Understanding the relationship between landscape and the dynamic nature of wetlands is essential to the assessment of wetland functions and values. Wetlands are constantly adjusting to disturbances occurring within them and within the surrounding landscape. It is important to recognize to what extent various disturbances affect wetlands when assessing disturbance and impact, and when considering wetland protection options (Clearwater et al., 1998).
Types of Wetland Impacts
Types of Wetland Impacts
- Direct impacts result from disturbances that occur within the wetland. Common direct impacts to wetlands include filling, grading, removal of vegetation, building construction and changes in water levels and drainage patterns. Most disturbances that result in direct impacts to wetlands are controlled by State and Federal wetland regulatory programs.
- Indirect impacts result from disturbances that occur in areas outside of the wetland, such as uplands, other wetlands or waterways. Common indirect impacts include influx of surface water and sediments, fragmentation of a wetland from a contiguous wetland complex, loss of recharge area, or changes in local drainage patterns. Given that most indirect impacts are beyond the authority of State and Federal wetland regulatory programs, wetland protection can be provided by a watershed management plan under local implementation.
- Cumulative impacts are those impacts resulting from combined direct and indirect impacts to the wetland over time.
Human influences have caused significant changes in the function and quality of many wetlands. These changes have resulted from alteration of the physical, chemical and biological components of wetland ecosystems. Widespread land development and clearing have caused increased erosion in uplands areas leading to increased sedimentation in lowland wetlands. This increased accumulation of sediment can alter the chemical and hydrologic regime of the wetlands in a relatively short time. Other human acitivities which can have lasting effects on wetland ecosystems include stream channelization, dam construction, discharge of industrial wastes and municipal sewage (point source pollution) and runoff urban and agricultural areas (non-point source pollution). These activities contribute to changes in the flood regime of wetlands and the input and cycling of nutrients.
Sources of pollution have local and regional effects on the chemistry and quality of water flowing through wetlands. Point sources, such as municipal industrial sites, and non-point sources, such as agricultural lands and urban runoff, add materials to ground water and surface water that upset the balance of wetland water chemistry and the biogeochemical cycling of materials in wetland ecosystems. (Mitsch and Gosselink, 1993)
Maryland's wetlands have been traditionally used for hunting, trapping, fishing, berry and timber harvest, bird watching and livestock grazing. Most traditional uses of wetlands do not impose permanent impacts, with the exception of livestock grazing and timber harvest which can affect the functions such as water quality and habitat. Human uses of wetlands, such as drainage for agriculture and filling for industrial or residential development, can impose irreversible impacts to wetlands. In the past, the societal and ecological value of wetlands were not widely recognized and many wetlands were destroyed. (from Wetlands of Maryland, Tiner and Burke, 1995).
However, recent trends in federal and State laws governing wetlands have provided for preservation, protection, restoration and creation of wetlands throughtout Maryland.
Summary of Wetland Acreage and Gains, Losses and Net Change between 1991 and 2003
In Maryland, the gradual rise of sea level, approximately 3 to 4 millimeters per year, has the potential to both increase wetland acreage by periodically flooding low-lying uplands and to destroy vegetated wetlands by increasing water depths.
Coastal marshes and swamps are particularly vulnerable to rising sea level because they are mostly within a few feet of sea level. As the sea rises, the outer boundary of these wetlands will erode, and new wetlands will form inland as previously dry areas are flooded by the higher water levels. The amount of newly created wetlands, however, will be much smaller than the area of wetlands that are lost. The amount of dryland within a few feet above the wetlands is much less than the area of wetlands that would be lost if sea level rises a few feet. Moreover, developed areas will often be protected with bulkheads, dikes, and other structures that keep new wetlands from forming inland. Along sandy beaches, like wetland shores, property owners often erect these structures to halt erosion. Although these structuress protect property, they can eliminate beaches, particularly bay beaches, which are usually less than 10 feet wide. Beaches are used for fishing, recreation, transportation, and landing small crafts, in addition to their environmental importance in the evolution of coastal wetlands. (from: www.epa.gov).
Evolution of a Marsh as Sea Level Rises
The dry land, or uplands, within a few feet above high tide includes forests, farms, low parts of some port cities, parts of deltas, and the bay sides of barrier islands. Some of the most economically important vulnerable areas are the recreational resorts on the "coastal barriers" of the Atlantic and Gulf coasts. In many cases, the ocean-front block of these islands is 5 to 10 feet above high tide; but the bay sides are often less than two feet above high water and regularly flooded. Erosion threatens the high ocean sides of these densely developed islands and is generally viewed as a more immediate problem than inundation of their low bay sides. Many ocean shores are currently eroding 1 to 4 feet per year. Coastal engineers generally estimate that a 1 foot rise in sea level will cause beaches to erode 0.5 to 1 feet from New England to Maryland, 2 feet along the Carolinas, 1 to 10 feet along the Florida coast, and 2 to 4 feet along the California coast. Because many US recreational beaches are less than 100 feet wide at high tide, even a 1 foot rise in sea level would threaten homes in these areas. (from: www.epa.gov).
More information about Sea Level Rise in Maryland.
Storm and Flood Events
Changing climate also increases the vulnerability of coastal areas to flooding and erosion. A higher sea level raises the flood level from a storm of a given severity. A 3-foot rise in sea level (for example) would enable a 15-year storm to flood many areas that today are only flooded by a 100-year storm. The Federal Emergency Management Agency has estimated that a rise in sea level of one foot would increase the size of the 100-year floodplain in the United States from 19,500 square miles in 1990 to 23,000 square miles, and increase flood damages (and hence flood insurance rates) by 36-58 percent. Coastal flooding is also exacerbated by increasing rainfall intensity. Along tidal rivers and in extremely flat areas, floods can be caused by storm surges from the sea or by river surges.
A Method for the Assessment of Wetland Function, Fugro East, Inc., 1995, for Department of Natural Resources.
Wetlands of Maryland, Tiner and Burke, 1995, for U.S. Fish & Wildlife Service and
Maryland Department of the Environment.
A Comprehensive Nontidal Wetland Watershed Management Plan: A Guide for Local
Governments, Clearwater et al., 1998, for Maryland Department of the Environment.
Wetlands, Mitsch and Gosselink, 1993, Van Nostrand Reinhold, New York, 722p.