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.