Environmental Degradation

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Erosion is the wearing away of the land surface by running water, wind, ice, or other geologic agents, including such processes as gravitational creep.

The term geologic erosion refers to the normal, natural erosion caused by geological processes acting over long periods of time, undisturbed by humans. Accelerated erosion is a more rapid erosion process influenced by human, or sometimes animal, activities. Accelerated erosion in North America has only been recorded for the past few centuries, and in research studies, postsettlement erosion rates were found to be eight to 350 times higher than presettlement erosion rates.

Soil erosion has been both accelerated and controlled by humans since recorded history. In Asia, the Pacific, Africa, and South America, complex terracing and other erosion control systems on arable land go back thousands of years. Soil erosion and the resultant decreased food supply have been linked to the decline of historic, particularly Mediterranean, civilizations, though the exact relationship with the decline of governments such as the Roman Empire is not clear.

A number of terms have been used to describe different types of erosion, including gully erosion, rill erosion, interrill erosion, sheet erosion, splash erosion, saltation, surface creep, suspension, and siltation.In gully erosion, water accumulates in narrow channels and, over short periods, removes the soil from this narrow area to considerable depths, ranging from 1.5 ft (0.5 m) to as much as 82–98 ft (25–30 m).

Rill erosion refers to a process in which numerous small channels of only a few inches in depth are formed, usually occurring on recently cultivated soils. Interrill erosion is the removal of a fairly uniform layer of soil on a multitude of relatively small areas by rainfall splash and film flow.

Usually interpreted to include rill and interril erosion, sheet erosion is the removal of soil from the land surface by rainfall and surface runoff. Splash erosion, the detachment and airborne movement of small soil particles, is caused by the impact of raindrops on the soil.

Saltation is the bouncing or jumping action of soil and mineral particles caused by wind, water, or gravity. Saltation occurs when soil particles 0.1–0.5 mm in diameter are blown to a height of less than 6 in (15 cm) above the soil surface for relatively short distances. The process includes gravel or stones effected by the energy of flowing water, as well as any soil or mineral particle movement downslope due to gravity.

Surface creep, which usually requires extended observation to be perceptible, is the rolling of dislodged particles 0.5–1.0 mm in diameter by wind along the soil surface. Suspension occurs when soil particles less than 0.1 mm diameter are blown through the air for relatively long distances, usually at a height of less than 6 in (15 cm) above the soil surface. In siltation, decreased water speed causes deposits water-borne sediments, or silt, to build up in stream channels, lakes, reservoirs, or flood plains.

In the water erosion process, the eroded sediment is often higher (enriched) in organic matter, nitrogen, phosphorus, and potassium than in the bulk soil from which it came. The amount of enrichment may be related to the soil, amount of erosion, the time of sampling within a storm, and other factors. Likewise, during a wind erosion event, the eroded particles are often higher in clay, organic matter, and plant nutrients. Frequently, in the Great Plains, the surface soil becomes increasingly more sandy over time as wind erosion continues.

Erosion estimates using the Universal Soil Loss Equation (USLE) and the Wind Erosion Equation (WEE) estimate erosion on a point basis expressed in mass per unit area. If aggregated for a large area (e.g., state or nation), very large numbers are generated and have been used to give misleading conclusions. The estimates of USLE and WEE indicate only the soil moved from a point. They do not indicate how far the sediment moved or where it was deposited. In cultivated fields, the sediment may be deposited in other parts of the field with different crop cover or in areas where the land slope is less. It may also be deposited in riparian land along stream channels or in flood plains.

Only a small fraction of the water-eroded sediment leaves the immediate area. For example, in a study of five river watersheds in Minnesota, it was estimated that from less than 1–27% of the eroded material entered stream channels, depending on the soil and topographic conditions. The deposition of wind-eroded sediment is not well quantified, but much of the sediment is probably deposited in nearbyareas more protected from the wind by vegetative cover, stream valleys, road ditches, woodlands, or farmsteads.

While a number of national and regional erosion estimates for the United States have been made since the 1920s, the methodologies of estimation and interpretations have been different, making accurate time comparisons impossible. The most extensive surveys have been made since the Soil, Water and Related Resources Act was passed in 1977. In these surveys a large number of points were randomly selected, data assembled for the points, and the Universal Soil Loss Equation (USLE) or the Wind Erosion Equation (WEE) used to estimate erosion amounts. While these equations were the best available at the time, their results are only estimations, and subject to interpretation. Considerable research on improved methods of estimation is underway by the U.S. Department of Agriculture.

In the cornbelt of the United States, water erosion may cause a 1.7–7.8% drop in soil productivity over the next one hundred years, as compared to current levels, depending on the topography and soils of the area. The U.S.D.A. results, based on estimated erosion amounts for 1977, only included sheet erosion, not losses of plant nutrients. Though the figures may be low for this reason, other surveys have produced similar estimates.

In addition to depleting farmlands, eroded sediment causes off-site damages that, according to one study, may exceed on-site loss. The sediment may end up in a domestic water supply, clog stream channels, even degrade wetlands, wildlife habitats, and entire ecosystems.

Environmental Degradation; Gillied Land; Soil Eluviation; Soil Organic Matter; Soil Texture

Paddock, J. N., and C. Bly. Soil and Survival: Land Stewardship and the Future of American Agriculture. San Francisco: Sierra Club Books, 1987.

Resource Conservation Glossary. 3rd ed. Ankeny, IA: Soil Conservation Society of America, 1982.

Steinhart, P. "The Edge Gets Thinner." Audubon 85 (November 1983): 94–106+.

Brown, L. R., and E. Wolf. "Soil Erosion: Quiet Crisis in the World Economy." Worldwatch Paper #60. Washington DC: Worldwatch Institute, 1984.

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