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Inventories of U.S. Wetlands 1
W.E. Frayer2
Abstract-The U.S. Fish and Wildlife Service has a mandate to
conduct periodic assessments of wetlands in the United States.
The first national report was produced in 1983. The second national report was published in 1991. Several reports for states
and regions have also been produced. A stochastic process was
developed for projections offuture wetland areas. The procedures
of the inventories are discussed. Some results ofthe latest inventory
are given in this paper.
An inventory is needed periodically for a wetland that is
to be managed. An inventory of this type could be called a
"management inventory" because it would provide information needed to manage the specific wetland. However, it
would be impossible to have up-to-date inventories of all
wetlands in an area as large as the United States.
If a wetlands inventory is to cover a large area-such as
the United States-other tactics would be used. A welldesigned, periodically updated inventory would not produce
the information essential for management of specific wetlands; but, it can produce what is needed to determine if
management of wetlands is in fact necessary. This kind of
inventory has been given various descriptive titles; it is
sometimes called a "policy inventory".
What kind of inventory is needed? Both are usually necessary. Unless a broad inventory is conducted and the resulting information points out specific needs, there is little
chance of getting adequate support for a comprehensive
series of management inventories. What can and has been
done in the U.S. is to conduct a broad inventory of the
nation's wetlands. This was 'Conducted for the first time
about 15 years ago for the 48 contiguous states by the U.S.
Fish and Wildlife Service with the assistance of other agencies and contracts (Frayer, et aI., 1983; Tiner, 1984). It
consisted of studying approximately 3,500 sample units to
determine changes between the mid-1950s and 1970s. The
area sampled was less than 0.5 percent of the nation's areasomething quite different from a 100 percent sample common for a management inventory. The results are fairly
reliable and very useful.
A second inventory of the nation's wetlands was conducted several years ago (Frayer, 1991; Dahl and Johnson,
1991). The first inventory of Alaska's wetlands was also
conducted (Hall et aI., 1994). This paper presents the procedures used for these inventories and makes mention of a
projection procedure that promises to be useful in forecasting future trends.
Ipaper presented at the North American Science Symposium: Toward a
Unified Framework for Inventorying and Monitoring Forest Ecosystem
Resources, Guadalajara, Mexico, November 1-6, 1998.
2W.E. Frayer is Dean, School of Forestry and Wood Products, Michigan
Technological University, Houghton, MI, U.S.A. Phone: (906) 487-3604;
Fax: (906) 487-2915; e-mail: wefrayer@mtu.edu
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Classification ----------------------------In general terms, wetlands are lands where saturation
with water is the dominant factor determining the nature
of soil development and the types of plants and animal
communities living in the soil and on its surface. Technically, wetlands are lands transitional between terrestrial
and aquatic systems where the water table usually at or
near the surface or the land is covered by shallow water.
Wetlands must have one ore 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 sometime during the growing season of each year. Common terms
used to describe various wetlands include marshes, swamps,
bogs, small ponds, sloughs, potholes, river overflows, mud
flats, and wet meadows.
The classification and categories used are described by
Cowardin, et al. (1979). Groupings of categories were made
to accommodate 1) the special interests of the study and
2) the detail to which available aerial photography could be
interpreted. The kinds of wetland for which estimates were
produced include:
Marine intertidal
Estuarine intertidal nonvegetated
Estuarine intertidal emergent
Estuarine intertidal forested and scrub/shrub
Palustrine unconsolidated shore
Palustrine open water
Other palustrine nonvegetated
Palustrine emergent
Palustrine forested
Palustrine scrub/shrub
Within the classification structure that follows, wetlands
and deepwater habitats are grouped according to systems.
A system consists of environments of similar hydrological,
geomorphological, chemical and biological factors. Each
system is further divided following traditional ecological
concepts. Although deepwater habitats were included in
the study, they are not considered in this paper.
The Marine System extends from the outer edge of the
continental shelf to the high water of spring tides or to the
boundary of other systems as defined later. Marine Intertidal includes areas in which the substrate is exposed and
flooded by tides, including the associated splash zone.
The Estuarine System consists of deepwater tidal habitats and adjacent tidal wetlands which are usually semienclosed by land, but have open, partially obstructed, or
sporadic access to the open ocean and in which ocean water
is at least occasionally diluted by fresh water runoff from
the land. Offshore areas with typically estuarine plants and
animals, such as mangroves and oysters, are also included
Estuarine Intertidal is the portion exposed and flooded by
USDA Forest Service Proceedings RMRS-P-12. 1999
tides, including the associated splash zone. For the purpose
of this study, Estuarine Intertidal wetlands are shown by
the following groups: Nonvegetated, Emergent, and Forested
and Scrub / Shrub. Nonvegetated contains no emergent vegetation but does inciude vegetation in the form of aquatic
beds, while Emergent contains primarily those erect, rooted
herbaceous plants typically found in wet environments.
Forested is characterized by the presence of trees, and
Scrub / Shrub includes areas dominated by shrubs and small
or stunted trees.
The Lacustrine System includes some wetlands and
deepwater habitats situated in topographic depressions or
dammed river channels. Each area must exceed 20 acres or
have depths in excess of two meters or have an active waveformed or bedrock shoreline feature. The Riverine System
includes wetlands and deepwater habitats contained within
a channel. Lacustrine and Riverine wetlands were grouped
with and included in Palustrine wetlands in this study.
The Palustrine System includes all nontidal wetlands not
included within any of the other four systems. There are no
deepwater habitats included. For this study, Palustrine
wetlands are shown by the following groups: Unconsolidated Shore, Open Water, Other Palustrine Nonvegetated,
Emergent, Forested, and Scrub / Shrub. Unconsolidated
Shore includes wetlands generally having unstable substrates with less than 75 percent cover of stones, boulders, or
bedrock and little or no vegetation. Open Water includes
small inland open water bodies that are too small to be part
of the Lacustrine System. Other Palustrine Nonvegetated
includes other inh, 1 wetlands with little or no vegetation
other than aquatic beds, and the remaining terms are
defined as they were under the Estuarine System.
In addition to these, three more categories were used in
the study. Two are Urban and Agriculture; and together
with Other Surface Area (forests, rangeland, etc., not qualifying as wetland), they account for all other areas.
This is only a brief discussion of the classification used in
the study. It is difficult to differentiate the categories
further without introducing highly technical terms. For
those interested in detailed, exact definitions, the descriptions presented by Cowardin, et al., (1979) are available.
Survey Procedure _ _ _ _ _ __
The objectives of the latest national study were to develop
statistical estimates of acreage for categories of wetlands for
the lower 48 states during the 1970s, the 1980s, and changes
for the period.
A stratified random sample was used with the basic strata
being formed by state boundaries and the 35 physical subdivisions described by E.H. Hammond (1970). Additional
strata specific to the study are special coastal strata encompassing most of the marine and estuarine categories
used in the study and other strata encompassing the Great
Lakes. This resulted in over 200 strata for the study.
Sample units had been allocated to strata in a previous
study (Frayer et al., 1983) in proportion to the expected
amount of wetland and deepwater habitat acreage as estimated by earlier work (including Shaw and Fredine, 1956).
A total of 3629 sample units were used in the study.
Each sample unit is a four-square mile area, two miles on
each side. The units had been plotted on U.S. Geological
USDA Forest Service Proceedings RMRS-P-12. 1999
Survey topographic maps for the previous study. Also,
1:80,000 black and white aerial photography had been
obtained for the 1970s. The 1980s aerial photography was
then obtained, which was primarily 1:58,000 scale color
infrared transparencies. The mean years of photography
were 1974 and 1983, a nine-year period.
The 1980s photography was interpreted and annotated in
accordance with the classification system described earlier
and procedures developed by the U.S. Fish and Wildlife
Service's National Wetlands Inventory Project. The results
were compared to the 1970s photography, and any changes
in classification were annotated. Both the recent classification and the classification for the 1970s were recorded for
each change. If a change was human-induced, that was
recorded also.
Results _ _ _ _ _ _ _ _ _ __
Estimates for the 1970s, 1980s, and change during the
period were produced for the kinds of wetlands described
and are presented in Table 1. Some ofthe significant results
are described below by category.
Marine Intertidal Wetlands
Changes in marine intertidal wetlands since the 1970s
have been small, with estimated size in the 1980s being
104.3 thousand acres.
Estuarine Wetlands
Losses in estuarine wetlands occurred, with a primary
loss of 70.9 thousand acres of estuarine intertidal emergent
wetlands (coastal salt marshes) since the 1970s. States
having the largest losses in estuarine wetlands since the
1970s are Louisiana with 49.7 thousand acres loss and
Texas with 8.2 thousand acres loss. Most of the loss in
Louisiana was to estuarine subtidal deepwater habitats
(bay bottoms).
Palustrine Wetlands
The 1970s and 1980s estimates of palustrine wetlands
are 100.3 and 97.8 million acres, respectively. The average
annual net loss was 283.5 thousand acres.
Palustrine Nonvegetated Wetlands
Palustrine nonvegetated wetlands comprised 6.1 million
acres in the 1980s. Gains since the 1970s have occurred in all
four flyways and total 792.4 thousand acres. Almost all of
this increase was in palustrine unconsolidated bottoms
(primarily ponds), and most occurred on lands not currently
classified as wetlands or deepwater habitats.
Palustrine Vegetated Wetlands
There was an estimated 91.6 million acres of palustrine
vegetated wetlands in the 1980s compared to 95.0 million
acres in the 1970s. The average annual net loss was 371.6
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Table 1.-Area, in thousands of acres, by kind of wetland for the
conterminous United States 1
19705
Marine
Intertidal
Estuarine
Intertidal Nonvegetated
Estuarine
Intertidal Emergent
Estuarine Intertidal
Forested & Scrub/Shrub
Palustrine
Unconsolidated Shore
Palustrine
Open Water
Other Palustrine
Nonvegetated
Palustrine
Emergent
Palustrine
Forested
Palustrine
Scrub/Shrub
All
Wetlands
104.5
(.. )
678.2
(.. )
4,144.9
(... )
709.0
(..)
368.9
(.. )
4,781.4
(.. )
198.6
(.. )
24,312.8
(... )
55,151.2
(... )
15,505.6
(... )
105,955.1
( ... )
19805
Change
104.3
(.. )
689.8
(.. )
4,074.0
(... )
708.9
-D.2
(>50)
+11.6
(>50)
-70.9
(.. )
-D.1
(>50)
+18.4
(.. )
387.3
(.)
(.)
5,535.2
( .. )
218.8
(.. )
24,533.0
(... )
51,747.8
(... )
15,344.5
(... )
103,343.6
(... )
+753.8
(.)
+20.2
(.)
+220.2
(>50)
-,403.4
(... )
-161.1
(>50)
-2,611.5
(.. )
1 Entries
in parentheses are classes of standard error as a percentage of
the estimated total.
... Standard error is less than 10% of estimate .
.. Standard error is greater than 10% and less than 25% of estimate.
. Standard error is greater than 25% and less than 50% of estimate.
thousand acres. This reduction is due primarily to losses of
palustrine forested wetlands.
Palustrine Emergent Wetlands
There were several gains and losses of palustrine emergent wetlands. Gains were primarily from palustrine forested wetlands, and losses were primarily due to agriculture. States having gains include Louisiana, Arkansas,
Mississippi, Alabama and Michigan in the Mississippi
Flyway; and Georgia, South Carolina and Maine in the
Atlantic Flyway. States having losses include California in
the Pacific Flyway; North Dakota, South Dakota, Nebraska
and Texas in the Central Flyway; Minnesota in the
Mississippi Flyway; and Florida in the Atlantic Flyway.
Palustrine Forested Wetlands
There was a net loss of 3.4 million acres of palustrine
forested wetlands since the 1970s. The 1980s estimates are
51. 7 million acres of palustrine forested wetlands with an
average annual net loss of378.2 thousand acres. There were
some large changes to other wetland categories, primarily
palustrine emergent and palustrine scrub/shrub. However,
more than two million acres were converted to non-wetlands
in the period, with 1.0 million acres lost to agriculture.
The Mississippi and Atlantic flyways are dominant in
the losses since the 1970s. Mississippi Flyway states with
large losses include Louisiana (628.0 thousand acres loss),
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Arkansas (210.3 thousand acres), Mississippi (365.4 thousand acres), Alabama (97.4 thousand acres), Tennessee
(40.6 thousand acres) and Michigan (57.9 thousand acres).
Atlantic Flyway states with large losses include North
Carolina (985.8 thousand acres loss), South Carolina (124.8
thousand acres), Georgia 454.6 thousand acres), Florida
(184.1 thousandacres),Massachusetts (67.6 thousand acres),
and Maine (8.9 thousand acres).
Palustrine Scrub/Shrub Wetlands
There were both losses and gains in palustrine scrub/
shrub wetlands since the 1970s. Some significant losses
were offset by gains of a different nature. The losses included
conversion of over one-half million acres to non-wetlands.
The gains came primarily from other palustrIne vegetated
categories.
States with large losses since the 1970s include Minnesota, in which 214.6 thousand acres were lost; and North
Carolina, in which 205.3 thousand acres were lost. Gains
were posted in Alabama, Louisiana, Georgia, Maryland,
Pennsylvania and Massachusetts.
Conclusions __________
The results reported are based on a designed study of
the wetlands of the lower 48 states. The re::mlts of this
report document continued major net losses of wetlands and
provide insights to where these losses took place. The design
involved careful measurement of a sample of the nation's
surface area. In general, results are meaningful only at the
national level or for broad areas. Some of the results, however, have adequate reliability to be useful at flyway or state
levels. Intensification of the samples for selected areas in
future studies can provide useful results for those areas.
Some findings are very clear and involve large acreages.
Large decreases in wetlands continued to occur in the lower
Mississippi River states of Louisiana, Mississippi and
Arkansas. Other sates with large losses in the Mississippi
Flyway include Alabama, Tennessee, Minnesota and
Michigan. Dominate losers in the Atlantic Flyway include
North Carolina, South Carolina, Georgia, Florida and
Massachusetts. Other states cited in this report as having
significant wetland losses are California, North Dakota,
Sliuth Dakota, Nebraska, Texas and Maine. Additional
states having statistically significant wetland losses, since
the 1970s are Illinois, Wisconsin, Missouri, Kentucky, New
Hampshire, Vermont, Connecticut, New York, Pennsylvania,
New Jersey, Maryland, Delaware and Virginia. In the
palustrine forested category alone, an area approximately
equivalent to the entire state of Connecticut was lost between 1974 and 1983.
Other changes are also clear, but involve lesser acreages.
Importance of change is not necessarily reflected by area
alone. Some of the smaller wetlands-particularly along
the coastline of the United States-are extremely important
habitats for plant and animal life.
Significant increases occurred in small open water areas.
The important of these newly created habitats to fish and
wildlife populations is yet to be fully determined.
USDA Forest Service Proceedings RMRS-P-12. 1999
This report does not document the significant reduction
in quality of many wetlands. Some of the factors causing
this reduction in quality are: canals and inlets that cause
changes in water chemistry due to salt water intrusion,
polluted runoff from adjacent uplands or polluted inflow
from rivers and streams, urban encroachment, and dissection by transportation corridors.
Continual monitoring of surface area use and changes in
use is needed to provide the basis for wise decisions. This
report is the result of one such method of monitoring initia ted by the U. S. Fish and Wildlife Service. The results in this
report provide wetland information similar to the forest
and range information required by the Forest and Rangeland Renewable Resources
Planning Act and to soils information required by the
Soil and Water Resource Conservation Act. The results can
be updated in the future on the schedule required by those
Acts.
Although it would be preferable to remeasure the plots on
a periodic basis, a shortage of funding may necessitate a
different approach to obtaining new estimates. A procedure
for projecting wetland areas was first developed and described by Frayer (1987) and later refined by TerrazasGonzalez (1997).
USDA Forest Service Proceedings RMRS-P-12. 1999
Literature Cited
Cowardin, L. M., V. Carter, F. C. Golet, and E. T. LaRoe. 1979.
Classification of wetlands and deepwater habitats of the United
States. U.S. Fish Wildl. Servo 103 pp.
Dahl, T. E., and C. E. Johnson. 1991. Status and trends of
wetlands in the conterminous United States, mid-1970s to mid1980s. U.S. Fish Wildl. Servo 28 pp.
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, 1950s to 1970s. Colo. State
Univ. 32 pp.
Frayer, W. E. 1987. In the absence of concern: wetland projections
to the year 2000. In: Proceedings, Land and Resource Evaluation for National Planning in the Tropics. USDA Forest Servo
Pp 383-385.
Frayer, W. E. 1991. Status and trends of wetlands and deepwater
habitats in the conterminous United States, 1970s to 1980s.
Mich. Tech. Univ. 32 pp.
Hall, J. V., W. E. Frayer, and B. O. Wilen. 1994. Status of Alaska
wetlands. U.S. Fish Wildl. Servo 33 pp.
Hammond, E. H. 1970. Physical subdivisions of the United States.
In: National Atlas of the United States. U.S. Geol. Surv. 417 pp.
Shaw, S. P.,andC. G. Fredine. 1956. Wetlands of the United States.
U.S. Fish Wildl. Serv., Circ. 39. 67 pp.
Tiner, R. W. Jr. 1984. Wetlands of the United States: current status
and recent trends. U.S. Fish Wildl. Servo 59 pp.
Terrazas-Gonzalez. 1997. Evaluation of projection methods to predict wetlands area sizes: the wetlands inventory of U.S.A. PhD
dissertation. Colo. State Univ. 150 pp.
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