• 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.

    Silly smiles in silty sediment.

    Silt Fence
    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.

    adapt Verb

    to adjust to new surroundings or a new situation.

    agriculture Noun

    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.

    ancient Adjective

    very old.

    annual Adjective


    arsenic Noun

    chemical element with the symbol As.

    Buddhist Noun

    person who follows the teachings of Siddhartha Gautama (Buddha).

    canyon Noun

    deep, narrow valley with steep sides.

    Encyclopedic Entry: canyon
    circulate Verb

    to move around, often in a pattern.

    classify Verb

    to identify or arrange by specific type or characteristic.

    clay Noun

    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.

    crop Noun

    agricultural produce.

    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.

    deposit Verb

    to place or deliver an item in a different area than it originated.

    dust Noun

    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.

    farmer Noun

    person who cultivates land and raises crops.

    fertile Adjective

    able to produce crops or sustain agriculture.

    fertilizer Noun

    nutrient-rich chemical substance (natural or manmade) applied to soil to encourage plant growth.

    flood Noun

    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.

    glacier Noun

    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.

    habitat Noun

    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.

    hibernate Verb

    to reduce activity almost to sleeping in order to conserve food and energy, usually in winter.

    Hindu Noun

    religion of the Indian subcontinent with many different sub-types, most based around the idea of "daily morality."

    insulation Noun

    material used to keep an object warm.

    lake Noun

    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.

    loess Noun

    windblown soil or silt.

    Encyclopedic Entry: loess
    mineral Noun

    inorganic material that has a characteristic chemical composition and specific crystal structure.

    niche Noun

    role and space of a species within an ecosystem.

    particle Noun

    small piece of material.

    port Noun

    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.

    replenish Verb

    to supply or refill.

    retention Noun

    process of keeping or holding in place.

    river Noun

    large stream of flowing fresh water.

    Encyclopedic Entry: river
    river bank Noun

    raised edges of land on the side of a river.

    rock Noun

    natural substance composed of solid mineral matter.

    runoff Noun

    overflow of fluid from a farm or industrial factory.

    Encyclopedic Entry: runoff
    sand Noun

    small, loose grains of disintegrated rocks.

    sediment Noun

    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.

    slash-and-burn Noun

    method of agriculture where trees and shrubs are cleared and burned to create cropland.

    soil Noun

    top layer of the Earth's surface where plants can grow.

    thrive Verb

    to develop and be successful.

    toxic Adjective


    transport Verb

    to move material from one place to another.

    wetland Noun

    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.