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Term Project CE 394K: Geographic Information Systems in Water Resources
Using GIS to Determine Risks to Habitat in Corpus Christi Bay
and Evaluate Potential Sites for Constructed Wetlands
by Sarah Benedict
Naturally occurring coastal wetlands are one of the most productive and intensely used ecosystems on earth. Pressures on this resource will continue to be exacerbated by population growth and increasing demands for the absorption of greater amounts of waste. It has been estimated that about sixty percent of the world population lives within 100km of the coast. Population forecasts predict that current growth rates will continue and pressure on the surrounding coastal environment to absorb waste will be magnified. This objectives of this project are to:
Many coastal areas have already
exceeded existing infrastructure capabilities and do not provide adequate
wastewater treatment for a variety of inputs. This situation poses
a significant health risk to the community and the environment that can
reach epidemic proportions. There is an increasing requirement for sustainable
wastewater management strategies that directly address local conditions
and needs, whether it is on an urban or rural scale. Wastewater management
is especially crucial in coastal areas where much of the population directly
depends upon that water resource for their livelihood. Constructed wetlands
are a low-cost, low-technology and low-maintenance approach to help mitigate
water pollution and provide important habitat. Natural and constructed
wetlands have other benefits as well, such as flood and erosion control,
ground water recharge, nursery grounds and recreation.
In addition to municipal wastewater
treatment, wetlands can help to treat wastewater from a variety of other
uses as well. More frequently, wetlands are being used to filter and treat
wastewater from industrial plants, closed animal feed operations, aquaculture
farms, agricultural and highway runoff and seafood processing plants. Constructed
wetlands can benefit wildlife, the environment and current as well as future
generations. They are not always sufficient to meet the total wastewater
management needs of some larger urban areas, but when used in conjunction
with other treatment methods or in smaller communities they can be both
practical and successful.
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There are a vast number of
different wetland definitions, but all generally agree that wetlands are
a zone of transition between wet and dry regions. The U.S. Fish and Wildlife
Service (USFWS) defines wetlands as "lands transitional between terrestrial
and aquatic systems where the water table is usually at or near the surface,
or the land is covered by shallow water."(Rezendes and Roy, 1996)
Another definition simply states that wetlands "…are areas of land
which, either permanently or seasonally, are wet and support specially
adapted vegetation." (Cylinder et al., 1995) A third definition
proposed at the Ramsar Convention on Wetlands of International Importance
Especially as Waterfowl Habitat in Iran in 1971 is equally broad. The Ramsar
definition calls wetlands "areas of marsh, fen, peatland or water,
whether natural or artificial, permanent or temporary, with water that
is static or flowing, fresh, brackish or salt including areas of marine
water, the depth of which at low tide does not exceed 6 meters."(Finlayson
and Moser, eds., 1991) These definitions of what constitutes a wetland
are large in scope due to the extraordinary amount of diversity found within
and between wetlands. Such variety is not easily classified, but there
is growing need for a more strict definition due to increased litigation.
Wetlands do share certain characteristics
that help to define them. The hydrology, soils and vegetation found within
wetlands are unique. Hydrology influences the distribution and movement
of water within the wetland which in turn effects soils, water quality,
vegetation, other biological features and how effective the wetland is
at filtering toxic materials, pollutants and nutrients. Wetland soils are
characterized by slow drainage and high saturation levels. Saturation leads
to rapid oxygen loss and anaerobic conditions which result in an accumulation
of organic matter. These conditions promote the conversion of nutrients
useful to plants and microorganisms into useable forms. This conversion
yields dark gray or black soils high in organic content. (Cylinder et al.,
1995) The vegetation has adapted to the anaerobic soils by developing shallow
root systems in order to obtain limited oxygen. Coastal wetlands are additionally
influenced by the ebb and flow of the tides and are usually located within
estuaries or behind barrier beaches where they are semi-protected from
the impact of the ocean.
Historically wetlands were a
source of furs, hay, fuel, fiber, food, fish and game. Today wetlands are
still an important source of fish and shellfish and provide valuable habitat
for many species. In addition, wetlands offer essential flood protection
in many low lying areas due to their ability to store large amounts of
surface water. Wetlands also help to minimize erosion, subsidence and the
effects of sea level rise. They have also been called the "kidneys
of the landscape", (Mitsch and Gosselink, 1986) due to their ability
to immobilize and transform contaminants, thus improving water quality
and aiding in ground water recharge. Wetlands also serve to prevent the
build up of nitrates, that in large part come from agricultural fields
and cause eutrophication, by recycling them into harmless gasses. (Finlayson
and Moser, eds., 1991) Wetlands have been historically viewed as a negative
environment due to the fact that wetland soils are not suitable for farming
or construction, they have attracted unwanted birds and mosquitoes, they
can have a foul odor and excessive amounts of water. (Weller, 1981) However,
people are beginning to realize that wetlands are incredibly rich ecosystems
with productivity levels often compared to those of a rainforest. (Finlayson
and Moser, eds., 1991) Wetlands have also come to be recognized and appreciated
for their recreational, educational, wildlife habitat and aesthetic values.
Wetlands protection in the United States did not evolve until the 1960’s because the predominant attitude towards wetlands was not one of conservation, but rather one of conversion to a more useable type of land. The Swamp Lands Act of 1849, 1850 and 1860 allowed states with large amounts of land "wet and unfit for cultivation" (National Research Council, 1995) to drain and fill their swamp and overflowed lands for the purpose of agriculture. The Rivers and Harbors Act of 1899 gave the U.S. Army Corps of Engineers the responsibility of maintaining the navigability of U.S. waterways. The Corps became the regulatory body in charge of permitting dredging and filling operations. Amendments to the Water Pollution Control Act in 1972 gave the Corps and the Environmental Protection Agency the authority to regulate water pollution in U.S. waters, but does not explicitly mention wetlands in the definition of U.S. waters. In 1975 judicial interpretation of the Act extended federal jurisdiction to include nonnavigable tributaries of rivers. This extended protection to wetland areas regardless of their navigability. In 1977 the Water Pollution Control Act was revised and renamed the Clean Water Act. Section 404 was expanded to make provisions for individual states to start administering the permitting process and the only place that wetlands are mentioned specifically in this Act is in Section 404(g)(1).
While this legislative activity
was taking place in the early 1970’s it became increasingly clear that
a more exact definition for wetlands needed to be decided upon. The U.S.
Army Corps of Engineers proposed a definition in 1975 that classified wetlands
on the basis of their function alone. In 1977 the USACE issued a revised
and final definition of wetlands. "Those areas that are inundated
or saturated by surface or ground water at a frequency and duration sufficient
to support, and that under normal circumstances do support, a prevalence
of vegetation typically adapted for life in saturated soil conditions.
Wetlands generally include swamps, marshes, bogs and similar areas."
(NRC, 1995) This definition is still used by the USACE and the EPA. The
US Fish and Wildlife Service had also been working on a new definition
and in 1979 issued its final definition of wetlands:
"Wetlands are lands transitional between the terrestrial and aquatic
systems
where the water table is usually at or near the surface or the land is
covered
by shallow water. For purposed of this classification wetlands must have
one
or more of the following three attributes: (1) at least periodically, the
land
supports predominantly hydrophytes; (2) the substrate is predominately
undrained hydric soil; and (3) the substrate is nonsoil and is saturated
with
water or covered by shallow water at some time during the growing season
of each year." (NRC, 1995)
The National Wetlands Inventory of the USFWS further defines wetlands with a hierarchical classification system which is fully explained in the Classification of Wetlands and Deepwater Habitats of the United States. At the top level wetlands and deepwater habitats are divided into five major systems which are Marine (open ocean and the coastline), Estuarine (salt marshes and brackish tidal waters), Riverine (rivers, creeks and streams), Lacustrine (lakes and deep ponds) and Palustrine (shallow ponds, marshes, swamps and sloughs). These five systems are further classified into subsystems according to their hydrologic conditions. Each subsystem is then divided into classes based on vegetation or substrate. Vegetative subclasses are then classified in terms of life form and substrate classes in terms of composition. In addition, there are modifiers to describe hydrology, soils, water chemistry and human influences or activities. To learn more about the National Wetlands Inventory classification system, how to obtain data and the status of mapping, go to http://www.nwi.fws.gov/text.html .
In the late 19th century wetlands started to be acquired for the purpose of waterfowl sanctuaries. In 1971 The Convention on Wetlands of International Importance Especially as Waterfowl Habitat known as the Ramsar Convention met in Iran. Members worked out an intergovernmental treaty to aid in international cooperation for the conservation of wetlands. By 1991 more than 60 countries had joined the Convention, agreeing to designate wetlands as nature reserves, plan for wise use of the wetlands, maintain their ecological character and put wetlands of international importance on a Ramsar list. (Finlayson and Moser, eds., 1991) The early 1970’s mark the beginning of a real call for wetlands conservation. The Ramsar Convention has been responsible for the preservation of 74 million acres of wetland habitat. (Finlayson and Moser, eds., 1991) This preservation is indicative of the shift in how people view wetlands. They are increasingly viewed as a valuable resource that needs protection and management rather than a dismal wasteland that needs to be filled in.
Constructed wetlands can perform the same functions and provide the same uses as natural wetlands with the additional benefit of mitigating the impact of severely diminished wetland acreage. Most of the wetlands loss in the United States is a result of conversion into agricultural land. Other reasons for destruction are urban development, flood control, water diversion projects, excessive pumping of ground water and over grazing. (Cylinder et al., 1995) Activities farther away from the wetland such as up river contamination, high sediment deposition and the introduction of nonnative vegetation also contribute to overall loss. It has been estimated that at the time of colonization the United States had about 220 million acres of wetland or about 9% of the landscape. (NRC, 1995) Since the 1780’s 117 million acres of wetland have been lost in the lower 48 states (NRC, 1995) and more than 80% of that loss has been attributed to agriculture. (Finlayson and Moser, eds., 1991) The current rate of loss still exceeds 308,875 acres per year. (Finlayson and Moser, eds., 1991) Pressures on wetlands are intense, even though wetlands provide necessary functions and can add to the overall quality of life, the rates of loss are staggering.
Texas coastal wetlands are
important breeding, nesting, nursery and feeding grounds for many threatened
and endangered animals. They also provide habitat, both seasonal and permanent,
for 75% of North America’s bird species. (Calnan, 1994) In Texas it has
been estimated by the Texas Parks and Wildlife Department that 35% of the
state’s wetlands were lost between 1950 and 1979. (Calnan, 1994) Overall,
Texas has lost over 50% of it’s original 1.2 million acres of wetlands.
(TPWD, 1996) The increased use of water for domestic, agricultural and
industrial purposes associated with increases in population and urbanization
will continue to alter wetlands and other ecosystems. Most wetland loss
was originally a result of drainage. Now activities such as dredging, flood
control, mining, chemical discharge, subsidence, erosion, sea level rise,
drought, storms and modification for construction and development all contribute
to the loss and degradation of wetlands. The ecological, environmental
and economic values of wetlands are being severely compromised. Texas is
working hard to protect and acquire coastal wetlands through public and
private acquisition proposals, mitigation banking and regulatory measures.
In addition, Texas is very supportive of the use of constructed wetlands
to help aid in the treatment of wastewater.
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Constructed wetlands can be used as a sustainable development approach for the treatment of numerous varieties of wastewater in coastal communities and to help combat loss of habitat. As greater numbers of people move closer to the coast, the very aspects that draw them to that region are being compromised due to increasing pressure on resources. In 1990 37% or 93 million people of the United States population resided in coastal areas most of which were urban. (NRC, 1993) More than 1,400 municipal wastewater treatment plants, discharging 10 billion gallons of treated sewage a day, serve the coast of the U.S. (NRC, 1993) The constituents of wastewater that pose a risk to people and the environment are nutrients, pathogens, toxic organic chemicals, metals, hazardous materials, plastics, biological oxygen demand and solids. (NRC, 1993) Many of these same contaminants and others come directly or indirectly from landfills, superfund sites, permitted toxic release sites, industrial facilities and illegal dumping.
It appears unlikely that the influx of people living in and moving to coastal areas will end. There is a need for resource conservation and protection and at same time for a greater reliance on natural ecological processes and systems instead of energy and chemical intensive systems. While constructed wetlands are not completely natural, they do try to emulate or approximate natural systems. In addition, they are lower in cost, maintenance and educational requirements than the more mechanical chemically and biologically reliant methods of wastewater treatment. "Both freshwater and saltwater wetlands, due to their transitional location and reducing conditions, were found to have very significant roles in the natural cycling of organic and inorganic materials." (Moshiri, ed., 1993) Wastewater treatment systems should be designed to the characteristics, values and uses of the receiving environment and should be based on what combination of control measures can best achieve water quality objectives. It should be remembered that whatever management option is chosen, it should reflect societal goals and priorities, incorporate public inputs, be cost effective, consider the relative risks and achieve benefits at least commensurate with the costs of the controls. (NRC, 1993)
Constructed wetlands are macrophyte based wastewater treatment systems in which one or more species of aquatic macrophyte are located in one or more shallow ponds. Water can flow on the surface or subsurface and pollutants are removed by a variety of natural physical, chemical and biological processes. Macrophytes are excellent in treating wastewater because they can directly assimilate pollutants into their tissues and/or provide a suitable environment for microbial activity. (Moshiri, ed., 1993) The four major types of macrophyte based treatments are free floating, rooted emergent, submerged and multistage. Their applicability depends upon climate, quality requirements for the effluent, wastewater characteristics, availability and cost of land and conservation regulations.
In order for a constructed wetland to be an effective wastewater treatment method, there are several criteria that need to be met within the four major areas of physical layout, flora, weather and maintenance. Land area is one of the most important criteria for effectiveness. "…the wetland design that produces the best water quality is the one with the largest ratio of treatment area to base flow." (Moshiri, ed., 1993) A water depth of less than 18inches also aids in effectiveness as does the use of multiple cells that have a large length-to-width ratio. The farther the wastewater has to travel to reach an outlet and the slower the velocity it travels at, the longer the retention time and the more treatment it will necessarily undergo. What type of flora is used in the wetland is also critical. "Algae, bacteria, sphagnum moss (Sphagnum), cattails (Typha), bulrushes (Scirpus), and rushes (Juncus) have been shown to remove or support bacteria which remove nutrients, sulfate, and iron and other metals from polluted others also water." (Moshiri, ed., 1993) Storm events and the predominant weather patterns of the site need to be considered so that the runoff from those events can be considered into the capabilities of the wetland. Maintenance of constructed wetlands is relatively low, but over time the permeability of the substrate will be reduced and the effectiveness of the biomass will be reduced. Periodic dredging, build up of sediment and mowing of the impoudment structures is recommended. (Moshiri, ed., 1993) Soil type and depth to groundwater also need to be considered.
Constructed wetlands can
be built with a great deal of control, allowing managers to ascertain exactly
what methods work best given the inputs to be treated, goals and environmental
conditions. The advantages of using constructed wetlands for wastewater
treatment rather than natural wetlands or traditional technological methods
are low construction and maintenance costs, low energy requirements, low
technology levels so relatively untrained personnel can manage the system,
and greater flexibility. The disadvantages are increased land area requirements
and a possible decrease in performance during winter months when plant
productivity decreases.
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The Corpus Christi region
was chosen for the large amount of GIS data available for study of the
area. This area is currently under study by many organizations such as
Texas Parks and Wildlife Department, the General Land Office, the University
of Texas in Port Aransas, Texas A&M University in Corpus Christi and
the Corpus Christi National Estuary Program. The greater Corpus Christi
region was also of interest to me because it is a coastal urban area with
a great deal of industry located right on the estuary. In contrast, Mustang
Island, the protective barrier island, is relatively undeveloped and yet
impacted by what occurs upstream in Corpus Christi. Mustang Island is facing
intensified development as more people move to the coast and I am interested
in how ecosystem needs are balanced or managed with the needs of people
when development and infrastructure begin to encroach.
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1. The first step was to download all of the necessary data including information or metadata files from their respective sources. A complete listing of all coverages, their source and original projection can be found in the data dictionary.
2. Data that came in a PC zipped format was unzipped into the .e00 format using Win Zip. Then the data was imported from the .e00 format using ArcView Import 71.
3. The next step was to convert all of the coverages into the geographic projection with units of decimal degrees. You must know the original projection and units of the coverage. Note: It is possible to change the projection by going under View to Properties, selecting the projection box and changing the projection. However, this only changes the view and does not generate any new data sets. The method outlined below creates and saves a new theme with the new projection.
4. To do this, run ArcView. With the project window active go under File to Extensions and put a check mark next to Projector! And click OK. Now when the view window is active, you should see a button in the toolbar with an arrow inside a circle and a dot at each corner. This is your re-project button.
5. Open a new view and add to it the theme you would like to re-project, making it active. Go under View to Properties and make sure that the Map Units are the same as the ones of your theme in its original projection. Then click OK.
6. Now click on the re-project button in the tool bar. You will be prompted to select the input projection in the next dialog box. This dialog box is called Projection Properties. Select custom and enter all of the parameters of the original projection then click OK.
7. The next dialog box is called Projector! Select decimal degrees for the output units and click OK.
8. Select yes to recalculate and to add the new projected shape file as a theme to the view. It can be added to the view of your choice. I recommend naming the new theme something distinct so that if you are dealing with a lot of them, you can easily tell which have been re-projected and which have not later on in the project.
9. The blue bar at the bottom of the screen will show that it is "thinking". Although it is possible to re-project more than one theme with the same original projection at the same time by making them all active, I recommend only doing one at a time.
10. Check to make sure your new theme is different from the original by turning them both on and comparing their location, shape and the units in the upper right corner.
** Please note that I obtained the National Wetlands Inventory data
from the General Land Office Wet Net Data web site http://www.glo.state.tx.us/wetnet/data.html
, where it had already been converted to an Albers projection. If NWI data
is downloaded directly from the NWI http://www.nwi.fws.gov/text.html
, it must be unzipped, untarred and converted into an Arc/Info coverage.
The steps involved in this process are outlined in projects
by Connie Hinojos:
http://www.ce.utexas.edu/prof/maidment/tmpaper/spring97/hinojocm/hinojos.htm
and Aubrey Dugger:
http://www.ce.utexas.edu/prof/maidment/tmpaper/spring97/duggeral/title.html
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The analysis portion of this
project consisted mainly of generating a series of maps to illustrate the
potential risks to wetland habitats from hazardous waste sites, industrial
facilities landfills, toxic release sites and superfund sites as well as
nonpoint sources such as runoff from urban and agricultural areas. Buffers
around the colonies were drawn using a script called buffer.ave.
The resulting map with buffers illustrates the proximity of potential contaminants
to habitat and colonies. Below are a few of the maps that were produced.
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Future work entails
building on what was learned in this project to more accurately produce
a tool that would be useful in the determination of good sites for the
construction of wetlands. In order to search for environmental factors
that would enable the construction and operation of constructed wetlands
it is essential to know detailed information about the hydrology, soils
and vegetation of the site that is under consideration. Factors that
will influence the success of a constructed wetland include frequency of
flooding, annual precipitation, water table depth, the amount of clay in
the soils, percolatoin rates, slope, availability of open land, the proximity
of any naturally occurring wetlands and the chances for hydrophytic plants
to grow.
Data sets including STATSGO, precipitation,
vegetation, ecoregions, landuse/landcover and slope have been obtained,
but not yet evaluated for areas of potential constructed wetland success.
Criteria for each of the parameters still needs to be established.
It is hoped that this evaluation can then be linked to the preliminary
portion of this project and provide a useful way to determine habitats
that are at risk of contamination and the feasibility of using constructed
wetlands to mitigate these types of water quality and treatment problems.
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Due to the generalized nature
of the initial prtion of this project, not a lot of detailed analysis was
made. This is something to be worked on in the future. I did
gain great familiarity with obtaining and displaying data, changing projections,
and generating potentially useful maps for future analysis. The major
source of error in the project is a result of reprojecting
data sets without first changing their original spheroids. However,
this did not appear to influence the accuracy as much as I had thought
it would.
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Development practices that take a quick fix approach and rely heavily on the newest technology and infrastructure are in many situations detrimental to the sustainable growth and development of current and future generations. "The use of natural forces and processes in management is most likely to stimulate natural events, conditions, and results. In most cases, these techniques are the least expensive to use and have the greatest permanence." (Weller, 1981) Constructed wetlands are an example of such a natural system that works to the advantage of the community as well as the environment. "In general, we tend to expect too much from technologies and hope that they can save us from sins of the past or present, even replacing major losses of habitat." (Weller, 1981) Constructed wetlands are not a replacement for natural wetlands, nor does their construction in any way allow for natural wetlands to be altered or destroyed. It is important to remember that while constructed wetlands are not completely natural, neither are they dependent on, or intense users of, technology. The use of constructed wetlands for many different kinds of wastewater treatment is an appropriate and sustainable approach to development that serves many diverse, but interrelated purposes. However, there is no one constructed wetland design that will fit all possible sites. Constructed wetlands have to be tailored to the goals of the users, industrial or municipal, and the constraints of the environment it will be situated in.
The economies of many coastal
communities are still highly dependent upon the benefits that can be derived
from auqatic and wetland resources. However, this resource is threatened
by growing populations, industry, urbanization, a greater intensity of
use and an increase in the amount of waste it is expected to assimilate,
especially in coastal regions. Construction of wetlands and conservation
efforts will help to maintain this diverse habitat, but it will also require
interdisciplinary cooperation and governmental support. Constructed wetlands
are a relatively low cost, low technology and low maintenance solution
for the treatment of wastewater, the improvement of water quality and public
health and the creation of needed habitat. While they work best in areas
with small to medium populations and large amounts of land, they can benefit
larger communities as well if used in conjunction with other treatment
methods. Natural and constructed wetlands are "…the kind of nonhuman
environment most appropriate to a people whose traditional frontiers are
closed and who must begin to look closely at what they have rather than
what they might have, who must begin to think of what they are rather than
what they might be." (Fritzell, 1978) Our traditional frontiers of
unchecked growth and technological solutions are rapidly becoming unfeasible
and it is time to move towards more appropriate, feasible and sustainable
methods for development, such as constructed wetlands, that integrate the
needs of communities and wildlife with the health of the environment.
The original objectives of this
project as outlined in the introduction were met. I was able to show
the potential sources of wetlands contamination in Corpus Christi Bay.
By comparing the locations of potential contaminants, both point and nonpoint
source, with landuse and National Wetlands Inventory data I was able to
determine those wildlife and fisheries habitat areas that may be negatively
impacted. In addition, during the course of the project I realised
that there was much more that could be done in this area such as the evaluation
of environmental parameters for constructed wetlands. This is the
aspect that I would like to pursue in the future. It is my sincere
belief that constructed wetlands are a sustainable approach to treating
the many different kinds of wastewater that are generated in coastal communities
while at the same time minimizing risks to habitat and as an ancillary
benefit, adding to overall habitat acreage.
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Data Dictionary
References
Bontoux, J. and J. Bebin, Editors, 1992, Special Issue: "Wastewater Management in Coastal Areas," Water Science and Technology, 25 (12)
Calnan, T., 1994, Comprehensive Strategies for Protecting Texas Coastal Wetlands. Austin, TX.: Coastal Division of the Texas General Land Office.
Calnan, T., 1995, A Coastal Wetlands Acquisition Plan for Texas. Austin, TX.: Coastal Division of the Texas General Land Office.
Cowardin, L. et al., 1979, Classification of Wetlands and Deepwater Habitats of the United States. Washington, D.C.: U.S. Fish and Wildlife Service.
Cylinder, P., K. Bogdan, E. Davis, and A. Herson, 1995, Wetlands Regulation. Point Arena, CA.: Solano Press Books.
Finlayson, M. and M. Moser, Editors, 1991, Wetlands. New York, NY.: Facts On File Limited.
Haberl, R., Perfler, Laber and Cooper, Editors, 1997, Special Issue: "Wetland Systems for Water Pollution Control 1996," Water Science and Technology, 35 (5)
Hairston, A., Editor, 1992, Wetlands: An Approach to Improving Decision Making in Wetland Restoration and Creation. Washington, D.C.: Island Press.
Hammer, D., 1989, Constructed Wetlands for Wastewater Treatment: Municipal, Industrial and Agricultural. Chelsea, Michigan: Lewis Publishers.
Henze, M., Somlyody, Schilling and Tyson, Editors, 1997, Special Issue: "Sustainable Sanitation," Water Science and Technology, 35 (9).
Mitsch, W. and G. Gosselink, 1986, Wetlands. New York: Van Nostrand Reinhold Company.
Moshiri, G., Editor, 1993, Constructed Wetlands for Water Quality Improvement. Boca Raton, FL.: Lewis Publishers.
National Research Council, 1993, Managing Wastewater in Coastal Urban Areas. Washington, D.C.: National Academy Press.
National Research Council, 1995, Wetlands: Characteristics and Boundaries. Washington, D.C.: National Academy Press.
Rezendes, P. and P. Roy, 1996, Wetlands: The Web of Life. Hong Kong: Palace Press International.
Texas Parks and Wildlife, 1996, www.tpwd.state.tx.us/edu/texas/env95.htm