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Encyclopedic Entry

silt

silt

Encyclopedic entry. 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.

Grades

6 - 12+

Subjects

Earth Science, Geology, Geography, Physical Geography

Contents

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Morgan Stanley

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.

Vocabulary

Term Part of Speech Definition

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 land for growing crops (farming) or raising livestock (ranching).

Amazon River dolphin

noun

pink, aquatic mammal native to the Amazon River in South America.

ancient

adjective

very old.

annual

adjective

yearly.

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.

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.

delta

noun

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

noun

tiny, dry particles of material solid enough for wind to carry.

ecosystem

noun

community and interactions of living and nonliving things in an area.

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.

flood plain

noun

flat area alongside a stream or river that is subject to flooding.

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.

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.

harbor

noun

part of a body of water deep enough for ships to dock.

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.

landscape

noun

the geographic features of a region.

lead

noun

chemical element with the symbol Pb.

loess

noun

windblown soil or silt.

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.

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.

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.

sand

noun

small, loose grains of disintegrated rocks.

sediment

noun

solid material transported and deposited by water, ice, and wind.

silt

noun

small sediment particles.

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

poisonous.

transport

verb

to move material from one place to another.

wetland

noun

area of land covered by shallow water or saturated by water.

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.

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