recreational use of aquatic resources when they occur above specific 

concentrations. Microcystin is the algal toxin believed to be most common in lakes, and it was evaluated 

in the NWCA to determine how frequently and at what concentrations, it might occur in wetlands. At 

sites with standing water present, a composite sample from the water column, sediment, and 

vegetation was collected and analyzed to detect the presence and concentration of microcystin.

Buffer: The presence and condition of the habitat in the area surrounding a site can influence the 

ecological condition of the site. For example, natural vegetation cover in the buffer can protect the 

wetland by trapping and absorbing incoming sediments, nutrients, and pollutants before they reach the 

wetland. The buffer can also reduce wetland disturbance from activities in adjacent areas and mitigate 

stressors that may affect wetland condition. In contrast, human-mediated disturbances to the buffer can 

be indicators of stress to wetland condition, or may directly cause stress. An example of this would be 

surface hardening (e.g., pavement, soil compaction) in the buffer which could alter the hydrology of a 

wetland by limiting the natural ability of soils in the buffer (and potentially the wetland) to soak up 

stormwater, thus increasing the potential for flooding or erosion to the wetland. Field crews collected 
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National Wetland Condition Assessment 2011

observational data on the presence or absence of a variety of disturbance types within the assessment 

area and the wetland or upland area extending 100 meters around the assessment area (termed the 

buffer area).

Each of these categories of data collected by NWCA were evaluated to determine their utility in 

describing the ecological condition of wetlands or defining indicators of stress to condition at broad 

national and regional scales. The specific properties measured and how they were used to evaluate 

wetland stress and condition are discussed in the next section.

How were the NWCA core data used to report on ecological condition and 

stress?

The raw data from the field and laboratories were combined into a number of metrics and indices to 

evaluate NWCA 2011 data for this report. A metric is an individual measure of a particular property for 

an individual site, while an index is a combination of metrics used to generate a single score for a 

particular site. Indicators of condition or stress can be based on single metrics or on indices. An index of 

biological condition based on vegetation was created using plant species data collected at each site and 

information on plant traits. Indicators of stress were also developed using biological, chemical, or 

physical data collected for the NWCA.

Figure 2-4 illustrates the relationships of the core NWCA data types to the 1) development of indicators 

of condition and stress for wetlands, 2) calculation of extent estimates for wetland condition and for 

indicators of stress, and 3) calculation of relative and attributable risk associated with each of the 

indicators of stress. Core data types are indicated by gray boxes in the figure. The gray arrows represent 

analyses leading from these core data types to the development of final indicators of stress or condition 

used in NWCA 2011, and also show the sources of data for the NWCA indicators of condition and stress. 

Further documentation describing the technical aspects of the analysis process used in NWCA is 

available in the NWCA 2011 Technical Report (USEPA 2015).
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Figure 2-4. Core data types and relationships to how they are used to develop indicators of condition and 

stress. Core data types (gray boxes) and relationships to how they are used to develop indicators of condition (open green 

box) and stress (open yellow boxes), and how the indicators are used to determine wetland condition (green box) and stressor 

extent (purple box) estimates and relative and attributable risk (teal box).

After appropriate indicators of condition and stress were determined from the data and calculated, each 

probability site was assigned to a condition or stressor class, based on the value of the indicator for that 

site. For the indicator of biological condition, each site was assigned to a good, fair, or poor condition 

class. For the metrics or indices describing the indicators of stress, each site was assigned to a low, 

moderate, or high stressor level. The thresholds values used to assign sites to the different condition 

classes and stressor levels were defined using either a fixed or distribution based approach.

Fixed thresholds are based on accepted values from peer-reviewed, scientific literature and are wellestablished and/or widely and consistently used by government agencies. An example of the latter is the 

World Health Organization (WHO) risk levels for recreational exposure to the algal toxin, microcystin. 

Fixed thresholds are also sometimes based on the best professional judgment of scientific analysts by 

incorporating field and research experience with other knowledge from the collective scientific 

community.

Distribution based thresholds are determined using the distribution of values of a particular index or 

metric found at a designated set of reference sites. NWCA defines reference as “least-disturbed 

condition” and used field data collected from probability sites and additional handpicked sites believed 

to be in reference condition to identify a set of reference sites with the least amount of human 

disturbance. Data from this set of reference sites is then used to determine the distribution of values for 

each indicator of condition or stress. The threshold values for good, fair, and poor condition, or low, 

moderate, and high stressor level are set based on defined percentiles from the distribution of values 
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observed in the least disturbed sites (see text box below on “Use of Reference Site Approach” and 

NWCA 2011 Technical Report (USEPA 2015)).

The good, fair, and poor condition classes and the low, moderate, and high stressor levels reported by 

NWCA have no regulatory implications and are not replacements for the evaluation of the quality of 

wetlands with respect to water quality standards set by states and tribes.
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USE OF REFERENCE SITE APPROACH

To interpret the data collected by the NWCA field crews and to assess current ecological condition, scientists 

need to compare the collected data to benchmarks — an estimate of what scientists would expect to find in 

wetlands with the best natural condition. Because it is difficult to estimate historical conditions for many 

indicators, the NWCA’s benchmark, or reference, is characterized as the “least-disturbed condition”: the best 

available physical, chemical, and biological conditions given today’s state of the landscape. Least disturbed 

condition is defined based on data from sites selected according to a set of explicit screening criteria. These 

criteria vary from region to region and among wetland types to account for differences in natural variability 

and anthropogenic disturbance across the American landscape*. For the NWCA, separate screening criteria 

were defined based on combinations of the NWCA Aggregated Ecoregions and Aggregated Wetland Types. The 

screening criteria were developed with the goal of identifying reference sites with the least amount of human 

disturbance for each reporting group (e.g., estuarine herbaceous wetlands, Interior Plains inland woody 

wetlands). Reference criteria, in essence, allow identification of the set of sites with the least-disturbed 

condition across the target population of wetlands.

The NWCA compares specific physical, chemical, and biological stressor data collected at each site to the 

reference site screening criteria to determine whether any given site meets the definition of least disturbed 

condition for its combined Aggregate Ecoregion/Wetland Type. The group of sites passing all the screening 

criteria are considered to be in reference condition. Good ecological condition and low stressor levels are 

characteristic of reference sites. These reference sites are then used to set thresholds against which the 

broader population of wetlands can be compared and assigned to condition or stressor classes. The range of 

conditions found in the group of reference sites for an ecoregion describes a distribution of values expected 

for least-disturbed condition or stress. The thresholds used to define distinct condition classes (e.g., good, fair, 

poor) or stressor levels (e.g., low, moderate, high) are drawn from this reference condition distribution. The 5th

percentile of the reference distribution was used to separate the sites in poor biological condition from those 

in fair. Similarly, the 25th percentile of the reference distribution was used to distinguish between sites in fair 

biological condition from those assessed in good condition. For stressor classes, the 95th percentile of the 

reference distribution was used to separate the sites considered to have high stress from soil phosphorus 

concentrations from those with moderate, and the 75th percentile was used to separate the moderate and low 

stressor classes.

This approach for establishing reference condition is well documented and consistent with current science, 

EPA guidelines, state practice, and established protocols for ecological assessment (Bailey et al. 2004; Barbour 

et al. 1999; Carter and Resh, 2013; Hughes, 1995; Reynoldson et al. 1997; Stoddard et al. 2006; and USEPA, 

2011d).

*Within the reference site distribution, there are two sources of variability:

Natural variability includes a wide range of habitat types naturally found within each ecoregion. This range is 

captured in the reference sites representing those different habitats. For this reason, reference condition 

thresholds were set based on the distribution of least disturbed sites, rather than from a single site. Capturing 

natural variability in reference sites helps establish reference conditions that represent the range of natural 

environments in the ecoregions.

Human activities have altered habitats in the U.S., with natural landscapes transformed by cities, suburban 

and rural development, agricultural development, and resource extraction. The extent of those disturbances 

varies across regions. Some reference sites are in watersheds with little to no evidence of human impact, such 

as mountain streams or rivers in areas with very low population densities. Others have been highly influenced 

by human activities. The least-disturbed reference sites in regions with more human activity will usually have 

lower screening criteria than those in areas with little human disturbance.
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Evaluating Wetland Biological Condition

Vegetation is a fundamental component of wetlands. The composition and abundance of plant species 

at a site reflect and influence other ecological processes related to hydrology, water chemistry, and soil 

properties. Vegetation integrates different wetland processes and plants respond to physical, chemical, 

and biological disturbances. These properties make vegetation a particularly good indicator of wetland 

condition. Using field data describing species composition and abundance in combination with species 

trait information, numerous candidate metrics were developed and evaluated as potential components 

of a Vegetation Multimetric Index (VMMI). A national-scale VMMI was then developed as the indicator 

of biological condition for the NWCA.

The VMMI is called a multimetric index because it combines more than one metric. After careful 

screening of many candidate metrics, four were chosen for inclusion in the VMMI:

 A Floristic Quality Assessment Index (FQAI),

 Relative Importance of Native Plant Species,

 Number of Plant Species Tolerant to Disturbance, and

 Relative Cover of Native Monocot Species.

Each metric in the VMMI is based on combinations of data types describing species composition 

(identity and/or abundance) and species traits. These metrics were chosen because they 1) best reflect 

ecological condition of wetlands across the conterminous U.S. for the different NWCA Aggregated 

Ecoregions and NWCA Aggregated Wetland Types, 2) detect differences between the least and mostdisturbed sites, and 3) were not strongly related to one another (see NWCA 2011 Technical Report 

(USEPA 2015)). Each metric is scored from 0 to 10 for consistency in scaling the metric value ranges.

VMMI values are scaled from 0 to 100.

The FQAI is often considered as a standalone index for describing floristic quality, but is used here as 

one metric contributing to the VMMI. It captures information about plant community composition 

based upon all unique plant species occurring at a given site and a value given to each plant species 

based on its sensitivity to human-mediated disturbance. This value is known as the Coefficient of 

Conservatism or C-value. C-values range from 0 to 10 where a value of 0 is assigned to plant species that 

occur in highly disturbed habitats and a score of 10 is assigned to species found only in minimally 

disturbed habitats. C-values may vary by state or region to account for natural differences in habitat and 

plant community composition. C-values were compiled from existing state and regional lists of values, 

or, for states and regions where lists did not exist, by assigning values based on those of ecologically 

similar neighboring states or regions.

The second metric in the VMMI is the Relative Importance of Native Plant Species. Relative importance 

combines information on how much of the sampling location is covered by native wetland plants in 

relation to all plants present (e.g., relative cover) and how many occurrences of native plant species 

there are across a site compared to the number of all plant species occurrences (i.e., relative frequency). 

Native status of a given species-site occurrence was based on whether the species was indigenous to the 

state in which the sample site was located. As disturbance at a site increases, the native plant 

community is often altered—a change that is often related to declining condition.

The Number of Plant Species Tolerant to Disturbance is the third metric in the VMMI. The number of 

tolerant species increases with increasing disturbance, indicating a potential shift in plant community 

dynamics. Such a shift could reflect competitive pressure or other stress to disturbance-intolerant 
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species that are often indicative of good ecological condition. C-values are used to describe sensitivity or 

tolerance of plant species to disturbance. Tolerance can be indicated by the presence or abundance of 

plant species with low C-values. Species tolerant to disturbance are defined in NWCA as those with a Cvalue of 4 or less.

Finally, the fourth metric in the VMMI is the Relative Cover of Native Monocot Species. Monocots are 

one of two groups into which flowering plants are divided. They are common to many wetland types, 

and are represented by plants like grasses, sedges, rushes, lilies, irises, orchids, etc. Native monocots 

represent dominant natural components of many emergent (non-woody) wetland types. In other 

wetland types, they may reflect species that are indicative of relatively undisturbed conditions.

The final VMMI score is calculated based on the combination of values for all four metrics for each site, 

with the overall value indicating the level of biological condition. Good, fair, or poor condition 

thresholds were set using a distribution-based approach based on the VMMI values for reference sites 

(see “Use of Reference Sites” text box) in each combination of NWCA Aggregated Ecoregion by NWCA 

Aggregated Wetland Type. The 5th percentile of the reference distribution for each NWCA Aggregated 

Ecoregion and NWCA Aggregated Wetland Type combination was used to separate the sites in poor 

condition from those in fair condition. Similarly, the 25th percentile of the reference distribution was 

used to distinguish between sites in fair condition and those assessed as being in good condition (Figure 

2-5). Specific threshold values for each NWCA Aggregated Ecoregion and NWCA Aggregated Wetland 

Type combination are provided in the NWCA 2011 Technical Report (USEPA 2015).

Figure 2-5. Criteria for setting VMMI thresholds for good, fair, and poor condition classes based on VMMI 

values observed for Least Disturbed (Reference) sites.
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Highlight

Wisconsin’s Intensification Study: A Floristic Characterization of Wetlands in Eastern 

Wisconsin

Thomas Bernthal, Wisconsin Department of Natural Resources

Overview: Plant community composition can provide a detailed picture of wetland health, and the 

differences between a pristine and a degraded habitat may be partially characterized by looking at the 

presence and abundance of plant species. The main objective of the Wisconsin intensification study was 

to compare the Floristic Quality Index (FQI) 

currently used with an adjusted index to 

determine if it may be a better predictor of 

wetland ecosystem health.

Main Story: The Wisconsin Department of 

Natural Resources (DNR) chose to do a more 

intensive study of wetlands in the Lake 

Michigan basin of Eastern Wisconsin. As part 

of the 2011 NWCA, 34 unique sites were 

sampled in Wisconsin, 12 of which were in 

the Lake Michigan basin. An additional 38 

sites were sampled in 2012 for the 

intensification project, for a total of 50 sites in 

the project area (Figure 1).

Using vegetation data collected at each site, 

the Wisconsin DNR employed a Floristic 

Quality Assessment (FQA) framework to 

examine wetland health. The FQA uses 

species richness or the number of different 

species present (N) and the Coefficient of 

Conservatism (C) to calculate the Floristic 

Quality Index (FQI). The C is an assigned 

number between 0 and 10 for each plant 

species, which reflects the response to 

disturbance. A species that is considered 

tolerant of disturbance is assigned a lower C value. Conversely, a species that disappears from a 

community following a disturbance is given a higher C value. Invasive species are assigned a C value of 0.

A plant community with a high species count scores well on a FQI. In some habitats this high FQI score 

may not be indicative of healthy natural conditions, as excess nutrients like phosphorous and nitrogen 

can stimulate excessive plant growth and increase the number of plant species leading to an impaired 

community. Though these nutrients are basic building blocks of plants and are needed in a wetland plant 

community, in high concentrations these nutrients can degrade wetlands.

Figure 1. Intensification study project area in eastern 

Wisconsin.
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To counter the inflating effects of species richness on the FQI, the Wisconsin DNR employed a weighted 

mean Coefficient of Conservatism metric (wC ̅). This weighted metric accounts for species abundance, 

which may allow for a better picture of the composition of a plant community.

What They Found: The main objective of the study was to compare C̅and wC̅to determine which 

method would give a more accurate assessment of wetland ecological condition when incorporated into 

the FQI. Figure 2 shows comparison boxplots of the calculated C̅and wC̅for the wetland community 

types sampled in the intensification study, including Palustrine Emergent (PEM), Palustrine Forested 

(PFO) and Palustrine Shrub/Scrub (PSS). The PEM showed the greatest difference between C̅and wC̅. 

Among the other community types (PFO and PSS), where sampled wetlands were not dominated by 

invasive graminoid (i.e., grass or grass-like plant) species as often, the differences between C̅and wC̅

were less pronounced.

Wisconsin found that lower wC̅values were correlated with an increase in the relative cover of invasive 

graminoids, (R2=0.754), including reed canary grass (Phalaris arundinacea), hybrid cattail (Typha x 

glauca), narrow-leaf cattail (Typha angustifolia), and common reed (Phragmites australis). They found 

that the wC̅metric was especially useful for capturing differences between sites with sparse invasive 

plant populations and sites overrun by invasive species. More PEM sites were dominated by invasive 

reed canary grass or hybrid cattail stands than in the other community types, which could explain the 

difference between C̅and wC̅for the PEM wetlands, as noted in Figure 2.

Although wC̅is a useful way to describe some wetland plant communities in Wisconsin, it may not be the 

best metric for all communities. The conventional methods of calculating the FQI using C̅may be better 

suited for communities that are characteristically more species-rich, such as cedar swamps. Communities 

that tend to be naturally species-poor, such as bogs or muskegs, may be better characterized by using 

the wC̅. The conventional FQI used by Wisconsin currently and the adjusted index using wC̅were found 

to be valuable metrics to describe the vegetation condition in wetlands. Scientists and wetland managers 

may use the plant community attribute information gained in this intensification study in order to better 

evaluate Wisconsin’s wetlands based on vegetation condition.

To learn more, contact Thomas Bernthal (Thomas.Bernthal@wisconsin.gov), Wisconsin Department of 

Natural Resources.
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Evaluating Indicators of Stress

Indicators of stress are physical, chemical, or biological factors that have the potential to reflect 

anthropogenic impacts on wetland ecological condition. Indicators of stress do not necessarily directly 

cause ecological decline, but are often associated with changes in wetland condition. NWCA evaluated 

the extensive set of data collected in the field to identify and develop appropriate indicators of stress 

and quantify their national and regional extents. While these are expected to be associated with effects 

on wetland condition, the exact relationship cannot be explicitly determined from the data collected in 

NWCA. For simplicity of language, in this report “indicators of stress” are also referred to as “stressors.” 

It was not possible to evaluate all potential stressors affecting wetland condition. NWCA 2011 indicators 

of stress are derived from field data collected at the site-level within the assessment area and the 100 

meter buffer immediately surrounding it. Due to this, potential effects on condition from stressors 

occurring outside of the vicinity of the assessment area and buffer may not be fully considered in the 

NWCA reported results and could have significant influences on wetland condition.

Six physical, three chemical, and one biological indicator of stress were developed for the NWCA report. 

The following sub-sections summarize the development of the indicators pertaining to each of these 

three categories.

Physical Indicators of Stress

Wetlands can be impacted by human-mediated activities that cause physical changes to wetland 

systems. These may be stressors that are occurring in the wetland itself or many miles away. Due to the 

limitations of the survey, NWCA 2011 focused on physical stressors occurring in the assessment area 

and in the area immediately surrounding the assessment area (i.e., buffer). Physical indicators of stress 

include vegetation alterations that occur through removal or replacement, or hydrologic alterations that 

occur through damming, ditching, surface hardening, filling or erosion. These alterations can disrupt 

wetland structure and function. Data reflecting indicators of physical stress were evaluated as part of 

the hydrology and buffer protocols both within the wetland assessment area and the 100 meter buffer 

immediately surrounding it. The indicators evaluated were placed into different stress indicator groups 

based on whether they resulted primarily in vegetative or hydrologic alteration to a wetland.

Vegetation alteration included two indicators of stress:

• Vegetation Removal – any field observation related to loss, removal, or damage of vegetation 

(e.g., mowing/shrub cutting, herbicide use, highly grazed grasses, recently burned forest); and

• Vegetation Replacement – any field observation of a change in the plant species present due to 

anthropogenic activities (e.g., tree plantation, nursery, golf course, lawn/park, row crops, 

pasture/hay, rangeland).

Changes in vegetation at the sampling site and the surrounding area can indicate effects of activities 

that could impact the ecological condition of the wetland. Removal of vegetation may increase 

sediment, nutrient, and pollutant loads entering or residing in a wetland. Replacement reflects 

conversion from one vegetation type to another in the buffer and assessment area and can decrease 

biodiversity, simplify the vertical structure, and reduce habitat quality on the site. Not all instances of 

vegetation removal or replacement (e.g., moderate grazing by native wildlife, natural wildfire regimes in 

ecosystems adapted to fire) result in stress to a wetland or poor ecological condition. The relationship 

between vegetation alteration stressors and biological condition as indicated by the VMMI are 

evaluated through the concepts of relative and attributable risk later in the report.
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Hydrologic alteration included four indicators of stress:

• Damming – any field observation related to impounding or impeding water flow from or within 

the site (e.g., dikes, dams, berms, railroad beds);

• Ditching – any field observation related to draining water within the site (e.g., ditches, 

corrugated pipe, excavation-dredging);

• Hardening – any field observation related to soil compaction, including activities and 

infrastructure that primarily result in soil hardening (e.g., parking lots, suburban residential 

development, roads, pavement); and

• Filling/Erosion – any field observation related to soil erosion or deposition (e.g., soil loss/root 

exposure, fill/spoil banks, freshly deposited sediment).

Changes in how water moves in or out of a wetland, or water levels within the wetland resulting from 

hydrologic alterations can affect plant productivity, nutrient cycling in the soil and water, and the 

physical habitat, thereby impacting the overall ecological condition of the wetland. For example, a 

nearby parking lot or impervious surface could increase the volume of water entering the wetland. 

Higher water tables may limit the plant species that can grow in the wetland.

Stress-level thresholds were established for each stressor group (vegetation alteration and hydrologic 

alteration) and applied to each individual indicator of stress (see NWCA 2011 Technical Report (USEPA 

2015)). Thresholds reflecting low stress were set to indicate the complete absence of the given physical 

indicator of stress at the site. Thresholds reflecting high stress were set using best professional 

judgment. Indicator values between the established low and high threshold levels were placed into the 

moderate stress class. These thresholds were used across all NWCA sites (See Table 2-6 for a complete 

list of indicators and summary information on how thresholds were set).

Chemical Indicators of Stress

Chemical stressors that can impact the ecological condition of wetlands include excess nutrients, metals, 

organic toxins, and other chemical compounds that can disrupt nutrient cycles, affect plant and animal 

growth, and be detrimental to human health. Two chemical indicators of stress were developed for 

NWCA using data from soil samples collected at each site: a Heavy Metal Index and soil phosphorus

concentration. Another potential indicator of stress, microcystin (an algal toxin), was evaluated based on 

concentrations from a composite sample of surface water and algal scrapings from vegetation stems and 

leaves.

The Heavy Metal Index, developed for the NWCA as a chemical indicator of stress, was comprised of 12 

different heavy metals closely associated with anthropogenic activities: antimony (Sb), cadmium (Cd), 

chromium (Cr), cobalt (Co), copper (Cu), lead (Pb), nickel (Ni), silver (Ag), tin (Sn), tungsten (W), 

vanadium (V), and zinc (Zn). Stress-level thresholds were set for each metal using published estimates of 

natural background concentrations and best professional judgment (Table 2-3). It should be noted that 

the thresholds established for heavy metals do not reflect toxicity; rather, they are indicators of human 

disturbance. Sites where all 12 metal concentrations were equal to or below the stress-level threshold

established for each metal were placed into the low stress-level class. Sites where three or more metal 

concentrations were above the stress-level thresholds were placed into the high stress class. Sites not 

placed into either the low or high stress-level classes were put into the moderate stress class. The same 

thresholds were used across all NWCA sites.
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Table 2-3. Heavy metals included in the Heavy Metal Index as a chemical indicator of stress. Natural background 

concentrations are based on Alloway (2013). Thresholds were set based on natural background concentration and best 

professional judgment.

Metal

Primary Anthropogenic 

Association

Natural Background 

Concentration (mg/kg)

Stress-Level 

Threshold 

(mg/kg)

Silver (Ag) Industry 0.05 – 1.00 1.0

Cadmium (Cd) Agriculture 0.1 – 1.0 1.0

Cobalt (Co) Industry < 50 25

Chromium (Cr) Industry 0.5 – 250 125

Copper (Cu) Agriculture / Industry / Roads 2 – 50 50

Nickel (Ni) Industry / Agriculture 0.2 – 450 225

Lead (Pb) Roads / Industry Mean of 18 35

Antimony (Sb) Industry 0.1 – 1.9 1.0

Tin (Sn) Industry / Agriculture 1.7 – 50 17

Vanadium (V) Industry / Roads 36 – 150 150

Tungsten (W) Industry / Agriculture < 2 2.0

Zinc (Zn) Industry / Agriculture 10 – 150 150

Soil phosphorus concentrations were measured from samples collected at each site. Naturally-occurring 

soil phosphorus concentrations vary widely across wetlands due to differences in soil types, wetland 

types, climate, and other factors. Soil phosphorus concentration can also be influenced by and reflect 

human activity on the landscape. No nationally-accepted soil phosphorus criteria exist to assign stress 

classes. Therefore, NWCA used a distribution-based, or reference site approach, paralleling that of the 

VMMI, to set soil phosphorus concentration stress classes for each combination of NWCA Aggregated 

Ecoregion by NWCA Aggregated Wetland Type (see “Use of Reference Site Approach” text box). Soil 

phosphorus concentrations above the 95th percentile of reference sites were considered to have high 

stress, while those below the 75th percentile of reference sites were considered to have low stress 

(Table 2-4, see NWCA 2011 Technical Report (USEPA 2015) for further details).
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Table 2-4. Soil phosphorus concentration thresholds as chemical indicators of stress. Stress-level thresholds were set for 

estuarine wetland types nationally and for inland wetland types by NWCA Aggregated Ecoregion based on the 75th percentile 

(low stress) and 95th percentile (high stress) of reference sites for the group.

NWCA Reporting Group

(NWCA Aggregated Ecoregion, Wetland Type)

“Low” Threshold

(mg P/kg soil)

“High” Threshold

(mg P/kg soil)

Estuarine (Herbaceous and Woody) ≤ 519 > 969

Coastal Plains (Inland Herbaceous and Woody) ≤ 582 > 1,180

Eastern Mountain and Upper Midwest (Inland 

Herbaceous and Woody)

≤ 914 > 1,280

Interior Plains (Inland Herbaceous and Woody) ≤ 1,110 > 1,810

West (Inland Herbaceous and Woody) ≤ 1,140 > 2,090

Microcystin is a toxic substance produced by cyanobacteria, a group of microbes also called blue-green 

algae. Cyanobacteria are a natural part of aquatic ecosystems, but under certain environmental 

conditions, can proliferate into algal blooms that can be unsightly, smelly, and in some instances cause 

severe health issues for wildlife, people, and domestic animals.

EPA collected samples and tested them for the presence and concentration of microcystin. The results 

are categorized using a fixed threshold approach based on recreational exposure guidelines established 

by the World Health Organization (Table 2-5). Concentrations of microcystin have not been previously 

documented over a spatial area or for as many wetland types as sampled in the NWCA, consequently, 

more research is likely to be needed to appropriately interpret how these results relate to wetland 

condition.

Table 2-5. World Health Organization thresholds of risk associated with exposure to microcystin.

Indicator (units) Low Risk of Exposure

Moderate Risk of 

Exposure

High Risk of 

Exposure

Microcystin (μg/L) < 10 10 - ≤ 20 > 20

Biological Indicators of Stress

EPA used the presence and abundance of nonnative plants to develop a biological indicator of stress. 

Nonnative plants are often related to human-mediated disturbance, and can also be direct or indirect 

stressors to wetland ecosystems by competing with or displacing native plant species or communities, or 

by altering wetland structure and processes.

The Nonnative Plant Stressor Indicator (NPSI) was developed for the NWCA using collected plant data 

and information describing the native status of each species (see NWCA 2011 Technical Report (USEPA 

2015) for definition of nonnative plant concepts). The NPSI included three complementary nonnative 

species metrics that describe different avenues of potential impact to ecological condition:

• Relative Cover of Nonnative Species

• Richness of Nonnative Species (number of unique nonnative species)

• Relative Frequency of Occurrence of Nonnative Species
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National Wetland Condition Assessment 2011

Relative Nonnative Cover reflects preemption of space and resources, changes in species composition, 

and alteration of ecosystem processes. Higher values are often associated with decreases in ecological 

condition. Total Richness of Nonnative Species can be an indicator of potential risk for ecological 

impact; greater numbers of individual nonnative plant species increases risk that one or more may be or 

become invasive or cause ecosystem alterations. Greater Relative Frequency of Occurrence of 

Nonnative Species reflects increasing numbers of locations for further nonnative incursions and a 

decreasing proportion of the species composition that is native.

These three metrics are used together to assign low, moderate, high, and very high stress levels for the 

NPSI. Stress-level thresholds were set for each metric in the NPSI using best professional judgment. The 

addition of a “very high” stressor class was made because of the greater range in values for this stressor 

indicator compared to other stressor indicators. The same threshold values were used across all NWCA 

sites (see NWCA 2011 Technical Report (USEPA 2015) for details on NPSI development and application).
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National Wetland Condition Assessment 2011

Table 2-6. Summary of NWCA 2011 indicators of condition and stress and information on how thresholds were set.

Indicator Reference Approach General Assessment Notes

Biological Condition

Vegetation MMI Regionally specific

(NWCA ecoregion / 

wetland type)

distribution-based 

threshold

Data on specific plants and abundance collected from five plots 

systematically distributed in assessment area. Index developed based 

on species composition (presence and abundance) and species traits.

Indicators of Stress

Physical – Vegetation Alteration:

Vegetation Removal

Nationally consistent 

fixed threshold

Field observations related to loss, removal, or damage of vegetation 

(e.g., mowing / shrub cutting, herbicide use, highly grazed grasses, 

recently burned forest) collected from plots systematically distributed 

in assessment area and 100 meter buffer area.

Vegetation

Replacement

Field observations of a change in the plant species present due to 

anthropogenic activities (e.g., tree plantation, golf course, lawn/park, 

row crops, pasture/hay, rangeland) collected from plots systematically 

distributed in assessment area and 100 meter buffer area.

Physical – Hydrologic Alteration:

Damming

Nationally consistent 

fixed threshold

Field observations related to impounding or impeding water flow (e.g., 

dikes, dams, berms, railroad beds) collected in the assessment area and 

from plots systematically distributed in the 100 meter buffer area. 

Ditching

Field observations related to draining water within the site (e.g., 

ditches, corrugated pipe, excavation-dredging) collected in the 

assessment area and from plots systematically distributed in the 100 

meter buffer area.

Hardening

Field observations related to soil compaction, including activities and 

infrastructure that primarily result in soil hardening (e.g., roads, 

suburban residential development, pavement) collected in the 

assessment area and from plots systematically distributed in the 100 

meter buffer area. 

Filling/Erosion

Field observations related to soil erosion or deposition (e.g., soil 

loss/root exposure, fill/spoil banks, freshly deposited sediment)

collected in the assessment area and from plots systematically 

distributed in the 100 meter buffer area.

Chemical:

Heavy Metal Index Nationally consistent 

fixed threshold

Samples collected from upper 10 centimeter of soil in the assessment 

area. Measured concentrations compared to thresholds based on 

background concentrations of heavy metals. 

Soil Phosphorus Regionally specific

(NWCA ecoregion)

distribution-based 

threshold

Samples collected from upper 10 centimeter of soil in the assessment 

area. Measured concentrations compared to thresholds based on 

reference sites.

Microcystin Nationally consistent 

fixed threshold

Samples collected from surface water, sediment, and vegetation 

surfaces. Measured concentrations compared to World Health 

Organization (WHO) algal toxin threshold for recreation.

Biological:

Nonnative Plant 

Stressor Index

Nationally consistent 

fixed threshold

Data on specific plants and abundance collected from five plots 

systematically distributed in assessment area. Index developed based 

on species composition (presence and abundance) and species traits.
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National Wetland Condition Assessment 2011

Estimating the extent of wetland area for condition classes or stress-level 

classes

The NWCA 2011 results presented in the subsequent chapters of the report are calculated by estimating 

the extent of wetland area in each of the different condition classes or stressor levels established for 

each indicator. The process used to accomplish this is briefly described below for the NWCA indicators 

of condition and stress. More detailed information is provided in the NWCA 2011 Technical Report 

(USEPA 2015).

Condition: There are three different classes to describe wetland condition: good, fair, and poor. The 

VMMI thresholds for each condition class are used with site weights to calculate extent estimates of 

wetland area in good, fair, and poor condition using a two-step process:

1) Each NWCA probability site is assigned good, fair, or poor biological condition based on its 

VMMI value and the thresholds appropriate to the site.

2) Next, the site weights from the probability design, which reflect the number of acres each site 

represents across the total population of NWCA Target Wetland Types, are summed within 

condition class to estimate the wetland area in good, fair, and poor condition.

In Figure 2-4, this process is illustrated by the arrows between the Vegetation Data, VMMI, and Wetland 

Condition Extent Estimate boxes. The survey design also allows evaluation of the statistical certainty of 

these condition estimates.

Stress: For each stress indicator, its designated threshold values are used to assign a low, moderate, or 

high stress level to each site. An additional stress-level of very high was designated for the Nonnative 

Plant Stressor Index. The process for estimating the extent of wetland area with low, moderate, high, or 

very high stress for each of the NWCA indicators of stress, parallels the approach used for making 

condition estimates. In Figure 2-4, this process is illustrated by the arrow between each group of 

indicators of stress and the Stressor Extent Estimate box.

Other data collected as part of NWCA 2011 but not reported in national and 

regional results

Water Chemistry

The NWCA 2011 survey is the first national-scale survey of wetland surface water chemistry. Water 

chemistry data were collected for chlorophyll-a, conductivity, ammonia, nitrate-nitrite, total nitrogen, 

total phosphorus, and pH. Water temperature and dissolved oxygen levels also were measured at some 

sites at the option of the states involved. The objectives of the NWCA water chemistry data analyses 

were to examine the extent to which water chemistry could be sampled and evaluated across different 

wetland types, to explore any patterns found in water chemistry for wetlands across the nation and 

relate them to possible classification variables and natural and anthropogenic drivers, and to generate 

recommendations concerning further research and protocols for future NWCA assessments.

Water chemistry samples could only be collected from approximately 55% of the sites sampled in NWCA 

2011 due to a lack of sufficiently deep standing water at the sampling site. This confounded efforts to 

assess and report results for the water chemistry data using the analytic approaches adopted by NWCA 

2011 for its core data indicators. For this reason, water chemistry results are not presented in the NWCA 
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National Wetland Condition Assessment 2011

2011 report. Analysis and interpretation of the data collected is discussed in the NWCA 2011 Technical 

Report (USEPA 2015) and, as opportunities arise, will be further disseminated through scientific journals.

Algae Species Data

NWCA 2011 collected composite algae samples from the water column, sediment, and vegetation 

surfaces at sites with standing water greater than 15 centimeters deep and sites that had evidence of 

recent inundation. Samples were analyzed to characterize the algal communities present at sites and to 

investigate relationships between certain communities and ecological condition of wetlands. Though 

algae taxonomic data were obtained from approximately 80% of the sites sampled in NWCA 2011, issues 

with data consistency arising from sample collection in the field and taxonomic identification in the lab 

raised questions about the efficacy of evaluating wetland condition using this algae species data for the 

NWCA 2011 report.
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National Wetland Condition Assessment 2011

Highlight

Nebraska’s Intensification Project: Impacts of Land Use and Management on Wetland Condition

Ted LaGrange, Nebraska Game and Parks Commission

Overview and Goals: Through the intensification project, participating state, federal, and regional agencies 

wanted to learn more about 11 priority landscape regions in the state of Nebraska. Most of these regions have 

been identified in the state’s Wildlife Action Plan as Biologically Unique Landscapes, and the information 

gained from the project will help direct conservation and management efforts in these key landscapes found 

in Nebraska and throughout the Great Plains.

Since settlement, Nebraska has lost an estimated 35% of its total wetland area due to land-use conversion, 

and 97% of the remaining wetlands occur on private lands. In the Rainwater Basins (an important region 

within the Central Flyway as a major bird migration route in North America), wetland losses may be as high as 

90%. In order to obtain more data on these important and diverse wetland habitats, an intensified sampling 

effort was conducted in 11 wetland complexes in Nebraska. This included isolated playa wetlands in 

landscapes dominated by row-crop agriculture in the Rainwater Basin, Central Table Playas, and Southwest 

Playas; wet meadows and freshwater and alkaline marshes in the Nebraska Sandhills (includes the Elkhorn 

River Headwaters, Cherry County Sandhills Wetland, and Sandhills Alkaline Lake regions); wet meadows and 

alkaline wet meadows located along the North Platte (Western Alkaline), Platte (Central Platte), and Niobrara 

Rivers; Eastern Saline wetlands located near Lincoln, NE; and forested wetlands along the lower Missouri River 

(Figure 1).

Information Collected: 109 sampling locations were visited across the 11 regions during the growing seasons 

of 2011-2013. At each site, Level 1, 2, and 3 assessment data were collected using the 2011 NWCA protocols. 

Figure 1. Priority landscape regions identified for the intensification project.
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National Wetland Condition Assessment 2011

As part of the study, the team developed the Nebraska Wetland Rapid Assessment Method (NeW_RAM), 

which was tested at 40 of the sites in 2013 and compared to Level 3 assessments. Additionally, amphibian 

community surveys were conducted at 125 sites in the Rainwater Basin complex with the goal of developing a 

sampling protocol for long-term amphibian community monitoring and to assess how wetland management 

practices might affect the presence and health of frog and toad species. Wetlands surveyed for amphibians 

were grouped according to land use: Wildlife Management Areas (WMA), Waterfowl Production Areas (WPA), 

Wetlands Reserve Program (WRP), privately owned wetlands, and agricultural reuse pits.

What They Have Learned: Sample processing and data analysis is still underway, but preliminary results have 

already provided useful information about wetland condition across landscape regions in Nebraska. The draft 

NeW_RAM, developed in early 2013, was used at 40 sites sampled in 2013. Overall, the NeW_RAM worked 

well in the field and showed good agreement with level 3 measurements, including the FQAI. It is hoped the 

NeW_RAM will be a useful tool for agencies and organizations to quickly assess wetland condition, improving 

Nebraska wetland protection and restoration efforts.

Differences in wetland vegetation condition, measured by the FQAI, varied among landscape regions and with 

respect to reference condition within each region (Figure 2). Given the high diversity of Nebraska’s wetlands, 

reference wetland FQAI scores may be very different in each of the regions, and direct comparisons of FQAI 

scores between regions may not be 

appropriate.

Within each region, high and low FQAI 

scores with respect to reference condition 

may be linked to land use. In regions 

where sites scored well below reference, 

high quality sites may no longer exist on 

the landscape due to historical and 

contemporary land use change or 

disturbance. Two of the lowest scoring 

regions, the Southwest Playas (SWP) and 

Central Table Playas (CTP), included 

shallow playa wetlands in areas 

dominated by intensive agriculture and 

recently affected by a period of extreme 

drought. Low scores were also observed 

in wetlands along the lower Missouri 

River (MR), which had been highly altered 

during historic flooding in 2011. Slightly 

higher FQAI scores (although still below 

reference condition) were documented in 

the Western Alkaline (NPR) and Eastern Saline (SAL) regions, both areas that have been altered, but where 

some restoration efforts have occurred. The Central Platte (CP), Sandhills Alkaline Lakes (SALK), Rainwater 

Basin (RWB), Niobrara River (NR), Cherry County Sandhills Wetlands 

(CCWM), and Elkhorn River Headwaters (EHW) regions have large areas 

of unmodified land or have been the focus of intensive management 

and conservation efforts. FQAI scores in these regions tended to be 

closer to the reference standard, evidence that current management 

practices are effective in maintaining and restoring wetland condition. 

This project is providing baseline data of wetland condition within these 

regions of Nebraska, and offers insight towards the direction of future 

conservation efforts within the state.

Seven species of frogs and toads were observed at wetlands sampled in 

the Rainwater Basin, including the Chorus Frog, Pseudacris maculata. 

Figure 2. Floristic Quality Assessment Index (FQAI) scores for each of the 

wetland complexes studied. The top and bottom of the boxes show the 

75th and 25th percentile of scores and the line inside the box represents 

the median score for each region. Reference standard condition FQAI 

scores for each landscape region are shown as red dots.

Chorus Frog (Pseudacris maculata).
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National Wetland Condition Assessment 2011

While data analysis is continuing, a few general trends have started to emerge. Differences in amphibian 

species diversity among wetlands within WMAs, WPAs, WRP land, and privately owned land seem to be

minimal, but somewhat lower species diversity has been observed in the agricultural reuse pits. This may 

suggest that many of the amphibian species are generalists, adaptable to any habitat with water, but prefer

actual wetlands over the agricultural reuse pits. Once data analysis is complete, researchers hope to have a 

better idea of how land use impacts amphibian communities. The team hopes to continue monitoring 

amphibian communities in wetlands within the Rainwater Basin to document community trends and develop a 

long-term data set.

To learn more, contact Ted LaGrange (ted.lagrange@nebraska.gov; 402-471-5436), Nebraska Game and Parks 

Commission.

Nebraska’s Intensification Project was a joint effort among the Nebraska Game and Parks Commission, the University 

of Nebraska-Lincoln, U.S. EPA, U.S. Fish & Wildlife Service, U.S. Army Corps of Engineers, Natural Resources 

Conservation Service, U.S. Geological Survey, Nebraska Department of Environmental Quality, the Rainwater Basin 

Joint Venture, the Playa Lakes Joint Venture, the Saline Wetland Conservation Partnership, and The Nature 

Conservancy.
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National Wetland Condition Assessment 2011

Chapter 3: National Results

The goal of the CWA is to restore and maintain the “chemical, physical, and biological integrity” of the 

nation’s waters. NWCA examines these three aspects of the ecological quality of aquatic systems 

through a set of commonly used and widely accepted indicators. It does not include all aspects of 

ecological integrity or all possible chemical, physical, or biological stressors known to affect wetland 

systems.

This chapter presents the results from the NWCA 2011 using: 1) an indicator of biological condition, 2) 

physical, chemical, and biological indicators of stress, and 3) a ranking of the relative importance of the 

stressors in affecting biological condition. Results for each indicator are shown for wetlands nationally 

and for the four NWCA Aggregated Ecoregions established for reporting. Regional results are presented 

in Chapter 4.

Results for wetland condition are estimates of the extent of wetland area (presented as percent area 

and numbers of acres) in three condition classes (good, fair, and poor). Results for the indicators of 

stress are presented as estimates of the percent wetland area in a particular stressor-level class 

(generally, low, moderate, and high). See Chapter 2 of this report, and the NWCA 2011 Technical Report 

(USEPA 2015) for details on how these estimates are made. The estimated wetland area results are 

often referred to as population estimates, and each estimate is accompanied by a confidence interval 

that conveys the level of certainty or confidence in the estimate (see text boxes “How to Read and 

Interpret NWCA Results Figures” and “Confidence Interval” below).

HOW TO READ AND INTERPRET NWCA RESULTS FIGURES

Confidence Interval – displays level of 

certainty or confidence in the estimate

Wetland area, in acres, nationally or 

within an ecoregion in each of the 

condition or stress level categories

Proportion of wetland area nationally 

or within an ecoregion in each of the 

condition or stress level categories

Condition classes (this example) or stressor levels are indicated by colors:

Condition Class: Stressor Level:

Green = Good Green = Low

Yellow = Fair Yellow = Moderate

Red = Poor Red = High

Dark Red = Very High

Gray = Unassessed (area that could not be sampled)

National estimates of 

condition or stress level 

are shown by the first 

group of bars, followed 

by estimates for each of 

the NWCA Aggregated 

Ecoregions Number shows the value of the 

estimate represented by the bar

(e.g., 61% of wetland area in the 

West is in Poor condition)

This example figure displays national and 

ecoregion estimates of biological condition 

as measured by the VMMI 
Chapter 3 National Results

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National Wetland Condition Assessment 2011

Biological Condition Based on Vegetation MMI

Vegetation is a major component of the biodiversity found in wetlands and also provides habitat for a 

broad range of microbes, insects, amphibians, reptiles, birds, and mammals. The composition and 

abundance of plant species reflects, as well as influences, the hydrology, water chemistry, and soil 

properties of wetlands. Vegetation is a particularly good indicator of wetland condition because of its 

ability to integrate different wetland processes and because plants respond to physical, chemical, and 

biological disturbances at multiple temporal and spatial scales. Using field collected data and plant trait 

information, a national Vegetation Multimetric Index (VMMI) was developed. (See Chapter 2 in this 

document, and the NWCA 2011 Technical Report (USEPA 2015) for more details). The VMMI serves as 

the indicator of biological condition for the NWCA.

The condition of wetlands at national and ecoregional scales is shown in Figure 3-1. This figure presents 

both the percentage of wetland area and the number of wetland acres in different condition classes 

(good, fair, and poor). The NWCA found nationally, 48% of the wetland area (29,998,957 acres) is in 

good condition, while 20% (12,179,915 acres) is in fair condition. The remaining 32% (19,977,327 acres) 

of the wetland area is in poor condition.

CONFIDENCE INTERVALS

Confidence intervals convey the level of certainty or confidence in the estimates presented in this report. For 

example, for the VMMI, NWCA found that 48% of the nation’s assessed wetland acres are in good condition, 

with a confidence interval of +/- 5%. This means that there is a 95% certainty that the real value is between 

43% and 53%. The confidence interval is influenced by the number of sites sampled. As more wetland sites are 

sampled, the confidence interval becomes narrower, meaning there is more confidence in the findings. Figure 

3-1 shows an example of this pattern, in which the confidence interval for the national results (the largest 

sample size) is narrowest, whereas the confidence intervals for the NWCA Aggregated Ecoregions (smaller 

sample sizes), are generally broader. Ultimately the number of sites sampled is a tradeoff between the need 

for increased certainty to support management and policy decisions, and the cost in money and resources to 

perform more extensive monitoring activities. Note, confidence intervals are shown in the results figures for all 

NWCA population estimates presented in this report.
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National Wetland Condition Assessment 2011

Figure 3-1. Estimated extent of wetland biological condition by condition classes (good, fair, poor) based on the 

VMMI. Results are reported for the nation and by NWCA Aggregated Ecoregion.

The regional results apply to specific NWCA Aggregated Ecoregions, and care must be taken when 

comparing results across the four ecoregions. It is important to note that the level of human-mediated 

disturbance for reference (least disturbed) sites varied by region and wetland type. For example, in the 

Interior Plains and West, reference sites have the highest level of disturbance. Reference sites for 

estuarine wetlands, which are predominantly found in the Coastal Plain, have the lowest level of 

disturbance. See NWCA 2011 Technical Report (USEPA 2015) for details.

Wetland area varies significantly among the NWCA Aggregated Ecoregions. For example, in the NWCA 

survey, the West represents only 6% of the total assessed wetland area of the target wetland types 

sampled across the country. In the West, 61% of the wetland area (2,214,806 acres) is in poor condition,

representing a major proportion of the wetland area for that ecoregion, but a small proportion of the 

total wetland area nationally. The West has 21% and 18% of wetland area (782,525 and 649,729 acres) 

in good and fair condition, respectively.

The Coastal Plains, Eastern Mountains and Upper Midwest, and Interior Plains have a range of 44% to 

52% wetland area in good condition. The Eastern Mountains and Upper Midwest has 11% of wetland 

area (2,117,215 acres) in fair condition and 37% (7,462,851 acres) in poor condition. In the Coastal 

Plains, 21% of wetland area (6,620,942 acres) is in fair condition and 29% (8,808,894 acres) is in poor 

condition. In the Interior Plains, 36% of wetland area (2,792,028 acres) is in fair condition while 19% 

(1,490,777 acres) is in poor condition.
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As future NWCA surveys are implemented, we will be able to use the 2011 results as a point of 

comparison to track whether wetland areas are getting better (i.e., moving to good condition) or worse 

(i.e., moving to poor condition) within each aggregated ecoregion.

Indicators of Stress

The NWCA also includes measurements of indicators of stress, which are physical, chemical, and 

biological factors that have the potential to reflect human-mediated impacts on wetland condition. The 

indicators of stress do not necessarily directly cause ecological decline, but are often associated with 

changes in wetland condition. While these stress indicators are expected to be associated with effects 

on wetland condition, both in their presence and magnitude, the exact relationship between them and 

condition cannot be explicitly determined from the data collected in the NWCA. A goal of the NWCA was 

to characterize indicators of stress that are common in wetlands to help inform priorities for 

management actions. See Chapter 2 in this document, and the NWCA 2011 Technical Report (USEPA 

2015) for more detail on the development of each indicator of stress.

For simplicity of language, in the remainder of this chapter, “indicators of stress” are sometimes 

referred to as “stressors.” The results report the extent of the NWCA 2011 stress indicators by stressor 

levels (low, moderate, or high). Stressor extent is an estimate of how spatially common an indicator of 

stress is nationally or within each NWCA Aggregated Ecoregion.

Physical

Wetlands can be influenced by physical changes to the ecosystem and its immediate surroundings. For 

the NWCA, physical data collected by crews were categorized and assigned to one of six indicators 

representing alteration of wetland vegetation or hydrology: vegetation removal, vegetation 

replacement, damming, ditching, surface hardening, and filling/erosion (see Figure 2-4).

Vegetation Alteration

Vegetation alteration was identified by either the removal or replacement of vegetation. The vegetation 

removal indicator evaluated the loss, removal, or damage of vegetation either within the assessment 

area or the assessed buffer area immediately surrounding it. Nationally, the vegetation removal stressor

is low for 56% of the wetland area and high for 27% (Figure 3-2). However, this varied greatly by NWCA 

Aggregated Ecoregion. In the Coastal Plains and Eastern Mountain and Upper Midwest, the degree of 

vegetation removal for most of the wetland area fell into the low stressor level. In contrast, a large 

proportion of the wetland area in the Interior Plains and West have high stressor levels from vegetation 

removal, encompassing 44% and 61% of the wetland area in the Interior Plains and West, respectively.
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Figure 3-2. Estimated extent of vegetation alteration in wetlands by stressor levels as indicated by vegetation 

removal and vegetation replacement. Results are presented nationally and by NWCA Aggregated Ecoregion.

The vegetation replacement indicator documented major changes to the natural vegetation structure 

and composition due to anthropogenic activities (e.g., conversion of natural plant communities to golf 

course, lawn, park, row crops, nursery, etc.). Nationally, and within each of the NWCA Aggregated 

Ecoregions, wetland area is predominantly found at low stressor levels related to vegetation 

replacement (Figure 3-2). Moderate and high stressor levels associated with vegetation replacement are 

found in 18% and 10% of national wetland area, respectively.

Hydrologic Alteration

Information collected by crews pertaining to alterations of wetland hydrology were used to develop four 

stressor indicators: damming, ditching, hardening, and filling/erosion. For each of these four indicators, 

the majority of wetland area nationally have low stressor levels (Figure 3-3). Of these four, ditching and 

surface hardening are found to have the largest extent of wetland area at high stressor levels nationally,

23% and 27%, respectively. Among the ecoregions, the West has the largest proportion of wetland area 

at high stressor levels for damming, ditching, and surface hardening. In the other ecoregions, the 

majority of wetland area have low stressor levels for the evaluated hydrologic stressors.
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Figure 3-3. Estimated extent of hydrologic alteration in wetlands by stressor levels as indicated by damming, 

ditching, hardening, and filling/erosion. Results are presented nationally and by NWCA Aggregated Ecoregion.

Chemical

Chemical stressors that can impact the condition of wetlands include excess nutrients, metals, organic 

toxins, and other chemical compounds that can disrupt nutrient cycles, affect plant and animal growth, 

and be detrimental to human health. Two chemical indicators of stress were developed for NWCA using 

data from soil samples collected at each site: a Heavy Metal Index and concentration of soil phosphorus.

At some sites it was not possible to collect a soil sample because of ponded water or other site 

conditions on the sampling day. Additionally, at some sites in the Interior Plains ecoregion soil samples 

were not analyzed using NWCA laboratory protocols. As a result, these sites could not be evaluated for 

the chemical stressors. Another potential indicator of stress, microcystin (an algal toxin) concentration, 

is discussed separately at the end of this chapter.
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Figure 3-4. Estimated extent of chemical indicators of stress in wetlands by stressor levels as indicated by a 

Heavy Metal Index and soil phosphorus concentrations. Results are presented nationally and by NWCA Aggregated 

Ecoregion.

Concentrations of twelve heavy metals found in soils and closely associated with human activities 

comprised the Heavy Metal Index. Heavy metals have measured concentrations below background

levels across most sites. Consequently, the majority of assessed wetland area nationally, 73%, is at low 

levels for this stressor (Figure 3-4). Moderate levels are found in 18% of wetland area nationally, and 

high levels in only 2%. Soil samples could not be collected or were not analyzed for heavy metal 

concentrations for 7% of wetland area nationally. Of the four NWCA Aggregated Ecoregions, Eastern 

Mountains and Upper Midwest and West have the greatest estimated percentage of wetland area at 

high stressor levels (4% and 5%, respectively).

Soil phosphorus is a necessary plant nutrient, but at high levels can indicate human-mediated impacts. 

Soil phosphorus concentration thresholds were determined using a distribution-based approach (see 

Chapter 2). Nationally, the majority of wetland area, 69%, is at low stressor levels (Figure 3-4), while 19% 

of wetland area is at moderate stressor levels. Soil phosphorus concentrations are not assessed for 7% 

of the wetland area due to difficulties in collecting soil samples or because some soil samples were not 

analyzed using NWCA lab protocols. Among the NWCA Aggregated Ecoregions, wetland area with soil 

phosphorous concentrations at low stressor levels ranges from 63 to 88%. The greatest estimated extent 

of wetland area at high stressor levels is found in the Eastern Mountain and Upper Midwest (13%) and 

in the Coastal Plains (3%).
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Biological

In addition to looking at the chemical and physical stressor indicators that can impact wetland systems, 

the NWCA developed a Nonnative Plant Stressor Indicator (NPSI) to assess the level of biological stress 

in wetlands. Nonnative plants can have numerous direct and indirect effects on native vegetation and 

ecosystem components. This indicator used three metrics: the relative cover of nonnative species, 

richness of nonnative species (the number of unique nonnative species), and relative frequency of 

occurrence of nonnative species. Wetlands were assessed as having low, moderate, high, or very high 

stressor levels based on the potential impact of nonnative plants on the native vegetation of the site.

Nationally, 61% (37,709,004 acres) of the wetland area are estimated to have low stressor levels from 

nonnative plants, but results are not uniform across the country (Figure 3-5). In the Eastern Mountains 

and Upper Midwest and the Coastal Plains stressor levels based on the NPSI are low for 74% (14,761,495 

acres) and 66% (20,358,855 acres) of the wetland area, respectively. In the Interior Plains and West, the 

extent of wetland area at low stressor levels is much smaller, 27% and 14%, respectively. In the Interior 

Plains, the extent of wetland area is distributed more uniformly across the four NPSI stressor levels, with 

relatively similar proportions of area having low, moderate, high, and very high stressor levels. In the 

West, the majority of wetland area, 71% (2,602,079 acres) is estimated to have high or very high 

stressor levels as indicated by the NPSI.

Figure 3-5. Estimated extent of biological stress in wetlands by stressor levels as indicated by the Nonnative 

Plant Stressor Indicator. Results are presented nationally and by NWCA Aggregated Ecoregion.
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Ranking of Stressors

An important function of the NWCA is to provide information for sound policy and decision-making to 

support the maintenance and improvement of wetland ecological quality. It is important for resource 

managers to understand not only which stressors are present nationally and in various regions, but to 

have information on the potential impact of stressors on ecological condition and to be able to rank 

those stressors in terms of the estimated improvements we might expect by reducing or eliminating 

them. To meet these objectives, the NWCA uses relative extent, relative risk, and attributable risk to 

consider how stressors could influence the ecological condition of wetlands. An overview of these three 

concepts is provided here. Further details on their calculation can be obtained in the NWCA 2011

Technical Report (USEPA 2015, Chapter 9).

Relative extent ranks the stressor indicators in terms of the amount of wetland area affected by a high 

level of each stressor indicator evaluated. Relative risk is the probability or likelihood of having poor 

biological condition when the magnitude of a stressor indicator is high relative to when it is low. Relative 

risk analysis is commonly employed in medicine where it has been used to describe the risk of having a 

health problem relative to a potential cause or its indicator. For example, a person who smokes has a 

greater risk of developing lung cancer. Often this is presented as a relative risk ratio; for example, a 

person who smokes is 15 to 30 times more likely to get or die of lung cancer than someone who does 

not (CDC 2015). Similarly, the relative risk value for an ecological stressor measures the likelihood that a 

wetland will have poor ecological condition if the wetland has high levels of the stressor rather than if 

the wetland had low or moderate levels of the stressor. Finally, calculation of attributable risk provides 

an estimate of the proportion of the wetland area in poor condition that could be reduced if the effects 

of a particular stressor were eliminated.

It is important to note that while the NPSI was reported with the other indicators of stress in the 

previous section, it is not used in the analyses to describe risk. Because relative and attributable risk 

specifically relate stressors to condition, and both the NPSI and VMMI use related data, it is not 

appropriate to include the NPSI in reporting relative and attributable risk. Although nonnative plant 

species likely confer risk to wetland condition (see the wetland area extent estimates for the NPSI in the 

previous section), this risk cannot be evaluated using the relative and attributable risk approach.

National results for relative extent, and relative and attributable risk for each stressor indicator are 

presented in the following sections of this chapter. Specifics on stressor indicators for each of the NWCA 

Aggregated Ecoregions are given in Chapter 4.

Relative Extent and Relative Risk

Relative extent is a way to evaluate how widespread and common a high stressor level for each stressor 

is across the wetland area. A stressor with a high relative extent suggests a national concern. Figure 3-6

(left panel) shows the proportion of wetland area in the U.S. with high stressor levels for each of the 

stressor indicators. Vegetation removal, surface hardening, and ditching are the most pervasive 

stressors across the nation. High levels of vegetation removal and surface hardening stressors are found 

for 27% of the wetland area, while 23% of wetland area has high levels of the ditching stressor.

NARS and the NWCA use the concept of relative risk to estimate the severity of stressor effects. Relative 

risk is the probability or likelihood of having poor resource condition when a stressor level is high 

relative to when the stressor is low or moderate. For the NWCA, biological condition is described by the 

VMMI. A relative risk value of 1 indicates that the stressor indicator has no effect on condition. Relative 
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risk values greater than 1 suggest that the stressor has a greater impact on biological condition (e.g., the 

vegetation removal indicator has a relative risk of 1.9, indicating that the likelihood of having a poor 

VMMI score is 90% greater in wetlands with high levels of vegetation removal).

Figure 3-6 (central panel) shows the relative risk, or relative effect, of each stressor on wetland 

condition. At the national level, vegetation removal, surface hardening (e.g., soil compaction), ditching, 

damming, filling/erosion, and vegetation replacement are highly associated with poor biological

condition in wetlands with relative risk ranges from about 1.5 to 1.6. There does not appear to be a clear 

relationship between high soil phosphorus or heavy metal concentrations and poor biological condition.

Figure 3-6. National level estimates for relative extent of stressor indicators when stressor level is high, relative 

risk associated with each stressor indicator, and attributable risk for each stressor indicator relative to wetland 

biological condition.

Attributable Risk

NWCA uses the calculation of attributable risk to estimate the proportion of wetland area in poor 

condition that could be reduced if the effects of a particular stressor indicator were eliminated.

Attributable risk combines relative extent and relative risk into one value to evaluate the impact of a 

stressor across the assessed area. The calculation of attributable risk assumes that the stressor causes 

poor condition, the effects of the stressor can be reversed, and that the stressor’s impact on condition is 

independent of other stressors. These assumptions are difficult to meet with survey data like those 

collected in the NWCA. Despite these limitations, estimates of attributable risk can provide general 

guidance as to what stressors are affecting condition and to what degree (relative to the other stressor 

indicators evaluated). This information can provide an indication of how policymakers and resource 

managers could prioritize actions and the use of limited resources by stressor, geographic region, and/or 

wetland type.

Figure 3-6 (right panel) shows the attributable risk for each of the stressor indicators across the nation. 

The stressors are ranked according to their attributable risk. The stressor categories with the highest 

attributable risk values are vegetation removal and surface hardening, with 19% and 18%, respectively, 

of the wetland area potentially affected by each. This estimate suggests, for example, that if high levels 
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of vegetation removal are eliminated as a stressor, one would expect to see 19% of wetland acres 

improve to good or fair biological condition. Ditching has the next highest relative risk, potentially 

affecting 12% of the wetland area. It is also important to note that some stressors can have large 

relative risk values but small attributable risk estimates because the relative extent is small. For 

example, filling/erosion is found at high levels in only 10% of the wetland area. Although the relative risk 

number is about 1.6, the attributable risk estimate indicates that nationally only about 6% of wetland 

acres may improve to good or fair biological condition by eliminating filling/erosion.

These attributable risk estimates indicate the need to continue efforts to reduce the impact of 

vegetation removal, surface hardening, and ditching nationally. Although some stressors, such as 

filling/erosion, might not be as widespread nationally, that does not mean that localized management 

actions targeting these stressors are not needed.

Microcystin Presence and Risk

Microcystins are one group of naturally occurring toxins produced by various cyanobacteria (blue-green 

algae) that are common in surface waters. Microcystins have been detected nationally in lakes and 

reservoirs and are considered to be the most commonly occurring class of cyanobacteria toxins 

(cyanotoxins). Three main exposure scenarios are of potential concern regarding microcystins and 

wetlands: direct ecological impacts on plants and animals, human consumption of exposed organisms, 

and direct human exposure through recreational contact. As in other NARS assessments, the microcystin 

results reported here focus on risks associated with recreational contact. Information on other exposure 

risks is discussed in the NWCA 2011 Technical Report (USEPA 2015, Chapter 10).

Although there are relatively few documented cases of human health effects from exposure to 

cyanotoxins through recreational activities, exposure to cyanobacteria or their toxins may produce 

allergic like reactions such as skin rashes, eye irritations, respiratory symptoms, and in some cases 

gastrointestinal illness, liver and kidney damage, and in rare cases even death. During recreational 

activities, exposure for humans may occur through accidental ingestion, inhalation or direct contact. 

Cyanotoxins can also be a concern in drinking water.

2

In addition to human impacts, livestock, pets and 

wildlife are also exposed to cyanotoxins when consuming scum or drinking cyanotoxins-contaminated 

water. The probability of adverse recreational health effects for humans due to cyanobacteria and/or 

microcystin exposure is frequently assessed based on World Health Organization (WHO) guidance 

thresholds (see Table 2-2). Many states have developed harmful algal bloom (HAB) guidance thresholds 

in the event of a cyanobacterial bloom and cyanotoxins in recreational waters.

The occurrence of microcystins in wetlands at national and ecoregional scales is shown in Figure 3-7. 

This figure presents both the percentage of wetland area and the number of wetland acres where 

microcystin is detected, not detected, or unassessed (i.e., no microcystin sample was collected). The 

NWCA found that, nationally, 12% of wetland area has detectable concentrations of microcystin. 

Microcystin is not detected at 27% of wetland area, nationally, and 61% of wetland area is unassessed. 

The large percentage of unassessed wetland area is due to the large number of wetland sites where 

water depth was not deep enough for a microcystin sample to be collected. At the ecoregional scale, 

NWCA finds that the Interior Plains has the highest percent of wetland area with detectable 

 

2

In June 2015, EPA announced health advisory values for microcystin in drinking water. The Agency is using a nonregulatory option of health advisories to address the growing public health threat of cyanotoxins in drinking water. 

The health advisory values are 0.3 micrograms per liter (μg/L) for children younger than six and 1.6 μg/L for 

children six and older and adults.
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concentrations of microcystin at 34%. This is much larger than the other NWCA Aggregated Ecoregions, 

which each have ranges closer to the national estimate.

Figure 3-7. Estimated occurrence of microcystin in wetlands. Results are presented nationally and by NWCA 

Aggregated Ecoregion.

The level of risk associated with recreational exposure to microcystin in wetlands at national and 

ecoregional scales is shown in Figure 3-8. This figure presents both the percentage of wetland area and 

number of wetland acres in different risk categories (low, moderate, high) based on the WHO 

thresholds. Nationally, and across the NWCA Aggregated Ecoregions, NWCA finds very little wetland 

area (< 1%) at either moderate or high risk levels. Most wetland area is at low risk levels or could not be 

assessed.
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Figure 3-8. Estimated extent of recreational health risk from exposure to microcystin by risk category (low, 

moderate, and high) based on WHO guidelines. Results are presented nationally and by NWCA Aggregated Ecoregion.
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Highlight

Alaska’s Arctic Wetlands Assessment

Terri Lomax, Alaska Department of Environmental Conservation

In 2011, as part of EPA’s National Wetland Condition Assessment (NWCA) the Alaska Department of 

Environmental Conservation (DEC) completed a wetland condition assessment in the Arctic Coastal 

Plain region of the National Petroleum Reserve-Alaska. Due to Alaska’s size and limited 

infrastructure statewide surveys are not feasible, leaving regional assessments as a more realistic 

option for sampling in Alaska. DEC partnered with the University of Alaska Anchorage and the North 

Slope Borough to complete this survey.

Freshwater emergent wetlands are the 

dominant wetland type in the region, and 

were selected from the National Wetland 

Inventory within the sample region. Within

this sample frame 57,188 freshwater 

emergent wetland polygons exist, totaling

more than 5 million acres. To relieve 

community concerns with potential helicopter 

interference of subsistence hunting activities 

we created a buffer around high subsistence 

use areas and excluded sites in the sample 

frame that fell within the buffer. From the 

remaining population, 40 random sites were 

selected according to NWCA protocols. Alaska’s limited infrastructure, small population base, and 

the remote nature of most of the state drives the selection of sites in random surveys, typically only 

selecting sites with reference or near reference condition. In prior DEC surveys this proved to be 

problematic as range of disturbance is needed to understand condition and develop metrics based 

on stress. To overcome this we added 10 targeted sites to our survey, these sites with known or 

potentially impacted freshwater emergent wetland sites within the sample area.

Wetlands in this region differ significantly from wetlands surveyed in the contiguous United States 

and therefore required modifications to national methods. The Arctic Coastal Plain, the land of the 

midnight sun and the polar night, is a vast treeless area underlain by continuous permafrost. In 

cooperation with EPA, DEC modified the national methods to include non-vascular plant 

identification, modified soil methods to account for permafrost conditions, and excluded analysis 

requiring short hold times.

Overall, assessment areas averaged 33 cm of water depth and 16% of the area was covered with 

surface water. Sites averaged an 89 meter vegetated buffer out of the 100 meters evaluated. The 

plant community was comprised of short woody and emergent species. 65% of sites were 

dominated by Carex aquatilis, Eriophorum vaginatum was the second most common species 

Arctic Wetlands. Photo courtesy of Alaska DEC.
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identified. Other common plants included several species of Salix, Eriophorum russeolum and 

angustifolium, and Betula nana, all typical of arctic emergent wetlands. Permafrost was 

encountered at all sites, at an average depth of 43 cm.

Results were evaluated for patterns across the landscape, overall, and in targeted verses random 

sites. Differences were observed between random verses targeted sites, but otherwise no significant 

patterns were observed. The percentage of sites with stressors identified in the buffer and

assessment area was greater in targeted sites, additionally plant and diatom richness was greater at 

targeted sites than random sites. Targeted sites were typically adjacent to previous military 

installments with one to several meters of fill forming a pad on the tundra. Water chemistry and soil 

profiles did not demonstrate significant differences.

As expected, stressors were more often observed in targeted sites and typically involved changes in 

the soil (soil compaction, recent fill, and grading). The next most common stressor identified was 

off-road vehicle use. Mild to moderately severe stressors were observed in the assessment area of 

100% of targeted sites and mild stressors were observed in 37% of random sites. Similar stressors 

were observed in the buffer of all targeted and none of the random sites. We are continuing to 

evaluate the data in relation to lake, river, stream and coastal surveys in the same region.

The success of our survey is attributed to our partnerships. Working with local tribal governments, 

federal and state agencies, and the University was crucial to overcoming numerous challenges. 

During field work we experienced freezing weather, gale force winds, equipment malfunctions, 

medical emergencies, and major logistical hurdles. In spite of these challenges, we were able to 

sample 41 out of the 50 wetlands selected.

To learn more, contact Terri Lomax (terri.lomax@alaska.gov; 907-269-7635), Alaska Department of 

Environmental Conservation, or visit: https://dec.alaska.gov/water/wqsar/monitoring/AKMAP.htm
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Chapter 4: Ecoregion and Wetland Type Results

Ecoregions are geographic areas with similar environmental characteristics, such as climate, vegetation, 

type of soil, and geology. EPA has defined ecoregions at various scales, from large areas to finer units of 

the landscape (Omernik 1987, USEPA 2011a). The EPA ecoregions have been widely used in assessing 

resource status, establishing water quality and biological condition criteria, and setting management 

goals for resource protection. NARS uses the various levels (scales) of these EPA ecoregions because 

they were developed to support decision-making for aquatic resources, and it is important to consider 

aquatic and other natural resources within the context of their ecological setting.

Wetlands in an ecoregion tend to have more similar natural characteristics to each other than with 

wetlands in other parts of the country. Forested wetlands in the Upper Midwest, for example, share 

more similar traits with other forested wetlands in this region than they do with forested wetlands in 

the Coastal Plain. Additionally, different wetland types have distinct characteristics that distinguish them 

from other wetland types. Wetland types and ecoregions are useful in evaluating and understanding 

results describing ecological condition and stress because of these patterns.

Previous NARS studies have combined Level III ecoregions into nine major ecoregion groups in order to 

evaluate the data at ecoregional scales. In the NWCA, to ensure enough sample points within a 

reporting group to present statistically valid results, it was necessary to combine ecoregions and 

wetland types for data analysis (see Chapter 2, How are the NWCA results presented?).

The four NWCA Aggregated Ecoregions (Figure 4-1) are based on a combination of smaller scale EPA 

ecoregions:

• Coastal Plains

• Eastern Mountains & Upper Midwest

• Interior Plains

• West

The seven NWCA wetland types were combined into four NWCA Aggregated Wetland Types:

• Estuarine Herbaceous

• Estuarine Woody, which represents both scrub-shrub and forested wetlands.

• Inland Herbaceous, which represents emergent, unconsolidated bottom/aquatic bed, and 

farmed wetlands not in crop production.

• Inland Woody, which represents both forested and scrub-shrub wetlands.
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Figure 4-1. The NWCA Aggregated Ecoregions. Chapter 2 of this report and the NWCA 2011 Technical Report 

(USEPA 2015) detail the compilation of these four ecoregions.

In this chapter, condition and stress results are reported by each major NWCA ecoregion group and for 

NWCA aggregated wetland types:

• Results for each of the four NWCA ecoregions are presented in two ways based on 1) all NWCA 

wetland types occurring in a region; and 2) aggregated inland herbaceous and inland woody 

wetland types within a region.

• Results for estuarine herbaceous and estuarine woody wetland types are presented in the 

Estuarine Wetlands section of this chapter and include all NWCA ecoregions where estuarine 

wetlands occur (Coastal Plains, Eastern Mountain and Upper Midwest, and West).

NWCA results should not be extrapolated to an individual state or specific wetland within the ecoregions 

because the study is not intended or designed to characterize conditions at these finer scales. A number 

of states worked with EPA to design and implement statistically-based assessments at the state scale in 

order to characterize the condition of wetland populations within their states. Short highlights 

discussing these studies appear throughout the report.
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Coastal Plains Ecoregion

Landscape setting of the ecoregion

The Coastal Plains ecoregion includes the Mississippi Delta 

and Gulf Coast, runs north along the Mississippi River to the 

Ohio River, and includes Florida and eastern Texas, and the 

Atlantic seaboard from Florida to Cape Cod. 

The Coastal Plains is relatively flat and much of the area is 

less than 500 feet above sea level. Most of the region is 

composed of sedimentary rocks or unconsolidated sediments 

of marine origin, with some areas made up of younger alluvial 

deposits. A large proportion of the natural vegetation is 

mixed pine and hardwood forests. Along the western edge of 

the region (coastal Texas and Louisiana), post oak savanna and prairie vegetation is more common. The 

Mississippi River Valley is dominated by bottomland deciduous forests.

The climate in the Coastal Plains is temperate wet to subtropical, with average annual temperatures 

ranging from 50 to 80°F. Average annual precipitation ranges from 30 to 79 inches.

Although many areas had been previously cleared for agricultural purposes, much of the region has 

been reforested or is used for lumber and pulpwood production. Agricultural crops, pasture, and 

rangeland is found throughout the region, but production of cotton, soybeans, rice, and sugarcane 

remains a dominant land use in the Mississippi Valley.

The types of wetlands found throughout the region are 

diverse. Tidal salt and brackish marshes and tidal and non-tidal 

freshwater marshes are found along the Gulf and Atlantic 

coasts. In the most southern part of the region (central Florida 

and southward) mangrove swamps are found in association 

with coastal marshes. In the interior portions of the Coastal 

Plains, bottomland hardwood forests and swamps occur along 

rivers, streams, and their headwaters. Other wetlands found 

throughout the Coastal Plains region are locally referred to as 

flatwoods, pineywoods, pine savannas, pine barrens, flatlands, 

and coastal prairies. These wetlands can be dominated by 

herbaceous plants, hardwoods, pines, or a mixture of 

vegetation. Freshwater bogs, called pocosins, are found from 

southern Virginia to northern Florida.

In the Coastal Plains, there are several well-known wetland 

complexes, including the Florida Everglades, the Okefenokee 

Swamp in southeastern Georgia and northeastern Florida, the 

Great Dismal Swamp in southeastern Virginia and northeastern North Carolina, and the Louisiana Delta 

(a complex of forested wetlands, freshwater marshes, salt marshes, and shallow coastal lakes).

Swamp in Florida (site NWCA11-1258). 

Photo courtesy of University of Florida.
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Summary of findings

A total of 513 randomly selected sites were sampled in the Coastal Plains ecoregion during the 2011 

field season, representing 30,893,305 acres. Of the total number of Coastal Plains sites sampled, 62 are 

inland herbaceous wetlands, representing 3,750,551 acres, and 163 are inland woody wetlands, 

representing 21,859,265 acres. Estuarine wetlands in the Coastal Plains include 288 sites, representing

5,283,489 acres. Detailed results for estuarine wetland types are reported in the Estuarine Wetlands 

section of this chapter.

Biological Condition

For all wetland types assessed in the Coastal Plains (Figure 4-2), 50% of the estimated wetland area is in 

good condition; 21% is in fair condition and 29% is in poor condition based on the Vegetation 

Multimetric Index (VMMI) (see Chapter 2 for details on the VMMI). The proportion of wetland area in 

good (50%), fair (26%), and poor (25%) is similar for inland woody wetlands in this ecoregion. The NWCA 

found that the proportion of inland herbaceous wetlands in poor condition, however, is much larger, 

59%, than the proportion of inland woody wetlands in poor condition (Figures 4-3 and Figure 4-4).

Indicators of Stress

For all wetland types assessed in the Coastal Plains, vegetation removal, ditching, and surface hardening 

are the indicators with the greatest proportion of wetland area at high stressor levels (Figure 4-2). 

However, the majority of wetland area throughout the ecoregion has low levels for each of the

stressors. Key findings include:

• Vegetation removal is high for 25% of the wetland area compared to moderate for 21% and low 

for 54% of wetland area.

• Hardening levels are high in 23% of wetland area, while 9% and 69% of wetland area have 

moderate or low stressor levels, respectively.

• Ditching is high in 21% of the wetland area, moderate in 5%, and low in 74%.

For inland wetland types assessed within the Coastal Plains ecoregion (Figures 4-3 and 4-4), data show:

 More than half of the area assessed for inland herbaceous wetlands has high stressor levels 

associated with vegetation removal (61%), hardening (57%), and ditching (52%).

 The most prevalent stressors at high levels for inland woody wetlands are also vegetation 

removal (24% of wetland area), hardening (20%), and ditching (16%), but these high stressor

levels are a smaller proportion of wetland area compared to the herbaceous wetlands.

 The extent of high levels of vegetation removal, damming, ditching, hardening, and 

filling/erosion stressors are greater in inland herbaceous wetlands than in inland woody 

wetlands. Very high and high levels for the nonnative plant stressor indicator are also greater in 

herbaceous wetlands.
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Figure 4-2. NWCA 2011 survey results for the wetlands (i.e., all target wetland types) across the Coastal Plains.

Bars show the percentage of wetland area within a condition or stressor class. Error bars represent 95% confidence intervals.

Figure 4-3. NWCA survey results for the inland herbaceous wetland type across the Coastal Plains. Bars show 

the percentage of wetland area within a condition or stressor class. Error bars represent 95% confidence intervals.
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Figure 4-4. NWCA 2011 survey results for the inland woody wetland type across the Coastal Plains. Bars show 

the percentage of wetland area within a condition or stressor class. Error bars represent 95% confidence intervals.
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Highlight

Southeast Wetlands Monitoring and Assessment Intensification Study: North Carolina, 

South Carolina, Alabama, and Georgia

Rick Savage and Kristie Gianopulos, N.C. Department of Environment and Natural Resources

Overview: Recognizing that natural ecosystems are not confined within state boundaries, four neighboring 

states embarked on an unprecedented collaboration to study the condition of forested wetlands in the 

Southeastern United States. North Carolina, South Carolina, Georgia, and Alabama performed a regional 

analysis of forested wetlands within the Piedmont and Coastal Plain ecoregions, using both NWCA survey 

methodologies and additional indicators of wetland health (i.e., amphibian, macroinvertebrates, and overall 

landscape measures). The regional intensification project included 110 forested wetlands across the four 

states involved in the study. Although the final analysis has not been completed, preliminary data reveal 

differences in condition between the ecoregions, as well as areas of possible concern for amphibian 

populations.

Sampling and Protocols: There 

were 45 wetland sites sampled in 

the Piedmont Region and 65 

wetland sites sampled in the 

Coastal Plain Region. At each of 

these sites, NWCA data were 

collected as well as additional 

indicators, including for amphibians 

and macroinvertebrates. This 

intensification study developed a 

composite scoring system from 

several data indicators gathered at 

each location in order to determine 

the overall wetland condition at 

each site. The preliminary wetland 

condition index integrated several 

important measurements into one 

total, including the following 

individual biotic and abiotic 

measures: the vegetation mean C; 

vegetation invasive species cover, 

the Amphibian Quality Assessment 

Index (AQAI), macroinvertebrate 

diversity, the buffer Landscape 

Development Intensity Index (LDI), the Ohio Rapid Assessment Method score, water quality nutrients, and soil 

metals information. Mean C is a commonly used metric based on Coefficient of Conservatism values of the 

plant species. Species are rated from 0 to 10 based on their fidelity to natural habitat; a high C value indicates 

nearly all species occurrences are in pristine habitat and low C values are assigned to weedy species which are 

Figure 1. Preliminary composite rankings for forested wetland sites; low rank 

numbers indicate best quality and high rank numbers indicate lowest quality,

based on a variety of factors.
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tolerant of human-caused disturbance. 

The LDI scores were developed using 

GIS analysis of land uses and their 

aerial extent surrounding sampled 

wetland points. The AQAI values were 

calculated using Coefficient of 

Conservatism ratings developed for 

amphibian species and an equation 

identical to that used commonly for 

floristic quality assessments (FQA). 

Low AQAI values indicate low species 

diversity at a given site, while higher 

numbers correspond with increasing 

quality.

What They Have Learned: Preliminary 

data analysis reveal differences 

between the ecoregions and have 

identified areas of possible concern for 

amphibian populations. Preliminary 

composite rankings (Figure 1) for 

forested wetland sites within the 

region indicate that forested wetlands 

in the Southeastern Coastal Plain 

(riverine swamp forests) are in better condition than those in the Piedmont ecoregion (bottomland hardwood 

forests). This may not be surprising since the Piedmont has been heavily farmed and is more densely 

populated.

In addition to the overall composite score, specific measures like the AQAI provide insight into the condition of 

wetlands and management implications for amphibian habitat. Amphibians are considered sentinel indicators 

because of their sensitivity to environmental stress, making them ideal for regional biomonitoring. While all of 

these results are preliminary, the AQAI scores derived from sampling site data are largely consistent with the 

overall composite indicator of wetland condition.

Generally, greater numbers of amphibian species were found in 

Coastal Plain sites, where unique species counts ranged from 3 to 12 

(mean = 4.9). In the Piedmont ecoregion, the number of amphibian 

species found at each site ranged from 1 to 7 species (mean = 3.8). 

These numbers correspond to AQAI values in the good score (green), 

fair (yellow), and poor score (red) categories as seen in Figure 2. 

Similar to the findings of the overall composite scoring, AQAI values in 

the Piedmont Region were relatively low or poor compared to AQAI 

scores for the Coastal Plain wetlands. The marbled salamander is a 

species particularly sensitive to anthropogenic disturbances, and is 

found mainly in the highest quality wetland sites. Marbled salamander 

were found in twice as many Coastal Plain sites (12) as Piedmont sites (6).

Overall, this type of wetland condition information can aid state and regional management decisions about 

wetland services such as flood control, water quality filtering, or the identification of restoration needs and 

opportunities. When the full analysis is complete, the improved knowledge of wetland condition in the 

Southeast region will help state agencies and conservation organizations to target scarce resources toward 

improving wetland condition in an efficacious way.

To learn more, contact Rick Savage (rick.savage@ncdenr.gov; 919-707-8792) or Kristie Gianopulos 

(kristie.gianopulos@ncdenr.gov; 919-707-8796), N.C. Department of Environment and Natural Resources.

Figure 2. Wetland condition based on Amphibian AQAI indicator; low 

numbers indicate low species quality.

Marbled Salamander (Ambystoma 

opacum).
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Eastern Mountains and Upper Midwest

Landscape setting of the ecoregion

The NWCA Eastern Mountains and Upper Midwest ecoregion 

combines the Northern Appalachians, Southern Appalachians, 

and the Upper Midwest NARS ecoregions. The region includes 

portions of Minnesota, Wisconsin, Michigan, northeastern 

Ohio, and virtually all of the New England states. It also 

includes New York, Pennsylvania, West Virginia, most of 

Kentucky, a significant part of Tennessee, as well as portions 

of East Coast states interior to the Coastal Plains ecoregion. 

The southern part of the ecoregion extends into northeastern 

Alabama and the Ouachita Mountains in Arkansas, Missouri, 

and Oklahoma. 

Dominant landscape features of the Eastern Mountains and Upper Midwest ecoregion include the 

Appalachian Mountains and the Great Lakes Basin. Retreating ice during the last glacial period and karst 

topography were important in shaping the region’s landscape. The northern portion is composed of 

glaciated terrain, with expansive plains and hills in the area surrounding the Great Lakes. In some areas 

(particularly northern Minnesota), extensive peatlands have formed on glacial lake plains. Most of the 

Northern Appalachians was also glaciated, resulting in mountainous or hilly terrain with intermixed 

plains. The Southern Appalachian region is also hilly, with wide valleys, plateaus, and irregular plains. 

Northern boreal and broadleaf deciduous forests transition to broadleaf and needleleaf forest 

communities toward the south.

The climate of the Eastern Mountains and Upper Midwest is characterized by a range of temperatures 

and precipitation. Cold winters and relatively short summers with a mean of 20 to 47 inches of 

precipitation characterize the areas surrounding the Great Lakes. Moving eastward, the climate is 

slightly warmer with increased annual precipitation totals ranging from 35 to 60 inches. Average annual 

temperatures range from 39 to 49oF. The southern portion of the ecoregion is both warmer and wetter, 

with annual precipitation of about 40 to 80 inches and average annual temperatures ranging from 52 to 

59oF.

Forests in the Eastern Mountains and Upper Midwest were extensively cleared in the 18th and 19th 

centuries for agricultural and industrial production. Today, much of the area within this ecoregion is 

highly populated and major manufacturing, including chemical, steel and power production may be 

found in metropolitan areas throughout the ecoregion. However, much of the northern part of the 

region remains forested and relatively undeveloped.
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Wetlands are relatively abundant in the northern portion of the ecoregion, owing to the climate and 

glaciated terrain. In contrast, wetlands are not as dominant a landscape feature in the southern portion. 

Forested wetlands are the most common type found throughout the ecoregion, and include forested 

swamps, bottomland hardwoods, wet flatwoods in 

the plains of the Great Lakes, and boreal coniferous 

forested bogs. Depressional wetlands, such as small 

ponds, kettle depressions, and vernal pools, form in 

low-lying areas left from retreating glaciers, areas 

with karst topography, and other landscape 

characteristics. Freshwater marshes are common 

around the Great Lakes and other water bodies 

throughout the ecoregion, while salt marshes may be 

found along the New England coastline.

Well known wetland complexes in the Eastern 

Mountains and Upper Midwest include the Boundary 

Waters area of Northern Minnesota, Horicon Marsh 

in Wisconsin, and Montezuma Swamp in the Finger 

Lakes region of New York.

Summary of findings

A total of 152 randomly selected sites were sampled in the Eastern Mountains and Upper Midwest 

ecoregion during the 2011 field season, representing 19,956,668 acres. Of the total number of Eastern 

Mountains and Upper Midwest sites sampled, 55 are inland herbaceous wetlands, representing 

3,762,089 acres, and 83 are inland woody wetlands, representing 16,165,406 acres. Estuarine wetlands 

in the Eastern Mountains and Upper Midwest included 14 sites, representing 29,173 acres. Detailed 

results for estuarine wetland types are reported in the Estuarine Wetlands section of this chapter.

Biological Condition

For all wetland types assessed in the Eastern Mountains and Upper Midwest (Figure 4-5), 52% of the 

estimated wetland area is in good condition; 11% is in fair condition, and 37% is in poor condition based 

on the VMMI. Inland herbaceous wetlands have 62% of assessed wetland area in good condition, 17% in 

fair condition, and 22% in poor condition (Figure 4-6). Compared to the inland herbaceous wetlands, a

slightly lower proportion of inland woody wetlands are in good and fair condition, 50% and 9%, 

respectively, while a greater proportion of inland woody wetlands are in poor condition, 41% (Figure 4-

7).

Indicators of Stress

For all wetland types assessed in the Eastern Mountains and Upper Midwest, surface hardening, 

vegetation removal, ditching, and soil phosphorus concentration are the indicators with the greatest 

proportion of wetland area at high stressor levels (Figure 4-5). However, the majority of wetland area 

throughout the ecoregion had low levels for each of the stressors. Key findings include:

• Hardening has high stressor levels for 22% of wetland area, while 23% and 55% have moderate 

or low levels, respectively.

• Vegetation removal stressor levels are high in 17% of wetland area, moderate in 15% of wetland 

area, and low in 68% of wetland area.

Forested wetland in New Hampshire (site NWCA11-

2163). Photo courtesy of New Hampshire Department 

of Environmental Services.
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National Wetland Condition Assessment 2011

• Ditching is high in 15% of the wetland area, moderate in 8%, and low in 77%.

• Soil phosphorus concentrations in this ecoregion are at high stressor levels for 13% of wetland 

area, moderate for 22% and low for 63% of wetland area. Soil phosphorus could not be assessed 

for 1% of the wetland area due to difficulties collecting soil samples at some sites.

For inland wetland types assessed within the Eastern Mountain and Upper Midwest ecoregion (Figures 

4-6 and 4-7), data show:

 The most prevalent stressors at high levels for inland herbaceous wetlands are soil phosphorus 

(35% of wetland area), vegetation removal (18%), ditching (14%), hardening (12%), and 

damming (12%).

 The most prevalent stressors at high levels for inland woody wetlands are hardening (25% of 

wetland area), vegetation removal (17%), and ditching (15%).

 High stressor levels for soil phosphorus are found at a greater proportion of wetland area for 

inland herbaceous wetlands (35%) than inland woody wetlands (8%). It should be noted that 8% 

of the herbaceous wetlands could not be assessed for soil phosphorus due to difficulties 

collecting soil samples at some sites. Less than 1% of woody wetlands were not assessed.

 Hardening is a more prevalent stressor for inland woody wetlands, with 25% of wetland area

having high-stressor levels compared to 12% for inland herbaceous wetlands.

Figure 4-5. NWCA 2011 survey results for the wetlands (i.e., all target wetland types) across the Eastern 

Mountains and Upper Midwest. Bars show the percentage of wetland area within a condition or stressor class. Error bars 

represent 95% confidence intervals.
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National Wetland Condition Assessment 2011

Figure 4-6. NWCA 2011 survey results for the inland herbaceous wetland type across the Eastern Mountains 

and Upper Midwest. Bars show the percentage of wetland area within a condition or stressor class. Error bars represent 

95% confidence intervals.

Figure 4-7. NWCA 2011 survey results for the inland woody wetland type across the Eastern Mountains and

Upper Midwest. Bars show the percentage of wetland area within a condition or stressor class. Error bars represent 95% 

confidence intervals.
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National Wetland Condition Assessment 2011

Highlight

Establishing a Baseline for Ohio’s Valuable Wetland Resources: A National Wetland 

Condition Assessment Intensification

Brian Gara, Ohio EPA

Objectives: According to previous estimates, Ohio has lost approximately 90% of its original, presettlement wetland habitat (Dahl 1990). The main goal of this intensification project was to perform a 

statewide survey to determine the current ecological condition of wetlands in Ohio that will serve as a 

baseline for future studies of this valuable and diminishing resource. A randomly-selected sample of 50 

wetlands were studied between 2011 and 2014 to generate the data necessary to determine condition 

and to:

(1) Compare and contrast the results NWCA 2011 with results generated from Ohio’s Level 1, 2, and 3 

wetland assessment methodologies.

(2) Identify differences in how NWCA and Ohio 

field methods assess wetland conditions that 

will help inform monitoring protocols for future 

state and national wetland condition 

assessments.

(3) Develop a plan to consistently repeat this 

statewide analysis on a regular schedule to 

illustrate long-term trends in both wetland 

quantity and quality in Ohio.

Protocols and Methods Used: Eleven wetland 

sites were sampled in Ohio for the 2011 NWCA, 

following the national NWCA protocols. An 

additional 39 wetland sites were sampled over 

the course of four field seasons (2011-2014) 

using the national protocols and other wetland 

methodologies previously developed by Ohio 

EPA. Because the sample design is areaweighted, a large proportion of the sites 

included in Ohio’s intensification study are 

located in the northeastern part of the state, 

where a majority of the remaining wetland 

area is contained (Figure 1). The other 

assessment methodologies included the Ohio 

Rapid Assessment Method for Wetlands 

(ORAM), a simplified soil sampling protocol, 

and expanded identification and collection of 

all unique bryophyte species found within 

NWCA vegetation plots to calculate metrics for bryophytes. From these data, an ORAM score, a 

Figure 1. All Ohio NWCA and intensification sites included in 

the study.
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Vegetation Index of Biotic Integrity (VIBI), and a recently developed vegetation analysis called the VIBIFloristic Quality (VIBI-FQ), were calculated for each site. The VIBI-FQ is a simplified analysis of vegetation 

based on two equally-weighted metrics calculated for diversity and dominance (Gara 2013). Ohio utilized 

proposed bryophyte metrics in order to explore the utility of bryophyte taxonomic group systems as a 

potential measure of ecological condition. The metrics include (1) the proposed Moss Quality Assessment 

Index (Moss QAI); (2) the number of bryophyte species; and (3) the number of bryophyte genera.

What They Found: The 2011-2014 

intensification project provided a unique 

opportunity to survey a random sample of 

wetlands in Ohio. Using the data 

collected, Ohio has completed ORAM, 

VIBI and VIBI-FQ assessments, which all 

suggest that Ohio wetland resources are 

in generally good ecological condition. 

Both VIBI and VIBI-FQ assessments 

resulted in approximately half of the 

wetlands within the “Excellent” or “Good” 

ecological condition ranges, while the 

ORAM assessment resulted in over half of 

wetlands within that upper range (Table 

1). This is higher than expected, given the 

amount of wetland loss experienced 

historically.

This study further allowed for the comparison of VIBI-FQ with results from both ORAM and VIBI for all 50 

sites. VIBI-FQ is shown to be highly correlated to both the level 2 (ORAM) and level 3 (VIBI) assessments 

that have been part of the Ohio EPA wetland regulatory program for more than 10 years (Figure 2, Figure 

3). Ohio EPA is considering making this new tool a preferred assessment technique for monitoring certain 

wetland restoration projects.

The bryophyte metrics have been calculated for all 50 surveyed sites, and preliminary results have 

shown a strong 

statistical correlation 

between the overall 

diversity of bryophytes 

present and the 

ecological condition of 

the wetland vascular 

plant community. 

Developing a 

bryophyte index of 

biotic integrity could be 

extremely beneficial to 

the state wetland 

regulatory program 

because most 

bryophytes are present 

year-round, unlike 

most other indicators 

for detailed biological 

Wetland 

Condition ORAM VIBI VIBIFQ

“Poor” 7 (14%) 11 (22%) 8 (16%)

“Fair” 15 (30%) 14 (28%) 16 

(32%)

“Good” 17 (34%) 7 (14%) 16 

(32%)

“Excellent” 11 (27%) 18 (36%) 8 (16%)

Table 1. ORAM, VIBI, and VIBI-FQ score for all Ohio NWCA 

intensification wetlands, based upon unweighted data and by 

approximate ecological condition ranges which correspond to 

Ohio’s wetland anti-degradation categories.

Figure 2. VIBI-FQ vs. ORAM.
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National Wetland Condition Assessment 2011

assessments. Ohio is still 

analyzing the soils data, 

with plans to compare 

results to those from the 

NWCA; additionally, statespecific protocol results 

will be compared with 

those generated from 

NWCA’s USA-RAM, VMMI, 

buffer stressors, etc.

The Ohio intensification of 

the NWCA has proven to 

be a valuable first step 

toward characterizing the 

state’s remaining wetland 

resources. Several “lessons 

learned” will allow Ohio to 

streamline field protocols 

when conducting future 

state surveys, and they have been considered in the context of future NWCA protocol revisions. Ohio 

hopes to replicate this probabilistic wetland survey on a regular basis in order to track temporal trends in 

both wetland quality and quantity, preferably in conjunction with future cycles of NWCA.

To learn more, contact Brian Gara (brian.gara@epa.ohio.gov; 614-836-8787), Ohio EPA, Division of 

Surface Water, Wetland Ecology Group.

References:

Dahl, Thomas E. Wetlands losses in the United States, 1780's to 1980's. Report to the Congress. No. PB91-169284/XAB. National Wetlands Inventory, St. Petersburg, FL (USA), 1990.

Gara, Brian. 2013. The Vegetation Index of Biotic Integrity “Floristic Quality” (VIBI-FQ). Ohio EPA 

Technical Report WET/2013-2. Ohio Environmental Protection Agency, Wetland Ecology Group, 

Division of Surface Water, Columbus, Ohio.

Ohio Statewide Imagery Program (OSIP). 2006-2007. Ohio Office of Information Technology, Ohio 

Geographically Referenced Information Program (OGRIP). http://ogrip.oit.ohio.gov/.

Soil Survey Staff, Natural Resources Conservation Service (NRCS), U.S. Department of Agriculture. Soil 

surveys for each Ohio County available online from 

http://soildatamart.nrcs.usda.gov/Survey.aspx?State=OH [Accessed 2009].

U. S. Fish and Wildlife Service. 2010. National Wetlands Inventory for Ohio. U.S. Department of the 

Interior, Fish and Wildlife Service, Washington, D.C. http://www.fws.gov/wetlans/.

Figure 3. VIBI-FQ vs. VIBI.
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Highlight

Minnesota’s Intensification Project: NWCA Intensification Survey Helps Reveal 

Important Regional Variation in Minnesota

Michael Bourdaghs, Minnesota Pollution Control Agency

Minnesota is known as the “land of 10,000 lakes,” but wetlands also cover 10.62 million acres of the 

state (Kloiber and Norris 2013). By any measure it is a water rich state, however, surface water resources 

(and the pressures they face) do not occur evenly throughout.

As with many other states, a variety of landscapes occur in Minnesota, with three widely recognized 

ecoregions present. As described by Omernik level II ecoregions they are (Figure 1):

 Mixed Wood Shield: Covering the northeast and 

north-central areas of the state, the Mixed Wood 

Shield is characterized by a mix of conifer and 

hardwood forests. Wetlands are extensive and 

agricultural and urban development is very low 

compared to the rest of the state, with forestry and 

mining as top industries.

 Mixed Wood Plains: This ecoregion occupies a central 

transitional zone between the drier/warmer former 

prairies to the south and west and the wetter/cooler 

northern forests. Historically, much of the ecoregion 

was covered by hardwood forests. Currently, 

agricultural development is widespread and the 

majority of Minnesota’s population is concentrated 

here.

 Temperate Prairies: Once covered by tallgrass prairie, 

oak savanna, and aspen parkland, the Temperate 

Prairies ecoregion is now predominantly used for 

agricultural production.

Across the state, approximately 50% of pre-settlement wetlands remain. However, development and 

wetland drainage history vary by ecoregion. Most of the pre-settlement wetlands remain in the Mixed 

Wood Shield, with counties in the ecoregion retaining 92% of wetland acreage on average (Anderson and 

Craig 1984). Remaining wetland acreage is much lower in the Mixed Wood Plains, with counties 

averaging 40%. Wetland losses are even greater in the Temperate Prairies, where on average only 5% of 

each county’s pre-settlement wetlands remain.

The Minnesota Pollution Control Agency took the opportunity to conduct an intensification survey in 

conjunction with the NWCA. Considering the known variation in development, drainage history, and 

wetland quantity between ecoregions, Minnesota chose a sampling approach that reduced field sampling 

Figure 1. Omernik Level II ecoregions in Minnesota.
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National Wetland Condition Assessment 2011

at individual sites and focused on vegetation condition. This allowed them to increase the number of 

sites sampled and achieve a representative sample for each of the ecoregions.

Statewide, wetland vegetation condition was high overall (Figure 2), with an estimated 49% of wetlands 

assessed as Exceptional and another 18% in Good condition. Correspondingly, degraded vegetation 

conditions were low with 23% assessed as Fair and only 10% in Poor condition. These are encouraging 

Figure 2. Wetland vegetation condition in Minnesota (statewide and by ecoregion).
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results. The clear majority of Minnesota’s wetlands have either no detectable or only minor impacts to 

their vegetation.

The ecoregion results, however, reveal a disparity of vegetation conditions (Figure 2) that correspond 

with the broad patterns of human development. Wetland vegetation condition in the largely intact 

Mixed Wood Shield was very high, with an estimated 64% in Excellent condition. Conversely, in the 

heavily developed Mixed Wood Plains and Temperate Prairies, vegetation was predominately degraded–

about 80% of wetlands were assessed as Fair or Poor.

The predominant stress indicator in these two ecoregions was non-native invasive plant species that can 

tolerate higher degrees of human impacts, out-compete native species, and produce persistent litter 

thereby altering native plant communities and driving out native species (Galatowitsch 2012). The most 

widespread non-native invasives were Reed canary grass (Phalaris arundinacea) and the invasive Cattails 

(Typha angustifolia and Typha x glauca). High non-native cover was strongly associated with all types of 

stressors estimated in the survey. For example, non-native cover was high at approximately 95% of 

wetlands that had at one time been plowed and left to revert back to natural wetland (about 14-16% of 

the wetland area in the Mixed Wood Plains and Temperate Prairie ecoregions). However, 9%-12% of the 

wetland area had moderate to high non-native species cover when all other stressor categories were low 

or absent, suggesting that non-native invasives may also be acting independent of other types of human 

impacts.

Approximately 75% of Minnesota’s wetlands occur in the Mixed Wood Shield and the high level of 

condition found there drives the statewide results, largely masking the degraded conditions in the Mixed 

Wood Plains and Temperate Prairies. While estimating overall wetland condition on a statewide basis is a 

fundamental scale for monitoring the long term status and trends of wetlands, in Minnesota, the regional 

scale provides a much more complete story.

To learn more, contact Michael Bourdaghs (Michael.Bourdaghs@state.mn.us; 651-757-2239), Minnesota 

Pollution Control Agency, Surface Water Monitoring Division.

References:

Anderson, J.P. and W.J. Craig. 1984. Growing energy crops on Minnesota’s wetlands: The land use 

perspective. University of Minnesota, Minneapolis, MN.

Galatowitsch, S.M. 2012. Why invasive species stymie wetland restoration. Society of Wetland 

Scientists Research Brief. No. 2012-0001.

Kloiber, S.M. and D.J. Norris. 2013. Status and Trends of Wetlands in Minnesota: Wetland Quantity 

Trends from 2006 to 2011. Minnesota Department of Natural Resources, St. Paul, MN.
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Interior Plains

Landscape setting of the ecoregion

The NWCA Interior Plains ecoregion combines the Northern 

Plains, Temperate Plains, and Southern Plains NARS 

ecoregions. The region extends from northern Montana to 

southern Texas and from the Rocky Mountains east to 

western Ohio.

The terrain consists of smooth and irregular plains 

interspersed with tablelands and low hills. The northern 

portion has also been shaped by glacial deposits from the last 

Ice Age. Great prairie grasslands were once a dominant 

feature of this region, but have been replaced by other 

vegetation as land was developed for other uses. Mixed prairie and forest communities occur along the 

eastern edge of the region.

The climate is generally dry and temperate. Annual precipitation ranges from 10 to 43 inches, with the 

eastern Temperate Plains being wetter than the Northern and Southern Plains. Temperatures vary more 

widely with average annual temperatures between 45°F and 79°F in the Southern Plains and 36°F and 

45°F in the Northern Plains.

Farming and livestock production is an important and dominant economic activity throughout the 

region. Mining, petroleum and natural gas production have a long history in the southern portion of the 

region and are increasingly prevalent in the north.

Terrain, climate, and land use have influenced the types of wetlands commonly found in the region 

today. In the north and central parts of the region, depressional wetlands called prairie potholes are a 

prevalent feature of the landscape. Prairie potholes typically have a mix of wetland vegetation, varying 

from submerged and floating plants found in areas 

with deeper water near the center of the pothole, to 

bulrush, cattails, and other marsh plants occurring 

along the edge. In the southern parts of the region, a 

type of depressional wetland called a playa wetland 

(also commonly known as a playa lake) is prevalent. 

Prairie potholes and playa wetlands provide 

important habitat for migrating waterfowl and other 

wildlife by, among other things, providing a source of 

water and food in these drier parts of the country. 

Prairie potholes and playa wetlands also are 

important sources of groundwater recharge. Wet 

meadows, marshes, and forested wetlands 

associated with major river systems like the Missouri 

River are also commonly found within this region.

Well known wetland complexes in the Interior Plains 

include the Prairie Pothole Region, the Sandhills and 

Prairie pothole sampled in North Dakota (site NWCA11-

ND-5032). Photo courtesy of NDSU.
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Rainwater Basin in Nebraska, the Cheyenne Bottoms in Kansas, as well as a well-defined region of playas 

that includes parts of Nebraska, Colorado, Kansas, Oklahoma, Texas, and New Mexico.

Summary of findings

A total of 156 randomly selected sites were sampled in the Interior Plains during the NWCA 2011 field 

season, representing 7,659,166 acres. Of the total number of Interior Plains sites sampled, 115 are

inland herbaceous wetlands, representing 4,598,831 acres, and 41 are inland woody wetlands, 

representing 3,060,335 acres.

Biological Condition

For all wetland types assessed in the Interior Plains (Figure 4-8), 44% of the estimated wetland area is in 

good condition; 36% is in fair condition, and 19% is in poor condition based on the VMMI. The 

proportion of wetland area in good condition for inland herbaceous wetlands is much larger, 60%, than 

it is for inland woody wetlands, where only 20% are in good condition (Figures 4-9 and 4-10). The 

proportion of woody wetlands in fair condition, however, is much larger, 59%, than it is for herbaceous 

wetlands, 21%. The proportion of area in poor condition is similar for both herbaceous (18%) and woody

(21%) wetlands.

Indicators of Stress

For all wetland types assessed in the Interior Plains, vegetation removal, surface hardening, ditching, 

and nonnative plants are the indicators with the greatest wetland area at high stressor levels (Figure 4-

8). While the majority of stressors in this ecoregion are at low levels, both vegetation removal and 

nonnative plant stressors have a greater proportion of wetland area at high and very high levels than 

low. Key findings include:

 Vegetation removal is at high stressor levels in 44% of the wetland area, moderate levels in 14%, 

and low levels in 42%.

 Hardening is high in 35% of wetland area, compared to 15% and 49% of wetland area at 

moderate or low stressor levels, respectively.

 Ditching stressor levels are high in 28% of the wetland area, moderate in 10%, and low in 62%.

 Damming is at high stressor levels in 26% of the wetland area, at moderate levels in 4%, and at 

low levels in 70%.

 The nonnative plant stressor has very high and high stressor levels in 21% and 25% of the 

wetland area, respectively, moderate levels in 26% of wetland area, and low levels in 27%.

 Soil phosphorus concentration is at low stressor levels, for most of the wetland area in the 

region, 71%. However, it is not assessed for 23% of the area due to difficulties in collecting soil 

samples or because some soil samples for this ecoregion were not analyzed using NWCA lab 

protocols. Similarly, heavy metal concentrations are at low stressor levels for 61% of wetland 

area, but is not assessed for 23% of the area.

For inland wetland types assessed within the Interior Plains ecoregions (Figure 4-9 and 4-10), data show:

 Inland herbaceous wetlands have several indicators at high stressor levels for large proportions 

of assessed wetland area. These include hardening (56% of wetland area), vegetation removal 

(55%), ditching (41%), and vegetation replacement (30%). In addition, 63% of inland herbaceous 

wetlands have high or very high levels for the nonnative plant stressor.
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 The indicators of stress with the largest proportion of wetland area at high stressor levels for 

inland woody wetlands are damming (37%) and vegetation removal (28%). Wetland area for 

woody wetlands at high or very high levels for the nonnative plant stressor is 20%.

 For most of the stressors measured, a greater proportion of herbaceous wetland area has high 

stressor levels relative to the woody wetlands. The exception is the damming stressor, which is

at high levels in 37% of woody wetlands compared to 18% of herbaceous wetlands.

 While the majority of herbaceous wetlands have low stressor levels for soil phosphorus (55% of 

wetland area) and heavy metals (60%), it should be noted that 38% of the wetland area was not 

assessed due to difficulties in collecting soil samples or because some soil samples for this 

ecoregion were not analyzed using NWCA lab protocols.

Figure 4-8. NWCA 2011 survey results for the wetlands (i.e., all target wetland types) across the Interior 

Plains. Bars show the percentage of wetland area within a condition or stressor class. Error bars represent 95% confidence 

intervals.
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Figure 4-9. NWCA 2011 survey results for the inland herbaceous wetland type across the Interior Plains. Bars 

show the percentage of wetland area within a condition or stressor class. Error bars represent 95% confidence intervals.

Figure 4-10. NWCA 2011 survey results for the inland woody wetland type across the Interior Plains. Bars show 

the percentage of wetland area within a condition or stressor classes. Error bars represent 95% confidence intervals.
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West Ecoregion

Landscape setting of the ecoregion

The NWCA West ecoregion combines the Western Mountain 

and Xeric NARS ecoregions. The region includes the western 

parts of Texas, New Mexico, Colorado, Wyoming, and 

Montana to the Pacific Coast.

The West ecoregion is topographically diverse, including large 

extensive mountain ranges, plateaus and high-relief 

tablelands, lowland plains with hills and low mountains, 

isolated mountains, and intermountain basins and valleys. 

Coastal mountains are bordered by coastal plains and include 

important estuaries along the margins of the Pacific Ocean. 

The topographic and climatic diversity of the West ecoregion results in diverse plant communities. In the 

drier parts of the region, native vegetation is dominated by grasses and shrubs, with relatively few large 

trees. Desert and shrub-steppe ecosystems are found in the rain shadow of large mountain ranges, but 

within the mountain ranges, vegetation communities include alpine tundra, mountain meadows, valley 

grasslands, shrublands, and hardwood riparian systems. Foothills and mountain ranges are often 

dominated by expansive forests.

The climate varies widely across the West ecoregion. In the Xeric region, conditions tend to be hot and 

dry with a long summer dry season. Average annual temperatures range from 32 to 75°F. Average 

annual precipitation throughout the region ranges from 2 to 40 inches but varies widely both spatially 

and temporally. The Western Mountain region tends to be cooler and more humid than the Xeric region, 

with average annual temperatures ranging from 32 to 55°F and average annual precipitation ranging

from 16 to 240 inches. At higher elevations, most precipitation falls as snow.

Much of the region is federally-owned land, some of which is used for recreation purposes. Grazing is 

widespread and timber production is a leading industry in forested areas. Agricultural production varies 

widely across the region with climate—ranging from citrus, subtropical, and tropical fruits to vegetables 

and horticultural crops, to irrigated and dry-farmed 

grain and forage crops.

Wetlands comprise a relatively small, but important, 

proportion of land area in the West, and vary widely 

with geography and climate. They support a variety 

of habitats, including the salmon fishery, and are key 

contributors to water supplies, especially in the drier 

regions. In relatively dry basins and valleys, wetlands 

are concentrated along rivers, streams, and in 

abandoned river channels. Additionally, wet 

meadows and emergent marsh complexes are found 

in large basins and valleys. Snowmelt- and 

groundwater-fed wetlands such as wet meadows, 

fens, seeps, and forested wetlands are found in 

mountain ranges. Tidal salt and freshwater marshes, 

wet meadows and forests are found along the coast.

Wet meadow along riparian corridor in Yellowstone 

National Park, WY (site NWCA11-2790). Photo courtesy 

of Colorado Natural Heritage Program.
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Large estuaries, such as the San Francisco Bay and Puget Sound, are familiar to many people, but 

extensive wetland complexes are also associated with major rivers and lakes throughout the region 

(e.g., the Colorado, Columbia, Sacramento, and Snake Rivers, the Great Salt Lake).

Summary of findings

A total of 146 randomly selected sites were sampled in the West ecoregion during the NWCA 2011 to 

characterize the condition of 3,647,060 acres. Of the total number of West sites sampled, 70 are inland

herbaceous wetlands, representing 1,488,139 acres, and 51 are inland woody wetlands, representing 

1,985,936 acres. Estuarine wetlands in the West total 25 sites, representing 172,985 acres. Detailed 

results for estuarine wetland types are reported in the Estuarine Wetlands section of this chapter.

Biological Condition

For all wetland types assessed in the West (Figure 4-11), 21% of the estimated wetland area is in good 

condition; 18% in fair condition, and 61% in poor condition based on the VMMI (see Chapter 2 for 

details on the VMMI). Inland herbaceous wetlands have an estimated 25% of wetland area in good 

condition, 32% in fair condition, and in 43% poor condition (Figure 4-12). Inland woody wetlands have

an estimated 21% of wetland area in good condition, 8% in fair condition, and 71% in poor condition 

(Figure 4-13).

Indicators of Stress

For all wetland types assessed in the West, ditching, nonnative plants, surface hardening, and 

vegetation removal are the indicators with the greatest wetland area at high stressor levels (Figure 4-

11). Each of these stressors have a greater proportion of wetland area at high levels than low. Key 

findings include:

 Ditching is at high stressor levels for 76% of wetland area, moderate for 1% and low for 22%.

 The nonnative plant stressor is at very high or high stressor levels for 72% of the wetland area, 

and at moderate and low levels for 15% and 14%, respectively.

 Hardening stressor levels are high for 70% of wetland area, moderate for 7%, and low for 23%.

 Vegetation removal stressor levels are high for 61% of the wetland area, moderate for 6%, and 

low for 33%.

 Damming is at high stressor levels for 32% of wetland area, moderate for 1%, and low for 67%.

For inland wetland types assessed within the West ecoregion (Figures 4-12 and 4-13), data show:

 The indicators of stress with the greatest percent wetland area at high levels for inland 

herbaceous wetlands are ditching (78% of wetland area), hardening (70%), damming (61%), and 

vegetation removal (47%). High or very high levels for the nonnative plant stressor total 73% of 

wetland area.

 The indicators of stress with the greatest percent wetland area at high levels for inland woody 

wetlands are vegetation removal (76% of wetland area), ditching (75%), and hardening (75%). 

High or very high levels for the nonnative plant stressor total 74% of wetland area.

 Inland herbaceous and woody wetland area have similar percentages of wetland area at high 

stressor levels for ditching, hardening, and nonnative plant indicators. Both wetland types also 

have large percentages of wetland area at low stressor levels for vegetation replacement, 

filling/erosion, and soil phosphorus indicators.
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 Inland woody wetlands have a greater proportion of wetland area at high stressor levels for 

vegetation removal, 78%, than inland herbaceous wetlands, which have 47%.

 High stressor levels for damming are found in 61% of inland herbaceous wetland area, but only 

10% of inland woody wetland area.

 The majority of inland herbaceous wetlands have low stressor levels for heavy metals (80% of 

wetland area), which is greater than the percentage for inland woody wetlands. Among woody 

wetlands, only 20% of wetland area have low stressor levels, while 69% have moderate and 4% 

have high stressor levels for the heavy metals indicator.

Figure 4-11. NWCA 2011 survey results for the wetlands (i.e., all target wetland types) across the West. Bars 

show the percentage of wetland area within a condition or stressor class. Error bars represent 95% confidence intervals.
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Figure 4-12. NWCA 2011 survey results for the inland herbaceous wetland type across the West. Bars show the 

percentage of wetland area within a condition or stressor class (good, fair, and poor. Error bars represent 95% confidence 

intervals.

Figure 4-13. NWCA 2011 survey results for the inland woody wetland type across the West. Bars show the 

percentage of wetland area within a condition or stressor class. Error bars represent 95% confidence intervals.
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Estuarine Wetlands

Landscape setting for estuarine wetlands

Estuarine wetlands occur along the coastal areas of 

the conterminous U.S. They can be found in three of 

the NWCA Ecoregions: Coastal Plains, Eastern 

Mountains and Upper Midwest, and the West.

Estuarine wetlands—tidal systems that are saline or 

brackish—may be dominated by herbaceous 

emergent vegetation or by shrubs or trees. Estuarine 

wetlands occur predominantly along coastal areas of 

the Coastal Plains Ecoregion, while fewer estuarine 

wetlands occur along the west and northeastern 

coasts of the U.S. Because few NWCA 2011 sites 

occurred in western and northeastern estuarine 

wetlands, results for estuarine wetlands are 

presented only by wetland type—estuarine 

herbaceous and estuarine woody.

Tidal salt and brackish marshes are found along the 

nation’s coasts. Shrub/tree dominated wetlands are 

often located in high intertidal zones along coasts 

where they are exposed to less saltwater and there 

are greater influxes of freshwater from the 

surrounding landscape. In the most southern part of 

the Eastern coast, mangrove swamps are found in 

association with coastal marshes.

Summary of findings

The 327 randomly selected estuarine wetland sites that were sampled in the NWCA represent an 

estimated 5,485,646 acres. Estuarine wetlands are reported separately for herbaceous and woody 

types. Estuarine herbaceous wetlands (salt marshes) are evaluated based on 258 randomly selected 

sites, which represented an estimated 4,987,824 acres. Estuarine woody wetlands (deciduous or 

evergreen woody dominated wetlands, mangrove swamps) represent a smaller proportion of the 

wetland area, 497,821 acres, based on evaluation of 69 sites.

Biological Condition 

Estuarine herbaceous wetlands have an estimated 58% of wetland area in good condition, 17% in fair 

condition, and 26% in poor condition (Figure 4-14) based on the VMMI (see Chapter 2 for details). 

Estuarine woody wetlands have an estimated 59% of wetland area in good condition, 20% in fair 

condition, and 22% in poor condition (Figure 4-15).

Indicators of Stress

Stressor level is generally low for all indicators of stress for both estuarine herbaceous and estuarine 

woody wetlands. For the estuarine herbaceous wetlands (Figure 4-14) the indicators of stress with the 

greatest estimated wetland area at high stressor levels are ditching (18%), surface hardening (11%), and 

Salt marsh in Florida (site NWCA11-3069). Photo 

courtesy of University of Florida.
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damming (10%). In addition, high or very high levels for the nonnative plant stressor total 24% of 

estuarine herbaceous wetland area. Soil phosphorus stressor levels are low for 37% of herbaceous 

wetland area, moderate for 28%, and high for 2%. Soil phosphorus and soil heavy metal stressors are

not assessed for 33% of the wetland area, due to difficulties collecting soil samples at some sites.

In the estuarine woody wetlands (Figure 4-15), the indicators of stress with the greatest estimated 

wetland area at high stressor levels are ditching (18% of wetland area) and hardening (13%). The heavy 

metals stressor is at low levels for 44% of wetland area, but at moderate levels for 55%. Less than 1% of 

estuarine woody wetland area is at high stressor levels for heavy metals.

Figure 4-14. NWCA 2011 survey results for estuarine herbaceous wetland types across all coastal areas of the 

conterminous U.S. Bars show the percentage of wetland area within a condition or stressor class. Error bars represent 95% 

confidence intervals.
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Figure 4-15. NWCA 2011 survey results for estuarine woody wetland types across all coastal areas of the 

conterminous U.S. Bars show the percentage of wetland area within a condition or stressor class. Error bars represent 95% 

confidence intervals.
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Chapter 5: Summary and Next Steps

Summary of Major Findings and Implications

The NWCA 2011 is the first national assessment of wetland ecological condition. This accomplishment

required an extraordinary amount of effort and cooperation between state, tribal, and federal partners

throughout its design and implementation. During the 2011 field season, more than 50 field crews 

sampled 1,179 wetland sites across the country, using standardized protocols to collect information to 

characterize wetland condition. Previous wetland monitoring and assessment studies have been 

conducted at local and regional scales, or have focused on specific wetland types or ecological 

properties, but none have evaluated wetland condition for a full range of wetland types across the 

entire country. Thanks to the efforts of the field crews and many other partners, the NWCA collected 

the most comprehensive set of biological, physical, and chemical data on wetlands across the U.S. This 

national data set will provide valuable and previously unavailable information on the ecological 

condition of a broad range of wetlands to policy makers, land managers, and scientists. This includes

important insight on wide-spread stressors impacting wetland biological condition, and the potential 

improvement that could be seen nationally by reducing these stressors.

The NWCA 2011 found 48% of wetland area is in good condition, based on the national Vegetation 

Multimetric Index (VMMI) developed for NWCA, while 32% of wetland area is in poor condition. Of the 

four major ecoregions reported on by NWCA, the West had the lowest percentage of wetland area in 

good condition at 21%. The Coastal Plains (50%), Eastern Mountains and Upper Midwest (52%), and 

Interior Plains (44%) have similar percentages of area in good condition as the national estimates.

Nationally, vegetation removal, surface hardening (e.g., pavement, soil compaction), and ditching are

the most widespread of the indicators of stress evaluated in NWCA. Vegetation removal and surface 

hardening stressor indicators are high for 27% of wetland area, while the ditching stressor is high for 

23% of wetland area. NWCA 2011 further found that wetlands with high stressor levels from vegetation 

removal and surface hardening are about twice as likely to have poor biological condition as those with 

low or moderate levels. Additional analysis that looks at how condition might improve if these two 

stressors are reduced, called attributable risk, suggests a possible 20% reduction in wetland area with 

poor biological condition if the stressor level was reduced from high to moderate or low.

Stressor levels for both of the soil indicators of chemical stress are low for the majority of wetland area 

nationally. However, moderate stressor levels for heavy metals are found in 47% of wetland area in the 

West and 31% of wetland area in the Eastern Mountains and Upper Midwest. Soil phosphorous stressor 

levels are also moderate or high for 22% and 13% of wetland area, respectively, in the Eastern 

Mountains and Upper Midwest.

NWCA conducted the first national study of algal toxins in wetlands. Microcystin, a toxin that can harm 

people, pets, and wildlife, was detected in 12% of wetland area nationally. However, based on 

recreational exposure risk levels established by the World Health Organization, very little wetland area is

found to be at either moderate or high risk levels.

Nationally, 61% of wetland area has low stressor levels from the nonnative plant indicator, but stressor 

levels varied by ecoregion. In the West, the majority of wetland area, 71%, has high or very high stressor 

levels from the nonnative plant indicator. In the Interior Plains, nearly half of the wetland area (46%) has

high or very high stressor levels.
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Advancing Wetland Science

The contributions of the NWCA go beyond the development of the first national assessment of wetland 

condition. EPA, states, tribes, and other federal agency efforts to implement the NWCA resulted in a 

robust national program and contributed to the development and enhancement of state and tribal 

wetland monitoring programs. For example, Minnesota used vegetation data from a larger sample set 

within the state to identify regional variations in wetland condition and potential stressors (see state 

highlight, “Minnesota’s Intensification Project: NWCA Intensification Survey Helps Reveal Important 

Regional Variation in Minnesota”).).

The NWCA 2011 survey led to the first national vegetation multimetric index developed for wetlands. 

This index provided an assessment of the biological condition of wetlands nationally, and the methods 

and assessment tools can also be used at a regional level. In addition, several indicators of stress based 

on readily collected field data were developed and used to evaluate the relationship between common 

stressors and biological condition. Research into a number of other potential indicators of wetland 

condition was conducted, and while not part of this report, will help inform future scientific studies.

NWCA scientists, for example, attempted to develop indicators of wetland condition based on the 

presence, abundance, and diversity of algal species, recognizing their role in wetland ecology. The 

difficulty in collecting uniform samples and identifying species presence limited the development of such 

an indicator. However, the data collected provides valuable information to scientists to further research 

relationships between algal communities and wetland ecosystem health.

Water chemistry has been widely used in monitoring and assessment programs in aquatic habitats. 

While the NWCA collected water chemistry data from 631 sites, its use was somewhat limited in the 

NWCA 2011 because of the variability in surface water presence, both within a wetland and among 

wetland types. Future efforts will focus on protocol improvements that would allow for better and more 

complete sampling of wetland surface and/or subsurface water. Additionally, analysis of data collected 

in 2011 may reveal relationships between water quality and other measures of wetland condition and 

ecosystem function that can be further developed in future studies.

Rapid assessment methods (RAMs) are widely used at state and regional levels to evaluate wetlands and 

play a key role in the implementation of many state wetland monitoring and assessment programs. As 

part of the NWCA, EPA developed and tested a national rapid assessment method named USA-RAM. 

This was an integral component of the methods used in the 2011 survey. Initial analysis of the data 

indicates that USA-RAM provided measures of wetland stress and condition that correlated with several 

individual metrics from the more intensive assessment methods used in the NWCA. Further research 

into the data is needed to fully understand how the rapid assessment methods work across regions and 

wetland types and to verify and refine USA-RAM. Many states are conducting their own field 

assessments of the USA-RAM protocols, and adapting these protocols to meet state-specific wetland 

assessment and management needs.

Microbes play an essential role in the breakdown of organic carbon compounds in soils. Their activity is 

of particular interest in wetland soils, which have the capacity to store (or sequester) large amounts of 

carbon. Carbon storage or release occurs through microbial respiration and is affected by a number of 

factors including hydrology, climatic conditions, and the microbial community. Amid growing concerns 

regarding greenhouse gases and climate change, scientists are interested in better understanding the 

factors that affect carbon cycling in wetlands and their role in the global carbon cycle. As part of the 

NWCA 2011, soil samples from 936 wetland sites across the U.S. were analyzed for microbial enzyme
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activity and respiration. By analyzing the specific types of enzymes present and their relative proportions 

in the soil, researchers hope to identify nutrient imbalances in the wetland that may limit microbial 

growth. These data will also be used to assess the impact of other factors, such as water and soil 

chemistry, land use and related anthropogenic stressors, and atmospheric deposition of nutrients, on 

microbial growth. Scientists also hope to use this information to estimate rates of carbon decomposition 

and how changes in environmental factors might change microbial communities and decomposition 

rates.

As part of NWCA 2011, analysts measured carbon concentrations in the soil, which provides important 

information on the amount of carbon that is stored in wetland soils. This is helpful as scientists work to 

understand carbon cycling in various wetland types and regions. For instance, scientists will be using soil 

data collected for NWCA 2011 to inform the development of baseline estimates of carbon storage in 

coastal wetlands as they work to include coastal wetlands into the U.S. Greenhouse Gas Emissions and 

Sinks Inventory under the Intergovernmental Panel on Climate Change. This report is prepared annually 

by the EPA and cooperating agencies to track greenhouse gas emissions and sinks associated with 

anthropogenic activities and land uses. Coastal wetlands (i.e., tidal marshes, mangroves, and sea 

grasses) have an important role in greenhouse gas cycling, however they have not yet been included in 

the inventory.

NWCA is an initial step in our endeavors to assess wetland condition. As the first survey of its kind, the 

2011 study identified biological, chemical, and physical indicators of condition and stress for wetlands 

and developed appropriate metrics to assess ecological condition at national and regional scales. The 

indicators measured provide new information regarding the health of wetlands nationally, and on 

relationships between indicators of ecosystem stress or disturbance and wetland condition. Subsequent 

studies and research by EPA, states, and other partners will continue to build upon the knowledge 

resulting from the NWCA and allow us to further explore and evaluate the condition of wetlands at 

multiple scales. We will be better able to answer important policy and management questions about the 

overall health of this critical resource, and design effective strategies to fulfill the objectives of the CWA.

Next Steps: Preparing for the 2016 Assessment

Plans are underway for the NWCA 2016. NWCA scientists are using the findings of the 2011 survey to 

guide preparations for the next round of the national assessment. NWCA methods will be further 

refined and developed with the goal of increasingly effective and efficient assessment of national 

wetland condition.

As an example, a number of different physical, chemical, and biological measures indicative of soil 

quality were collected as part of the 2011 survey and were used to indicate potential stressors. Soils are 

an important component of wetland ecosystems and the NWCA is interested in further exploring how 

these indicators could be used to make assessments about soil health in wetlands. Using both the data 

collected and the insights of research scientists and 2011 field crews, the NWCA is working to develop 

new methods and further refine existing protocols that will assess the condition of wetland soils and link 

soil properties to other measures of wetland health.

The NWCA would not have been possible without the assistance and collaboration of hundreds of 

dedicated scientists working for state, federal, and tribal agencies and universities across the country. 

These scientists helped plan and design the survey, select indicators, develop and pilot assessment 

protocols, train field crews, conduct sampling, track samples, review data for quality control, analyze 

data, and review and write up the findings. Future wetland surveys will continue to rely on this close 
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collaboration between EPA and its partners, as we further develop our abilities to study and assess 

wetlands at multiple scales. We will also continue to build upon the considerable baseline of information 

on wetland condition and work to ensure its use in evaluating our progress in protecting and restoring 

the quality of our nation’s wetlands.
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Highlight

The U.S. Fish and Wildlife Service’s Wetlands Status and Trends Program

Mitch Bergeson, U.S. Fish and Wildlife Service

The U. S. Fish and Wildlife Service (FWS) is the principal Federal agency that provides information to the 

public on the extent and status (quantity and type) of the Nation’s wetlands. The Wetlands Status and 

Trends (S&T) component has had a history of success in providing scientific information to resource 

managers and decision makers about wetlands resource trends. The scientific integrity of the Wetlands 

S&T study is unchallenged as it represents the most comprehensive and contemporary effort to track 

wetlands acreage on a national scale. The information in the National Wetlands S&T Report and the 

NWCA Report will complement each other providing the most comprehensive nation-wide picture of 

wetland resources.

In 1986, the Emergency Wetlands Resources Act (Public Law 99-645) was enacted to promote the 

conservation of our Nation’s wetlands. Congress recognized that wetlands are nationally significant 

resources and that these resources have been affected by human activities. Under the provisions of this 

Act, Section 401 requires the FWS to conduct wetland S&T studies of the Nation’s wetlands at periodic 

intervals. Reports on the S&T of wetland area were produced by the FWS in 1983/84, 1990, 1991, 2000, 

2006, 2008, 2011 and 2013.

The goal of Wetlands Status and Trends is to provide the Nation with current scientifically valid 

information on the status and extent of wetland, riparian and related aquatic resources and 

monitor trends of these resources over time.

The S&T studies have provided the nation with information on wetland quantity. The FWS worked closely 

with the EPA in preparation for the NWCA study which was designed to address the quality of wetlands 

across the nation. This partnership arose because the FWS’s Wetlands S&T data set offered one of the 

best starting points for a probabilistic national wetlands sampling design. This sample design included a 

population of stratified random sample plots across the nation that contained wetlands defined with 

Cowardin et al. (1979) wetland classes. These two studies will prove to be invaluable tools by providing 

resource managers, agencies and the public with information on both quantity and quality of wetlands of 

the Nation.

Wetland Definition and Classification: The FWS uses the Cowardin et al. (1979) definition of wetland. This 

definition is the standard for the agency and is the national standard for wetland mapping, monitoring, and 

data reporting as determined by the Federal Geographic Data Committee and is separate from the 

definition of a wetland found in Section 404 of the CWA regulatory program. The Cowardin et al. definition 

of wetlands is described 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 non-soil and is saturated with water or 

covered by shallow water at some time during the growing season of each year.
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As noted in this definition, plant community composition, soil morphology, and site wetness (hydrology) 

are the principal indicators of whether a site is a wetland for ecological purposes. Site wetness (i.e., the 

presence of water) while central to the concept of wetland, is often the most difficult indicator to assess 

accurately because it is more dynamic (temporally variable) than plant community composition or soil 

properties. Plants and soil tend to reflect the prevailing degree of wetness at a site over time. For this 

reason, they frequently are excellent indicators of relative wetness, and this is why they are listed first as 

indicators of wetlands.

Ephemeral waters, which are not recognized as a wetland type, and certain types of “farmed wetlands” 

as defined by the Food Security Act and that do not coincide with the Cowardin et al. definition were not 

included in this study. The definition and classification of wetland types are consistent between every 

S&T study conducted by the FWS and the focus of reporting is on the Nation’s wetlands regardless of 

ownership. Federal, state, and local regulatory agencies with jurisdiction over wetlands may define and 

describe wetlands in a different manner than that used in the Wetland S&T studies.

Wetland Classification Applications: The FWS has made adaptations to the Cowardin classification 

system to accommodate the use of remotely sensed imagery as the primary data source. For example, 

water chemistry, water depth, substrate size and type and even some differences in vegetative species 

cannot always be reliably ascertained from imagery. Image analysts must rely primarily on physical or 

spectral characteristics evident on high altitude imagery, in conjunction with collateral data, to make 

decisions regarding wetland classification and deepwater determinations3.

The delineation of wetlands and deepwater habitat features through image analysis forms the 

foundation for deriving all subsequent products and data results. The wetlands are interpreted from the 

image by photo interpreters using key concepts of tone, size, shape, texture, pattern, shadow, location 

and association. The FWS makes no attempt to adapt or apply the products of these techniques to 

regulatory or legal authorities regarding wetland boundary determinations, jurisdiction or land 

ownership, but rather uses the information to assist in making trends estimates characterizing wetland 

habitats.

Study Design: The S&T studies were designed to be a quantitative measure of the areal extent of all 

wetlands in the conterminous U.S. The approach used is a stratified random sampling of plots. These are 

examined, with the use of remotely sensed data in combination with field work, to determine wetland 

change.

To monitor changes in wetland area, the 48 conterminous U.S. are stratified or divided by state 

boundaries and 37 physiographical subdivisions described by Hammond (1970) and shown in Figure 1. 

Zone 36 was added by the FWS to include coastal wetlands and nearshore features. In 2008, Zone 37 was 

added to intensify the coastal wetlands along the Pacific coast of conterminous U.S.

To permit even spatial coverage of the sample and to allow results to be computed easily by sets of 

states, the 37 physiographic regions formed by the Hammond subdivisions and the coastal zone stratum 

are intersected with state boundaries to form 220 subdivisions or strata. An example of this stratification 

approach and the way it relates to sampling frequency is shown for Georgia (Figure 2).

Within the physiographic strata described above, weighted, stratified sample plots are randomly 

allocated in proportion to the amount of wetland acreage expected to occur within each stratum. Each 

sample area is a plot 2 miles (3.218 km) on a side or 4 square miles of area equaling 2,560 acres (1,036 

ha). The study includes all wetlands regardless of land ownership.

 

3

Analysis of imagery is often supplemented with limited field work and ground observations.
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The advantages to this design are that it was developed by an interagency group of spatial sampling 

experts specifically to monitor wetland changes. It can be used to monitor conversions between 

ecologically different wetland types, as well as, measure wetland gains and losses.

All habitats, including wetlands, uplands and deepwater, were mapped in each plot using imagery at two 

different dates. The dates of imagery were selected based on availability of imagery closest to the start 

and end dates of the sample period for each study. All wetland change was also recorded whether it was 

considered the result of either natural change, such as the natural succession of emergent wetlands to 

shrub wetlands, or human induced change. The analysis of this data provides accurate estimates of 

wetland acreage or status at the start and end of the sample period and also provides estimates of 

observed changes over time by wetland type.

Data from S&T studies provide important long-term trend information about specific changes and the 

overall status of wetland quantity in the U.S. The FWS has documented this information by producing six 

national reports. With the release of the EPA’s NWCA report the nation will have documented 

information on the quality and condition of these same wetlands. These reports, used in conjunction, will 

provide Federal and State agencies, the scientific community and conservation groups information on 

both quantity and quality of the Nation’s wetlands to assist in planning, decision making and wetland 

policy formulation and assessment.

To learn more, contact Mitch Bergeson (mitch_bergeson@fws.gov, 608-238-9333 ext 112), U.S. Fish and 

Wildlife Service

Figure 1. Physiographic regions of the conterminous U.S. as used for stratification in the Wetlands S&T study 

(adapted from Hammond 1970).
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References:

Cowardin, L.M, V. Carter, F.C. Golet and E.T. LaRoe. 1979. Classification of wetlands and deepwater 

habitats of the United States. Department of the Interior. U.S. Fish and Wildlife Service, Washington, 

D.C. 131 p.

Dahl, T.E. 1990. Wetlands losses in 

the United States 1780s to 

1980s. Department of the 

Interior, U.S. Fish and Wildlife 

Service, Washington, D.C. 21 p.

Dahl, T.E. and C.E. Johnson. 1991. 

Status and trends of wetlands in 

the conterminous United States, 

mid-1970s to mid-1980s. U.S. 

Department of the Interior. U.S. 

Fish and Wildlife Service, 

Washington, D.C. 28 p.

Dahl, T.E. 2000. Status and trends of 

wetlands in conterminous 

United States 1986 to 1997. U.S. 

Department of the Interior, Fish 

and Wildlife Service, 

Washington, D.C. 82 p.

Dahl, T.E. 2006. Status and trends of 

wetlands in the conterminous 

United States 1998 to 2004. U.S. 

Department of the Interior, Fish 

and Wildlife Service, Washington, D.C. 112 p.

Dahl, T.E. 2011. Status and trends of wetlands in the conterminous United States 2004 to 2011. U.S. 

Department of the Interior, Fish and Wildlife Service, Washington, D.C. 108 p.

Dahl T.E. and S.M. Stedman. 2013. Status and trends of wetlands in the coastal watersheds of the 

Conterminous United States 2004 to 2009. U.S. Department of the Interior, Fish and Wildlife Service 

and National Oceanic and Atmospheric Administration, National Marine Fisheries Service. 46 p.

Frayer, W.E., T.J. Monahan, D.C. Bowden, and F.A. Graybill. 1983. Status and trends of wetlands and 

deepwater habitats in the conterminous United States, 1950’s to 1970’s. Colorado State University, 

Fort Collins, CO. 31 p.

Hammond, E.H. 1970. Physical subdivisions of the United States of America. In: U.S. Geological 

Survey. National Atlas of the United States of America. Department of the Interior, Washington, 

D.C. 61 p.

Stedman, S.M. and T.E. Dahl. 2008. Status and trends of wetlands in the coastal watersheds of the 

Eastern United States 1998 to 2004. National Oceanic and Atmospheric Administration, National 

Marine Fisheries Service and U.S. Department of the Interior, Fish and Wildlife Service. 32 p.

Tiner, R.W. Jr. 1984. Wetlands of the United States: Current status and recent trends. Department of 

the Interior. U.S. Fish and Wildlife Service. Washington, D.C. 59 p.

Figure 2. Physiographic strata in Georgia.
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Glossary of Terms

Anthropogenic: Made by people or resulting from human activities. Usually used in the context of 

environmental impacts that are a result of human activities.

Assessment Area: A 0.5 hectare area that represents the wetland sampling point where data were 

collected for the NWCA.

Attributable Risk: An estimate of the proportion of the population in poor biological condition that could 

be reduced if the effects of a particular stressor were eliminated.

Buffer: A defined area immediately adjacent to and surrounding the NWCA Assessment Area, extending 

100 meters from the Assessment Area.

Coefficient of Conservatism (C-value): An assigned value describing the tendency of an individual plant 

species to occur in disturbed versus pristine conditions. Values are state or regionally specific and scaled 

from 0 (widespread, generalist species that thrive under disturbed conditions) to 10 (occur in specific 

habitats that are minimally disturbed).

Condition: The ecological state of a wetland. The NWCA used indicators of condition, such as the 

Vegetation Multimetric Index, to describe ecological condition.

Conterminous United States: The United States exclusive of Alaska and Hawaii.

Cowardin Classification System: A national wetland classification system developed by the U.S. Fish and 

Wildlife Service (FWS) to describe ecological units with similar natural attributes and provide uniform 

concepts and terminology for describing and mapping wetlands and deepwater habitats. The units are 

arranged in a hierarchal system to aid resource management decisions.

Cowardin Definition of a Wetland: 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. Wetlands 

categorized using the Cowardin System must have one or more of the following three attributes: (1) at 

least periodically, the land supports predominantly hydrophytes (i.e., plants that have adapted to living 

in saturated conditions); (2) the substrate is predominantly undrained hydric soils; and (3) the substrate 

is non-soil and is saturated with water or covered by shallow water at some time during the growing 

season of each year. This biologically-based definition is used by the NWCA to define the target 

population or group of wetlands of the survey.

Damming Stressors: Indicators of stressors that cause hydrologic alterations in a wetland by impounding 

or impeding water flow from or within the wetland. Stress was measured based on observations of 

features or activities that could restrict water flow from a site, such as dikes, dams, berms, or railroad 

beds.

Ditching Stressors: Indicators of stressors that cause hydrologic alterations in a wetland by draining 

water within the wetland. Stress was measured based on observations of features or activities that 

potentially drain water from a site, such as ditches, corrugated pipe, excavation, or dredging.
Glossary of Terms

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Ecoregion: Geographical areas that are similar in climate, vegetation, soil type, and geology. Water 

resources within a particular ecoregion have similar natural characteristics and similar responses to 

stressors.

Ecosystem services: The direct and indirect benefits that people, society, and the economy receive 

through the goods and services provided by nature.

Estuarine Ecosystem: As defined by the Cowardin system, deep water tidal habitats and adjacent tidal 

wetlands that are usually semi-enclosed by land but have open, partly obstructed, or sporadic access to 

the open ocean, and in which ocean water is at least occasionally diluted by freshwater runoff from the 

land.

Eutrophication: The enrichment of water bodies by inorganic plant nutrients (e.g., nitrogen, 

phosphorus). It may occur naturally but can also be the result of human activity (e.g., fertilizer runoff,

sewage discharge).

Filling/Erosion Stressors: Indicators of stressors that cause hydrologic alterations in a wetland by 

removing or depositing soil or sediment. Stress was measured based on observations of features caused 

by erosion or deposition, such as soil loss, root exposure, fill/spoil banks, or freshly deposited sediment.

Floristic Quality Assessment Index (FQAI): A two metric calculation that captures information about plant 

community composition based upon the total number of unique plant species at a given site, known as 

species richness (diversity), and the tolerance of each species to human-mediated disturbance. One of 

four metrics included in the Vegetation Multimetric Index.

Geomorphology: The science and study of landforms on the Earth’s surface, their evolution over time, 

and the interpretation of landforms as a record of geologic history.

Hardening Stressors: Indicators of stressors that cause hydrologic alterations in a wetland by prohibiting 

or restricting the movement of water or air into or through the soil. Stress was measured based on 

observations of activities that result in surface hardening or compaction, such as parking lots, suburban 

residential development, or roads.

Heavy Metal Index: Indicator of stressors that can cause elevated heavy metal concentrations in wetland 

soils. The index evaluated concentrations of 12 heavy metals associated with anthropogenic activities 

relative to their natural background concentrations in soils.

Hydrogeomorphic (HGM) Classification System: A wetland classification system developed by the U.S. 

Army Corps of Engineers that describes and categorizes wetlands based on geomorphic setting 

(landscape location), water source(s), and hydrodynamics.

Hydrodynamics: The movement of groundwater and surface water.

Hydrology: The movement, distribution, and physical and chemical characteristics of surface and 

subsurface water.

Hydrophytes: Plants that have adapted to living in saturated conditions.

Indicators of Condition: Physical, chemical, and biological factors that describe the ecological condition 

of a wetland. The NWCA uses a Vegetation Multimetric Index as a biological indicator of condition.
Glossary of Terms

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Index: A combination of metrics used to generate a single score evaluating the condition or prevalence 

of stressors at a site.

Indicators of Stress (Stressors): Physical, chemical, and biological factors that can result from, and 

therefore, can be used to identify stressors or anthropogenic activities that have impacted wetland 

condition.

Least-disturbed: A disturbance class used to describe sites that represent the best available physical, 

chemical, and biological habitat conditions in the current state of the landscape within an NWCA 

Reporting Group; represents approximately 15-25% of the sites within a Reporting Group and used as 

Reference Condition for the purposes of the NWCA Survey.

Metric: An individual measure of a particular property used to evaluate condition or stressors at an 

individual site.

Microcystin: A potentially toxic substance produced by cyanobacteria (a group of microbes also called 

blue-green algae).

Monocot: One of two groups of flowering plants characterized by seedlings that have one seed-leaf (e.g., 

grasses, sedges, rushes, lilies, irises, and orchids).

Most-disturbed: A disturbance class used to describe sites that have the worst physical, chemical, and 

biological habitat condition in the current state of the landscape within an NWCA Reporting Group; 

represents approximately 20-30% of the sites within a Reporting Group.

Native Plant Species: Plant taxa that are indigenous to the state in which they occur.

Nonnative Plant Species: Plant taxa that are indigenous or native to areas outside of the U.S., but not 

indigenous to the location of occurrence (introduced), or species that are indigenous or native to some 

areas or states in the conterminous U.S., but not indigenous to the location of occurrence (adventive).

Nonnative Plant Stressor Indicator (NPSI): Indicator of stress caused by the presence of nonnative plants 

in a wetland. The Index is comprised of three metrics: Relative Cover of Nonnative Species, Richness of 

Nonnative Species, and Relative Frequency of Occurrence of Nonnative Species.

Number of Plant Species Tolerant to Disturbance: The number of plant species at the sampling location

with a Coefficient of Conservatism (C-value) indicating a relatively high tolerance for disturbance. One of 

four metrics included in the Vegetation Multimetric Index.

Nutrients: Mineral substances that are absorbed by the roots of plants for nourishment. These 

substances (e.g., nitrogen, phosphorus) are essential to life, but in excess concentrations can overstimulate the growth of algae and other plants in water. Excess nutrients in aquatic resources can come 

from agricultural and urban runoff, leaking septic systems, sewage discharges, and similar sources.

NWCA Aggregated Ecoregions: Refers to the four ecoregions (Coastal Plains, Eastern Mountain and 

Upper Midwest, Interior Plains, and West) used for NWCA analysis and reporting. The Aggregated 

Ecoregions are combinations of nine aggregated ecoregions used in other NARS studies.

NWCA Aggregated Wetland Types: Refers to the four general wetland types (Estuarine Herbaceous; 

Estuarine Woody; Inland Herbaceous; and Inland Woody) used for NWCA analysis and reporting. The 
Glossary of Terms

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Aggregated Wetland Types are combinations of Cowardin Wetland Types that were part of the NWCA 

target population.

Palustrine Ecosystem: As defined by the Cowardin system, includes all nontidal wetlands dominated by 

trees, shrubs, persistent emergent, emergent mosses or lichens, and all such wetlands that occur in tidal 

areas where salinity due to ocean-derived salts is below 0.05%. It also includes wetlands lacking such 

vegetation, but with all of the following four characteristics: (1) area less than 8 hectares (20 acres); (2) 

active wave-formed or bedrock shoreline features lacking; (3) water depth in the deepest part of basin 

less than 2 meters at low water; and (4) salinity due to ocean-derived salts less than 0.05%.

Population Estimates: An approximation (reported as a percent of the total area or number of acres) of 

the entire group of wetlands in the conterminous U.S. that were the target of the study. The NWCA 

focus is on wetlands as groups or populations, rather than individual wetlands.

Probability Based Design: A type of random sampling technique in which every element of the 

population has a known probability of being selected for sampling.

Reference Condition: The least-disturbed condition available in an ecological region; determined based 

on specific criteria and used as a benchmark for comparison with other sample sites in the region.

Relative Cover of Native Monocot Species: The proportion of the sampling location covered by native

monocot species in relation to all plant species present. One of four metrics included in the Vegetation 

Multimetric Index.

Relative Cover of Nonnative Species: The proportion of the sampling location covered by nonnative 

plants relative to all plant species present. One of three metrics included in the Nonnative Plant Stressor 

Index (NPSI).

Relative Extent (Stressor Extent): An estimate (by percent of the resource or relative ranking of 

occurrence) of how spatially common a stressor is based on the population design.

Relative Frequency of Occurrence of Nonnative Species: The occurrence of nonnative plant species at a 

site compared to the total number of species. One of three metrics included in the Nonnative Plant 

Stressor Index (NPSI).

Relative Importance of Native Plant Species: A measurement of the proportion of the sampling location 

covered by native plants relative to all plants present (i.e., relative cover) and the occurrence of native 

plant species at a site compared to the total number of species (i.e., relative frequency). One of four 

metrics included in the Vegetation Multimetric Index.

Relative Risk: The probability or likelihood of having poor ecological condition when the magnitude of a 

stress indicator is high relative to when it is low. This is often presented as a relative risk ratio.

Richness of Nonnative Species: The number of nonnative plant species at the sampling location. One of 

three metrics included in the Nonnative Plant Stressor Index (NPSI).

Soil Phosphorus: An essential plant nutrient. However, high concentrations, due to runoff from 

agricultural and urban runoff, sewage discharges, leaking sewer systems, and other sources, can lead to 

eutrophication in wetlands and other water bodies. For the NWCA, the concentration of total 

phosphorus in the soil was used as an indicator of stressors that cause elevated soil phosphorus 
Glossary of Terms

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concentrations. Soil phosphorus concentrations were assessed relative to a threshold set using the 

reference site distribution approach.

Stressors: Factors, activities, or land uses that adversely affect, and therefore degrade, aquatic 

ecosystems. The NWCA measures stress using chemical, physical, and biological indicators (e.g., high soil 

phosphorus concentrations, ditches, relative cover of nonnative plants).

Vegetation Multimetric Index (VMMI): A national indicator of biological condition developed for the 

NWCA based on plant species composition (presence and abundance) at sampling locations and plant 

species traits. It combines four metrics: a Floristic Quality Assessment Index (FQAI), Relative Importance 

of Native Plant Species, Number of Plant Species Tolerant to Disturbance, and Relative Cover of Native 

Monocot Species.

Vegetation Removal Stressors: Indicators of stressors that result in losses, removals, or damage of the 

vegetation community in a wetland. Stress was measured based on observations of activities that result 

in vegetation removal, such as mowing, shrub cutting, herbicide use, intensive grazing, and recently 

burned forest.

Vegetation Replacement Stressors: Indicators of stressors that result in changes to the plant species 

present in a wetland. Stress was measured based on observations of activities or land uses that would 

alter the composition of the plant community, such as tree plantations, nursery, golf courses, lawns, 

parks, row crops, pasture, hay fields, or rangeland.
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List of Abbreviations and Acronyms

Ag Silver

Cd Cadmium

cm Centimeter

Co Cobalt

Cr Chromium

Cu Copper

C-value Coefficient of Conservatism

CWA Clean Water Act

EPA U.S. Environmental Protection Agency

 ̊F degree Fahrenheit

FQAI Floristic Quality Assessment Index

FWS U. S. Fish and Wildlife Service

GRTS Generalized Random Tessellation Stratified

GIS Geographic Information System

ha Hectare

HAB Harmful Algal Bloom

HGM Hydrogeomorphic Class

kg Kilogram

L Liter

m Meter

mg Milligram

MMI Multimetric Index

NARS National Aquatic Resource Survey

Ni Nickel

NPSI Nonnative Plant Stressor Indicator

NRCS Natural Resources Conservation Service

NWCA National Wetland Condition Assessment

ORD Office of Research and Development

ORISE Oak Ridge Institute for Science and Education

OW Office of Water

P Phosphorus

Pb Lead

RAM Rapid Assessment Method

Sb Antimony

S&T Status and Trends

Sn Tin

spp Multiple species

μg microgram

USA-RAM U.S.A. Rapid Assessment Method

USDA U.S. Department of Agriculture

V Vanadium

VMMI Vegetation Multimetric Index

W Tungsten

WHO World Health Organization
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Zn Zinc