Wetlands:According to the EPA, a wetland is an area where water covers the soil, or is present either at or near the surface of the soil all year or for varying periods of time during the year, including during the growing season. Wetlands act as the transition between the land and the water. The hydrology of the site or the amount that the water is saturated plays an integral part in the composition of the soil and on the aquatic life that lives there. Wetlands are unique ecosystems as they support both terrestrial and aquatic life. The prolonged presence of water creates conditions that favor the growth of specially adapted plants (hydrophytes) and promote the development of characteristic wetland (hydric) soils.
Wetland (hydric) soils are saturated long enough in the growing season to create an anaerobic (low oxygen) state in the soil horizon (the layers of soil found as you dig a hole) The wetlands soil becomes so saturated with water that it cannot hold much, if any, oxygen.
Wetlands are often called "nurseries of life", meaning they support thousands of species, both terrestrial and aquatic. But they do more than just provide a habitat for these animals. When rivers overflow, wetlands help absorb the flood waters, which can help alleviate property damage and loss. The heavy spongy vegetation helps absorb the overflow by providing a place to store this water.
Rain water runs off and brings exposed soil sediment (or particles) down towards larger bodies of water. If there is no wetland all the sediment will run into the water making the water depth shallower. The sediment also destroys the spawning grounds for fish and may make them suffocate. If a wetland is present the sediment hits the plants and the low depression and slows down the water allowing the sediment to settle in the wetland. This helps prevent the sedimentation (or mud-clogging) of streams, lakes or rivers from erosion.
Runoff entering wetlands contains much more than just sediments. Pesticides, excess nutrients from fertilizers, bacteria, salts from winter road maintenance, and other chemicals also wash from the land and can enter our water ways. Scientists have found that after this polluted water has flowed through a wetland it becomes much cleaner. Wetlands, with all those plants and unique anaerobic (without oxygen) environment can protect lakes and streams from these substances by using the extra nutrients for their plants, and storing and breaking down the chemicals. This filtrating process improves the quality of the water for wildlife and humans.
About 75% of the nation's commercially important species of marine fish and shellfish and 80-90% of recreationally important species are dependent for their survival (for spawning, nursery, migration and feeding areas) on shallow inshore waters that include bays, estuaries and rivers flowing to the sea (Vymazal, 2007). New OrleansThe coastal area of New Orleans experienced the importance of wetlands first hand through hurricane Katrina. Just since World War II an area of land the size of Rhode Island has turned to water between New Orleans and the Gulf of Mexico, most of it former marshland. And every 2.7 miles of marshland reduces a hurricane surge tide by a foot, dispersing the storm's power. Simply put, had Katrina struck in 1945 instead of 2005, the surge that reached New Orleans would have been as much as 5-10 feet less than it was. These marshes, as well as the barrier islands, were created by the sediment-rich flood waters of the Mississippi River deposited over thousands of years. But modern levees have prevented this natural flooding, and the existing wetlands, starved for new sediments and nutrients, have eroded and "subsided" and just washed away. Every ten months, even without hurricanes, an area of Louisiana land equal to Manhattan turns to water. That's 50 acres a day. A football field every 30 minutes (Tidwell, 2005). Three Gorges Dam in ChinaIn China, The Yangtze River branches out into a broad estuary that stretches 655 kilometers into the East China Sea, and forms one of the largest continental shelves in the world. Over half of the Yangtze's annual sediment load is deposited in the estuary. The health of the estuary depends on the delivery of this sediment because a significant relationship exists between inter tidal wetland growth rate and riverine sediment supply. Yet, due to the Three Gorges project and other dams, the sediment accumulation rate in all reservoirs on the river has increased from close to zero in 1950 to more than 850 * 106 tons per year in 2003. This is causing erosion of the wetland habitat there, which provides nurseries for fish and resting areas for migratory birds and is considered one of the world's most important wetland ecosystems. There is also concern about the impact the project will have on biological diversity. The baiji dolphin, the ancient river sturgeon and the finless porpoise depend on the Yangtze for their survival. The population of Siberian cranes in Poyang Lake will also be affected by the dam (Cleveland 2007). Sources:Vymazal, J. (2007). Removal of nutrients in various types of constructed wetlands_. S{_}cience of the Total Environment. Vol. 380, no. 1-3, pp. 48-65.Tidwell, Mike. (2005). Goodbye New Orleans. Alter New. http://www.alternet.org/katrina/29274 /?page=entire.
Cleveland, Cutler (Lead Author); Brian Black (Topic Editor). 2007. "Three Gorges Dam, China." In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in September 21, 2006; Last revised January 17, 2007; Retrieved November 18, 2007]. http://www.eoearth.org/article/Three_Gorges_Dam,_China>