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Sediment and Sedimentation

Environmental Impacts Of Sedimentation

Erosion, weathering, and sedimentation constantly work together to reshape the Earth's surface. These are natural processes that sometimes require us to adapt and adjust to changes in our environment. However, too many people and too much disturbance of the land surface can drastically increase sedimentation rates, leading to significant increases in the frequency and severity of certain natural disasters. For example, disturbance by construction and related land development is sometimes a contributing factor in the mudflows and landslides that occur in certain areas of California. The resulting damage can be costly both in terms of money and lives.

It is reported that the world's rivers carry as much as 24 million tons of sediment to the ocean each year. About two-thirds of this may be directly related to human activity, which greatly accelerates the natural rate of erosion. This causes rapid loss of fertile topsoil, which leads to decreased crop productivity.

Increased sedimentation also causes increased size and frequency of flooding. As stream channels are filled in, the capacity of the channel decreases. As a result, streams flood more rapidly during a rainstorm, as well as more often, and they drain less quickly after flooding. Likewise, sedimentation can become a major problem on dammed rivers. Sediment accumulates in the lake created by the dam rather than moving farther downstream and accumulating in a delta. Over time, trapped sediment reduces the size of the lake and the useful life of the dam. In areas that are forested, lakes formed by dams are not as susceptible to this problem. Sedimentation is not as great due to interception of rainfall by the trees and underbrush.

Vegetative cover also prevents soil from washing into streams by holding the soil in place. Without vegetation, erosion rates can increase significantly. Human activity that disturbs the natural landscape and increases sediment loads to streams also disturbs aquatic ecosystems.

Many state and local governments are now developing regulations concerning erosion and sedimentation resulting from private and commercial development. Only by implementing such measures can we hope to curb these and other destructive side effects, thereby preserving the environment as well as our quality of life.

Resources

Books

Dixon, Dougal, and Raymond Bernor. The Practical Geologist. New York: Simon and Schuster, 1992.

Hancock P.L. and Skinner B.J., eds. The Oxford Companion to the Earth. Oxford: Oxford University Press, 2000.

Leopold, Luna. A View of the River. Cambridge: Harvard University Press, 1994.

Middleton, Gerard V., and Celestina V. Cotti Ferrero. Encyclopedia of Sediments & Sedimentary Rocks. Boston: Kluwer Academic Publishers, 2003.

Siever, Raymond. Sand. Scientific American Library Series. New York: W.H. Freeman, 1988.

Skinner, Brian J., and Stephen C. Porter. The Dynamic Earth: An Introduction to Physical Geology. 4th ed. John Wiley & Sons, 2000.

Westbroek, Peter. Life as a Geological Force: Dynamics of the Earth. New York: W. W. Norton, 1991.

Clay Harris

KEY TERMS

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Bedload: —The portion of sediment that is transported by rolling, skipping, and hopping along the stream bed at any given time because it is too heavy to be lifted by flowing stream water. It stands in contrast to suspended load.
Bedrock: —The unweathered or partially weathered solid rock layer, which is exposed at the Earth's surface or covered by a thin mantle of soil or sediment.
Clay: —The finest of sediment particles, less than 1/256 of a millimeter in diameter.
Delta: —A landform that develops where a stream deposits sediment at the edge of a standing body of water (lake or sea).
Floodplain: —The flat, low-lying area adjacent to a river or stream that becomes covered with water during flooding; flood waters deposit sand, silt and clay on this surface.
Geochemical cycle: —A number of interrelated environments or settings through which a chemical can move as a result of changes in state or incorporation into different compounds.
Grain size: —The size of a particle of sediment, ranging from clay to boulders; smaller size sediment is called fine grained, larger sediment is coarse grained.
Mass wasting: —Movement of large masses of sediment primarily in response to the force of gravity.
Outcrop: —A natural exposure of rock at the Earth's surface.
Pebbles: —Coarse particles of sediment larger than sand (2 mm) and smaller than boulders (256 mm).
Sand: —Sediment particles smaller than pebbles and larger than silt, ranging in size from 1/16 of a millimeter to 2 millimeters.
Sediment: —Soil and rock particles that wash off land surfaces and flow with water and gravity toward the sea. On the sea floor, sediment can build up into thick layers. When it compresses under its weight, sedimentary rock is formed.
Sedimentation: —The process by which sediment is removed from one place, and transported to another, where it accumulates.
Silt: —Soil particles derived mainly from sedimentary materials that range between 0.0002–0.05 mm in size.

Additional topics

Science EncyclopediaScience & Philosophy: Jean-Paul Sartre Biography to Seminiferous tubulesSediment and Sedimentation - Weathering, Water, Wind, Glacial Ice, Sediment Erosion, Sediment Size, Sediment Load - Erosion and transport, Agents of erosion and transport, Deposition