Geographically isolated wetlands a preliminary assessment of their characteristics and status in selected areas of the United States

              Geographically Isolated Wetlands
A Preliminary Assessment of Their Characteristics and Status
in Selected Areas of the United States
U.S. Fish and Wildlife Service
March 2002
Ralph W. Tiner, Herbert C. Bergquist,
Gabriel P. DeAlessio, and Matthew J. Starr.
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This report should be cited as follows:
Tiner, R.W., H. C. Bergquist, G. P. DeAlessio, and M. J. Starr. 2002.
Geographically Isolated Wetlands: A Preliminary Assessment of their Characteristics and Status in
Selected Areas of the United States. U.S. Department of the Interior, Fish and Wildlife Service,
Northeast Region, Hadley, MA.
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Contact: h_bergquist@fws.gov
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Geographically Isolated Wetlands
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Section 1. Introduction
Wetlands occur on many landscapes across America. They form in low-lying areas along rivers, streams,
lakes, and estuaries where they are periodically flooded (e.g., floodplains), on slopes in areas of
groundwater discharge (e.g., seepage slopes), on broad flat interstream divides where soil drainage is
poor (e.g., flatwoods), in geographically isolated depressions where precipitation collects (e.g., potholes,
playas, vernal pools, and ponds), in paludified landscapes in cold, wet climates where peat moss grows
over the land (e.g., blanket bogs), and in other seasonally wet sites. Wetlands therefore may be
connected with waterbodies or surrounded by dry land. Although the latter appear to be separated from
surface waters, many "isolated" wetlands are actually linked hydrologically to other wetlands or streams
by subsurface flows.
Purpose and Organization of this Report
The U.S. Fish and Wildlife Service prepared this report to provide an introduction to isolated wetlands.
A primary objective was to present ecological and geographic information to assist resource managers
and the general public in gaining a better perspective and understanding of these wetlands. There was no
intent to address jurisdictional questions about isolated wetlands. This report provides an overview of
many types of isolated wetlands and their functions and values, along with general estimates of the
number and acreage of isolated wetlands in a variety of physiographic settings across the country.
Available geospatial data from the Service's National Wetlands Inventory (NWI) Program and the U.S.
Geological Survey facilitated analysis and mapping of isolated wetlands in selected locations. Although
the report is not an exhaustive treatment of isolated wetlands, it provides readers with a basic
understanding of the relative extent of these wetlands and their significance. This report is arranged in
six sections: 1) introduction, 2) overview of isolated wetlands, 3) extent of isolated wetlands in selected
areas, 4) summary, 5) acknowledgments, and 6) references cited.
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Table of Contents
Preface
Executive Summary
Section 1. Introduction
l   Purpose and Organization of this Report
Section 2. Overview of Isolated Wetlands
l   What is an Isolated Wetland?
l   Overview of Isolated Wetland Functions
l   Profiles of Selected Isolated Wetland Types
n   Prairie Potholes
n   Playas
n   Rainwater Basin Wetlands
n   Nebraska Sandhills Wetlands
n   Salt Flat and Salt Lake Wetlands
n   Wetlands of Washington's Channeled Scablands
n   Desert Springs and their Wetlands
n   Glaciated Kettle-hole Wetlands
n   Delmarva Potholes
n   Coastal Plain Ponds
n   Carolina Bay Wetlands
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n   Pocosin Wetlands
n   Cypress Domes
n   Sinkhole Wetlands
n   Former Floodplain Wetlands
n   West Coast Vernal Pools
n   Woodland Vernal Pools
n   Coastal Zone Dune Swale and Deflation Plain Wetlands
n   Great Lakes Alvar Wetlands
Section 3. Extent of Isolated Wetlands in Selected Areas
l   Study Methods
l   Study Limitations
l   Map Interpretation
l   Study Results
Region 1 (Washington, Oregon, Idaho, California, Nevada, and Hawaii)
Region 2 (Arizona, New Mexico, Oklahoma, and Texas)
Region 3 (Minnesota, Wisconsin, Michigan, Ohio, Indiana, Illinois, Iowa, Missouri)
Region 4 (North Carolina, South Carolina, Georgia, Florida, Alabama, Mississippi,
Louisiana,
Arkansas, Tennessee, and Kentucky)
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Region 5 (Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, Connecticut,
New York, New Jersey, Pennsylvania, Delaware, Maryland, Virginia, and West
Virginia)
Region 6 (North Dakota, South Dakota, Nebraska, Kansas, Colorado, Wyoming, Utah, and
Montana)
Region 7 (Alaska)
Section 4. Summary
Section 5. Acknowledgments
Section 6. References Cited
Footnotes
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Preface
The mission of the Fish and Wildlife Service is to conserve, protect, and enhance fish, wildlife, plants
and their habitats for the continuing benefit of the American people. Wetlands are among the Nation's
most valuable natural resources, providing crucial habitat for a wide variety of fish and wildlife. Their
landscape position, hydrology, and vegetation also make wetlands especially important for water quality
renovation, flood water storage, shoreline stabilization, and production of food and fiber. Because of
these and other values, wetlands and their associated waters receive much attention for resource
protection, restoration, and management.
The Service's National Wetlands Inventory Program is responsible for mapping the Nation's wetlands,
and reporting to the Congress at ten-year intervals on the national status and trends of these important
habitats. This information aids planners, resource managers, decision-makers, and others in better
understanding these valuable habitats, and in improving the status of wetlands.
Wetlands surrounded by upland - "geographically isolated wetlands" - are vital habitats for numerous
wildlife species (including endangered and threatened plants). In many areas, these wetlands serve as
oases for wildlife, especially in arid and semi-arid regions. Being surrounded by upland and often small
in size also have made them perhaps the most vulnerable wetlands in the country. Development of
adjacent uplands has posed significant threats both direct and indirect to these wetlands and their
associated wildlife.
Given the wildlife significance of these wetlands, the Service prepared this report to provide an
ecological and geographic overview of isolated wetland resources of the United States. For the purposes
of this report, isolated wetlands have no apparent surface water connection to perennial rivers and
streams, estuaries, or oceans. Ecological profiles were developed to provide the public with a better
understanding of 19 types of wetlands that are often considered geographically isolated wetland habitats.
The report also included the results of a study that estimated the extent of potentially isolated wetlands at
72 selected sites in 44 States. Available National Wetlands Inventory map data were combined with U.S.
Geological Survey hydrology data through geographic information system analysis to develop these
estimates. The report should increase public awareness of these significant and vulnerable wetlands.
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Section 2. Overview of Isolated Wetlands
This section provides an introduction to isolated wetlands. It begins with a general discussion that
explains different definitions of isolated wetlands and reviews major functions of isolated wetlands. The
section concludes with brief profiles of individual wetland types that have been traditionally viewed as
isolated from the geographic or landscape perspective or other wetland types that include isolated forms.
These profiles are based on readily available materials and do not represent a comprehensive literature
review of isolated wetlands. The objective was to provide readers with sufficient background to better
understand the nature, functions, and values of so-called "isolated wetlands."
What is an Isolated Wetland?
The term "isolated" is a relative term. There is no single, ecologically or scientifically accepted
definition of isolated wetland because this issue is more a matter of perspective than scientific fact.
Nonetheless, it is a question of particular relevance for wetlands since it may affect their future
well-being.
Webster’s New World Dictionary of the American Language (Guralnik 1972) defines "isolate" as "to set
apart from others; place alone." An isolated wetland, therefore, would be one that is separated from
other wetlands or other waters – standing alone. Isolation can be viewed from a number of perspectives.
Two common viewpoints are based on landscape or geomorphic differences and hydrologic interactions.
Others may consider ecological relationships.
Wetlands surrounded by upland may be considered isolated, since they are separated from other wetlands
by dry land. This is isolation from a geographic, landscape, or geomorphic perspective. Examples of
geographically isolated wetlands include prairie pothole wetlands, cypress domes, playas, and kettle-hole
bogs. This type of isolation is easy to determine because, in most cases, these isolated wetlands are
depressional wetlands surrounded by upland (Figures 2-1 and 2-2).
Figure 2-1. Prairie pothole wetlands surrounded by upland cropland. (R. Tiner photo)
Figure 2-2. Sketch showing isolated wetlands surrounded by upland. (Source: Tiner 1996)
From a hydrologic standpoint, isolated wetlands may be defined as wetlands not connected to other
wetlands or waterbodies by surface water or ground water. Hydrologically isolated wetlands might be:
1) wetlands with no surface water connection to other wetlands or waters, or 2) wetlands lacking a
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hydrologic connection to other wetlands or waters considering both surface and subsurface flows.
Examples of hydrologically isolated wetlands are Southwest playas and Nebraska's Rainwater Basin
wetlands. Many wetlands considered isolated from the landscape or geographic perspective are
connected hydrologically via groundwater to other wetlands and to rivers and streams. For example,
Figure 2-3 shows subsurface flows from South Dakota prairie pothole wetlands to a regional
groundwater system that eventually empties into a stream. Similar groundwater connections take place
in other areas (see numerous illustrations in Fretwell et al. 1996; some of which are presented elsewhere
in this section). In karst regions, many geographically isolated wetlands are hydrologically connected to
underground waterways. Other geographically isolated wetlands may become hydrologically linked to
other wetlands during extremely wet years as surface water overflows from one depressional wetland to
another. So time may also be a factor when considering whether or not a given wetland is hydrologically
isolated.
Figure 2-3. General subsurface flows between different South Dakota potholes and riverine wetlands. (Source: Sando
1996)
Isolation from an ecological perspective is even more difficult to define since one would have to identify
physical, chemical, or other barriers to the exchange of genetic material, for example. Barriers that affect
seed dispersal, animal movements, and reproductive success would have to be evaluated. Isolation in
this context may be best defined relative to a particular organism, since some features that pose barriers
to some organisms do not restrict others. From a wetland-dependent animal's standpoint, isolation is a
function of the number of neighboring wetlands, the distance between them, the nature of the matrix land
cover, and the mobility of the particular species (Gilberto Cintron, pers. comm.). Determining ecological
isolation might also require identification of discontinuities in biological characteristics like genotypes or
phenotypes of certain species. In the view of some landscape ecologists, an isolated system is one that
has no exchange of energy or matter with its surrounding environment (Forman and Godron 1986).
From this perspective, it may not be possible for any wetland to be truly isolated.
Overview of Isolated Wetland Functions
Wetlands provide a host of functions that benefit ecosystems as well as society (see Mitsch and
Gosselink 2000). Many of the functions are synergistic in producing services or materials that are valued
by people (Table 2-1). Wetlands largely operate as a holistic or integrated system within a watershed,
waterfowl flyway, or ecoregion (Tiner 1998). Individual wetlands working together provide valued
functions and the value of a network of wetlands (e.g., within a watershed or flyway) is greater than the
sum of its individual parts. A collection of wetlands on the landscape may be the vital ecological unit for
some animals, while others require a combination of wetlands and uplands for survival and
reproduction.
The following discussion is a brief overview of some of the functions of isolated wetlands. It is not
intended to be exhaustive, but is designed to acquaint readers with the potential roles isolated wetlands
play. For additional information about functions of specific types of isolated wetlands, readers are
referred to the next subsection and to publications listed in the References Cited section.
Table 2-1. Major wetland functions and some of their values. (Source: Tiner 1998)
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Function Some Values
Water storage
Flood- and storm-damage protection, water source during dry seasons, groundwater recharge, fish and
shellfish habitat, water source for fish and wildlife, recreational boating, fishing, shellfishing, waterfowl
hunting, livestock watering, ice skating, nature photography, and aesthetic appreciation
Slow water release
Flood-damage protection, maintenance of stream flows, maintenance of fresh and saltwater balance in
estuaries, linkages with watersheds for wildlife and water-based processes, nutrient transport, and
recreational boating
Nutrient retention and cycling
Water-quality renovation, peat deposits, increases in plant productivity and aquatic productivity,
decreases in eutrophication, pollutant abatement, global cycling of nitrogen, sulfur, methane, and carbon
dioxide, reduction of harmful sulfates, production of methane to maintain Earth’s protective ozone layer,
and mining (peat and limestone)
Sediment retention
Water-quality renovation, reduction of sedimentation of waterways, and pollution abatement (retention
of contaminants)
Substrate for plants and animals
Shoreline stabilization, reduction of flood crests and water’s erosive potential, plant-biomass
productivity, peat deposits, organic export, fish and wildlife habitats (specialized animals, including rare
and endangered species), aquatic productivity, trapping, hunting, fishing, nature observation, production
of timber and other natural commodities, livestock grazing, scientific study, environmental education,
nature photography, and aesthetic appreciation
Water Storage
Depressional wetlands, whether isolated or not, store precipitation that could otherwise rapidly flow
downstream, creating potential flooding of low-lying areas. Since isolated basins have no natural outlets,
all water entering them is retained (including groundwater recharge). This is valuable for flood
reduction, since such water does not contribute to local or regional flooding (Carter 1996). When an area
contains thousands of isolated depressional wetlands, the surface water storage capacity can be
enormous. For example, pothole wetlands in North Dakota’s Devils Lake Basin can store as much as 72
percent of the total runoff from a 2-year frequency storm and about 41 percent from a 100-year storm
(Ludden et al. 1983). In many cases, this water storage function facilitates an isolated wetland’s
potential role in groundwater recharge and streamflow maintenance (through contribution to regional
groundwater supplies) and at the same time, provides valuable waterfowl and waterbird habitat. The
multiple benefits of temporary water storage are considerable.
By holding water for long periods, isolated wetlands serve as water sources that benefit fish and wildlife,
domestic livestock, and people. An abundance of water creates wetland habitats for native fish and
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wildlife that provide recreational opportunities for many people (e.g., hunting and fishing) and help
support local economies. Isolated wetlands within rangeland are often used as watering holes for
livestock, while similar wetlands in agricultural settings may serve as sources of irrigation water. These
two uses can have adverse effects on wildlife and the habitat quality of these wetlands. The wettest of
isolated wetlands may provide fish habitat that may be a valuable resource for local landowners.
Recreational fishing and commercial harvest of fish may take place in some isolated wetlands. For
example, fathead minnows are caught in Minnesota potholes and sold as baitfish (Hubbard 1988).
Slow Water Release
Many wetlands that appear isolated from surface waters are actually vital components of regional water
systems, since they contribute to local and regional aquifers (Stone and Lindley Stone 1994). Examples
of isolated wetlands contributing water to regional aquifers that ultimately support stream flow are shown
in Figures 2-4 and 2-5. Isolated wetlands hold water until it is removed by evapotranspiration, seepage
(percolation contributing to groundwater supplies), irrigation devices, or drainage structures. During
extreme high water events, for example, water-filled isolated basins often contribute to groundwater
supplies (including regional aquifers) as water enters more permeable adjacent soils and moves
downward to underlying aquifers. Such groundwater may flow laterally to contribute to streamflow
critical for supporting aquatic life and their respective ecosystems. Playa lakes are major recharge sites
in the Southern High Plains (Wood and Osterkamp 1984 as reported in Carter 1996).
Figure 2-4. Generalized subsurface flows between isolated wetlands and rivers and estuaries. (Source: McPherson 1996)
Figure 2-5. Generalized subsurface flows between sinkhole wetlands, springs, creeks, and rivers. (Source: McPherson
1996)
Nutrient Retention and Cycling
The role of wetlands in nutrient retention and cycling is well known. Wetlands can be sources, sinks, or
transformers of chemicals and the range of hydrologies creating and maintaining wetlands has a great
impact on biogeochemical processes. Mitsch and Gosselink (2000) reference numerous examples of
wetlands serving as sinks for a host of chemicals and summarize various applications of constructed
wetlands for wastewater treatment. Two potentially isolated wetland types - ombrotrophic bogs and
cypress domes - are cited as natural wetlands that are biogeochemically closed systems which recycle
nutrients internally (i.e., intrasystem cycling). These and other isolated wetlands should retain most of
the chemicals entering them from surrounding areas and, therefore, appear to serve as sinks for a variety
of chemicals. Because of these properties, artificial wetlands are purposely constructed to treat
wastewater of various kinds (e.g., municipal wastewater, mine drainage runoff, stormwater and
nonsource pollution, landfill leachate, and agricultural wastewater) and improve water quality (Mitsch
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and Gosselink 2000).
Sediment Retention
Isolated depressional wetlands are sediment traps. Given their landform and landscape position, they
should retain all sediments and other particulates entering them. In fact, the volume of many such
wetlands is reduced over time due to this process, especially in agricultural areas. Luo and others (1999)
reported on sediment deposition in playa wetlands. Most of these sediments are water-borne materials
originating from local watersheds. Coarser materials settle out first, so sand content is higher at the
margins of playas, while finer particles are carried further and settle out near the center of the basins
where clay content is greater.
Substrate for Plants and Animals
Wetlands provide substrates that support plant growth and colonization by thousands of animals ranging
from microscopic invertebrates to large vertebrates. By doing this, wetlands provide habitat for plants
and animals. The variety of wetland types is a major contributor to the Nation's biodiversity (see Tiner
1999 for examples of wetland plant communities).
From an ecological standpoint, isolated wetlands are among the country’s most significant biological
resources. In some areas, isolation has led to the evolution of endemic species vital for the conservation
of biodiversity. In other cases, their isolation and sheer numbers in a given locality have made these
wetlands crucial habitats for amphibian breeding and survival (e.g., woodland vernal pools and cypress
domes) or for waterfowl and waterbird breeding (e.g., potholes). In arid and semi-arid regions, many
isolated wetlands are veritable oases – watering places and habitats vital to many wildlife that use them
for breeding, feeding, and resting, or for their primary residence. Many of these wetlands may be small
in size, but their value to wildlife is far greater than their size alone would suggest.
The high density of isolated marshes and wet meadows has made the Prairie Pothole Region, North
America’s leading waterfowl production area. This region produces half of the continent’s waterfowl in
an average year (Smith et al. 1964). Forty-one percent of the continent’s breeding dabbling ducks use
this area (Bellrose 1979). Macroinvertebrates produced by the pothole marshes are a protein-rich food
source required by nesting hens (Hubbard 1988).
Regions with a high density of isolated wetlands may provide a series of "stepping stones" for migrating
waterfowl and waterbirds. For example, isolated wetlands east of the Rocky Mountains provide feeding
and resting areas for millions of birds that overwinter along the Gulf Coast and migrate to northern
breeding grounds. These wetlands produce an abundance of macroinvertebrates and plant life –
nourishment required by these species to successfully make the migratory journey essential for
maintaining their populations. Playas may be important intermediate stopover sites for migrating
shorebirds (Davis and Smith 1998), while Rainwater Basin wetlands are stopover areas for millions of
birds. Nearly all of the midcontinental population of greater white-fronted goose (Anser albifrons) stage
in the Rainwater Basin every year (U.S. Fish and Wildlife Service 1985).
A high density of small wetlands is also vital for other animals. Semlitsch and Bodie (1998) described
the importance of small wetlands to amphibians. The abundance of small isolated wetlands supports a
diverse assemblage of amphibian species, produces large numbers of juveniles (necessary to maintain
populations), and serves as "stepping stones" to aid in dispersal and recolonization of suitable habitats
(Semlitsch 2000). Local populations of wetland-dependent organisms are vulnerable to extinction due to
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several factors including natural events (e.g., prolonged droughts and changing vegetation), disease,
inbreeding, and habitat destruction. A study of wetlands in central Maine by Gibbs (1993) suggests that
a high number of small wetlands increases the number of sources of potential colonists for wetlands that
have lost populations due to chance extinction. The presence of a high number of small wetlands
therefore increases the chances of survival of local populations over time.
Reducing the number of small wetlands in a given area increases overland migration distances and
exposure of migrants (e.g., salamanders) to predators. This may place local populations at the risk of
extinction. For example, Semlitsch and Bodie (1998) found that eliminating all natural wetlands less
than 10 acres in size (in a South Carolina study area) would increase the nearest-wetland distance from
1,570 feet to 5,443 feet – a distance that would take most amphibian species several generations or more
to travel. This type of loss would increase the probability of local population extinction for some
amphibians.
Isolated wetlands with fluctuating water levels provide unique habitats for certain species, especially
those that are vulnerable to fish predation. Much of the value of woodland vernal pools to amphibians is
due to the absence of fish, which cannot survive periodic drawdowns. The presence of fish would
eliminate or severely reduce the reproductive success of amphibians that breed in these pools.
Isolation and periodic drawdowns also promote endemism - the development of unique species.
Increased number of species adds to the country's biological richness. Some examples of wetlands that
are particularly important in this regard are West Coast vernal pools, desert spring wetlands, and Coastal
Plain ponds (see discussion in following subsection).
Profiles of Selected Isolated Wetland Types
Regional differences in climate, physiography, hydrology, and other factors have led to the formation of
a diverse assemblage of wetlands across the country. A number of distinct wetland types are typically
isolated (e.g., playas, potholes, vernal pools, and interdunal swales), while others (e.g., Carolina bays and
kettle-hole wetlands) may be either isolated or connected to streams and other surface waters. Isolated
wetlands on former floodplains (e.g., oxbow lakes) were at one time periodically inundated by seasonal
river flows but due to changes in river courses are now left isolated beyond the active floodplain. In
other cases, the isolation of former floodplain wetlands has been caused by construction of levees to
prevent overbank flooding to provide flood protection or by upstream dams that reduce flow regimes.
Many other isolated wetlands were also created by human actions. Most of them are ponds built for a
variety of reasons including aesthetic appreciation, livestock watering, irrigation, aquaculture, and
stormwater management. Other isolated wetlands have been created by fragmentation from
development; they represent remnants of once larger wetland complexes.
The following review describes the range and types of wetlands that have been considered isolated. For
ecological discussion purposes, 19 types of isolated wetlands are described: 1) prairie potholes, 2) playas,
3) Rainwater Basin wetlands, 4) Nebraska Sandhills wetlands, 5) salt flat and salt lake wetlands, 6)
wetlands of Washington's Channeled Scablands, 7) desert springs and their wetlands, 8) glaciated
kettle-hole wetlands, 9) Delmarva potholes, 10) Coastal Plain ponds, 11) Carolina Bay wetlands, 12)
pocosin wetlands, 13) cypress domes, 14) sinkhole wetlands, 15) former floodplain wetlands, 16) West
Coast vernal pools, 17) woodland vernal pools, 18) coastal zone dune swale and deflation plain wetlands,
and 19) Great Lakes alvar wetlands. The discussions are intentionally brief and readers are encouraged
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to consult the cited references for additional information.
Prairie Potholes
The Prairie Pothole Region extends from Iowa and South Dakota northward into south-central Canada
(Figure 2-6). Glacial activity in this area created millions of shallow depressions now called "prairie
pothole wetlands" (Figure 2-7). Most of these depressions have been commonly viewed as isolated
wetlands, since they occur as separate basins on the land surface. Despite this, many of these wetlands
are hydrologically connected (Figure 2-8). Prairie wetlands serve as both recharge and discharge areas,
contributing to both local groundwater flow and regional flow (Lissey 1971). Water is recharged at
topographic highs (wetlands at higher elevations) and discharged to regional lows (e.g., lakes and other
wetlands) and eventually to local rivers and streams (Winter 1989). Seasonal changes in functions may
occur as some wetlands contribute to groundwater during high water periods (recharge), yet receive
groundwater inputs during the dry season due to evapotranspiration.
Figure 2-6. Location of the Prairie Pothole Region. (Source: Hubbard 1988)
Figure 2-7. Aerial view showing high density of prairie pothole wetlands. (U.S. Fish and Wildlife Service photo)
Figure 2-8. Generalized subsurface flows between different pothole wetlands. (Source: Berkas 1996)
The existence of millions of isolated basins in this region provides considerable surface water storage
capacity. For example, the approximately 50,000 pothole wetlands in the Devils Lake Basin (North
Dakota) that cover only 15 percent of the local landscape can store up to 72 percent of the total runoff of
a 2-year storm event and 41 percent of a 100-year storm (Ludden et al. 1983). Water trapped within
these basins is lost mainly through evapotranspiration and groundwater seepage. In their undrained state,
these wetlands do not contribute to runoff (Wiche et al. 1990). Yet drainage of these basins and
connection to the surface water network emptying into streams makes them contributing sources of
stream flow, thereby exacerbating flooding problems. Artificial drainage increases the watershed runoff
area and decreases the water storage capacity of potholes (Moore and Larson 1979).
A basin (depressional) landform and its effect on hydrology create a zonation of vegetation within
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individual potholes. Potholes are often described by the hydrology of the deepest part of their basins
(i.e., permanent, semipermanent, seasonal, temporary, and ephemeral) (Stewart and Kantrud 1971).
Concentric rings of vegetation zones are typical with aquatic bed species such as widgeon-grass (Ruppia
maritima), pondweeds (Potamogeton spp.), and duckweeds (Lemna spp. and Spirodela polyrhiza) in the
permanently flooded zone; robust emergents like cattails (Typha spp.) and bulrushes (Scirpus spp.) in the
semipermanently flooded zone; other emergents including spikerush (Eleocharis palustris), giant
bur-reed (Sparganium eurycarpum), water plantain (Alisma plantago-aquatica), slough sedge (Carex
atherodes), hydrophytic grasses (e.g., Phalaris arundinacea, Glyceria grandis, Scolochloa festucacea,
and Beckmannia syzigachne), and water smartweed (Polygonum coccineum) in the seasonally flooded
zone; and graminoids such as salt grass (Distichlis spicata), squirrel-tail (Hordeum jubatum), prairie
cordgrass (Spartina pectinata), baltic rush (Juncus balticus), and sedges (e.g., Carex sartwellii, C.
lanuginosa, and C. praegracilis) in the driest zone (temporarily flooded). Note that all potholes do not
contain all zones (Figure 2-7). Smaller potholes may have only one or two zones (Figure 2-9).
Figure 2-9. Pothole marsh. (R. Tiner photo)
Although it only accounts for 10 percent of North America’s waterfowl breeding area, the Prairie Pothole
Region produces half of the continent’s waterfowl in an average year (Smith et al. 1964). These pothole
wetlands are primary breeding habitats for mallard (Anas platyrhynchos), northern pintail (A. acuta),
American wigeon (A. americana), gadwall (A. strepera), northern shoveler (A. clypeata), green-winged
teal (A. crecca), blue-winged teal (A. discors), canvasback (Aythya valisineria), redhead (A. americana),
and ring-necked duck (A. collaris). It is important to note that successful breeding requires that
waterfowl have a variety of wetland habitats available because no single wetland type satisfies all their
reproductive needs through the breeding season (Swanson and Duebbert 1989). The availability of large
numbers of small wetlands allows the birds to disperse over the landscape, thereby making them less
vulnerable to predation and diseases like avian cholera (Kantrud et al. 1989).
While pothole wetlands are noted for their waterfowl production, many other animals use these
wetlands. Migratory birds that nest in the Arctic use potholes for feeding along their route. Many other
birds breed or feed in pothole wetlands, including horned grebe (Podiceps auritus), eared grebe (P.
nigricollis), pied-billed grebe (Podilymbus podiceps), black-crowned night heron (Nycticorax
nycticorax), American bittern (Botaurus lentiginosus), northern harrier (Circus cyaneus), Virginia rail
(Rallus limicola), sora (Porzana carolina), American coot (Fulica americana), killideer (Charadrius
vociferus), willet (Catoptrophorus semipalmatus), marbled godwit (Limosa fedoa), American avocet
(Recurvirostra americana), Wilson's phalarope (Phalaropus tricolor), black tern (Chlidonias niger),
marsh wren (Cistothorus palustris), yellow-headed blackbird (Xanthocephalus xanthocephalus),
red-winged blackbird (Agelaius phoenicus), and savannah sparrow (Passerculus sandwichensis)
(Kantrud and Stewart 1984). Ring-necked pheasants (Phasianus colchicus) depend on many pothole
wetlands for winter cover (Kantrud et al. 1989). Muskrat (Ondatra zibethicus) is the most conspicuous
mammal using potholes and their mounds may be used as nesting sites by waterfowl. Many other
mammals also use potholes, including shrews (Sorex arcticus, S. cinereus, and Microsorex hoyi),
jumping mice (Zapus hudsonius and Z. princeps), meadow vole (Microtus pennsylvanicus), weasels
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(Mustela frenata and M. nivalis), mink (Mustela vison), raccoon (Procyon lotor), red fox (Vulpes vulpes),
and white-tailed deer (Odocoileus virginianus). Garter snakes (Thamnophis radix and T. sirtalis),
American toad (Bufo americanus), Great Plains toad (B. cognatus), Rocky Mountain toad (B.
woodhousei), wood frog (Rana sylvatica), leopard frog (Rana pipiens), chorus frog (Pseudacris nigrita),
and tiger salamander (Ambystoma tigrinum) frequent potholes, with the latter three species being most
dependent on these wetlands (Kantrud et al. 1989). Kantrud and others (1989) provide a comprehensive
review of the ecology of pothole wetlands in the Dakotas, while Hubbard (1988) presents a summary of
the literature on pothole functions and values.
About half of the original potholes in the Dakotas have been destroyed (60% in North Dakota and 40% in
South Dakota; Tiner 1984). Over half were altered by agriculture, irrigation, and flood control projects.
More than 99 percent of Iowa’s original marshes have been lost, while 9 million acres of potholes in
western Minnesota have been drained. Destruction of pothole wetlands and alteration of natural
vegetated buffers around remaining wetlands have significantly reduced valuable waterfowl nesting and
rearing areas. Pothole drainage also eliminates or severely reduces their surface water storage function
and makes potholes and their local watersheds contributing sources of potential flood waters. Use of
pesticides poses problems for waterfowl, since many insecticides are highly toxic to aquatic invertebrates
(important food source) or to birds directly (Grue et al. 1986). Wetland restoration projects have been
initiated to help bring back lost wetland functions and values in this region.
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Playas
Playas are nearly circular, shallow basins formed in semi-arid and arid regions (prairies to deserts)
(Figure 2-10). Between 25,000 to 30,000 playas occur from southeastern Colorado and southwestern
Kansas to west Texas (Haukos and Smith 1997). While playas are found in other countries, the world's
highest density of playas is found in the southwestern United States - in the Southern High Plains of the
Texas panhandle and eastern New Mexico (Figure 2-11) (Haukos and Smith 1994). Nearly 22,000 playa
basins exist in this area (19,340 in Texas and 2,460 in New Mexico; Guthery and Bryant 1982).
Figure 2-10. Aerial photo showing numerous playas (some marked by arrow).
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Figure 2-11. Location of playa region in the Southern Great Plains, with playa density indicated. (Source: Nelson et al.
1983)
Most playas derive water from rainfall and local runoff; very few (e.g., playa lakes) are linked to
groundwater (Haukos and Smith 1994). Playas are usually dry in late winter, early spring, and late
summer. Multiple wet-dry cycles during a single growing season are common (Figure 2-12). These
fluctuating water levels promote nutrient cycling and biological productivity (Bolen et al. 1989).
Figure 2-12. A playa wetland during a wet phase. (U.S. Fish and Wildlife Service photo)
According to Haukos and Smith (1994), playas are the only remaining native habitat in the Southern
High Plains. From a wildlife standpoint, playas are perhaps most important as wintering grounds for
waterfowl. More than 90 percent of the region’s overwintering waterfowl depends on the playas:
600,000 to over 3 million ducks and geese (Nelson et al. 1983; U.S Fish and Wildlife Service 1988).
More than 90 percent of the midcontinental population of sandhill cranes (Grus canadensis) overwinters
here and many cranes frequent larger playas as well as salt lakes (Iverson et al. 1985). Migrating
shorebirds feed heavily on aquatic invertebrates produced in the playas.
Playas also serve as vital habitats for amphibians. Spotted chorus frog (Pseudacris clarkii), Blanchard’s
cricket frog (Acris crepitans blanchardi), Plains leopard frog (Rana blairi), Great Plains narrow-mouth
toad (Gastrophyrne olivacea), Great Plains toad, Texas toad (Bufo speciosus), Woodhouse’s toad (B.
woodhousei woodhousei), Plains spadefoot (Scaphiopus bombifrons), New Mexico spadefoot (S.
multiplicatus), Couch’s spadefoot (S. couchii), and tiger salamander depend on playas (Anderson and
Haukos 1997). Given the variability in playa wetness, amphibian community composition and
populations fluctuate from year to year (Anderson et al. 1999). Haukos and Smith (1994) provide a
summary of wildlife use of playas and stress the significance of playas to local landscape heterogeneity
and regional and continental biodiversity. Additional information on playas can be found in "Playa
Lakes - Symposium Proceedings" (U.S. Fish and Wildlife Service 1981) and "Playa Wetlands and
Wildlife on the Southern Great Plains: A Characterization of Habitat" (Nelson et al. 1983).
Negative impacts to playas appear to be related most to water pollution. Playas receive poor quality
water from a number of sources: 1) runoff from adjacent cropland (e.g., pesticides and herbicides), 2)
discharge of contaminated water from oil fields, and 3) effluents from livestock operations such as cattle
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feedlots (Haukos and Smith 1994). The second source has led to widespread bird mortality. Playas
adjacent to feedlots are often used as wastewater retention ponds (Bohlen et al. 1989). Other impacts to
playas include sedimentation from farmland, pit construction (for irrigation), and overgrazing of playa
vegetation.
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Rainwater Basin Wetlands
The Rainwater Basin is located in southern Nebraska (Figure 2-13). Wetlands have formed in
depressions underlain by clay on a rolling silty loess plain. Wind-formed depressional wetlands typify
this region. They are essentially isolated wetlands (closed basins with internal drainage) that depend on
overland runoff from precipitation for their water supply (Figure 2-14) (Gersib 1991; Gilbert 1989;
Starks 1984). Water is lost primarily through evapotranspiration with little seepage to underlying water
tables (Figure 2-15) (Frankforter 1996). The presence of a clay lens in these Rainwater Basin wetlands
restricts seepage to underlying water tables (50-100+ feet below wetlands); groundwater recharge
potential appears to be limited in most cases (Ellis and Dreeszen 1987; Keech and Dreeszen 1959, 1968).
Figure 2-13. Major wetland regions in Nebraska. (Source: Frankforter 1996)
Figure 2-14. Aerial photo showing Rainwater Basin wetlands (some marked by arrow).
Figure 2-15. Generalized flow of water from Rainwater Basin wetlands. (Source: Frankforter 1996)
The Rainwater Basin is one of Nebraska’s wetland complexes of international importance (Gersib 1991).
From 5 to 7 million ducks and geese visit these wetlands annually. Nearly all of the midcontinental
population of 300,000 greater white-fronted geese stage in the Basin every year (U.S. Fish and Wildlife
Service 1985). According to a functional assessment performed in 1989 (Gersib et al. 1989), all or most
Rainwater Basin wetlands have a high probability of providing the following functions: wildlife habitat,
food chain support, nutrient retention, flood storage, sediment trapping, and shoreline anchoring. An
abundance of fish and aquatic invertebrates produced in Rainwater Basin wetlands provides important
food for migrating waterfowl in spring (Gersib et al. 1990). Such food helps waterfowl build body
reserves for successful reproduction in northern breeding grounds.
At pre-European settlement, 94,000 acres of wetlands existed in the Rainwater Basin. By 1981, less than
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10 percent of this acreage remained (Farrar 1982). Agricultural activities such as drainage, clearing, and
ground water pumping have exacted a tremendous toll on these wetlands. Losses of Basin wetlands have
forced ducks and geese to concentrate in remaining wetlands. In dry years with late winter storms,
migrating waterfowl crowd into Basin wetlands. Such concentrations increase the likelihood for spread
of diseases like avian cholera. In 1980, avian cholera killed about 80,000 waterfowl in the Basin – this
was the second largest reported cholera die-off in the country.
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Nebraska Sandhills Wetlands
The Sandhills region of north-central and northwestern Nebraska is the largest sand dune area in the
Western Hemisphere, covering about 20,000 square miles (Figure 2-16; Bleed and Flowerday 1990).
This expansive grassland overlies the Ogallala Aquifer, to which most wetlands in the region owe their
existence. Groundwater is a major water source for Sandhills wetlands in the eastern portion of the
region and for subirrigated meadows (Chuck Elliott, pers. comm.).
Figure 2-16. Aerial photo showing isolated basins in the Sandhills.
According to Ginsberg (1985), wet meadows characteristic of this region commonly have surface
outlets. Yet many wet meadows in the western Sandhills have little or no surface outflow (Figure 2-17)
(Frankforter 1996). Despite this apparent lack of surface outflow, most of these wetlands are
interconnected with a regional groundwater system (Figure 2-17) (LeBaugh 1986; Winter 1986).
Figure 2-17. Generalized flow of water between Sandhills wetlands. Note subsurface flow in north to south direction.
(Source: Frankforter 1996)
The Sandhills are among three major wetland resource areas in Nebraska that provide spring staging
areas, breeding areas, migration and wintering habitat for endangered and threatened species, including
whooping crane (Grus americana) and bald eagle (Haliaeetus leucocephalus) and for millions of
migratory waterfowl (Elliott 1991; Gersib 1991). Two percent of the mallard duck breeding population
of the north-central flyway depends on these wetlands (Novacek 1989).
Losses of these wetlands are due mostly to agriculture. The grassland economy of the Sandhills is based
primarily on cattle grazing. Ditching of wet meadows has created large acreages of subirrigated
meadows with water tables near the surface for cattle grazing and hay production. Wetland loss has
resulted mainly from center-pivot irrigation operations, with associated drainage, land-leveling, filling,
and lowered groundwater levels from deep well irrigation. These activities are largely responsible for
about 30 percent of the loss of original Sandhills wetlands (Erickson and Leslie 1987).
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Salt Flat and Salt Lake Wetlands
Salt lakes and associated salt flat wetlands are found in arid and semi-arid regions and are characteristic
of the Great Basin region. The Great Basin is a vast area of mountains and desert basins that includes
most of Nevada and western Utah (Figure 2-18). It lies between Utah’s Wasatch Mountains in the east,
the Sierra Nevada range in the west, southern Idaho and southeastern Oregon in the north, and the
Colorado River Basin in the south. The Great Basin is characterized by interior drainage, meaning that
all waters entering the Basin stay in the Basin; there is no discharge to the sea. During the Pleistocene
Epoch, much of the region was inundated by two large lakes (Lake Bonneville in the east and Lake
Lahontan in the west) and many smaller ones. Today’s salt flats, playas, and lakes are vestiges of these
waterbodies. In more recent geologic time (after the last Ice Age 10,000-20,000 years ago), many of
today’s salt lakes and salt flats were larger lakes connected by rivers. Those of the Death Valley region
(southeastern California) may have flowed into the Colorado River, as suggested by taxonomic
similarities among the fishes of these areas (Soltz and Naiman 1978).
Figure 2-18. Location of Great Basin in the western United States. (Source: U.S. Geological Survey 1970)
The Great Basin receives less than 10 inches of rain annually. Salt flats contain water for short periods in
winter and spring and become "dry" plains in summer (Figure 2-19).
Figure 2-19. Salt flat wetlands. (U.S. Fish and Wildlife Service photo)
Salt lakes include the Great Salt Lake, Mono Lake, and the Salton Sea and all may be considered isolated
(or terminal aquatic ecosystems) since they do not discharge to the ocean. Wetlands along their margins
provide habitat for many species. These lakes and the salt flats provide food for wildlife. They are
critical stopover areas for many migratory birds. For example, the shallow-water wetlands of Mono
Lake produce brine shrimp (Artemia spp.) and alkali or brine flies (Ephydra riparia) – the key food
source for migrating waterbirds. Wilson’s phalaropes feed here on alkali flies, molting and doubling
their body weight, before making their 3-day 3,000-mile nonstop flight to South America. Likewise, 1.5
to 1.8 million eared grebe feed on brine shrimp, increasing their weight three-fold, before migrating
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southward. Other birds using salt flat wetlands include red-necked phalarope (Phalaropus lobatus),
western sandpiper (Calidris mauri), least sandpiper (C. minutilla), snowy plover (Charadrius
alexandrinus), cinnamon teal (Anas cyanoptera), northern pintail, redhead, tundra swan (Cygnus
columbianus), northern harrier, short-eared owl (Asio flammeus), and savannah sparrow (Ammodramus
savannarum).
The abundance of food sources available in salt lakes is also vital to the success of breeding birds. From
44,000 to 65,000 California gulls (Larus californicus) breed on an island in Mono Lake.1 The nation’s
largest colony of American white pelicans (Pelecanus erythrorhynchos) nests on an island in Pyramid
Lake, Nevada. Other breeding birds of salt lake and salt flat wetlands include American avocet,
black-necked stilt (Himatopus mexicanus), white-faced ibis (Plegadis chihi), spotted sandpiper (Actitis
macularia), common snipe (Gallinago gallinago), and willet (Jehl 1994). Wetlands along California's
Salton Sea are habitat for the federally endangered Yuma clapper rail (Rallus longirostris obsoletus = R.
longirostris yumanensis).
Most inland salt marshes occur in the interior of the Great Basin and are not subjected to extensive
development. Major threats to Great Basin wetlands are from road and utility construction. Salt flats in
urbanizing areas are at greater risk due to impacts from encroaching urban development and associated
disruption of drainage patterns (Dennis Peters, pers. comm.).
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Wetlands of Washington's Channeled Scablands
The Channeled Scablands area is on the east side of the Cascade Mountains in eastern Washington. This
"rain-shadow" location creates a semi-desert environment that receives 7 to 10 inches of rain annually
(U.S. Fish and Wildlife Service 1998a). The post-glacial Spokane Floods created channeled scablands
and outwash lakes about 12,000 to 15,000 years ago. Today, only three creeks drain this area: Rock,
Cow, and Crab Creeks. The rest of the area is pock-marked with isolated ponds, lakes, and cyclical
wetlands (i.e., present during wet years and absent during drought years) forming a mosaic of wetlands
across the landscape (see Lincoln County, Washington study area map in Section 4).
Although many of the wetlands occur in isolated depressions, they are often interconnected during high
precipitation years, creating large wetland complexes of varied types (U.S. Fish and Wildlife Service
1998a). Common plants in these wetlands (from wettest to driest zones) include fennel-leaved pondweed
(Potamogeton pectinatus), hornwort (Ceratophyllum demersum), common water milfoil (Myriophyllum
exalbescens), broad-leaved cattail (Typha latifolia), hard-stemmed bulrush (Scirpus acutus), spikerush
(Eleocharis macrostachya), common three-square (Scirpus pungens = S. americanus), Douglas' sedge
(Carex douglasi), baltic rush, salt grass, Nevada bulrush (Scirpus nevadensis), and alkali cordgrass
(Spartina gracilis). Some ponds contain the federally-threatened water howellia (Howellia aquatilis).
These wetlands are particularly valuable for waterfowl and other migratory birds, serving as staging
areas during migration (early spring and fall) and breeding and brood-rearing habitat in summer.
Migrants using these wetlands include tundra swan, trumpeter swan (Cygnus buccinator), Canada goose
(Branta canadensis; several subspecies), Pacific white-fronted goose (Anser albifrons frontalis), lesser
snow goose (A. caerulescens caerulescens), bufflehead (Bucephala albeola), common goldeneye (B.
clangula), greater scaup (Aythya marila), hooded merganser (Mergus cucullatus), red-breasted
merganser (M. serrator), and other waterfowl. Resident waterfowl include mallard, gadwall, northern
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pintail, green-winged teal, American wigeon, northern shoveler, wood duck (Aix sponsa), redhead, ruddy
duck (Oxyura jamaicensis), western Canada goose (Branta canadensis moffitti), common merganser
(Mergus merganser), coot (Fulica americana), and others. Nearly 100,000 individual birds may breed in
these wetlands. The main breeding ducks are mallard, blue-winged teal, redhead, and ruddy duck. Other
birds dependent on these wetlands are American white pelican, great blue heron (Ardea herodius),
black-crowned night heron, common snipe, various shorebirds, avocet, sandhill crane, and bald eagle.
Scabland wetlands occur in rangeland and impacts from livestock may be significant. For example,
cattle use ponds as wallows, which often interferes with waterfowl brood-rearing. Overgrazing of
palustrine emergent wetlands also occurs. Some large ponds have been drained and converted to
hayfields and pasture. The activity of carp has muddied many ponds, reducing their value to waterfowl.
Carp removal has been initiated in some areas (U.S. Fish and Wildlife Service 1998a).
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Desert Springs and Their Wetlands
In the desert, springs arise where groundwater from large underground reserves discharges to the land
surface via fractures in underlying rock strata (e.g., fault lines) or through porous materials (e.g.,
permeable carbonate rocks). Water discharging from springs may be quite old (8,000-12,000 years for
Ash Meadow Springs in Nevada; Soltz and Naiman 1978). Isolated springs may harbor unique
populations of endemic desert fishes (e.g., pupfish, Cyprinodon spp.), invertebrates, and plants. These
springs may support wetlands of variable sizes from small fringes of vegetated wetlands to extensive
bulrush and cattail marshes (Figure 2-20) (Minckley 1991). Some springs may be hot; often they are
called "thermal springs."
Figure 2-20. A spring-fed desert wetland. (Source: Minckley 1991; photo by J.N. Rinne)
Isolated springs and seepage areas support small marshes (cienagas), oases (in California and Arizona),
and other wetlands (Figure 2-21) (Bakker 1984; Bertoldi and Swain 1996). Saline wetlands form where
water supply is persistent and drainage is limited. While some springs are isolated, others are headwaters
of rivers like the Muddy River in Nevada.
Figure 2-21. Generalized flow of water between different wetland types in Southern California. (Source: Bertoldi and
Swain 1996)
Isolation has led to the development of unique populations of certain organisms. In the Death Valley
region, there were more than 20 isolated pupfish populations in the late 1970s (Soltz and Naiman 1978).
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These populations have been isolated for 12,000 to 20,000 years and are excellent examples of biological
adaptation and evolution (i.e., speciation). Some of the species have become extinct, such as the Ash
Meadow poolfish (Empetrichthys merriami) and Tecopa pupfish (Cyprinodon nevadensis calidae), while
others are endangered, like the Owens pupfish (C. radiosus), Devils Hole pupfish (C. diabolis), and
Warm Springs pupfish (C. nevadensis pectoralis) (Soltz and Naiman 1978; Sada 1990). Some desert
springs and their adjacent wetlands also provide habitats for other threatened and endangered species or
species of concern, such as Ash Meadows centaurium (Centaurium namophilium), Ash Meadows
gumplant (Grindelia fraxino-pratensis), Ash Meadows montane vole (Microtus montanus nevadensis),
Devils Hole warm springs riffle beetle (Stenelmis calida calida), and endemic springsnails.
Pumping of groundwater for agriculture in California and urban and energy development in Nevada pose
the most serious threats to these species and the desert spring wetlands. Withdrawals may lower water
levels and expose areas used for pupfish spawning as was occurring in Devils Hole, an isolated spring
(the sole habitat for the Devils Hole pupfish) in the 1970s; this spring is now protected (Sada 1990). The
Pahrump Ranch poolfish (Empetrichthys latos pahrump) was eliminated because its springs dried up due
to groundwater withdrawals.
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Glaciated Kettle-hole Wetlands
The last continental glaciation created many depressions on the North American landscape (Figure
2-22). When the continental glacier receded (10,000-15,000 years ago), ice blocks of variable sizes were
left on the land. Over time, these ice blocks melted forming lakes and ponds. Vegetated wetlands
became established in the shallow water zone of these waterbodies. Gradually, many of these
waterbodies became filled with plant remains and eventually became vegetated wetlands (aquatic beds to
emergent wetlands to shrub bogs to forested wetlands). This process or chain of events leading to the
formation of vegetated wetlands in former waterbodies is called "hydrarch succession." While many of
these wetlands have outlets and are sources of streams, others are isolated. Their vegetation is quite
variable ranging from aquatic beds to shrub and forested bogs (see discussion on Coastal Plain ponds).
Figure 2-22. Map showing extent of last glaciation. (Source: U.S. Geological Survey 1970)
Kettle-hole bogs are wetlands found in glacial landscapes from northern New Jersey and New England to
Michigan and Wisconsin to Washington and north to Canada and Alaska. In Alaska, isolated bogs are
common in the southeast, south-central, and interior regions (Figure 2-23) (Hall et al. 1994). Kettle-hole
wetlands are also common in parts of the northeastern and north-central U.S. (Figure 2-24). They are
less common in the Pacific Northwest.
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Figure 2-23. Alaskan kettle-hole bog.
Figure 2-24. Aerial photo showing a series of kettle-hole ponds and associated wetlands in South Kingstown, Rhode
Island.
Damman and French (1987) list three hydrological conditions under which bogs form in the northeastern
United States. One of these conditions is an undrained basin, while the other two are drained basins.
The former type is an isolated wetland type that derives its water mainly from precipitation. Vegetation
may be dominated by species such as leatherleaf (Chamaedaphne calyculata), highbush blueberry
(Vaccinium corymbosum), and bluejoint (Calamagrostis canadensis) in more southern locations. They
suggest that this type is uncommon further north. More northerly kettle-hole bogs are dominated by
typical bog vegetation including ericaceous shrubs (especially leatherleaf), evergreen trees such as black
spruce (Picea mariana), balsam fir (Abies balsamea), white pine (Pinus strobus), and pitch pine (P.
rigida), and the deciduous conifer, larch (Larix laricina) (Figure 2-25). These bogs occur on Cape Cod,
in the New York Adirondacks, and elsewhere in New England (Johnson 1985).
Figure 2-25. Northeast kettle-hole bog. (R. Tiner photo)
Bogs in several northeastern States are at the southern limits for many boreal plants including hare’s tail
(Eriophorum spissum), dragon’s mouth (Arethusa bulbosa), bog rosemary (Andromeda glaucophyllum),
and Labrador tea (Ledum groenlandicum). Kettle-hole bogs and similar mountain bogs harboring these
species are particularly important sites for the conservation of biodiversity.
Threats to bogs include peat mining, drainage, and conversion to open waterbodies (e.g., recreational
lakes) or to commercial cranberry bogs. The quality of remaining kettle-hole bogs may be further
jeopardized by development of adjacent uplands. Introduction of nutrients from lawn runoff, for
example, could change the plant composition of nutrient-poor bogs over time.
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Delmarva Potholes
In the center of the Delmarva Peninsula, thousands of depressional, pothole-like wetlands called
"Delmarva bays" or "Delmarva potholes" pockmark broad drainage divides or interfluves (Figure 2-26).
Although these wetlands occur throughout the Peninsula, they are most abundant in a 20-mile swath
along the Maryland-Delaware border from the headwaters of the Sassafras River to the Nanticoke River.
They are particularly abundant near the towns of Sudlersville and Millington, Maryland and Clayton,
Kenton, and Hartly, Delaware.
Figure 2-26. Aerial view of pothole wetlands (dark depressional features) on the Upper Delmarva Peninsula.
Vegetation is variable from open glades (e.g., sedge marshes) to buttonbush shrub swamps to forested
wetlands (Figure 2-27). Potholes support 68 percent of the amphibians of the Delmarva Peninsula and
61 rare vascular plants including the federally endangered Canby’s dropwort (Oxypolis canbyi) (Sipple
and Klockner 1984; Sipple 1999).
Figure 2-27. Delmarva pothole wetland in spring. Flooded shrubs are buttonbush (Cephalanthus occidentalis). (R. Tiner
photo)
Given their abundance, Delmarva potholes undoubtedly aid in temporary storage of surface water and
thereby help reduce local flooding. During the wet season, they receive groundwater discharge and
precipitation and during the dry season, flow can be reversed, with these wetlands recharging regional
groundwater supplies (Phillips and Shedlock 1993). This underground water may eventually discharge
into Coastal Plain streams and contribute to base flows (Figure 2-28).
Figure 2-28. Generalized flow of water between Coastal Plain wetlands, showing subsurface hydrologic connections
between Delmarva bay (pothole) wetlands and other wetlands and streams. (Source: Hayes 1996)
Threats to these wetlands are from drainage usually associated with agriculture or silviculture. Some
wetlands are vulnerable to development, including houses and commercial facilities.
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Coastal Plain Ponds
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Along the Atlantic-Gulf Coastal Plain, isolated ponds have formed in depressions where groundwater
flows to the land surface and rainwater collects. In glacial areas (e.g., New England and Long Island,
New York), these ponds developed in kettle-holes or in shallow depressions in outwash plains. These
isolated ponds may be hydrologically linked by groundwater, while others may be connected to small
streams (Reschke 1990). In non-glaciated areas of the Coastal Plain (from New Jersey south), Coastal
Plain ponds formed on broad flats. Some examples of Coastal Plain ponds are Calverton Ponds and
Tarkill Pond on Long Island (New York), Clermont Bog and Bennett Bogs on Cape May (New Jersey)
(U.S. Fish and Wildlife Service 1997), and ponds on barrier islands of the Florida Panhandle, such as St.
Vincent, St. George, and Dog Islands (Wolfe et al. 1988).
Water levels fluctuate seasonally and annually, inducing significant changes in vegetation (U.S. Fish and
Wildlife Service 1997). Periodic high water levels eliminate woody seedlings that may have colonized
these ponds during drawdowns. Vegetation patterns are similar to prairie pothole wetlands, with
concentric bands of vegetation following different water regimes (permanently flooded/intermittently
exposed to semipermanently flooded to seasonally flooded zones – from wettest to driest).
The fluctuating water levels and isolated nature of coastal ponds have resulted in each pond hosting some
unique species as well as possessing many common species. These ponds may be characterized by
aquatic bed species such as water-shield (Brasenia schreberi), white water lily (Nymphaea odorata),
water milfoil (Myriophyllum humile), naiad (Najas flexilis), waterweed (Elodea spp.), and pondweeds,
and by shallow-water emergent species like bayonet rush (Juncus militaris) and spikerush (Eleocharis
robbinsii) (Reschke 1990). Rare species may be associated with some ponds. For example, in the New
York Bight region, four globally rare species - quill-leaf arrowhead (Sagittaria teres), pine barren
bellwort (Uvularia puberula var. nitida), rose tickseed (Coreopsis rosea), and creeping St. John’s-wort
(Hypericum adpressum) - may occur in these ponds (Zaremba and Lamont 1993; U.S. Fish and Wildlife
Service 1997). Rare dragonflies, damselflies, butterflies, and moths may also be found in these wetlands:
lateral bluet (Enallagma laterale), painted bluet (E. pictum), Barrens blue damselfly (E. recurvatum),
pink sallow (Psectraglaea carnosa), violet dart (Euxoa violaris), and chain fern borer moth (Papaipema
stenocelis). Vertebrates of special concern living in these ponds include the Pine Barrens treefrog (Hyla
andersonii), Cope’s gray treefrog (H. chrysoscelis), eastern spadefoot (Scaphiopus holbrookii
holbrookii), spotted salamander (Ambystoma maculatum), tiger salamander, and spotted turtle (Clemmys
guttata). Plant species of concern found in Coastal Plain ponds include red-rooted flatsedge (Cyperus
erythrorhizos), several spikerushes (Eleocharis brittonii, E. equisetoides, E. melanocarpa, E. tricostata,
and E. tuberculosa), slender blue flag (Iris prismatica), stargrass (Aletris farinosa), Pine Barrens boneset
(Eupatorium resinosum), several bladderworts (Utricularia biflora, U. fibrosa, U. juncea, U. olivacea,
and U. radiata), awned meadowbeauty (Rhexia aristosa), and Pine Barrens gerardia (Agalinis virgata).
Barrier island coastal ponds on the Florida Panhandle may be fringed with persimmon (Diospyros
virginiana), while similar ponds on the mainland are ringed with titi (Cyrilla racemiflora) (Wolfe et al.
1988). The former ponds are especially important because they often provide the only source of
freshwater on barrier islands for local wildlife and migratory birds.
Periodic drying out of coastal ponds eliminates fish from many ponds, thereby making them excellent
breeding areas for amphibians. The regionally rare tiger salamander is one of several species (which
include many vernal pool-breeding amphibians) using coastal ponds in the Northeast for reproduction
(U.S. Fish and Wildlife Service 1997). Coastal ponds in Florida may contain fish, but still serve as
breeding grounds for the southern toad (Bufo terrestris), southern leopard frog (Rana utricularia), and
pig frog (R. grylio) (Wolfe et al. 1988).
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Coastal development poses significant threats to these ponds. Disturbances that may be adversely
impacting the remaining ponds include waste dumping, all-terrain vehicle driving on pond shores, water
withdrawals, and water pollution from adjacent development, such as lawn, agricultural field, and road
runoff (U.S. Fish and Wildlife Service 1997).
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Carolina Bay Wetlands
Somewhat egg-shaped (elliptical) basins called Carolina bays have formed on the Atlantic Coastal Plain
from southeastern Virginia to Florida (Figure 2-29). They are most abundant in mid-coastal South
Carolina and southeastern North Carolina. Carolina bays vary greatly in size, ranging from less than 150
feet long to more than 5 miles in length (Sharitz and Gibbons 1982). These bays commonly have a
northwest to southeast orientation, with a conspicuous sandy rim (Figure 2-30). Most of the Carolina
bays are hydrologically isolated nutrient-poor (oligotrophic) ponds or "naturally isolated habitats" that
derive water mainly from rainfall (Sharitz and Gresham 1998). They are depressional wetlands often
surrounded by flatwood wetlands and upland forests in undisturbed areas, or by farmland and urban land
in developed areas.
Figure 2-29. Distribution of Carolina bay wetlands (larger than 800 feet long). The greatest concentration of bays are
located within the dashed area. (Source: Sharitz and Gibbons 1982)
Figure 2-30. Aerial photo showing several Carolina bay wetlands in Bladen County, North Carolina. (Note: Arrows mark
some bays for reference, but many others can be seen.)
Carolina bays are forested or shrub-dominated palustrine wetlands. Predominant trees are pond pine
(Pinus serotina), loblolly bay (Gordonia lasianthus), sweet bay (Magnolia virginiana), red bay (Persea
borbonia), swamp bay (P. palustris), pond cypress (Taxodium distichum var. nutans), and swamp black
gum (Nyssa sylvatica var. biflora). Dominant shrubs include ericaeous species such as fetterbush
(Lyonia lucida), leucothoe (Leucothoe spp.), zenobia (Zenobia pulverulenta), and highbush blueberry,
plus titi, sweet pepperbush (Clethra alnifolia), and hollies or gallberries (Ilex spp.) (Sharitz and Gibbons
1982). Their vegetation is quite similar to that of neighboring pocosins.
Many Carolina bays include seasonal ponds (e.g., vernal pools). Like other vernal pools, their vegetation
changes with the seasons. During dry periods, these ponds may be dominated by maidencane (Panicum
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hemitomon), little bluestem (Schizachyrium scoparium = Andropogon scoparius), and club-head cutgrass
(Leersia hexandra), while aquatic plants (e.g., bladderworts, Utricularia spp.) are characteristic of the
wet phase. Cypress savannas occur in Carolina bays with clay bottoms. Many rare species are
associated with these savannas (Sharitz and Gresham 1998).
Amphibians are particularly abundant in Carolina bays. Thousands of amphibians were counted in a
2.5-acre Carolina Bay called "Sun Bay" at the Savannah River Plant (Georgia) in 1979: 500 ornate
chorus frogs (Pseudacris ornata), 5,000 southern leopard frogs, and 500 mole salamanders (Ambystoma
talpoideum) (Sharitz and Gibbons 1982). Over a two-year period, researchers captured more than 72,000
amphibians including nine species of salamanders and 16 species of frogs (Gibbons and Semlitsch 1981
as reported by Sharitz and Gresham 1998). Other species common in these wetlands included the
southern toad, spadefoot toad, red-spotted newt (Notophthalmus viridescens viridescens), spring peeper
(Hyla crucifer crucifer), and green frog (Rana clamitans).
Carolina bays, especially the larger ones, are known to provide crucial aquatic habitat during droughts.
At these times, they become refuges for turtles and other animals. In areas of urban and agricultural
development, highest densities of amphibians, reptiles, and small mammals may be associated with
Carolina bay wetlands (Sharitz and Gibbons 1982). The federally endangered wood stork (Mycteria
americana) feeds heavily in Carolina bays (Sharitz and Gresham 1998).
Many Carolina bay wetlands have been drained for crop production, mainly for corn and soybeans
(Sharitz and Gresham 1998). In South Carolina, 71 percent of its Carolina bays have been altered by
agriculture, while about one-third of the original bay wetlands have been disturbed by timber harvest
(Bennett and Nelson 1991).
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Pocosin Wetlands
Pocosins (reportedly an Algonquin Indian term for "swamp-on-a-hill;" Tooker 1899) are southern bogs
with organic soils located in interfluves between major river systems (Figure 2-31). They receive all or
most of their water from precipitation (Sharitz and Gresham 1998). Their vegetation consists of a
mixture of evergreen trees, such as pond pine, loblolly pine (Pinus taeda), red bay, and sweet bay, and
broad-leaved evergreen shrubs including titi, zenobia, fetterbush, wax myrtle (Myrica cerifera),
leatherleaf, and gallberries (Ilex coriacea and I. glabra) (Kologiski 1977; Richardson et al. 1981).
Figure 2-31. Depressional pocosin wetland. (R. Tiner photo)
Pocosins occur along the Atlantic Coastal Plain from southern Virginia to Florida. They are most
abundant in North Carolina where they represent about half of the State’s wetlands (Richardson et al.
1981). About 70 percent of the Nation’s pocosins wetlands are located in North Carolina (Figure 2-32).
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Figure 2-32. Distribution of pocosin wetlands in North Carolina. Includes both isolated and non-isolated pocosins.
(Source: Sharitz and Gibbons 1982)
Specific definitions of pocosins may vary. For example, a forester’s definition might include
pine-dominated flatwoods of very wet sites, while a hydrologist’s definition might restrict the term to
shrub bogs on broad, undrained interstream areas (Sharitz and Gibbons 1982). In a textbook on
pocosins, Richardson and others (1981) define the typical pocosin ecosystem, in part, as "…
waterlogged, acid, nutrient poor, sandy or peaty soils located on broad, flat topographic plateaus, usually
removed from large streams and subject to periodic burning." While many pocosins are contiguous with
coastal and other non-isolated wetlands, many others may be located away from streams and
drainageways. In their classification of pocosins, Weakley and Schafale (1991) identify at least one
isolated type – the "small depression pocosin."2 Other pocosins may also be isolated as they occur in
swales (e.g., in the Sandhills) and in seasonally saturated interfluves.
One of the major functions of pocosins is to temporarily hold water that would otherwise run off the land
more quickly into adjacent estuaries. This water is then slowly released to the estuaries. This pocosin
function benefits the estuaries by giving them more time to assimilate the fresh water without rapid and
drastic fluctuations in water quality (Daniel 1981). Yet when drained and connected to a coastal stream,
the value of this buffering capacity is lost as ditched pocosins contribute more water (and possibly
enriched water) to stream flow. Landscape-level ditching can, therefore, have significant detrimental
effects on coastal waters.
Rare animals may live in pocosins. Examples include Hessel’s hairstreak butterfly (Mitoura hesseli) and
the Pine Barrens tree frog (Sharitz and Gresham 1998). The federally endangered red-cockaded
woodpecker (Picoides borealis) inhabits mature pond pines of pocosins, but is more abundant in mature
pine flatwoods. Wildlife of tall or forested pocosins is typical of Coastal Plain forests and include
species such as white-tailed deer, gray fox (Urocyon cineroargenteus), raccoon, opossum (Didelphis
virginiana), short-tailed shrew (Blarina brevicauda), cotton mouse (Peromyscus gossypinus), meadow
vole (Microtus pennsylvanicus), pileated woodpecker (Dryocopus pileatus), Acadian flycatcher
(Empidonax virescens), vireos (Vireo flavifrons and V. olivaceus), prothonotary warber (Protonotaria
citrea), and worm-eating warbler (Helmitheros vermivorus). In low shrubby pocosins, birds like
common yellowthroat (Geothlypis trichas), eastern wood-pewee (Contopus virens), and eastern towhee
(Piplio erythrophthalmus) may be abundant.
Forestry and agriculture have had major impacts on pocosins. Of the 2.5 million acres of pocosins that
once existed in North Carolina, roughly one million remained in natural condition by 1980 (Richardson
et al. 1981). At least 33 percent of the original acreage has been converted to agriculture or managed
forests (i.e., pine plantations), while 36 percent has been partially drained, cleared, or planned for
development. Since the 1980s, more acreage has been converted to managed forests as nearly half of the
pocosins are owned by forest companies.
Since drainage increases timber productivity, some pocosins that were naturally isolated from other
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wetlands and waters have been ditched and now are contributing sources for stream flow. Timber yields
may also be increased by fertilization; so planted pines on former pocosins are fertilized (Sharitz and
Gresham 1998). Former pocosins are cropped for soybeans and corn, but cultivation of remaining
pocosins may have decreased recently due to removal of farm subsidies. Agricultural conversion of
pocosins has some significant consequences: 1) lowered salinity in adjacent estuaries, particularly during
heavy rainfall periods due to introduction of more fresh water (cropland drainage), 2) increased peak
flow rates (up to 3 or 4 times that of undrained areas) and decreased flow durations, 3) increased
turbidity (ditches had 4 to 40 times greater turbidity than that of natural streams in pocosin areas,
depending on development phase – less at post-development), and 4) increased concentration of
phosphate, nitrate, and ammonia in streams and adjacent estuaries (Sharitz and Gresham 1998). From
this information, it is evident that human use of pocosins has adversely impacted water quality of
adjacent streams and estuaries. Drainage of pocosins and its effects on estuarine salinity (decreased
salinity) may be having a negative impact on North Carolina’s brown shrimp (Street and McClees 1981).
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Cypress Domes3
Cypress swamps found in nearly circular isolated depressions are called cypress domes due to the
appearance of the trees which are tallest in the center of the pond. Cypress domes are usually small in
size, usually 2.5-25 acres and often form an ecological mosaic within the pine flatwoods (Mitsch and
Gosselink 2000). They are abundant in Florida and southern Georgia.
Cypress domes receive water from precipitation, groundwater flow, and sometimes runoff. Most of the
water arrives with summer rains in southern Florida or with winter and summer rains in northern Florida
and southern Georgia (Ewel 1998).
Pond cypress and swamp black gum are the dominant trees of cypress domes (Mitsch and Gosselink
2000). Other trees include slash pine (Pinus elliottii), swamp bay, and sweet bay. The shrub layer may
be comprised of fetterbush, wax myrtle, red maple (Acer rubrum), buttonbush (Cephalanthus
occidentalis), and Virginia willow (Itea virginica). Common herbs include Virginia chain fern
(Woodwardia virginica), lizard's tail (Saururus cernuus), and red root (Lachnanthes carolinana). Slash
pine co-dominates with pond cypress in partly drained domes in north-central Florida (Mitsch and Ewel
1979).
These wetlands are important for maintaining regional biodiversity. In particular, many domes are
significant amphibian-breeding areas, much like their vernal pool counterparts. Species such as the
carpenter frog (Rana virgatipes) may reproduce in these isolated swamps (McDiarmid 1978, as reported
in Ewel 1990). Also given that cypress domes hold water for long periods, they help prevent flooding of
local areas and aid in groundwater recharge. Drainage of domes could lead to increased flooding (Ewel
1998).
Although timber management has been performed in cypress dome-pine flatwood ecosystems for
hundreds of years, the most detrimental effect people have on this ecosystem is development, including
conversion of natural habitat to residential subdivisions, commercial sites, and golf courses. Virtually all
cypress ponds in northern Florida have been cut over, but many have regenerated (Ewel 1990). Drainage
of cypress domes causes organic soils (peats) to oxidize and the land to subside. A drying out of these
swamps increases their susceptibility to destruction by fire. Other species may then colonize these sites.
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Sinkhole Wetlands
Isolated depressional wetlands are common features in karst landscapes. Major karst regions are shown
on the map in Figure 2-33. Dissolution of underlying limestone causes a slumping of the land surface,
thereby creating distinct basins which may or may not be connected to surface water or ground water.
Wetlands that form in depressions in karst topography are commonly referred to as sinkhole wetlands.
Lost streams (e.g., streams that disappear underground) and underground caverns may be common in
karst areas.
Figure 2-33. Location of major karst regions in the United States. (Source: U.S. Geological Survey 1970)
Some sinkhole wetlands receive groundwater discharge from underlying limestone deposits (e.g., in karst
valleys). Others simply occur in basins formed by the dissolution of underlying limestone (Figure
2-34). Many cypress domes formed in such basins (see previous discussion).
Figure 2-34. Generalized water flow patterns between wetlands in a karst region. (Source: Haag and Taylor 1996)
Karst lakes and associated marginal wetlands may also be geographically isolated features on this type of
landscape. Many areas in Florida are pock-marked with isolated depressional wetlands and lakes due to
the abundance of limestone on the peninsula (Figure 2-35). Some lakes drain into streams connecting to
larger ones flowing to the sea, while many do not.
Figure 2-35. Aerial photo showing karst lakes and wetlands in northern Florida.
The vegetation of sinkhole wetlands varies geographically and in response to different hydrologies and
other factors. The wetter ones may be ponds and marshes with cattail (Typha sp.), water plantain, water
parsnip (Sium suave), St. John’s-worts (Hypericum spp.), sedges (Carex spp.), bulrushes, beak-rushes
(Rhynchospora spp.), spikerushes (Eleocharis spp.), manna-grasses (Glyceria spp.), rice cut-grass
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(Leersia oryzoides), and bluejoint, and two hydrophytic shrubs - buttonbush and swamp rose (Rosa
palustris). Other shrubs and trees may colonize drier sites: green ash (Fraxinus pennsylvanica) and
willows (Salix spp.) in New York and red maple, black birch (Betula lenta), persimmon, pin oak
(Quercus palustris), black gum (Nyssa sylvatica), green ash, black willow (Salix nigra), and common
winterberry (Ilex verticillata) in western Maryland and West Virginia (Reschke 1990; Bartgis 1992). In
western Maryland, Bartgis (1992) found 56 species in sinkhole ponds including the federally endangered
northeastern (barbed-bristle) bulrush (Scirpus ancistrochaetus). He noted that these ponds formed on
sandstone and are not in direct contact with the underlying limestone deposit. Lentz and Dunson (1999)
reported that "geographically isolated" ponds in central Pennsylvania supported northeastern bulrush.
The federally-endangered swamp pink (Helonias bullata) and federally-threatened Virginia sneezeweed
(Helenium virginicum) have been found in sinkhole wetlands in western Virginia (Buhlmann et al.
1999). Pond cypress is common in sinkhole ponds in Florida. Rare plants, such as smooth-barked St.
John’s-wort (Hypericum lissophloeus) and karst pond xyris (Xyris longisepala), may be found along the
shores of some of Florida’s karst lakes (Wolfe et al. 1988).
Sinkhole ponds may be productive amphibian breeding grounds. For example, a one-half acre pond in
Alabama had more than 1,500 adult amphibians: 527 mole salamanders, 127 tiger salamanders, 269
gopher frogs (Rana capito), 241 leopard frogs, and 191 ornate chorus frogs (Pseudacris ornata) (Bailey
1999). Marbled salamander (Ambystoma opacum), spotted salamander (A. maculatum), red-spotted newt
(Notophthalmus viridescens), wood frog (Rana sylvatica), spring peeper (Pseudacris crucifer), green
frog (Rana clamitans), and cricket frog (Acris crepitans) breed in sinkhole ponds in the Virginia's
Shenandoah Valley (Buhlmann et al. 1999). Ponds with peat moss covering the bottom may support the
four-toed salamander (Hemidactylium scutatum) in this area, while spring-fed permanent sinkhole ponds
may be breeding grounds for the spring salamander (Gyrinophilus porphyriticus). Bullfrogs (Rana
catesbeiana), pickerel frogs (R. palustris), spotted turtles (Clemmys guttata), snapping turtles (Chelydra
serpentina), painted turtles (Chrysemys picta), northern water snakes (Nerodia sipedon), and many
invertebrates (e.g., dragonflies, water beetles, and fairy shrimp) also frequent in these wetlands.
There is an entire world of organisms beneath the earth in underground caves. Specially-adapted cave
animals (troglobites) such as blind salamanders (several genera of the family Plethodontidae) live in the
subterranean pools and streams.
Some threats to these ecosystems are: 1) water pollution from lawn, agricultural field, and road runoff or
from direct discharge of wastes, 2) groundwater withdrawals, 3) impoundment of local streams, 4) timber
harvest of adjacent forests (e.g., habitat for the pond-breeding amphibians), 5) fish stocking of sinkhole
ponds, and 6) agricultural and residential development (Wolfe et al. 1988; Buhlmann et al. 1999).
Activities that negatively impact local water tables may pose the most insidious threat.
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Former Floodplain Wetlands
Major shifts in river courses over time have left some wetlands isolated on former floodplains. For
example, the Mississippi River has changed its course many times over thousands of years. In addition
to the natural processes affecting floodplains, humans have constructed levees to prevent flooding, so
that such lands could be used for agriculture, development, or other purposes. Consequently, many
wetlands that were flooded seasonally during high water periods are now separated from the river and not
overflowed during flood stages.
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Former floodplain wetlands are very common in Alaska. Rivers such as the Yukon and Kuskokwim
have migrated back and forth in broad valleys over the course of thousands of years. As a result of these
shifts, many oxbow channels and meander scars are now isolated - sometimes miles away from the active
river channel. In some areas, the historic floodplain of the Yukon River is over 15 miles wide (Jon Hall,
pers. comm.). An outstanding example illustrating these types of isolated former floodplain wetlands can
be seen along Alaska’s Porcupine River (Figures 2-36 and 2-37). These wetlands and waterbodies are
havens for waterfowl. Isolated wetlands and lakes in the Yukon Flats are among Alaska’s most
important waterfowl nesting areas, with an average breeding population over one million ducks (Lensink
and Derksen 1986).
Figure 2-36. Map showing isolated wetlands that were former floodplain wetlands along Alaska’s Porcupine River. Red
areas are isolated wetlands.
Figure 2-37. Aerial photo showing mostly former floodplain wetlands (e.g., meander scars and oxbows) along Alaska’s
Porcupine River.
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West Coast Vernal Pools4
West Coast vernal pools are cyclical wetlands that exhibit a marked seasonal shift in herbaceous
vegetation from hydrophytic species to drier-site species (Tiner 1999). Their vegetation may change
drastically within years and between years in response to changing environmental conditions (e.g.,
precipitation patterns).
West Coast vernal pools occur from southern Oregon to northern Baja Mexico. Many vernal pools and
associated seasonally flooded wetlands form a complex of depressional wetland and swale features that
are hydrologically linked during wet periods (Figure 2-38). They are typically filled with water by
winter rains, characteristic of the region’s Mediterranean climate. They may be flooded for weeks or
months in some years (Baskin 1994). They achieve their greatest size in extremely wet years when
individual depressions coalesce to form enormous complexes (Zedler 1987). When the pools form such
complexes, they may drain into intermittent streams, ditches, or perennial streams.
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Figure 2-38. Aerial photo showing Southern California vernal pool complex (Miramar Naval Air Station, San Diego).
Note white areas are tops of mounds and darker areas between them are interconnected vernal pool swales.
The isolated nature and unpredictable flooding promote endemism in vernal pool plants and animals
thereby creating unique flora and fauna. This makes West Coast vernal pools vital sites for the
conservation of biodiversity (Baskin 1994). Numerous federally-listed threatened and endangered
species as well as state-endangered and rare species are among the characteristic flora of West Coast
vernal pools. Some of the federally endangered species are San Diego mesa mint (Pogogyne abramsii),
Otay mesa mint (P. nudiuscula), several species of Orcutt grasses (Orcuttia spp.), Solano grass (Tuctoria
mucronata), San Diego button-celery (Eryngium aristulatum var. parishii), and Burke’s goldfields
(Lasthenia burkei). All are amphibious species that are found in the aquatic phase and the drying phase
of vernal pool ecosystem system development (Zedler 1987) (Figure 2-39). Vernal pools also possess
endangered and rare invertebrates such as the endangered delta green ground beetle (Elaphrus viridis)
(Morris 1988). Due to endemism, numerous vernal pool regions in California have been designated in
recognition of their unique flora.
Figure 2-39. Jepson Prairie, a large vernal pool, near Sacramento, with goldfields in bloom. (R. Tiner photo)
Historically, vernal pool areas were used for grazing and agriculture. Grazing may have relatively little
adverse effect on these ecosystems in contrast to the destruction of vernal pools brought about by tillage
and planting crops (Zedler 1987). More recently, population growth in California and corresponding
urbanization have exacted a great toll on these ecosystems, while agriculture continues to play a major
role in their demise (Keeler-Wolf et al. 1998). Many of these ecosystems have been destroyed, with the
largest remaining complexes often found in the open lands surrounding military airports and facilities
(e.g., Miramar Naval Air Station and Camp Pendleton). See Keeler-Wolf and others (1998), Witham and
others (1998), and Jain (1976) for more information on California vernal pools.5
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Woodland Vernal Pools
Virtually every forested region possesses examples of woodland vernal pools. The variety of vernal pool
wetlands is considerable due to differences in climate, geology, hydrology, and other factors. Vegetation
colonizing such sites and the aquatic species living there vary regionally. There is no single reference
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describing this variability. Although the following discussion focuses on these wetlands in the
northeastern United States, the same principles apply to all woodland vernal pools (regarding their
importance to amphibians), despite regional differences in species composition.
Vernal pools are temporary or ephemeral ponds that are inundated during the wet season, usually from
late fall to mid- or late-summer in the Northeast (Figure 2-40). They range in size from a hundred square
feet or less to several acres. Vernal pools may dry out every year or less often. The fluctuating water
levels preclude the establishment of fish populations. The aquatic habitat and lack of predatory fish
make these pools desirable and extremely productive sites for amphibian reproduction. Species
dependent on vernal pool for breeding include marbled salamander (Ambystoma opacum), spotted
salamander, Jefferson salamander (A. jeffersonianum), blue-spotted salamander (A. laterale), wood frog,
and gray treefrog (Hyla versicolor). Other aquatic species that also reproduce in these ponds include
spring peeper, American toad, green frog, and red-spotted newt. Spotted turtles frequent vernal pools
after winter hibernation to obtain an easy source of food such as amphibian eggs and aquatic
invertebrates (Kenney and Burne 2000).
Figure 2-40. Massachusetts woodland vernal pool. (R. Tiner photo)
While many amphibians use vernal pools for reproduction and growth of larvae, the adults of most
species spend the rest of their lives in the surrounding woodland either as burrowing vertebrates or
arboreal species. This makes the vernal pools plus the surrounding forest vital habitats for their survival.
In addition, each pool is often used by multiple species for breeding (e.g., marbled salamanders in fall,
spotted salamanders and wood frogs in early spring, followed by spring peepers and gray treefrogs).
Thousands of larvae may be produced from a single pool. For example, in a one-acre pond in eastern
Massachusetts, nearly 14,000 adult amphibians were counted and a two-acre pond in western
Massachusetts had 5,000 to 10,000 spotted salamanders and several times as many wood frogs and
spring peepers (Tiner 1998). Vernal pools and their adjacent woodlands are vitally important for the
conservation of biodiversity. In Massachusetts, the intricate fairy shrimp (Eubranchipus intricatus) is
known to occur in only 10 pools, and the eastern spadefoot has been reported at only 40 sites statewide
(Kenney and Burne 2000).
Since vernal pools are small and surrounded by upland, they are often destroyed by development (e.g.,
construction of houses, shopping malls, and commercial facilities). Pools located along roads may be
used as stormwater detention basins. Others receive drainage from agricultural fields or residential lawns
that degrade pool water quality. Mosquito spraying of pools and pool drainage also jeopardize vernal
pool wildlife. Groundwater withdrawals for private and public wells may drawdown vernal pool waters
prematurely and not allow for complete development of amphibian larvae (Kenney and Burne 2000).
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Coastal Zone Dune Swale and Deflation Plain Wetlands
Sandy beaches and sand dunes have formed along much of the U.S. coastline (e.g., Atlantic Ocean, Gulf
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of Mexico, Pacific Ocean, and the Great Lakes). Aeolian processes have created a rolling terrain of
ridges and relatively narrow swales and in some cases, broader deflation plains. The ridge-and-swale
complex forms dunefields of variable dimensions. The low depressions are often called dune swales and
many are wetlands. They occur as a series of low valleys between dune ridges or may be more randomly
dispersed on the land (Figure 2-41).6 These wetlands intersect the local groundwater tables and support a
variety of hydrophytic plants.
Figure 2-41. Map showing dune swale wetlands near the tip of Cape Cod, Massachusetts (Provincetown). Red areas
designate isolated wetlands among the sand dunes.
Although most dune swales are isolated landforms surrounded by dry sand dunes, some are
hydrologically connected to adjacent waters. The closer the swale is to the nearby waterbody, the higher
the likelihood for a hydrologic linkage. For example, dune swales along the shores of the Great Lakes
have water tables controlled by lake levels, while those further away are wet due to groundwater seepage
(Albert 2000).
Vegetation in the swales and deflation plains is variable, depending on the hydrology and geography.
The wettest swales are ponds and marshes.7 They are rich aquatic habitats supporting numerous aquatic
invertebrates and vertebrates alike. An abundance of aquatic insects in spring provides food for
migratory birds. The drier swales are wet meadows, shrub swamps, and forested wetlands. Dune swale
wetlands are colonized by many wide-ranging hydrophytic plants. Along the Atlantic Coast, wet dune
swales may be colonized by common three-square, Canada rush (Juncus canadensis), marsh fern
(Thelypteris thelypteroides), wool-grass (Scirpus cyperinus), cranberries (Vaccinium macrocarpon and V.
oxycoccos), salt hay cordgrass (Spartina patens), northern bayberry (Myrica pensylvanica), red
chokeberry (Aronia arbutifolia), and wax myrtle (Tiner and Burke 1995; Ralph Tiner, personal
observations). Albert (2000) reported buttonbush, willows, alders (Alnus spp.), northern white cedar
(Thuja occidentalis), and larch in similar habitats along the Great Lakes. Bogs, with bog laurel (Kalmia
polifolia), cranberries, leatherleaf, and Labrador tea predominating, may form in the wet swales along
Lake Superior (Albert 2000). For wet dune swales and broad deflation plains in the Pacific Northwest,
several distinct communities have been reported (Wiedemann 1984). Dominant species in these
communities included sickle-leaved rush (Juncus falcatus), springbank clover (Trifolium wormskjoldii),
slough sedge (Carex obnupta), broad-leaved cattail, mountain Labrador-tea (Ledum glandulosum),
Douglas’ spiraea (Spiraea douglasii), hooker willow (Salix hookeriana), Pacific wax myrtle (Myrica
californica), lodgepole pine (Pinus contorta), and western red cedar (Thuja plicata). Where deposition
of wind-blown sand is heavy and dune migration is active, dune swale wetlands may become uplands
when covered by thick sand deposits.
Dune marshes and ponds are vital habitats for many species (Wiedemann 1984). Dune marshes along
the Oregon coast are vital habitat for 61 bird species, 17 mammals, 5 amphibians, and two reptiles (Akins
1973). They also provide winter habitat for 49 species of waterfowl, shorebirds, and wading birds.
Fowler’s toad (Bufo woodhousei fowleri) breeds in wet dune swales along the Northeast coast (Kenney
and Burne 2000).
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Some unique species are associated with wet dune swales. Plants found nowhere else in some States
(e.g., Indiana) are found in these locations (Hiebert et al. 1986). Houghton’s goldenrod (Solidago
houghtonii), a federally threatened species, occurs in dune swales along Lakes Huron and Michigan,
while State-rare species such as Lapland buttercup (Ranunculus lapponicus) and round-leaved orchid
(Amerorchis rotundifolia) may be found in Michigan’s interdunal forested swales (Albert 2000). The St.
Andrew beach mouse (Peromyscus polionotus peninsularis), a federally endangered species, may
frequent wet dune swales of Florida’s Panhandle, although the swales are not the species’ primary habitat
(U.S. Fish and Wildlife Service 1998b). Blanchard’s cricket frog (rare in Michigan) lives in shallow
interdunal ponds. Other rare dune swale plants include butterwort (Pinguicula vulgaris) in Michigan
(Albert 2000) and horned bladderwort (Utricularia cornuta) and arrow-grass (Triglochin maritimum) in
Indiana (Doug Wilcox, pers. comm.; Hiebert et al. 1986).
Threats to dune swale wetlands include residential housing, golf courses, and resort development.
Invasion by introduced species may pose problems in some areas (e.g., Michigan; Albert 2000).
Back to Top
Great Lakes Alvar Wetlands
Alvar wetlands are a rare and unfamiliar wetland type. Alvars include both wetlands and terrestrial
habitats that occur in open landscapes on exposed, flat limestone/dolomite bedrock pavement in humid
and sub-humid climates (Reschke et al. 1999). They are rock garden-like environments with thin soils
over horizontal bedrock outcrops. These exposed pavements are usually surrounded by forest (the
dominant plant community in the region). Alvars are classified as globally imperiled habitats by The
Nature Conservancy (Reid 1996).
Alvars have recently received considerable study along the Great Lakes in both the U.S. and Canada
(Reschke et al. 1999). Most wet alvars are subjected to flooding in spring. While most dry out by early
summer, some alvar wetlands remain flooded for weeks (Reid 1996). Ponding comes mainly from
snowmelt and precipitation. The wetter alvars may occur as isolated depressions within larger drier
alvars (uplands). Hydrophytes, including spikerushes and sedges, characterize these rocky wetlands.
Reschke (1990) reported slender spikerush (Eleocharis elliptica var. elliptica), balsam groundsel or
balsam ragwort (Senecio pauperculus), Crawe’s sedge (Carex crawei), and mosses (Bryum cespiticium
and Drepanocladus spp.) as species characteristic of wet alvar grasslands in New York. A technical
report on Great Lakes alvars listed the tufted hairgrass wet alvar grassland community (Reschke et al.
1999). Dominant plants included tufted hairgrass (Deschampsia cespitosa), Crawe’s sedge, and
flat-stemmed spikerush (Eleocharis compressa). Average soil depth for this community is less than 4
inches. This alvar type is saturated or flooded in spring and fall and very dry in midsummer.
Rare species are typical of alvars. In Michigan, some State-rare species associated with small
depressional wet alvar grassland are flat-stemmed spikerush and the sedge Carex scirpoides (Dennis
Albert, pers. comm.).
Threats to alvars in general include quarrying, rural development, all-terrain vehicles (ATVs), and
invasive plants. Construction of cottages, vacation homes, and trailer parks poses problems for many
alvars, especially those along the Great Lakes shore. ATVs driven across alvars disrupt hydrologic
patterns, rut alvar surfaces, and favor the spread of invasive plants. Common buckthorn (Rhamnus
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cathartica), St. John’s-wort (Hypericum perforatum), honeysuckles (Lonicera tatarica and L. morrowii),
Canada bluegrass (Poa compressa), and rough-fruited cinquefoil (Potentilla recta) are among the more
problematic invasive species (Reschke et al. 1999).
Back to Top
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Footnotes
1 Data source: www.monolake.org/naturalhistory/birds.
2 Meador (1996) cites Sharitz and Gibbons (1982) as characterizing pocosins and Carolina bays "as examples of isolated
wetlands…."
3 The Dade City study area had 7,754 acres of wetlands dominated by cypress. Of these, about 57 percent (4,403 acres)
were classified as isolated.
4 Examples of West Coast vernal pool complexes can be seen on the maps for three study areas: Sacramento, Bird
Landing, and La Mesa (Region 1).
5 See also the California Wetlands Information System: ceres.ca.gov/wetlands.
6 An example of these wetlands can be seen on the map of the Coquille River study area (Region 1).
7 These ponds may be included in the category - Coastal Plain Ponds - described previously.
8The latter study areas may or may not be typical of these regions, since determining "typical" would require some form of
statistical analysis. They simply represent areas where data were collected and analyzed.
9 In evaluating these connections for this study, we relied on river and stream locations depicted on U.S. Geological
Survey topographic maps and represented in digital forms on digital line graphs (DLGs) or digital raster graphics (DRGs).
Consequently, connections by unmapped intermittent streams could not be detected. This would require extensive field
work or review of large-scale aerial photographs when trees are in their leaf-off condition.
10 Wetlands along isolated navigable lakes such as the Great Salt Lake (in the Great Basin) and wetlands along streams
draining into these lakes have traditionally been viewed as non-isolated. We also considered them non-isolated. See
Rockport Lake study area (Region 6).
11 Examples of this can be observed in numerous study areas including Edgemere, Porcupine Mountain, Epping,
Frederick, and Newton (Region 5).
12 See Dade City and Crystal Lake study areas (Region 4) for examples.
13 Affected study areas included Four Mile Flat (NV), Black Thunder (WY), Rainwater Basin (NE), Hill Lake (NE), and
Rockport Lake (UT).
14 Road-fragmented wetlands were labeled manually.
15 Most NWI maps have target mapping units of 1-3 acres, so the smallest wetlands typically mapped fall within that
range. Smaller wetlands are routinely mapped in some areas, such as the Prairie Pothole Region.
16 These wetlands may be regulated due to their close proximity to rivers and their periodic flooding.
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Section 3. Extent of Isolated Wetlands in Selected Areas
There are no existing documents that identify the extent of isolated wetlands across the country. In the
absence of national numbers, the Service decided to initiate a relatively small-scale study to determine
the extent of isolated wetlands in selected areas. This could be accomplished because the Service has
produced National Wetlands Inventory (NWI) maps for 90 percent of the conterminous United States, 35
percent of Alaska, and all of Hawaii. Many of these maps have been digitized to create data that can be
used for geospatial analyses through the use of geographic information system (GIS) technology. While
the Service's wetland classification system (Cowardin et al. 1979) does not have a descriptor for
identifying geographically isolated wetlands, the Service has developed a set of hydrogeomorphic-type
descriptors that can be added to the NWI data to, among other things, separate geographically isolated
wetlands from wetlands with other water flow paths (i.e., throughflow, outflow, inflow, and bidirectional
flow) (Tiner 2000). Such descriptors have been added to NWI digital data in various watersheds to better
characterize wetlands and to produce preliminary assessments of wetland functions (see Tiner et al. 1999,
2000 for examples). With this information, technology, and experience, the Service designed a study to
use existing digital data to produce estimates of potentially isolated wetlands in numerous areas of the
country. This section describes the study and its findings.
Study Methods
Selection of Study Areas
Study areas fall into two broad categories: 1) areas with an expected high percentage of isolated
wetlands and 2) areas associated with major physiographic regions.8 Areas with extensive coastal
wetlands and broad floodplains (e.g., Mississippi delta) were generally avoided because their wetlands
are typically not isolated, although a few such areas were included in the study.
Since the analysis would be done through the use of geographic information system (GIS) technology,
potential study areas were limited by available digital data. Ideally, candidate sites must have had the
following data available: 1) NWI digital data, 2) U.S. Geological Survey digital line graphs (DLGs) for
hydrology, and 3) U.S. Geological Survey digital raster graphics (DRGs). Regional Wetland
Coordinators from the Service's seven regions were consulted to choose potential study sites
(approximately 4-8 quads in size) for their regions, considering both areas where isolated wetlands were
expected to occur and areas representing other physiographic regions. We reviewed their selections
based on the availability of the above geospatial data. Where all the above data were available, a 4-quad
study area was usually selected. Two much larger areas (i.e., Devils Lake, North Dakota and Horry
County, South Carolina) were evaluated in the early stages of the analysis when testing the methods.
Where the three digital data sources were not available, we provided the Coordinators with information
on possible alternative areas in the vicinity of their original sites where such data were available and
asked them to select study areas accordingly. In the Southwest, NWI digital data were only available for
the Texas coastal zone, so potential study areas included areas where NWI maps and DLGs or DRGs
were available.
In selecting study areas, the objective was to have a minimum of six study areas for each Fish and
Wildlife Service Region in the conterminous U.S. and a minimum of three sites for Alaska. Seventy-two
study sites were evaluated in 44 States across the country: eight in U.S. Fish and Wildlife Service
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Region 1 (totaling 1,934 square miles), nine in Region 2 (2,012 square miles), 10 in Region 3 (2,181
square miles), 12 in Region 4 (3,903 square miles), 19 in Region 5 (4,267 square miles), 11 in Region 6
(3,716 square miles), and three in Region 7 (742 square miles) (Figure 3-1). In total, the analysis
covered nearly 19,000 square miles. From an ecoregion standpoint, the study areas fell into more than 20
ecoregions (see Figures 3-2 and 3-3). Study sites were located in all major U.S. watersheds (Figure 3-4),
with at least one site per watershed.
Figure 3-1. Location of study areas by U.S. Fish and Wildlife Service Region and State. Study area names are given.
Figure 3-2. Location of study areas in the conterminous U.S. by Bailey ecoregions. (Source: Bailey 1995)
Figure 3-3. Location of study areas in Alaska by Bailey ecoregions. (Source: Bailey 1995)
Figure 3-4. Location of study areas in major watersheds.
Definition of Isolated and Non-isolated Wetlands for the Study
We used a landscape-based or geographic definition of isolated wetlands for this study that allowed us to
readily and consistently extract information from existing digital data sources for tabulation and
reporting purposes. Isolated wetlands were defined as wetlands with no apparent surface water
connection to perennial rivers and streams9, estuaries, or the ocean. Streamside wetlands where the
stream disappeared underground or entered an isolated (no surface water outflow) lake or pond, as in
karst topography, were classified as isolated. Wetlands associated with most isolated waterbodies were
also identified as isolated.10 Geographically isolated wetlands may be linked hydrologically to other
wetlands or streams via subsurface flows (e.g., prairie potholes and Nebraska Sandhills wet meadows) or
infrequent overflows (e.g., West Coast vernal pools), but this was impossible to determine using existing
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digital data. For this study, these wetlands were considered geographically isolated because: 1) they lack
an apparent surface water connection, or 2) hydrologic linkages to streams or other waterbodies could not
be determined using the available digital datasets. Readers should note that "isolated wetlands"
referenced in this report are best described as "potentially isolated," since an accurate determination of
isolation (as defined in this report or elsewhere) requires field verification in many, if not most, cases.
For this study, non-isolated wetlands included wetlands located: 1) along perennial rivers and streams
and their intermittent tributaries (with some exception for karst regions – see remarks above) (Figure
3-5), 2) along the shores of lakes with outlets to rivers and streams, 3) along the margins of very large
isolated lake systems (e.g., Great Salt Lake; see footnote 10), 4) along estuaries (Figure 3-6), and 5)
along the shores of oceans and seas. Due to these landscape positions, many non-isolated wetlands are
subjected to periodic flooding during high water stages. Others are groundwater-fed wetlands with
surface drainage. Headwater wetlands serving as sources of streams were considered non-isolated since
their connection to streams is apparent.
Figure 3-5. Non-isolated freshwater wetlands along a meandering river. (R. Tiner photo)
Figure 3-6. Non-isolated brackish coastal marsh. (R. Tiner photo)
Data Compilation and Analysis
This study involved compiling existing data, creating new digital data, and geoprocessing digital data.
Existing digital data sources included: 1) NWI polygon data, 2) digital line graph (DLG) hydrology
coverages for study area quads, and 3) digital raster graphics (DRGs) for study quads. The NWI polygon
data served as the prime source of wetland and deepwater habitat data, while the DLG hydrology layer
was the major source of stream data. DRGs were used as collateral data to evaluate wetlands that were
not readily identified as isolated or non-isolated. Note that the wetland digital data used for this analysis
did not include NWI linear or point coverages. The analysis was strictly a GIS operation as no field
work was performed.
The DLG hydrology data represented a consistent dataset for the analysis of wetland-stream connectivity
and isolation. Preference was given to NWI polygon data for wetlands and to the DLG hydrology data
for streams. In some cases, when reviewing the draft map products from the data compilation, gaps
between the two sources were detected (e.g., wetland smaller on NWI than swamp symbols on the DLG
and so stream did not intersect NWI wetland). These areas were reviewed to insure proper wetland
classification. In hilly or mountainous terrain with prominent dendritic drainage patterns, most of the
gaps were artifacts, so the upstream wetlands were classified as non-isolated.11 In karst regions where
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perennial streams "disappear" as they go underground (e.g., "lost streams") and appear spatially isolated,
wetlands along these stream segments were considered as isolated, since they lack a surface water
connection to other streams (as per our definition).12 In the few areas where DLG hydrology data were
not available, we created a hydro-line coverage from the DRG by culling out the blue line features
(representing streams) from the digital data.13 The result was a crude hydro-line that was used for the
analysis.
Stream data were buffered (e.g., stream width expanded) because linear (line-width) streams from the
DLG may not intersect NWI wetlands that are actually streamside wetlands, due to differences in spatial
accuracy (i.e., linking the two digital datasets). Buffering the streams helped remedy this situation. For
the analysis, stream buffers of two-widths were used: 1) 20 meters and 2) 40 meters. The 20m buffer
was initially considered sufficient to intersect the two data layers. However, in reviewing some of the
preliminary draft maps, some headwater wetlands (sources of streams) were not selected as being
associated with streams because of a data gap. A second buffer width - 40m - was chosen to capture
these wetlands. Wetlands in the 20-40m zone were highlighted on the maps and tabulated separately. In
this way, two scenarios could be produced: one that included them as isolated (when a 20m buffer was
used) and another that included them as non-isolated or connected to streams (when the 40m buffer was
used). Readers could then see whether including these wetlands as isolated or not would significantly
affect the totals for isolated wetlands in each study area.
In reviewing draft maps (combined NWI and DLG coverages), we also noticed that wetlands separated
by roads had been designated as isolated even when the adjacent wetland was classified as non-isolated.
Subsequently, we added another category to the maps to highlight these wetlands. Although they are
called "road-fragmented wetlands," they also included wetlands fragmented by railroads.14 We suspect
that many of these wetlands are connected to the non-isolated wetlands by a culvert but could not be
certain, hence the specific classification. In this analysis, "road-fragmented wetlands" were included as
non-isolated in two scenarios and isolated in one scenario to give readers a range of the estimated extent
of isolated wetlands (see discussion of scenarios below).
In the Southwest, only the Texas coastal zone had digital data required for the isolated wetland analyses
using the methods described above. Since we wanted to have a few study sites in each part of the
country (including the playa region), we had to take an alternative approach where NWI digital data were
not available. In this case, we scanned and vectorized the NWI maps, separated wetlands from
deepwater habitats, labeled the general type (wetland or deepwater habitat), then performed the typical
analysis using the DLG hydro data. Since we did not label the polygons to the specific NWI
classification, we only reported quantitative data for these study areas.
Certain wetlands occurred along the edges of maps, extending into maps outside the study area. In
general, we evaluated the larger map-edge wetlands to determine their status as isolated or not. Smaller
ones were typically not reviewed and were labeled as "map-edge" wetlands. These unclassified wetlands
were not included in the analysis. They represented only a minute fraction of the study area wetlands.
Their totals, however, are listed on a detailed statistical data sheet for each study area.
Scenarios for Data Presentation
Three scenarios were chosen to generate estimates of isolated wetlands due to possible interpretations of
"isolated wetlands" through GIS analysis. Estimates of isolated wetlands were compiled for each study
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area following three basic scenarios, ranging from restrictive to broad interpretations of isolated
wetlands.
Scenario 1, the most restrictive scenario, designated only those wetlands that
had the highest likelihood of not being connected to a river, stream, or estuary
as isolated (shown in red on the maps). This scenario produced the most
limited extent of isolated wetlands (i.e., the red wetlands on the maps).
Scenario 2 used a slightly broader interpretation of isolated wetlands, including
wetlands in the 20-40m buffer (colored orange on the maps) as isolated. Under
this scenario, both the red and orange-colored wetlands were considered
isolated.
Scenario 3, the broadest interpretation, added the road-fragmented wetlands
(colored brown on the maps) to the isolated wetland category. The connection
of these wetlands to neighboring non-isolated wetlands was not evident. This
scenario generated the highest number and greatest extent of isolated wetlands
for most study sites (i.e., the red, orange, and brown wetlands were considered
isolated).
Thus a range of estimates for the number and acreage of isolated wetlands was generated for each study
area. These data are most useful for describing isolated wetlands in relative, not absolute, terms.
GIS-generated Products
Thematic maps and accompanying estimates were produced for each study area. For most areas, a set of
four 1:24,000 maps were tiled together to produce a study area map at 1:50,000. For other areas, the map
scale varied to provide a satisfactory graphic representation of the information.
The maps were designed to highlight geographically isolated wetlands. The following aquatic features
were depicted on the maps: 1) isolated wetlands (red polygons; wetlands with the highest probability of
being geographically isolated), 2) wetlands 20-40 meters from streams (may be isolated or may be
connected to stream; orange polygons), 3) road-fragmented wetlands (possibly connected but uncertain;
brown polygons), 4) non-isolated wetlands (green polygons), 5) map-edge wetlands (not counted in the
statistical analysis; salmon-colored polygons), 6) streams with a 20 meter buffer (light blue polygons), 7)
deepwater habitats (blue polygons), and 8) isolated deepwater habitats (purple polygons). Source data
for each map were listed in the map legend. For example, road data came from either the DLG or
TIGER (Topologically Intergrated Geographic Encoding and Referencing from U.S. Census Bureau)
data, with the former being the preferred source.
Analytical results generated for most study areas included tabulations of: 1) qualitative data on wetlands
and deepwater habitats (frequency of polygons and acreage for each type as classified on the NWI maps),
2) three scenarios presenting a range of values for isolated wetlands (e.g., acreage and number of
wetlands for each type, and corresponding percentages), and 3) qualitative data on isolated wetlands
(Scenario 2) (see blank data sheet, Figure 3-7). For a few areas where NWI digital data do not exist, no
qualitative data were reported.
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Figure 3-7. Blank Data Sheet
Study Limitations
Results presented here are estimates of the numbers or acreages of potentially isolated wetlands and
corresponding percentages versus the rest of the wetlands in the study areas. This analysis was intended
to provide improved information on the number and acreage of isolated wetlands for 72 study sites across
the Nation. These results provide perspective on the potential extent of isolated vs. non-isolated
wetlands. This geospatial analysis employed GIS technology and has inherent limitations associated
with source data limitations. There are also constraints to developing protocols for identifying isolated
wetlands from available data (either maps or digital data). Moreover, the data are not intended to be
expanded to physiographic regions, states, or other areas to predict the extent of isolated or non-isolated
wetlands for larger geographic regions.
Source Data Limitations
Data sources used for this analysis do not contain every wetland and every small creek (intermittent or
perennial). For example, NWI maps have limitations that are inherent in any map produced through
remote sensing techniques. In general, NWI maps tend to underestimate the extent of wetlands for
several reasons, including: 1) scale and quality of aerial photography (affects both minimum wetlands
mapped and ability to separate wetlands from nonwetlands, especially for small isolated wetlands and
narrow fringing types) and 2) the difficulty of identifying certain wetlands (e.g., drier types and