A definition for what is a wetland often depends who is asking the question and what development or study is proposed for a particular wetland site. The fact that wetlands may dry out from time to time complicates the attempt to define wetlands in a simple fashion. In fact, some wetlands may be dry more often than they are wet. With applications ranging from urban real estate to scientific nature preserve, a great many points of view may be expressed for proper definitions, classification, and management techniques for wetland environments.
Many definitions for wetlands have been proposed and utilized over the years--see FW definitions. Among the most widely accepted definitions is that of Cowardin et al. (1979), which was adopted by the U.S. Fish and Wildlife Service.
|Bird's eye view over Nigula Bog, southwestern Estonia. Notice the abundance of pools that indicate ground water is near the surface throughout the bog. A mineral island (left rear) rises above the bog and supports deciduous forest. Kite aerial photo date 9/01, © J.S. Aber.|
|Bird's eye view over Cheyenne Bottoms, central Kansas. View of soil exposed in mudflat during late-winter, low-water conditions. A - moist soil of mudflat, B - salty crust on dry mudflat, C - cattail marsh. Cattle tracks crisscross the open mudflat. Kite aerial photo date 3/02, © J.S. Aber|
|Spike rush growing in a small pothole surrounded by prairie grass and forbs. Flint Hills, east-central Kansas. Photo date 6/02, © J.S. Aber|
|Horsetails (dark shoots) growing beside a spring-fed stream. Spring Creek, near Cuchara, Sangre de Cristo Mountains, southern Colorado. Photo date 8/02, © J.S. Aber|
This triad is the modern approach for wetland definition under many circumstances that include greatly different environments. Notice that water quality is not specified--salinity varies from fresh, to brackish, to marine, to hypersaline. Acidity may span the entire range of naturally occuring pH values. Depth of standing water in pools and hollows is usually too deep to walk through but too shallow to swim in. However, tidal flats are flooded daily, and many wetlands experience deeper floods from time to time. Emergent vegetation ranges from heavily forested swamps to nearly bare playas and mudflats. The inclusion of rice, the world's most important crop, shows that wetlands may be cultural as well as natural features.
Wetlands are present in all climatic and topographic settings around the world. Wetlands are relatively common in tropical and temperate lowlands. Alpine wetlands are also fairly common, as mountains typically receive more precipitation than do adjacent lowlands. Even deserts have wetlands supported by drainage from adjacent mountains, ground water, or infrequent storm runoff. Much of the Arctic region turns to wetland during the brief period of summer melting. The rich diversity of wetland environments requires a flexible definition.
|U.S. postage stamp sheet depicting the Pacific Coast rain forest.|
Forested wetland showing typical plants, animals, and water.
|U.S. postage stamp sheet depicting the Arctic tundra environment.|
Lichens and low-growing plants beside a shallow stream channel.
|High-energy, rocky, marine shoreline at Pacific Grove, California. Photo date 11/02, © J.S. Aber.|
|Red mangroves on the lee side of Key Largo, Florida. These mangroves grow in seawater in Flordia Bay on the protected northwestern side of the Florida Keys. The stilt roots are exposed here at low tide. Photo date 12/75, © J.S. Aber.|
|Spring-fed stream at the head of Trinchero Creek, Sangre de Cristo Mountains, southern Colorado. Mounds of brook cress in full flower line the banks of the spring. Photo date 7/03, © J.S. Aber.|
|Reed beds grow in fresh, shallow water on the margin of Endla Lake, east-central Estonia. Bare birch trees occupy mineral soil in the background of this autumn view. Photo date 9/00, © J.S. Aber.|
|Cattails and alkali bulrush on the margin of a marsh at Stillwater National Wildlife Refuge, Carson Sink, Nevada. Photo date 7/03, © J.S. Aber.|
The primary emphasis of this course deals the palustrine system of wetland environments. This system includes several classes that are recognized on the basis of substrate conditions or dominant vegetation cover (Schot 1999). Palustrine wetland classes include rock bottom, unconsolidated bottom, aquatic bed, unconsolidated shore, moss/lichen wetland, emergent wetland, scrub/shrub wetland, and forested wetland. Common wetland terms and types and described in the following table.
|Bog||Peat accumulation usually dominated by moss. Receives only direct precipitation; characterized by acid water, low alkalinity, and low nutrients.|
|Fen||Peat accumulation; may be dominated by sedge, reed, shrub or forest. Receives some surface runoff and/or ground water, which has neutral pH and moderate to high nutrients.|
|Mire||Used mainly in Europe to include any peat-forming wetland (bog or fen).|
|Marsh||Permanently or periodically inundated site characterized by nutrient-rich water. In Europe, must have a mineral substrate and lack peat accumulation.|
|Playa||Shallow, ephemeral ponds or lagoons that experience significant seasonal changes in semi-arid to arid climates. Often have high salinity or may be completely dry.|
|Slough||Widely used term for wetland environment in a channel or series of shallow lakes. Water is stagnant or may flow slowly on a seasonal basis. Synonym--bayou.|
|Swamp||Characterized by forest, shrub, or reed cover (fen). Particularly a forested wetland in North America. Depends on nutrient-rich ground water derived from mineral soils.|
|Wet meadow||Open prairie, grassland or savannah with waterlogged soils but without standing water for most of the year.|
|Open water||Deeper, normally perennial pools within wetlands and shallow portions of lakes and rivers. Typically home to submerged macrophytes.|
Peat is intrinsic to many wetlands around the world. Peat is partly decomposed plant remains that consist of more than 65% organic matter (dry weight). Moss, grass, herbs, shrubs and trees may contribute to the buildup of organic remains, including stems, leaves, flowers, seeds, nuts, cones, roots, bark and wood. Through time, the accumulation of peat creates the substrate, influences ground-water conditions, and modifies surface morphology of the wetland. Several factors are considered important for classification of peatland types (Charman 2002).
|Vertical airphoto over Teosaare Bog, a small raised ombrotrophic bog in east-central Estonia. Reddish-brown vegetation is moss (Sphagnum sp.) in hollows and around pools. Dwarf pines grow on hummocks. White survey marker at bottom center of view is one meter square. Kite aerial photo date 9/01, © J.S. Aber.|
|Closeup view of blanket bog at Rj˙pnafell, southern Iceland. Moss forms a spongy cushion over volcanic rocks. Swiss army knife for scale. Photo date 8/94, © J.S. Aber.|
|Spring-fed fen, Sangre de Cristo Mountains, near Cuchara, southern Colorado. Oligotrophic conditions prevail, as ground water is derived from silica-rich sandstone bedrock. Note the diversity of vegetation. Photo date 8/02, © J.S. Aber.|
Return to wetlands syllabus.
Return to wetlands syllabus.