Wetlands of Maryland
Definition of Wetlands
As summarized in Wetlands of Maryland (Tiner and Burke, 1995), wetlands are areas that hold water for significant periods during the year and are characterized by anaerobic (low oxygen) conditions favoring the growth of specific plant species and the formation of specific soil types. The U.S. Fish and Wildlife Service developed a scientifically-based definition of the Nation’s wetlands for resource management purposes and to help ensure accurate and consistent wetland determinations. This definition emphasizes three key attributes of wetlands: 1) hydrology – the degree of flooding or soil saturation, 2) wetland vegetation (hydrophytes), and 3) hydric soils. This further defines wetlands as all areas having enough water at some time during the year to stress plants and animals not adapted for life in water or saturated soils.
Wetlands may be permanently flooded by shallow water, permanently saturated by groundwater, or periodically inundated or saturated for varying periods during the growing season in most years. Many wetlands are the periodically flooded lands that occur between uplands and salt or fresh waterbodies (ie., lakes, rivers, streams and estuaries). Other wetlands may be isolated in areas with seasonally high water tables that are surrounded by upland or occur on slopes where they are associated with groundwater seepage areas or drainageways. Wetlands are important natural resources providing numerous values to society, including fish and wildlife habitat, flood protection, erosion control and water quality preservation. Wetlands comprise a range of environments within interior and coastal regions of Maryland.
For more information about wetlands, follow these links to the EPA Wetlands website:
Wetland Types: http://www.epa.gov/students/
Wetlands in America: http://www.epa.gov/students
Overview of Wetland Surveys
Several surveys of wetland acreage have been done in Maryland since the early 1900s. Survey methods and wetland definitions have varied over the years, making an estimate of wetland trends nearly impossible. The most recent statewide estimate is from the U.S. Fish and Wildlife Service National Wetlands Inventory (NWI), using high altitude aerial photography. Wetland maps for Maryland were created during the early to mid 1980’s at a scale of 1 inch = 2000 feet based on NWI data. According to the National Wetlands Inventory survey (1995), Maryland possesses roughly 600,000 acres of vegetated wetlands. About 9.5 percent of the state’s land surface is covered by wetlands (NWI, 1995). Nearly 99.3% of the State’s wetlands are two main types, estuarine and palustrine. The most abundant type is palustrine or freshwater wetlands, representing 57.3% of the State’s total wetlands, equivalent to 342,626 acres. Palustrine wetlands may be either tidal or nontidal. Most palustrine wetlands - 88.7% - are nontidal wetlands. Estuarine wetlands (salt and brackish wetlands) represent 42 percent of the State’s total wetlands, equivalent to 251,542 acres.
Digital orthophoto quarter quadrangle (DOQQ) maps (scale of 1 inch = 600 feet) are available for the entire State and are produced by the Department of Natural Resources. While these DNR wetland maps are generally more accurate than National Wetland Inventory (NWI) maps, it is recommended that both be used to get the most complete picture of wetland coverage. Actual in-the-field wetland delineations generally find more wetland acreage than the digital maps. Therefore, the maps should only be used to get a rough feel for wetland coverage.
Vegetated tidal wetlands are also mapped by the State. State maps have been used since 1972 to identify the regulatory boundaries of wetlands under the jurisdiction of the Maryland Tidal Wetlands Act. Maps, at a scale of 1 inch =200 feet, are considered more accurate than both the State DOQQ maps and the NWI maps. According to the state maps, there are approximately 200,000 acres of vegetated tidal wetlands. Tidal wetlands include both fresh and brackish systems, with emergent, shrub, and forested vegetation. More recent aerial photographs, from the 1980’s and 1990’s, are used for guidance purposes.
Distribution of Wetlands by County
About 10 percent of the state is classified as wetland. Wetlands are most abundant on the Eastern Shore of the Coastal Plain, occupying 16 percent of the land area. Figure 3 gives an overview of the distribution of Maryland’s wetlands acreage by county and Table 1 summarizes the total acreage and percent acreage in each county, by wetland type. The counties with the most wetlands acreage in the State are Dorchester County, with 28.3 percent, and Somerset County, with 13.6 percent. Baltimore City, a substantially urbanized area, has the least wetland acreage with 0.04 percent. Of the coastal wetlands of Maryland, more than one third (36.4%) are located in Dorchester County and more than one quarter (26.0%) are located in Somerset County (McCormick and Somes, 1982).
Table 1. Wetland acreage for each county in Maryland as of 1981/1982, including wetland type, total wetland acreage and total percent of state. Totals have been rounded off to the nearest acre. (after Tiner and Burke, 1995)
Note: Acreages of palustrine wetlands may be conservative, especially for Eastern Shore Counties where temporarily flooded and seasonally saturated wetlands are difficult to identify.
Distribution of Wetlands by Watershed
As part of the National Wetland Inventory (NWI), U.S. Geological Survey hydrologic units (U.S.G.S, 1974) were used to determine the total acreage of wetlands throughout the State. This system defines 23 major watersheds in Maryland and names them based on the major rivers draining each geographical area (Tiner and Burke, 1995). This information is illustrated in Figure 4 and total wetland acreage for each watershed is summarized in Table 2.
Table 2. Total wetland acreage in Maryland, by watershed, as defined by U.S. Geological Survey hydrologic units (U.S.G.S, 1974). Data presented are from National Wetland Inventory (NWI) maps and do not include acreage of the narrow streams and wetlands mapped as linear features or wetland and waterways too small to depict on NWI maps. (after Tiner and Burke, 1995)
* The majority of the Christina watershed lies in south-central Pennsylvania
and only a small portion is located within the state of Maryland.
Based on the State designation, the twenty major watersheds of Maryland are illustrated in Figure 5. Many of these watersheds correspond with the U.S.G.S. hydrologic unit designations with a few exceptions where smaller watersheds have been combined. Like the U.S.G.S designations, the Maryland designations are named after the primary river drainage(s) within the geographical area.
As shown in Table 3, 66.4 percent of the coastal wetlands in Maryland are located in the Pokomoke and Nanticoke River Basins (both part of the Lower Eastern Shore watershed) and the Choptank River Basin on the Eastern Shore.
Table 3. Total acreage and percent acreage of coastal wetlands in the major watersheds of Maryland. (after McCormick and Somes, 1982)
Tidal wetlands are abundant on the lower Eastern Shore of the Coastal Plain and cover extensive areas Figure 6. Tidal wetlands are distinguished by their flood regime: wetlands flooded at least once per day are considered “low marsh” and those flooded less than once per day are considered “high marsh.” High marshes are typically flooded by high spring or storm tides. During the current post-glacial period, the gradual rise of sea level has resulted in the conversion of vegetated tidal wetlands to open water areas, and the conversion of forested nontidal wetlands to tidal marsh. Sea level rise has also inundated 16,721 acres of estuarine forested wetlands, equivalent to 6.7 percent of Maryland’s total estuarine wetlands acreage.
Eighty-two percent, 205,815 acres, of estuarine wetlands are emergent, thus making it the most common estuarine wetland type. Non-vegetated estuarine wetlands include 10.5 percent of the total acreage of estuarine wetlands. These coastal wetlands are extremely important to the Chesapeake Bay ecosystem and the economy of Maryland (Figure 7).
The following is a summary the predominant type(s) of wetland in each watershed. The Upper Eastern Shore (including the Chester and Elk River basins) contains mostly freshwater marshes but also some brackish high marshes. The Lower Eastern Shore (including the Nanticoke and Pokomoke River basins) contains a good amount of brackish high and low marshes, and submerged aquatic wetlands. The Choptank watershed contains mostly brackish high marshes and submerged aquatic wetlands. The Upper Western Shore (including the Bush, Gunpowder and Lower Susquehanna River Basins) and Patapsco watersheds predominately contain freshwater marshes. The Lower Western Shore, or West Chesapeake, watershed contains brackish high marshes and submerged aquatic wetlands. The Patuxent watershed contains almost equal proportions of freshwater marsh and brackish high marshes. The Lower Potomac contains mostly brackish high marshes. The Middle Potomac or Washington-Metro watershed contains mostly brackish high marshes, but also contains the highest percent of coastal wooded swamps in the state (26.8%). There are no coastal wetlands in the Upper Potomac watershed.
Generally, the Eastern Shore nontidal wetlands are characteristically low and flat. These nontidal wetlands are often difficult to identify and delineate due to the minor variations in regional topography and the similarity of wetland vegetation to vegetation found in surrounding uplands. On the Lower Eastern Shore, the wetlands may cover broad areas. Predominantly clay rich soils, which have slow drainage and form confining layers, help to retain ground water in these wetlands. Landscapes on the Upper Eastern Shore have steeper grades, and wetlands tend to be less extensive and have better drainage. Caroline, Kent, and Queen Anne’s Counties have the most abundant numbers of a unique wetland type commonly called a Delmarva Bay. These wetlands are usually isolated from surface water drainage systems and are elliptical in shape with sandy rims. Rare plant species are often found in these wetlands on the Eastern Shore including Bald cypress and Atlantic white cedar swamps.
On the Western Shore of the Coastal Plain, wetlands have more varied topography and are generally easier to delineate in comparison to wetlands on the Eastern Shore. These wetlands are often located near streams, although the prevalence of long-term overbank flooding is rare in these areas. Most Western Shore wetlands are supported by a localized, perched water table than by shallow groundwater.
Nontidal Wetlands of Special State Concern
Nontidal wetlands of Special State Concern are the best example of Maryland’s nontidal wetland habitats and are designated for special protection under the State’s nontidal wetland regulations. These 365 wetland sites have exceptional ecological and educational value and offer landowners opportunities to observe and safeguard the beauty and natural diversity of Maryland’s best remaining wetlands. Many of these special wetlands contain populations of rare and endangered native plants and animals. Other nontidal wetlands of Special State concern represent examples of unique wetland types and collective habitats for species that thrive in specialized environments.
Examples of these special types of wetlands are bogs, Delmarva bays and coniferous swamp forests. Bogs are highly acidic wetlands that lack the nutrients most common plants require and, therefore, provide habitat for specific communities of plants and animals. The Delmarva bays are depressions on the Eastern Shore that fill with water in the winter and spring, and dry in the late summer and fall. Because these environments are self-contained, they support many rare and unique species. Coniferous swamp forests are uncommon to Maryland and found in areas such as Garrett County.
Although Maryland has lost 45-65 percent of its original wetlands, many of which were drained for agricultural purposes, wetlands remain quite abundant. Increased Federal and State efforts in wetland restoration may eventually help achieve a net gain in wetlands, provided wetland regulatory programs maintain effective control of existing wetland resources (Tiner and Burke, 1995). Government regulatory programs have improved wetland conservation by providing for better protection of wetlands than at anytime before. As populations expand, there will be increased demand for development of commercial, resort, and residential real estate that will undoubtedly place additional pressure on remaining wetlands. To date, the public has supported wetland protection efforts, by recognizing the important water quality, flood storage, wildlife habitat, and other functions that wetlands perform. It is likely this trend of government and public support will continue (Tiner and Burke, 1995).
In addition, wetlands can be negatively impacted by water quality problems throughout the State. Although control of point sources of water pollution, such as industrial effluents and municipal wastewater treatment plants, is improving the quality of many of Maryland’s waterways, urban and agricultural runoff continue to degrade water quality. Improved techniques for storm water discharge treatment, riparian habitat management (e.g. streamside fencing) and employment of best management practices on farmland and managed forests, may further enhance water and wetland quality (Tiner and Burke, 1995).
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