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What is Silt?

By Alan Rankin
Updated May 17, 2024
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Silt is very fine sediment that is formed by the process of erosion. It is usually found in or near bodies of water or where bodies of water once existed. It is often extremely fertile and an aid to human agriculture. Changes in an ecosystem, however, can lead to the rapid alteration in the presence or absence of silt in a given location. These changes are particularly noticeable in the mouths of great rivers such as the Nile and the Mississippi.

Geological processes such as erosion divide rock fragments over time into tiny particles. Classification systems such as that used by the United States Department of Agriculture define silt as particles that are smaller than one-thousandth of an inch (0.05 mm) in size, even smaller than common sand. The small size and weight of these particles means that bodies of water such as rivers can carry them long distances. All rivers carry some quantity of silt, although the presence of solid surfaces in the water can impede this process. When this happens, the particles will drop out of the water and accumulate against the surface, whether it is a shore, natural body or foreign object.

This process has had a profound impact on the development of human culture. Many early civilizations grew up around river mouths, or deltas, that offered fertile land for agriculture. In Egypt, for example, the Nile River regularly floods and then recedes, leaving vast deposits of silt upon its shores. The ancient Egyptian society, one of the world’s first empires, thrived for thousands of years because of this agricultural boon. Other ancient cultures, such as Mesopotamia, China, and India, also benefited from this process.

The accumulation of silt, or the lack of the same, can be affected by human activity. In North America’s Mississippi River, for example, levees and dams built to regulate the river’s flooding activity can also cause silt to accumulate. This prevents the sediment from being deposited at the river’s delta in the Gulf of Mexico. Natural barrier islands and sandbars have deteriorated as a result. Unfortunately, these structures are home to many creatures that have consequently been displaced and endangered.

Human activity on land can have an opposite but equally detrimental effect. Expanding populations often clear away vegetation for housing developments or agriculture. As a result, rainfall and erosion carry silt and other forms of soil into nearby bodies of water. In regions such as Madagascar and the Amazon rain forest, this reduces the fertility of the remaining soil, making it less useful for food production. Meanwhile, the resulting change in the ecosystem of nearby rivers and wetlands can reduce fish populations, in turn affecting the creatures, including humans, who rely on fish supplies for survival.

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Discussion Comments

By bythewell — On Nov 18, 2013

@umbra21 - The problem is that it isn't that simple. There are many different ecosystems and some of them depend on silt being washed down to the ocean and spread along the river banks. Silt soil is some of the richest soil for plants and it also helps to prevent bigger floods from happening.

It's one of the reasons people are so worried about the Mississippi River. It's been so changed and diverted from the original path it's a natural disaster waiting to happen.

By umbra21 — On Nov 17, 2013

@browncoat - Well, it does depend on the river, although you are definitely right about erosion.

The good news is that it's not too late in most places. I saw a documentary about water and siltation recently where the banks of a river were all planted up with flax and trees and fenced so that livestock couldn't trample the riverbank.

There had been hardly any shellfish in the estuary at the end of the river for decades because of all the silt and how it disrupted the ecosystem, and they expected it to take years before that would change. But it only took a couple of years before the shellfish started to recover.

And it's not all that hard or expensive to plant a few trees. If more communities took responsibility for this, the world would be a better place very quickly. We'd have more access to cheap seafood as well!

By browncoat — On Nov 17, 2013

It always makes me sad to walk along a river and see how cloudy it is after a rain. They generally aren't supposed to change color like that. We just think it's normal because we've seen it so often, since so much of our land has been cleared of trees and plants.

If the land around the river has been properly planted for silt control and there isn't much erosion, then the river will remain relatively clear.

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