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.
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.
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.
A silt fence is a barrier made of wire and fabric. The silt fence is used to catch silt and runoff from areas prone to erosion, to keep silt from getting in streams and homes.
Truckloads of Silt
Every year, the U.S. Army Corps of Engineers removes about 400,000 truckloads of sediment from the Great Lakes, mostly from the Toledo, Ohio, area of Lake Erie. Silt clogs vital shipping channels.
Term Part of Speech Definition Encyclopedic Entry accumulate Verb
to gather or collect.
to adjust to new surroundings or a new situation.
the art and science of cultivating the land for growing crops (farming) or raising livestock (ranching).
Encyclopedic Entry: agriculture Amazon River dolphin Noun
pink, aquatic mammal native to the Amazon River in South America.
chemical element with the symbol As.
person who follows the teachings of Siddhartha Gautama (Buddha).
deep, narrow valley with steep sides.
Encyclopedic Entry: canyon circulate Verb
to move around, often in a pattern.
to identify or arrange by specific type or characteristic.
type of sedimentary rock that is able to be shaped when wet.
to obstruct or prevent travel.
to press together in a smaller space.
Encyclopedic Entry: crop delta Noun
the flat, low-lying plain that sometimes forms at the mouth of a river from deposits of sediments.
Encyclopedic Entry: delta deplete Verb
to use up.
to place or deliver an item in a different area than it originated.
tiny, dry particles of material solid enough for wind to carry.
Encyclopedic Entry: dust ecosystem Noun
community and interactions of living and nonliving things in an area.
Encyclopedic Entry: ecosystem erode Verb
to wear away.
person who cultivates land and raises crops.
able to produce crops or sustain agriculture.
nutrient-rich chemical substance (natural or manmade) applied to soil to encourage plant growth.
overflow of a body of water onto land.
Encyclopedic Entry: flood flood plain Noun
flat area alongside a stream or river that is subject to flooding.
Encyclopedic Entry: flood plain fragment Noun
piece or part.
glacial activity Noun
process of a glacier moving and changing the landscape.
mass of ice that moves slowly over land.
Encyclopedic Entry: glacier gravel Noun
small stones or pebbles.
Great Plains Noun
grassland region of North America, between the Rocky Mountains and the Mississippi River.
environment where an organism lives throughout the year or for shorter periods of time.
Encyclopedic Entry: habitat harbor Noun
part of a body of water deep enough for ships to dock.
Encyclopedic Entry: harbor harvest Noun
the gathering and collection of crops, including both plants and animals.
to reduce activity almost to sleeping in order to conserve food and energy, usually in winter.
religion of the Indian subcontinent with many different sub-types, most based around the idea of "daily morality."
material used to keep an object warm.
body of water surrounded by land.
Encyclopedic Entry: lake landscape Noun
the geographic features of a region.
Encyclopedic Entry: landscape lead Noun
chemical element with the symbol Pb.
windblown soil or silt.
Encyclopedic Entry: loess mineral Noun
inorganic material that has a characteristic chemical composition and specific crystal structure.
role and space of a species within an ecosystem.
small piece of material.
place on a body of water where ships can tie up or dock and load and unload cargo.
Encyclopedic Entry: port prey Noun
animal that is hunted and eaten by other animals.
to supply or refill.
process of keeping or holding in place.
large stream of flowing fresh water.
Encyclopedic Entry: river river bank Noun
raised edges of land on the side of a river.
natural substance composed of solid mineral matter.
overflow of fluid from a farm or industrial factory.
Encyclopedic Entry: runoff sand Noun
small, loose grains of disintegrated rocks.
solid material transported and deposited by water, ice, and wind.
Encyclopedic Entry: sediment silt Noun
small sediment particles.
Encyclopedic Entry: silt siltstone Noun
sedimentary rock made of hardened silt.
method of agriculture where trees and shrubs are cleared and burned to create cropland.
top layer of the Earth's surface where plants can grow.
to develop and be successful.
to move material from one place to another.
area of land covered by shallow water or saturated by water.
Encyclopedic Entry: wetland wind Noun
movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.