Under the Clean Water Act, wetlands are defined as "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."
Within the Allegheny Watershed there are many different types of water-soaked soils that fall into the broad classification of wetlands: spring-fed meadows, shallow ponds, bogs, marshes, fens, old oxbow lakes, river backchannels, and forested wetlands. Some wetlands are always wet, and some are seasonally dry. Wet environments together with large amounts of nutrients are rich in plant species that, in turn, provide food, cover, shelter, and breeding grounds for birds, waterfowl, insects, amphibians, and mammals. Because wetlands are transitional ecosystems between land and water, they exhibit a richer diversity of plants and animals and greater biological productivity than non-wetland areas around them. Here we look at different types of wetlands and wetland interactions with water.
Wetland types
Scrub-shrub Wetlands are inhabited by shrubs such as spicebush, swamp honeysuckle, highbush blueberry, winterberry, alder, willows, and viburnum. This type of wetland also is dominated by small trees less than 20 feet in height. In 1979, 28 percent of the Commonwealth 's wetlands (139,000 acres) were scrub-shrub.
Emergent Wetlands are vegetated by grasses, sedges, rushes, and other herbaceous plants that emerge from the water or soil surface. Emergent wetlands are half as common as the scrub-shrub types. In 1979, about 14 percent of the state's wetlands (70,000 acres) were emergent. Between 1956 and 1979, 38 percent (42,500 acres) of these types of wetlands disappeared due to development.
Bogs are distinct, rare wetlands characterized by spongy peat deposits, evergreens, shrubs, and a floor covered by a thick carpet of sphagnum moss. They are characterized by extremely low nutrient levels and, acidic peat deposits. Their water source is rainwater only.[2] Bogs are a northern hemisphere phenomenon associated with low temperatures and short growing seasons, where ample precipitation and high humidity cause excessive moisture to accumulate. Bogs in the Allegheny Watershed occur only in the Glaciated Province. Titus Bog in the French Creek watershed is a good example.
Fens are rare and unique. They are open, peat-forming, freshwater marsh wetlands that receive most of their water from constant groundwater feeds. Unlike bogs, peats associated with fens are not acidic and have higher nutrient mineral levels, and therefore they are able to support much more diverse plant and animal communities.[3] These systems feature peat moss mounds and are covered elsewhere by grasses, sedges, cattails, wildflowers,ferns, blueberries, and alders. Fens in the Allegheny Watershed occur only in the Glaciated Province; an excellent example is Wattsburg Fen, protected by the Western Pennsylvania Conservancy.
Wetland functions
Flood Control
Water Quality and Pollution Control
Wetlands remove excessive nutrients from stormwater runoff too. Nitrogen and phosphorus, the two most common nutrients in lawn and agricultural fertilizers and livestock wastes, can be taken up and and transformed by wetland vegetation. Complex processes in plant tissues break down inorganic nutrients to an organic form that can be recycled within natural systems. For instance, nitrogen can be permanently removed through transformation to a gaseous form by processes called denitrification. Removing excessive nitrogen before it enters a stream can prevent accelerated growth of algae as well as the depletion of oxygen that is vital for fish and instream organisms.
Groundwater Recharge
Wetland Loss and Restoration
In the 200 years between 1780 to 1980, Pennsylvania lost over 56 percent of its original wetlands. Wetlands are now the most protected natural lands in Pennsylvania, but we are still losing them to agriculture and development (housing tracts, shopping malls, parking lots, airports, and industrial parks to name a few). Concerned citizens, however, can play an important part in protecting, preserving, and restoring wetlands. Wetland restoration itself is becoming a sophisticated environmental business. (See Conservation Efforts in the Allegheny Watershed in the Explore the Watershed section).
[1] phone interview 10-27-98 with Jeff Wagner, Community Ecologist, Western Pennsylvania Conservancy
[2] USEPA, Office of Wetlands, Oceans, and Watersheds (www.epa.gov/owow/wetlands/facts/bogs.html)
[3] USEPA, Office of Wetlands, Oceans, and Watersheds (www.epa.gov/owow/wetlands/facts/fens.html)
[4] wetland functions from:
This document is part of the Resource Center of the Watershed Atlas of the Allegheny River.
Forested Wetlands are wet habitats where large woody trees find a home. These trees are over 20 feet in height and include red or silver maples, river birch, blackgum, and green ash. Almost 45 percent of Pennsylvania's wetlands (221,000 acres) are forested. In the Allegheny Watershed, forested wetlands account for more than half of all wetlands.[1]
Different wetlands perform different water cleaning or retention functions. Some may for example, provide excellent flood control while offering little in the way of wildlife habitat. Below, several different wetland functions are briefly discussed.
Wetlands that are effective in reducing flood damage store flood waters and slowly release them. They act like natural reservoirs and can have a significant impact on reducing peak flow and flood damage within a small watershed. A wetland's ability to control floods is determined by the storage potential of the wetland: its size, soils, and position within the watershed; and the rate at which it can release stored waters.
Wetlands improve water quality, in part, because of their ability to trap sediments. Initially, plants slow the speed of incoming waters, allowing sediments to drop out among the vegetation. Sediments may contain physically or chemically attached toxins (heavy metals, pesticides, benzene from car exhaust, or oil from roads). These toxins may be "removed" by chemical breakdown in the wetland environment, burial in soil substrate, or assimilation into plant tissues. Wetlands that retain water for long periods of time and receive low-velocity waters are best for sediment trapping.
Commonly, wetlands occur in groundwater discharge areas. But many wetlands facilitate groundwater recharge by moving surface water to the water table where it becomes groundwater and may continue to move further down into bedrock fractures. Three conditions determine a wetland's ability to recharge groundwater: the elevation, or height, of the wetland relative to the groundwater (head pressure); the weight and pressure of the water within the wetland (atmospheric pressure); and the physical characteristics of the underlying soil strata (porosity, permeability, and storage capacity).[4] Groundwater moving through wetlands drops its sediment load, loses some or all of the previously attached chemicals, and has been cleaned by bacterias in the upper soil layers. Post-wetland water now may enter a stream, become groundwater, or end up in someone's well clean and ready to use.