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Wetlands of the United States

Their Extent and Their Value To Waterfowl and Other Wildlife

The 20 Wetland Types

This chapter describes the 20 wetland types in relation to their usefulness as habitat for waterfowl. Wetlands do not all fit neatly into definite type classifications. In this connection the Wetlands Classification Committee reports:

Because of the infinitely varied and intergrading physical and chemical conditions that underlie the complex of wetlands in this country, it would be impossible to create a useful classification system that completely avoids over-lapping of types recognized. Some degree of overlapping . . . is acknowledged, but it is believed that they are sufficiently distinct to serve satisfactorily in evaluations of wetlands. [9]

Wetland and its vegetation

In practical use, however, the system has served its intended purpose effectively.

The 20 types of wetlands are grouped under four categories: INLAND FRESH AREAS (Types 1 to 8), INLAND SALINE AREAS (Types 9 to 11), COASTAL FRESH AREAS (Types 12 to 14), and COASTAL SALINE AREAS (Types 15 to 20). In each category, the types are arranged in order of increasing water depths during the growing season. Types 6, 7, and 20, and oftentimes Type 8, are characterized by growths of shrubs or trees. From the standpoint of acreage, Type 1 (Seasonally flooded basins and flats) is the most abundant, and Type 14 (Coastal open fresh water) is the least abundant.

Figure 2 names each type in numerical order and indicates its area and value as waterfowl habitat. In this figure, and in the type maps included in the plates at the end of the report, Primary Importance refers to wetland areas rated as high or moderate in value to waterfowl in the State inventories. The total acreage of primary importance in the United States is 22.4 million. Since this figure is derived by totaling values for individual States, it should not be inferred that the percentages used in figure 2 apply uniformly throughout the country. It is generally true, however, that the values of most individual types tend to be rather consistent from one part of the country to another.

Figure 3 shows the United States divided into eight flyway areas. The heavy lines running north and south are administrative boundaries of the four flyways commonly referred to in waterfowl management. The heavy east and west lines roughly divide each flyway into northern and southern halves. The eight resulting areas are convenient units for studying the abundance and importance of wetland types.

Most waterfowl breeding in the United States occurs in the four northern flyway areas, and most of the important wintering grounds are in the four southern ones. Although there is considerable overlapping of these seasonal activities, particularly in States along the line between north and south, wetlands in the four northern areas generally make their most important contribution to breeding waterfowl, and those in the southern areas are used principally for wintering. Wetlands in all eight flyway areas serve as habitat during migration; in some instances their primary value lies in this heavy use by migrant birds.

There follows a brief description of each wetland type, with mention of its more important physical and vegetative characteristics and a table of its acreage by flyway areas. At the end of the report are 20 plates which include for each of the wetland types a map showing its general location, abundance, and waterfowl value, and a photograph of an area representative of the type. The type-distribution maps are not comparable in acres-per-dot representation; they vary from 1 dot for 500 acres in Type 14 to 1 dot for 50,000 acres in Type 1. This variation is necessary because of the tremendous range in acreage totals among the 20 wetland types (see table 5). The dots are based on county acreage data; each dot is located in or close to the county to which it applies. When acreages for two or more counties are combined in order to equal the amount represented by one dot, the dot is close to the geographic center of the counties involved.

The symbols used to show relative waterfowl values by flyway areas are standardized for all the type maps. These symbols show what proportion of the habitat of a particular type in a particular flyway area is judged to be of primary importance to waterfowl, as follows:

Wetland acreage rate of primary importance


Type 1 -- Seasonally flooded basins or flats (pl.1). The soil is covered with water, or is waterlogged, during variable seasonal periods but usually is well drained during much of the growing season. This type is found both in upland depressions and in overflow bottom lands. Along river courses, flooding occurs in late fall, winter, or spring. In the uplands, basins or flats may be filled with water during periods of heavy rain or melting snow.

Vegetation varies greatly according to the season and the duration of flooding. It includes bottom-land hardwoods as well as some herbaceous growths. Where the water has receded early in the growing season, smartweeds, wild millet, fall panicum, tealgrass, chufa, redroot cyperus, and weeds (such as marsh elder, ragweed, and cockleburs) are likely to occur. Shallow basins that are submerged only very temporarily usually develop little or no wetland vegetation.

Upland depressions included in the inventory are confined largely to the three Lake States, the two Dakotas, Montana, and the Panhandle of Texas. In the northern States the presence of this temporary water stimulates high waterfowl production by providing greater area for the establishment of territories by breeding pairs. When water occurs abundantly in the Panhandle, the temporarily flooded basins (playas) are used extensively by migrating and wintering waterfowl.

The overflow bottom lands in the southern part of the Mississippi Flyway provide a major wintering area for ducks as well as good shooting sites for hunters. Particularly in good mast years, feeding ducks use bottom lands when they are flooded. Although there remain more than 10 million acres of overflow lands in Missouri, Kentucky, Tennessee, Arkansas, Mississippi, and Louisiana, most of the wintering waterfowl in this flyway concentrate in certain key areas.

Flyway area:Acres
Pacific north102,600
Pacific south143,400
Central north357,700
Central south2,856,400
Mississippi north1,134,000
Mississippi south11,945,400
Atlantic north1,200
Atlantic south6,551,400

Type 2 -- Inland fresh meadows. (pl.2). The soil usually is without standing water during most of the growing season but is waterlogged within at least a few inches of its surface. Vegetation includes grasses, sedges, rushes, and various broad-leaved plants. In the North, representative plants are carex rushes, redtop, reedgrasses, mannagrasses, prairie cordgrass, and mints. In Florida, cordgrasses and various species of paspalums and beakrushes are common. Meadows may fill shallow lake basins, sloughs, or farmland sags, or these meadows may border shallow marshes on the landward side. Wild hay oftentimes is cut from such areas.

Fresh meadows are used somewhat in the North by nesting waterfowl, but in most of the country their value is mainly as supplemental feeding areas. If shallow water can be impounded on them, their value can be increased considerably.

Flyway area:
Pacific north43,200
Pacific south289,500
Central north578,800
Central south40,700
Mississippi north2,383,300
Mississippi south68,700
Atlantic north30,700
Atlantic south4,083,700

Type 3 -- Inland shallow fresh marshes (pl.3.) The soil is usually waterlogged during the growing season; often it is covered with as much as 6 inches or more of water. Vegetation includes grasses, bulrushes, spikerushes, and various other marsh plants such as cattails, arrowheads, pickerelweed, and smartweeds. Common representatives in the North are reed, whitetop, rice cutgrass, carex, and giant burreed. In the Southeast, maidencane, sawgrass, arrowhead, and pickerelweed are characteristic. These marshes may nearly fill shallow lake basins or sloughs, or they may border deep marshes on the landward side. They are also common as seep areas on irrigated lands.

Marshes of this type are used extensively as nesting and feeding habitat in the pothole country of the North Central States and elsewhere. In combination with deep fresh marshes (Type 4), they constitute the principal production areas for waterfowl. Florida and Georgia are the only States where the majority of the shallow fresh marshes are considered to be of lesser importance to waterfowl. Florida alone contains more than 2 million acres of this type.

Flyway area:
Pacific north33,700
Pacific south64,100
Central north817,600
Central south84,600
Mississippi north758,500
Mississippi south15,300
Atlantic north35,900
Atlantic south2,159,900

Type 4 -- Inland deep fresh marshes (pl.4). The soil is covered with 6 inches to 3 feet or more of water during the growing season. Vegetation includes cattails, reeds, bulrushes, spikerushes, and wildrice. In open areas, pondweeds, naiads, coontail, watermilfoils, waterweeds, duckweeds, waterlilies, or spatterdocks may occur. Water-hyacinth and waterprimroses form surface mats in some localities in the Southeast. These deep marshes may almost completely fill shallow lake basins, potholes, limestone sinks, and sloughs, or they may border open water in such depressions.

Deep fresh marshes constitute the best breeding habitat in the country, and they are also important feeding places. In the Western States they are heavily used by migrating birds, especially diving ducks. Florida and Texas are the only States in which the vast majority of these marshes are not rated as being of primary importance to waterfowl.

Flyway area:
Pacific north92,500
Pacific south62,500
Central north686,500
Central south46,800
Mississippi north427,700
Mississippi south21,500
Atlantic north25,700
Atlantic south984,100

Type 5 -- Inland open fresh water (pl.5). Shallow ponds and reservoirs are included in this type. Water is usually less than 10 feet deep and is fringed by a border of emergent vegetation. Vegetation (mainly at water depths of less than 6 feet) includes pondweeds, naiads, wildcelery, coontail, watermilfoils, muskgrasses, waterlilies, spatterdocks, and (in the South) water-hyacinth.

In the pothole country of the North Central States, Type 5 areas are used extensively as brood areas when, in midsummer and late summer, the less permanent marshes begin to dry out. The borders of such areas are used for nesting throughout the Northern States. Where vegetation is plentiful, they are used in all sections of the country as feeding and resting areas by ducks, geese, and coots, especially during the migration period.

Flyway area:
Pacific north40,500
Pacific south51,900
Central north676,800
Central south87,100
Mississippi north1,000,200
Mississippi south186,500
Atlantic north12,000
Atlantic south541,500

Type 6 -- Shrub, swamps (pl.6). The soil is usually waterlogged during the growing season, and is often covered with as much as 6 inches of water. Vegetation includes alders, willows, buttonbush, dogwoods, and swamp-privet. Shrub swamps occur mostly along sluggish streams and occasionally on flood plains. They are used to a limited extent for nesting and feeding in the North and for roosting and feeding in some of the Mississippi Alluvial Valley States. Elsewhere, shrub swamps are little used except in a few special situations.

Flyway area:
Pacific north11,900
Pacific south800
Central north700
Central south23,500
Mississippi north2,912,400
Mississippi south164,300
Atlantic north77,800
Atlantic south622,100

Type 7 -- Wooded swamps (pl.7). The soil is waterlogged at least to within a few inches of its surface during the growing season, and is often covered with as much as 1 foot of water. Wooded swamps occur mostly along sluggish streams, on flood plains, on flat uplands, and in very shallow lake basins. In the North, trees include tamarack, arborvitae, black spruce, balsam, red maple, and black ash. In the South, water oak, overcup oak, tupelo gum, swamp black gum, and cypress are dominant. In the Northwest, western hemlock, red alder, and willows are common. Northern evergreen swamps usually have a thick ground covering of mosses. Deciduous swamps frequently support beds of duckweeds, smartweeds, and other herbs.

Wooded swamps often occur in association with shrub swamps, and waterfowl often use the two types interchangeably. In the Southeast, Type 7 swamps become particularly important in years when lack of sufficient fall and early winter rains leave overflow areas dry. At such times, wooded swamps represent the only shallow water available over wide areas. This type is particularly useful to the wood duck throughout the range of this species.

Flyway area:
Pacific north18,200
Pacific south2,100
Central north--------
Central south39,000
Mississippi north2,906,700
Mississippi south2,813,800
Atlantic north556,000
Atlantic south10,473,200

Type 8 -- Bogs (pl.8). These are often called pocosins, bays, and savannahs in the South. The soil is usually waterlogged and supports a spongy covering of mosses. Bogs occur mostly in shallow lake basins, on flat uplands, and along sluggish streams. Vegetation is woody or herbaceous, or both. Typical plants are heath shrubs, sphagnum moss, and sedges. In the North, leather-leaf, Labrador-tea, cranberries, carex, and cottongrass are often present. In the South, cyrilla, persea, gordonia, sweetbay, pond pine, Virginia chainfern, and pitcher-plants are common. Scattered, often stunted, black spruce and tamarack may occur in northern bogs.

Bogs have the lowest waterfowl rating, countrywide, of all the 20 types. In northern New England, however, they assume considerable significance. In Maine alone, 25,500 acres are classed as being of primary importance to waterfowl.

Flyway area:
Pacific north1,400
Pacific south1,500
Central north100
Central south300
Mississippi north477,300
Mississippi south46,100
Atlantic north87,600
Atlantic south2,733,500


Type 9 -- Inland Saline flats (pl.9). The soil is without standing water except after periods of heavy precipitation, but it is waterlogged to within at least a few inches of the surface during the growing season. Vegetation (often sparse or patchy) consists of salt-tolerant plants such as seablite, saltgrass, Nevada bulrush, saltbush, and burro-weed. Type 9 wetlands occur in undrained sumps in many parts of the arid West. Sometimes they cover extensive areas.

Saline flats, under natural conditions, are used very little during most seasons, but ducks and geese feed extensively in flats that become flooded in the fall and winter.

Flyway area:
Pacific north43,600
Pacific south912,500
Central north3,900
Central south104,100
Mississippi north--------
Mississippi south--------
Atlantic north--------
Atlantic south--------

Type 10 -- Inland saline marshes (pl.10). The soil is usually waterlooged during the growing season and is often covered with as much as 2 or 3 feet of water. This type occurs mostly in shallow lake basins. Vegetation is mainly alkali or hardstem bulrushes, often with wigeongrass or sago pondweed in openings.

Saline marshes are used heavily by nesting and feeding ducks and geese. They are extremely valuable to waterfowl in both the Pacific and Central Flyways. Throughout the range of this type, 98 percent of the saline marshes are considered to be of primary importance to waterfowl.

Flyway area:
Pacific north98,500
Pacific south153,700
Central north5,600
Central south14,700
Mississippi north--------
Mississippi south--------
Atlantic north--------
Atlantic south--------

Type II. -- Inland open saline water (pl.11). These more permanent areas of shallow, saline water are often closely associated with Types 9 and 10. Depth of water is variable. Vegetation (mainly at water depths of less than 6 feet) includes sago pondweed, wigeongrass, and muskgrasses.

Type 11 is used very extensively by feeding ducks and geese wherever vegetation is plentiful. In the Pacific Flyway, where 93 percent of this type is located, it is of major importance during migration seasons. Throughout its range, 87 percent of these areas are considred to be of primary importance to waterfowl.

Flyway area:
Pacific north154,700
Pacific south107,600
Central north15,000
Central south3,600
Mississippi north--------
Mississippi south--------
Atlantic north--------
Atlantic south1,800

Type 12 -- Coastal shallow fresh marshes (pl.12). The soil is always waterlogged during the growing season. It may be coverd at high tide with as much as 6 inches of water. These marshes are on the landward side of deep marshes along tidal rivers, sounds, and deltas. Vegetation consists of grasses (reed, big cordgrass, maidencane), sedges (carex, spikerushes, threesquares, sawgrass), and various other marsh plants such as cattails, arrowheads, smartweeds, and arrow-arum.

Nationwide, these shallow fresh marshes rate the highest of the nine coastal types in their importance to waterfowl. They are used moderately for nesting in the North Atlantic and Pacific Coast States, and they constitute the most used wetland type along the Gulf Coast during the winter season.

Flyway area:
Pacific north300
Pacific south5,300
Central north--------
Central south318,300
Mississippi north--------
Mississippi south1,578,600
Atlantic north150,700
Atlantic south160,000

Type 13 -- Coastal deep fresh marshes (pl.13). The soil is covered at average high tide with 6 inches to 3 feet of water during the growing season. These marshes occur along tidal rivers and bays, mainly on the Atlantic and Gulf Coasts. Vegetation is mainly cattails, wildrice, pickerelweed, giant cutgrass, and spatterdocks, often with pondweeds and other submerged growths in marsh openings. In the Gulf region, water-hyacinth, alligatorweed, and waterlettuce may produce surface mats.

More than 85 percent of the total of this type is found in Louisiana, where 422,000 acres are of primary importance to waterfowl and 984,000 acres are of lesser importance. This type, where suitable vegetation dominates, is used much in fall and winter by feeding waterfowl.

Flyway area:
Pacific north10,000
Pacific south5,400
Central north--------
Central south43,800
Mississippi north--------
Mississippi south1,418,400
Atlantic north36,200
Atlantic south117,600

Type 14 -- Coastal open fresh water (pl.14). Included in this type are shallow portions of open water along fresh tidal rivers and sounds that are considered vulnerable to reclamation for agricultural or industrial uses. Vegetation is scarce, or absent, in stained or turbid waters. At depths of less than 6 feet, pondweeds, naiads, wildcelery, coontail, waterweeds, watermilfoils, and muskgrasses are common. In some localities of the Gulf region, water-hyacinth forms mats on the surface.

Nearly four-fifths of the acreage is on the Louisiana and Texas coasts, where 92,600 acres are of primary importance to waterfowl and 54,200 acres are of lesser importance. This type, although not abundant along the North Atlantic coast, is particularly valuable wherever present. It is also used heavily in the San Francisco Bay region.

Flyway area:
Pacific north--------
Pacific south4,500
Central north--------
Central south46,700
Mississippi north--------
Mississippi south118,500
Atlantic north10,600
Atlantic south17,000

Type 15 -- Coastal salt flats (pl.15). The soil is usually waterlogged during the growing season. Sites vary from those submerged only by occassional wind tides to those covered fairly regularly with a few inches of water at high tide. These areas are on the landward side of, or as islands or basins within, salt meadows and salt marshes. Vegetation is often sparse or patchy and consists mainly of glassworts, seablite, saltgrass, and, in the South, saltflat grass and saltwort.

Many salt flats were too small and too intermixed with other coastal saline types to be included as a separate type in the inventory. This is particularly true in the North Atlantic States where all salt flats necessarily were bypassed. Salt flats do not assume much importance, except in the Puget Sound and San Francisco Bay areas where they are used for feeding. They are abundant on the Texas coast (351,000 acres), where 14 percent are of primary importance to waterfowl.

Flyway area:
Pacific north2,200
Pacific south23,500
Central north--------
Central south351,000
Mississippi north--------
Mississippi south7,000
Atlantic north--------
Atlantic south39,600

Type 16 -- Coastal salt meadows (pl.16). The soil is always waterlogged during the growing season, but is rarely covered with tidewater. These meadows are on the landward side of salt marshes or bordering open water. Vegetation on the Atlantic and Gulf coasts includes mainly saltmeadow cordgrass, saltgrass, blackrush, and, in fresher parts, Olney threesquare and saltmarsh fleabanes. On the Pacific Coast, carex, hairgrass, and jaumea often are present.

Salt meadows are used as feeding areas in both the production and wintering zones. The presence of shallow potholes greatly increases the value of these meadows.

Flyway area:
Pacific north7,300
Pacific south--------
Central north--------
Central south177,000
Mississippi north--------
Mississippi south27,800
Atlantic north344,600
Atlantic south399,400

Type 17 -- Irregular flooded salt marshes (pl.17). The soil is covered by wind tides at irregular intervals during the growing season. These marshes are along the shores of nearly enclosed bays, sounds, and rivers on the Atlantic coast from Maryland southward, including the Gulf coast. Vegetation is dominantly needlerush. Pure stands of needlerush make poor waterfowl marshes, but where wigeongrass occurs in ponds or channels within the marsh, adjoining growths of needlerush provide protective cover to feeding ducks. Because of this interspersion of Type 17 with open water, these irregularly flooded salt marshes usually rate fairly high in value.

Flyway area:
Pacific north--------
Pacific south--------
Central north--------
Central south18,400
Mississippi north--------
Mississippi south33,300
Atlantic north53,000
Atlantic south593,900

Type 18 -- Regularly flooded salt marshes (pl.18). The soil is covered at average high tide with 6 inches or more of water during the season. These marshes are along the open ocean in eastern Virginia, southern South Carolina, Georgia, and eastern Louisiana. Elsewhere, the type is found mostly along sounds. Vegetation on the Atlantic and Gulf coasts is mainly salt-marsh cordgrass. On the Pacific coast, alkali, bulrush, glassworts, and arrowgrass dominate. Permanent, open water in these marshes may support wigeongrass, eelgrass, or sago pondweed.

This type is used very much by feeding ducks and geese, particularly along the Pacific and North Atlantic coasts where food-abundant ponds are present.

Flyway area:
Pacific north8,700
Pacific south83,600
Central north--------
Central south18,900
Mississippi north--------
Mississippi south539,200
Atlantic north104,400
Atlantic south821,300

Type 19 -- Sounds and bays (pl.19). This type includes those portions of salt-water sounds and bays that are considered shallow enough to be diked and filled. On the Pacific and North Atlantic coasts, all water landward from the average low-tide line was included. In Texas, because of the small range between tide extremes, water less than 3 feet deep was arbitrarily classified in this type. Vegetation includes eelgrass, wigeongrass, sago pondweed, muskgrasses, and, in the Southeast, shoalgrass, manateegrass, and turtlegrass.

Sounds and bays are of particular value to wintering waterfowl in the San Francisco Bay area of California, on the Texas coast, and along the New England coast.

Flyway area:
Pacific north145,400
Pacific south42,500
Central north--------
Central south412,900
Mississippi north--------
Mississippi south111,900
Atlantic north147,400
Atlantic south245,700

Type 20 -- Mangrove swamps (pl.20). The soil is covered at average high tide with 6 inches to 3 feet of water during the year-round growing season. This type is found only along the coast of the southern half of Florida; it is best developed on the western coast of that State from Cape Sable to Everglades City. Tree growth consists of much red mangrove and some black mangrove. Scattered areas of black mangrove in Louisiana were included with regularly flooded salt marshes.

The value of mangrove swamps for waterfowl is dependent on other wetland types associated with them. Except in localized situations where duck food is common, these swamps are sparsely used.
Flyway area:
Florida only523,000

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