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1 Your web browser (Safari 7) is out of date. For more security, comfort and the best experience on this site: Update your browser Ignore SILT For the complete encyclopedic entry with media resources, visit: Silt is a solid, dust-like sediment that water, ice, and wind transport and deposit. Silt is made up of rock and mineral particles that are larger than clay but smaller than sand. Individual silt particles are so small that they are difficult to see. To be classified as silt, a particle must be less than.005 centimeters (.002 inches) across. Silt is found in soil, along with other types of sediment such as clay, sand, and gravel. Silty soil is slippery when wet, not grainy or rocky. The soil itself can be called silt if its silt content is greater than 80 percent. When deposits of silt are compressed and the grains are pressed together, rocks such as siltstone form. Silt is created when rock is eroded, or worn away, by water and ice. As flowing water transports tiny rock fragments, they scrape against the sides and bottoms of stream beds, chipping away more rock. The particles grind against each other, becoming smaller and smaller until they are silt-size. Glaciers can also erode rock particles to create silt. Finally, wind can transport rock particles through a canyon or across a landscape, forcing the particles to grind against the canyon wall or one another. All three processes create silt. Silt can change landscapes. For example, silt settles in still water. So, deposits of silt slowly fill in places like wetlands, lakes, and harbors. Floods deposit silt along river banks and on flood plains. Deltas develop where rivers deposit silt as they empty into another body of water. About 60 percent of the Mississippi River Delta is made up of silt. 1 of 6

2 In some parts of the world, windblown silt blankets the land. Such deposits of silt are known as loess. Loess landscapes, such as the Great Plains, are usually a sign of past glacial activity. Many species of organisms thrive in slick, silty soil. Lotus plants take root in muddy, silty wetlands, but their large, showy flowers blossom above water. The lotus is an important symbol in Hindu, Buddhist, and ancient Egyptian religions. The lotus is the national flower of India and Vietnam. Many species of frog hibernate during the cold winter by burying themselves in a layer of soft silt at the bottom of a lake or pond. Water at the bottom of a body of water does not freeze, and the silt provides some insulation, or warmth, for the animal. Silty soil is usually more fertile than other types of soil, meaning it is good for growing crops. Silt promotes water retention and air circulation. Too much clay can make soil too stiff for plants to thrive. In many parts of the world, agriculture has thrived in river deltas, where silt deposits are rich, and along the sides of rivers where annual floods replenish silt. The Nile River Delta in Egypt is one example of an extremely fertile area where farmers have been harvesting crops for thousands of years. When there aren't enough trees, rocks, or other materials to prevent erosion, silt can accumulate quickly. Too much silt can upset some ecosystems. "Slash and burn" agriculture, for instance, upsets the ecosystem by removing trees. Agricultural soil is washed away into rivers, and nearby waterways are clogged with silt. Animals and plants that have adapted to live in moderately silty soil are forced to find a new niche in order to survive. The river habitats of some organisms in the Amazon River, such as the pink Amazon River dolphin, also called the boto, are threatened. River dolphins cannot locate prey as well in silty water. 2 of 6

3 Agricultural and industrial runoff can also clog ecosystems with silt and other sediment. In areas that use chemical fertilizers, runoff can make silt toxic. Toxic silt can poison rivers, lakes, and streams. Silt can also be made toxic by exposure to industrial chemicals from ships, making the silt at the bottom of ports and harbors especially at risk. When the city of Melbourne, Australia, decided to deepen its harbor in 2008, many people worried that disturbing millions of tons of silt, filled with chemicals like arsenic and lead, would threaten the waterway's ecosystem. Vocabulary Term accumulateverb to gather or collect. 3 of 6 adapt verb to adjust to new surroundings or a new situation. agriculture Amazon River dolphin ancient annual the art and science of cultivating the land for growing crops (farming) or raising livestock (ranching). pink, aquatic mammal native to the Amazon River in South America. adjectivevery old. adjectiveyearly. arsenic chemical element with the symbol As. Buddhist person who follows the teachings of Siddhartha Gautama (Buddha). canyon deep, narrow valley with steep sides. circulate verb to move around, often in a pattern. classify verb to identify or arrange by specific type or characteristic. clay type of sedimentary rock that is able to be shaped when wet. clog verb to obstruct or prevent travel. compress verb to press together in a smaller space.

4 Term crop agricultural produce. delta the flat, low-lying plain that sometimes forms at the mouth of a river from deposits of sediments. deplete verb to use up. deposit verb to place or deliver an item in a different area than it originated. dust tiny, dry particles of material solid enough for wind to carry. ecosystem community and interactions of living and nonliving things in an area. erode verb to wear away. farmer person who cultivates land and raises crops. fertile adjectiveable to produce crops or sustain agriculture. fertilizer nutrient-rich chemical substance (natural or manmade) applied to soil to encourage plant growth. flood overflow of a body of water onto land. flood plain flat area alongside a stream or river that is subject to flooding. fragment piece or part. glacial activity process of a glacier moving and changing the landscape. glacier mass of ice that moves slowly over land. gravel small stones or pebbles. Great grassland region of North America, between the Rocky Plains Mountains and the Mississippi River. habitat environment where an organism lives throughout the year or for shorter periods of time. harbor part of a body of water deep enough for ships to dock. harvest the gathering and collection of crops, including both plants and animals. 4 of 6

5 Term hibernate verb to reduce activity almost to sleeping in order to conserve food and energy, usually in winter. Hindu religion of the Indian subcontinent with many different subtypes, most based around the idea of "daily morality." insulation material used to keep an object warm. lake body of water surrounded by land. landscape the geographic features of a region. lead chemical element with the symbol Pb. loess windblown soil or silt. mineral inorganic material that has a characteristic chemical composition and specific crystal structure. niche role and space of a species within an ecosystem. particle small piece of material. port place on a body of water where ships can tie up or dock and load and unload cargo. prey animal that is hunted and eaten by other animals. replenish verb to supply or refill. retention process of keeping or holding in place. river large stream of flowing fresh water. river bank raised edges of land on the side of a river. rock natural substance composed of solid mineral matter. runoff overflow of fluid from a farm or industrial factory. sand small, loose grains of disintegrated rocks. sediment solid material transported and deposited by water, ice, and wind. silt small sediment particles. siltstone sedimentary rock made of hardened silt. slash-andburn and burned to create cropland. method of agriculture where trees and shrubs are cleared soil top layer of the Earth's surface where plants can grow. 5 of 6

6 Term thrive verb to develop and be successful. toxic adjectivepoisonous. transport verb to move material from one place to another. wetland area of land covered by shallow water or saturated by water. wind movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun. Articles & Profiles National Geographic News: Hurricane Rita Actually Helped Wetlands, Study Says USDA: Silt Fence Fact Sheet National Geographic Society. All rights reserved. 6 of 6