erosion

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Concept

Erosion is a broadly defined group of processes involving the movement of soil and rock. This movement is often the result of flowing agents, whether wind, water, or ice, which sometimes behaves like a fluid in the large mass of a glacier. Gravitational pull may also influence erosion. Thus, erosion, as a concept in the earth sciences, overlaps with mass wasting or mass movement, the transfer of earth material down slopes as a result of gravitational force. Even more closely related to erosion is weathering, the breakdown of rocks and minerals at or near the surface of Earth owing to physical, chemical, or biological processes. Some definitions of erosion even include weathering as an erosive process. Though most widely known as a by-product of irresponsible land use by humans and for its negative effect on landforms, erosion is neither unnatural nor without benefit. Far more erosion occurs naturally than as a result of land development, and a combination of weathering and erosion is responsible for producing the soil from which Earth's plants grow.

How It Works

Weathering

The first step in the process of erosion is weathering. Weathering, in a general sense, occurs everywhere: paint peels; metal oxidizes, resulting in its tarnishing or rusting; and any number of products, from shoes to houses, begin to show the effects of physical wear and tear. The scuffing of a shoe, cracks in a sidewalk, or the chipping of glass in a gravel-spattered windshield are all examples of physical weathering. On the other hand, the peeling of paint is usually the result of chemical changes, which have reduced the adhesive quality of the paint. Certainly oxidation is a chemical change, meaning that it has not simply altered the external properties of the item but also has brought about a change in the way that the atoms are bonded.

Weathering, as the term is used in the geologic sciences, refers to these and other types of physical and chemical changes in rocks and minerals at or near the surface of Earth. A mineral is a substance that occurs naturally and is usually inorganic, meaning that it contains carbon in a form other than that of an oxide or a carbonate, neither of which is considered organic. It typically has a crystalline structure, or one in which the constituent parts have a simple and definite geometric arrangement repeated in all directions. Rocks are simply aggregates or combinations of minerals or organic material or both.

Two and One-Half Kinds of Weathering

There are three kinds of weathering (or perhaps two and one-half, since the third incorporates aspects of the first two): physical or mechanical, chemical, and biological. Physical or mechanical weathering takes place as a result of such factors as gravity, friction, temperature, and moisture. Gravity may cause a rock to drop from a height, such that it falls to the ground and breaks into pieces, while the friction of wind-borne sand may wear down a rock surface. Changes in temperature and moisture cause expansion and contraction of materials, as when water seeps into a crack in a rock and then freezes, expanding and splitting the rock.

Minerals are chemical compounds; thus, whereas physical weathering attacks the rock as a whole, chemical weathering effects the breakdown of the minerals that make up the rock. This breakdown may lead to the dissolution of the minerals, which then are washed away by water or wind or both, or it may be merely a matter of breaking the minerals down into simpler compounds. Reactions that play a part in this breakdown may include oxidation, mentioned earlier, as well as carbonation, hydrolysis (a reaction with water that results in the separation of a compound to form a new substance or substances), and acid reactions. For instance, if coal has been burned in an area, sulfur impurities in the air react with water vapor (an example of hydrolysis) to produce acid rain, which can eat away at rocks. Rainwater itself is a weak acid, and over the years it slowly dissolves the marble of headstones in old cemeteries.

As noted earlier, there are either three or two and one-half kinds of weathering, depending on whether one considers biological weathering a third variety or merely a subset of physical and chemical weathering. The weathering exerted by organisms (usually plants rather than animals) on rocks and minerals is indeed chemical and physical, but because of the special circumstances, it is useful to consider it individually. There is likely to be a long-term interaction between the organism and the geologic item, an obvious example being a piece of moss that grows on a rock. Over time, the moss will influence both physical and chemical weathering through its attendant moisture as well as its specific chemical properties, which induce decomposition of the rock's minerals.

Unconsolidated Material

The product of weathering in rocks or minerals is unconsolidated, meaning that it is in pieces, like gravel, though much less uniform in size. This is called regolith, a general term that describes a layer of weathered material that rests atop bedrock. Sand and soil, including soil mixed with loose rocks, are examples of regolith. Regolith is, in turn, a type of sediment, material deposited at or near Earth's surface from a number of sources, most notably preexisting rock.

Every variety of unconsolidated material has its own angle of repose, or the maximum angle at which it can remain standing. Piles of rocks may have an angle of repose as high as 45°, whereas dry sand has an angle of only 34°. The addition of water can increase the angle of repose, as anyone who has ever strengthened a sand castle by adding water to it knows. Suppose one builds a sand castle in the morning, sloping the sand at angles that would be impossible if it were dry. By afternoon, as wind and sunlight dry out the sand, the sand castle begins to fall apart, because its angle of repose is too high for the dry sand.

Water gives sand surface tension, the same property that causes water that has been spilled on a table to bead up rather than lie flat. If too much water is added to the sand, however, the sand becomes saturated and will flow, a process called lateral spreading. On the other hand, with too little moisture, the material is susceptible to erosion. Unconsolidated material in nature generally has a slope less than its angle of repose, owing to the influence of wind and other erosive forces.

Introduction to Mass Wasting

There are three general processes whereby a piece of earth material can be moved from a high out-cropping to the sea: weathering, mass wasting, and erosion. In the present context, we are concerned primarily with the last of these processes, of course, and secondarily with weathering, inasmuch as it contributes to erosion. A few words should be said about mass wasting, however, which, in its slower forms (most notably, creep), is related closely to erosion.

Mechanical or chemical processes, or a combination of the two, acting on a rock to dislodge it from a larger sample (e.g., separating a rock from a boulder) is an example of weathering, as we have seen. If the pieces of rock are swept away by a river in a valley below the outcropping, or if small pieces of rock are worn away by high winds, the process is erosion. Between the out-cropping and the river below, if a rock has been broken apart by weathering, it may be moved farther along by mass-wasting processes, such as creep or fall.

Real-Life Applications

Mass Wasting in Action

One of the principal sources of erosion is gravity, which is also the force behind creep, the slow downward movement of regolith along a hill slope. The regolith begins in a condition of unstable equilibrium, like a soda can lying on its side rather than perpendicular to a table's surface: in both cases, the object remains in place, yet a relatively small disturbance would be enough to dislodge it.

Changes in temperature or moisture are among the leading factors that result in creep. A variation in either can cause material to expand or contract, and freezing or thawing may be enough to shake regolith from its position of unstable equilibrium. Water also can provide lubrication, or additional weight, that assists the material in moving. Though it is slow, over time creep can produce some of the most dramatic results of any mass-wasting process. It can curve tree trunks at the base, break or dislodge retaining walls, and overturn objects ranging from fence posts to utility poles to tombstones.

Other Varieties of Flow

Creep is related to another slow mass-wasting process, known as solifluction, that occurs in the active layer of permafrost—that is, the layer that thaws in the summertime. The principal difference between creep and solifluction is not the speed at which they take place (neither moves any faster than about 0.5 in. [1 cm] per year) but the materials involved. Both are examples of flow, a chaotic form of mass wasting in which masses of material that are not uniform move downslope. With the exception of creep and solifluction, most forms of flow are comparatively rapid, and some are extremely so.

Because it involves mostly dry material, creep is an example of granular flow, which is composed of 0% to 20% water; on the other hand, solifluction, because of the ice component, is an instance of slurry flow, consisting of 20% to 40% water. If the water content is more than 40%, a slurry flow is considered a stream. Types of granular flow that move faster than creep range from earth flow to debris avalanche. Both earth flow and debris flow, its equivalent in slurry form, move at a broad range of speeds, anywhere from about 4 in. (10 cm) per year to 0.6 mi. (1 km) per hour. Grain flow can be as fast as 60 mi. (100 km) per hour, and mud flow is even faster. Fastest of all is debris avalanche, which may achieve speeds of 250 mi. (400 km) per hour.

Other Types of Mass Wasting

Other varieties of mass wasting include slump, slide, and fall. Slump occurs when a mass of regolith slides over or creates a concave surface (one shaped like the inside of a bowl.) The result is the formation of a small, crescent-shaped cliff, known as a scarp, at the upper end—rather like the crest of a wave. Slump often is classified as a variety of slide, in which material moves downhill in a fairly coherent mass (i.e., more or less in a section or group) along a flat or planar surface. These movements are sometimes called rock slides, debris slides, or, in common parlance, landslides.

In contrast to most other forms of mass wasting, in which there is movement along slopes that are considerably less than 90°, fall occurs at angles almost perpendicular to the ground. The "Watch for Falling Rock" signs on mountain roads may be frightening, and rock or debris fall is certainly one of the more dramatic forms of mass wasting. Yet the variety of mass wasting that has the most widespread effects on the morphology or shape of landforms is the slowest one—creep. (For more about the varieties of mass wasting, see Mass Wasting.)

What Causes Erosion?

As noted earlier, the influences behind erosion are typically either gravity or flowing media: water, wind, and even ice in glaciers. Liquid water is the substance perhaps most readily associated with erosion. Given enough time, water can wear away just about anything, as proved by the carving of the Grand Canyon by the Colorado River.

Dubbed the universal solvent for its ability to dissolve other materials, water almost never appears in its pure form, because it is so likely to contain other substances. Even "pure" mountain water contains minerals and pieces of the rocks over which it has flowed, a testament to the power of water in etching out landforms bit by bit. Nor does it take a rushing mountain stream or crashing waves to bring about erosion; even a steady drip of water is enough to wear away granite over time.

Moving Water

Along coasts, pounding waves continually alter the shoreline. The sheer force of those walls of water, a result of the Moon's gravitational pull (and, to a lesser extent, the Sun's), is enough to wear away cliffs, let alone beaches. In addition, waves carry pieces of pebble, stone, and sand that cause weathering in rocks. Waves even can bring about small explosions in pockmarked rock surfaces by trapping air in small cracks; eventually the pressure becomes great enough that the air escapes, loosening pieces of the rock.

In addition to the erosive power of saltwater waves on the shore, there is the force exerted by running water in creeks, streams, and rivers. As the river moves, pushing along sediment and other materials eroded from the streambed or riverbed, it carves out deep chasms in the bedrock beneath. These moving bodies of water continually reshape the land, carrying soil and debris downslope, or from the source of the river to its mouth or delta. A delta is a region of sediment formed when a river enters a larger body of water, at which point the reduction in velocity on the part of the river current leads to the widespread deposition (depositing) of sediment. It is so named because its triangular shape resembles that of the Greek letter delta, Δ .

Water at the bottom of a large body, such as a pond or lake, also exerts erosive power. Then there is the influence of falling rain. Assuming ground is not protected by vegetation, raindrops can loosen particles of soil, sending them scattering in all directions. A rain that is heavy enough may dislodge whole layers of topsoil and send them rushing away in a swiftly moving current. The land left behind may be rutted and scarred, much of its best soil lost for good.

Just as erosion gives to the soil, it also can take away. Whereas erosion on the Nile delta acted to move rich, black soil into the region (hence, the ancient Egyptians' nickname for their country, the "black land"), erosion also can remove soil layers. As is often the case, it is much easier to destroy than to create: 1 in. (2.5 cm) of soil may take as long as 500 years to form, yet a single powerful rainstorm or windstorm can sweep it away.

Glaciers

Ice, of course, is simply another form of water, but since it is solid, its physical (not its chemical) properties are quite different. Generally, physical sciences, such as physics or chemistry, treat as fluid all forms of matter that flow, whether they are liquid or gas. Normally, no solids are grouped under the heading of "fluid," but in the earth sciences there is at least one type of solid object that behaves as though it were fluid: a glacier.

A glacier is a large, typically moving mass of ice either on or adjacent to a land surface. It does not flow in the same way that water does; rather, it is moved by gravity, as a consequence of its extraordinary weight. Under certain conditions, a glacier may have a layer of melted water surrounding it, which greatly enhances it mobility. Regardless of whether it has this lubricant, however, a glacier steadily moves forward, carrying pieces of rock, soil, and vegetation with it.

These great rivers of ice gouge out pieces of bedrock from mountain slopes, fashioning deep valleys. Ice along the bottom of the glacier pulls away rocks and soil, which assist it in wearing away bedrock. The fjords of Norway, where high cliffs surround narrow inlets whose depths extend many thousands of feet below sea level, are a testament to the power of glaciers in shaping the Earth. The fact that the fjords came into existence only in the past two million years, a product of glacial activity associated with the last ice age, is evidence of something else remarkable about glaciers: their speed.

"Speed," of course, is a relative term when speaking about processes involved in the shaping of the planet. A "fast" glacier, one whose movement is assisted by a wet and warm (again, relatively warm!) maritime climate, moves at the rate of about 980 ft. (300 m) per year. Examples include not only the glaciers that shaped the fjords, but also the active Franz Josef glacier in southern New Zealand. By contrast, in the dry, exceptionally cold, inland climate of Antarctica, the Meserve glacier moves at the rate of just 9.8 ft. (3 m) per year.

Wind

The erosion produced by wind often is referred to as an eolian process, the name being a reference to Aeolus, the Greek god of the winds encountered in Homer's Odyssey and elsewhere. Eolian processes include the erosion, transport, and deposition of earth material owing to the action of wind. It is most pronounced in areas that lack effective ground cover in the form of solidly rooted, prevalent vegetation.

Eolian erosion in some ways is less forceful than the erosive influence of water. Water, after all, can lift heavier and larger particles than can the winds. Wind, however, has a much greater frictional component in certain situations. This is particularly true when the wind carries sand, every grain of which is like a cutting tool. In some desert regions the bases of rocks or cliffs have been sandblasted, leaving a mushroom-shaped formation. The wind could not lift the fine grains of sand very high, but in places where it has been able to do its work, it has left an indelible mark.

The Dust Bowl and Human Contribution to Erosion

Though human actions are not a direct cause of erosion, human negligence or mismanagement often has prepared the way for erosive action by wind, water, or other agents. Interesting, soil itself, formed primarily by chemical weathering and enhanced by biological activity in the sediment, is a product of nature's erosive powers. Erosion transports materials from one place to another, robbing the soil in one place and greatly enhancing it in another.

This is particularly the case where river deltas are concerned. By transporting sediment and depositing it in the delta, the river creates an area of extremely fertile soil that, in some cases, has become literally the basis for civilizations. The earliest civilizations of the Western world, in Egypt and Sumer, arose in the deltas of the Nile and the Tigris-Euphrates river systems, respectively.

Erosion on the Great Plains

An extreme example of the negative effects on the soil that can come from erosion (and, ultimately, from human mismanagement) took place in Texas, Oklahoma, Colorado, and Kansas during the 1930s. In the preceding years, farmers unwittingly had prepared the way for vast erosion by overcultivating the land and not taking proper steps to preserve its moisture against drought. In some places farmers alternated between wheat cultivation and livestock grazing on particular plots of land.

The soil, already weakened by raising wheat, was damaged further by the hooves of livestock, and thus when a period of high winds began at the height of the Great Depression (1929-41), the land was particularly vulnerable. The winds carried dust to places as far away as the eastern seaboard, in some cases removing topsoil to a depth of 3-4 in. (7-10 cm). Dunes of dust as tall as 15-20 ft. (4.6-6.1 m) formed, and the economic blight of the Depression was compounded for the farmers of the plains states, many of whom lost everything.

Out of the Dust Bowl era came some of the greatest American works of art: the 1939 film Wizard of Oz, John Steinbeck's book The Grapes of Wrath and the acclaimed motion picture (1939 and 1940, respectively), as well as Dorothea Lange's haunting photographs of Dust Bowl victims. The Dust Bowl years also taught farmers and agricultural officials a lesson about land use, and in later years farming practices changed. Instead of alternating one year of wheat growing with one year in which a field lay fallow, or unused, farmers discovered that a wheat-sorghum-fallow cycle worked better. They also enacted other measures, such as the planting of trees to serve as windbreaks around croplands.

The Striking Landscape of Erosion

Among the by-products of erosion are some of the most dramatic landscapes in the world, many of which are to be found in the United States. A particularly striking example appears in Colorado, where the Arkansas River carved out the Royal Gorge. Though it is not nearly as deep as the Grand Canyon, this one has something the more famous gorge does not: a bridge. Motorists with the stomach for it can cross a span 1,053 ft. (0.32 km) above the river, one of the most harrowing drives in America.

Another, perhaps equally taxing, drive is that down California 1, a gorgeous scenic highway whose most dramatic stretches lie between Carmel and San Simeon. Drivers headed south find themselves pressed up against the edge of the cliffs, such that the slightest deviation from the narrow road would send an automobile and its passengers plummeting to the rocks many hundreds of feet below. These magnificent, terrifying landforms are yet another product of erosion, in this case, the result of the pounding Pacific waves.

Also striking is the topography produced by the erosion of material left over from a volcanic eruption. As discussed in the Mountains essay, Devils Tower National Monument in Wyoming is the remains of an extinct volcano whose outer surface long ago eroded, leaving just the hard lava of the volcanic "neck." Erosion of lava also can produce mesas. Lava that has settled in a river valley may be harder than the rocks of the valley walls, such that the river eventually erodes the rocks, leaving only the lava platform. What was once the floor of the valley thus becomes the top of a mesa.

Controlling Erosion

The force that shapes valleys and coastlines is certainly enough to destroy hill slopes, often with disastrous consequences for nearby residents. Such has been the case in California, where, during the 1990s, areas were dealt a powerful onetwo punch of drought followed by rain. The drought killed off much of the vegetation that might have held the hillsides, and when rains came, they brought about mass wasting in the form of mudflows and landslides.

Over the surface of the planet, the average rate of erosion is about 1 in. (2.2 cm) in a thousand years. This is the average, however, meaning that in some places the rate is much, much higher, and in others it is greatly lower. The rate of erosion depends on several factors, including climate, the nature of the materials, the slope and angle of repose, and the role of plant and animal life in the local environment.

Whereas many types of plants help prevent erosion, the wrong types of planting can be detrimental. The dangers of improper land usage for crops and livestock are illustrated by the Dust Bowl experience, which highlights the fact that the organism most responsible for erosion is humanity itself. On the other hand, people also can protect against erosion by planting vegetation that holds the soil, by carefully managing and controlling land usage, and by lessening slope angle in places where gravity tends to erode the soil.

Where to Learn More

Cherrington, Mark. Degradation of the Land. New York: Chelsea House, 1991.

"Coastal and Nearshore Erosion." United States Geological Survey (USGS) (Web site). <http://walrus.wr.usgs.gov/hazards/erosion.html>.

Dean, Cornelia. Against the Tide: The Battle for America's Beaches. New York: Columbia University Press, 1999.

Hecht, Jeff. Shifting Shores: Rising Seas, Retreating Coastlines. New York: Scribners, 1990.

Middleton, Nick. Atlas of Environmental Issues. Illus. Steve Weston and John Downes. New York: Facts on File, 1989.

Protecting Your Property from Erosion (Web site). <http://www.abag.ca.gov/bayarea/enviro/erosion/erosion.html>.

Rybolt, Thomas R., and Robert C. Mebane. Environmental Experiments About Land. Hillside, NJ: Enslow Publishers, 1993.

"Soil Erosion on Farmland." New Zealand Ministry of Agriculture and Forestry (Web site). <http://www.maf.govt.nz/MAFnet/publications/erosion-risks/httoc.htm>.

Weathering and Erosion (Web site). <http://vishnu.glg.nau.edu/people/jhw/GLG101/Weathering.html>.

Wind Erosion Research Unit. United States Department of Agriculture/Kansas State University (Web site). <http://www.weru.ksu.edu/>.

The result of processes that entrain and transport earth materials along coastlines, in streams, and on hillslopes. Wind and water are common agents through which forces are applied to resistant rocks, soils, or other unconsolidated materials. Erosion types often are designated on the basis of the agent: wind erosion, fluvial (water) erosion, and glacial erosion. Fluvial erosion usually has been regarded as the most effective type in shaping the land surface during recent geologic time. Under certain environmental conditions, however, wind erosion moves considerable quantities of earth materials, as demonstrated during the “dust bowl” years in the United States. Glacial erosion shaped much of the land surface during the Quaternary Period of geologic time. Each type of erosion produces distinctive landforms, contributing to the diversity of terrestrial landscapes. See also Desert erosion features; Eolian landforms; Fluvial erosion landforms; Geomorphology; Glaciology; Mass wasting; Quaternary; Stream transport and deposition.

Forces exerted by erosion processes must exceed resistances of earth materials for entrainment and transportation to occur. Environmental conditions determine the magnitude of the forces, the resistances, and the relations among them. Erosion rates are highly variable in time and space due to changing relations between forces and resistances. The major factors governing wind-erosion rates are wind velocity, topography, surface roughness, soil properties and soil moisture, vegetation cover, and land use. The major factors governing fluvial-erosion rates on hillslopes are rainfall energy, topography, soil properties, vegetation cover, and land use. The major factors governing fluvial-erosion rates in stream channels are depth and velocity of water flow, together with the size and cohesiveness of the bed and bank materials. The major factors governing glacial-erosion rates are the depth and velocity of ice flow, together with the hardness of the bed and side-wall materials.

Accelerated erosion by fluvial processes may be the most important environmental problem worldwide because of its spatial and temporal ubiquity. Erosion rates commonly exceed soil-formation rates, causing depletion of soil resources. The effects of erosion are insidious due to the removal of the fertile topsoil horizon, compromising food production. Sediment frequently is transported well beyond the source area to degrade water quality in streams and lakes, harm aquatic life, reduce the water-storage capacity of reservoirs, and increase channel-maintenance costs. See also Soil; Soil conservation.

Definition: deterioration
Antonyms: building, construction, rebuilding, strengthening

The removal of part of the land surface by wind, water, gravity, or ice. These agents can only transport matter if the material has first been broken up by weathering. Some writers use a very narrow interpretation of the word, claiming that erosion refers only to the transport of debris and that denudation includes the weathering as well as the transport of rocks.

1. The deterioration brought about by the abrasive action of fluids or solids in motion.
2. The gradual deterioration of a paint film due to degradation of the binder, which results in chalking, or to mechanical abrasion, such as foot traffic.

Removal of weathered rocks by moving water, wind, ice, or gravity.

LearnThatWord.com is a free vocabulary and spelling program where you only pay for results!

A type of weathering in which surface soil and rock are worn away through the action of glaciers, water, and wind.

An eating or gnawing away; a shallow or superficial ulceration; in dentistry, the wasting away or loss of substance of a tooth by a chemical process that does not involve known bacterial action.

The chemical or mechanicochemical destruction of tooth substance, the mechanism of which is incompletely known, which leads to the creation of concavities of many shapes at the cementoenamel junction of teeth. The surface of the cavity, unlike dental caries, is hard and smooth.

Erosion. (Sapp/Eversole/Wysocki, 2004)

• Processes, Phenomena, Events, and Techniques - erosion: wearing down of natural landforms by wind, water, ice, or gravity
• Disasters and Phenomena - erosion: wearing away of topsoil by wind and water
• Soils and Soil Management - erosion: gradual eating or wearing away of soil by water, wind, snow, or ice

For morphological image processing operations, see Erosion (morphology). For use of in dermatopathology, see Erosion (dermatopathology).

A natural arch produced by the erosion of differentially weathered rock in Jebel Kharaz, Jordan

Erosion is the process by which soil and rock are removed from the Earth's surface by natural processes such as wind or water flow, and then transported and deposited in other locations.

While erosion is a natural process, human activities have dramatically increased (by 10-40 times) the rate at which erosion is occurring globally. Excessive erosion causes problems such as desertification, decreases in agricultural productivity due to land degradation, sedimentation of waterways, and ecological collapse due to loss of the nutrient rich upper soil layers. Water and wind erosion are now the two primary causes of land degradation; combined, they are responsible for 84% of degraded acreage, making excessive erosion one of the most significant global environmental problems we face today.^[1][2]

Industrial agriculture, deforestation, roads, anthropogenic climate change and urban sprawl are amongst the most significant human activities in regards to their effect on stimulating erosion.^[3][4] However, there are many available alternative land use practices that can curtail or limit erosion—such as terrace-building, no-till agriculture, and revegetation of denuded soils.

Contents 1 Physical processes 1.1 Water erosion 1.1.1 Rainfall 1.1.2 Rivers and streams 1.1.3 Coastal erosion 1.1.4 Glaciers 1.1.5 Floods 1.1.6 Freezing and thawing 1.2 Wind erosion 1.3 Gravitational erosion 1.4 Exfoliation 2 Factors affecting erosion rates 2.1 Precipitation and wind speed 2.2 Soil structure and composition 2.3 Vegetative cover 2.4 Topography 3 Human activities that increase erosion rates 3.1 Agricultural practices 3.2 Deforestation 3.3 Roads and urbanization 3.4 Climate change 4 Global environmental effects 4.1 Land degradation 4.2 Sedimentation of aquatic ecosystems 4.3 Airborne dust pollution 4.4 Tectonic effects 5 Monitoring, measuring, and modeling erosion 6 Prevention and remediation 7 See also 8 Notes 9 Further reading 10 External links

Physical processes

Water erosion

Rainfall

A hillside covered in rills and gullies due to erosion processes caused by rainfall

There are three primary types of erosion that occur as a direct result of rainfall -- sheet erosion, rill erosion, and gully erosion. Sheet erosion is generally seen as the first and least severe stage in the soil erosion process, which is followed by rill erosion, and finally gully erosion (the most severe of the three).^[5][6]

The impact of a falling raindrop creates a small crater in the soil, ejecting soil particles. The distance these soil particles travel (on level ground) can be as much as 2 feet vertically, and 5 feet horizontally. Once the rate of rain fall is faster than the rate of infiltration into the soil, surface runoff occurs and carries the loosened soil particles down slope.^[7]

Sheet erosion is the transport of loosened soil particles by surface runoff that is flowing downhill in thin sheets.^[7]

Rill erosion refers to the development of small, ephemeral concentrated flow paths, which function as both sediment source and sediment delivery systems for erosion on hillslopes. Generally, where water erosion rates on disturbed upland areas are greatest, rills are active. Flow depths in rills are typically on the order of a few centimeters or less and slopes may be quite steep. This means that rills exhibit very different hydraulic physics than water flowing through the deeper, wider channels of streams and rivers.^{[citation needed]}

Gully erosion occurs when runoff water accumulates, and then rapidly flows in narrow channels during or immediately after heavy rains or melting snow, removing soil to a considerable depth.^[8][9][10]

Rivers and streams

Dobbingstone Burn, Scotland -- This photo illustrates two different types of erosion affecting the same place. Valley erosion is occurring due to the flow of the stream, and the boulders and stones (and much of the soil) that are lying on the edges are glacial till that was left behind as ice age glaciers flowed over the terrain.

Valley or stream erosion occurs with continued water flow along a linear feature. The erosion is both downward, deepening the valley, and headward, extending the valley into the hillside. In the earliest stage of stream erosion, the erosive activity is dominantly vertical, the valleys have a typical V cross-section and the stream gradient is relatively steep. When some base level is reached, the erosive activity switches to lateral erosion, which widens the valley floor and creates a narrow floodplain. The stream gradient becomes nearly flat, and lateral deposition of sediments becomes important as the stream meanders across the valley floor. In all stages of stream erosion, by far the most erosion occurs during times of flood, when more and faster-moving water is available to carry a larger sediment load. In such processes, it is not the water alone that erodes: suspended abrasive particles, pebbles and boulders can also act erosively as they traverse a surface.^{[citation needed]}

Bank erosion is the wearing away of the banks of a stream or river. This is distinguished from changes on the bed of the watercourse, which is referred to as scour. Erosion and changes in the form of river banks may be measured by inserting metal rods into the bank and marking the position of the bank surface along the rods at different times.^[11]

Thermal erosion is the result of melting and weakening permafrost due to moving water.^[12] It can occur both along rivers and at the coast. Rapid river channel migration observed in the Lena River of Siberia is due to thermal erosion, as these portions of the banks are composed of permafrost-cemented non-cohesive materials.^[13] Much of this erosion occurs as the weakened banks fail in large slumps. Thermal erosion also affects the Arctic coast, where wave action and near-shore temperatures combine to undercut permafrost bluffs along the shoreline and cause them to fail. Annual erosion rates along a 100-kilometer segment of the Beaufort Sea shoreline averaged 5.6 meters per year from 1955 to 2002.^[14]

Coastal erosion

Main article: Coastal erosion

See also: Beach evolution

Wave cut platform caused by erosion of cliffs by the sea, at Southerndown in South Wales

Shoreline erosion, which occurs on both exposed and sheltered coasts, primarily occurs through the action of currents and waves but sea level (tidal) change can also play a role.

Hydraulic action takes place when air in a joint is suddenly compressed by a wave closing the entrance of the joint. This then cracks it. Wave pounding is when the sheer energy of the wave hitting the cliff or rock breaks pieces off. Abrasion or corrasion is caused by waves launching seaload at the cliff. It is the most effective and rapid form of shoreline erosion (not to be confused with corrosion). Corrosion is the dissolving of rock by carbonic acid in sea water. Limestone cliffs are particularly vulnerable to this kind of erosion. Attrition is where particles/seaload carried by the waves are worn down as they hit each other and the cliffs. This then makes the material easier to wash away. The material ends up as shingle and sand. Another significant source of erosion, particularly on carbonate coastlines, is the boring, scraping and grinding of organisms, a process termed bioerosion.^{[citation needed]}

Sediment is transported along the coast in the direction of the prevailing current (longshore drift). When the upcurrent amount of sediment is less than the amount being carried away, erosion occurs. When the upcurrent amount of sediment is greater, sand or gravel banks will tend to form as a result of deposition. These banks may slowly migrate along the coast in the direction of the longshore drift, alternately protecting and exposing parts of the coastline. Where there is a bend in the coastline, quite often a build up of eroded material occurs forming a long narrow bank (a spit). Armoured beaches and submerged offshore sandbanks may also protect parts of a coastline from erosion. Over the years, as the shoals gradually shift, the erosion may be redirected to attack different parts of the shore.^{[citation needed]}

Glaciers

Glacial moraines above Lake Louise, in Alberta, Canada.

Glaciers erode predominantly by three different processes: abrasion/scouring, plucking, and ice thrusting. In an abrasion process, debris in the basal ice scrapes along the bed, polishing and gouging the underlying rocks, similar to sandpaper on wood. Glaciers can also cause pieces of bedrock to crack off in the process of plucking. In ice thrusting, the glacier freezes to its bed, then as it surges forward, it moves large sheets of frozen sediment at the base along with the glacier. This method produced some of the many thousands of lake basins that dot the edge of the Canadian Shield. These processes, combined with erosion and transport by the water network beneath the glacier, leave moraines, drumlins, ground moraine (till), kames, kame deltas, moulins, and glacial erratics in their wake, typically at the terminus or during glacier retreat.^{[citation needed]}

Floods

At extremely high flows, kolks, or vortices are formed by large volumes of rapidly rushing water. Kolks cause extreme local erosion, plucking bedrock and creating pothole-type geographical features called Rock-cut basins. Examples can be seen in the flood regions result from glacial Lake Missoula, which created the channeled scablands in the Columbia Basin region of eastern Washington.^[15]

Freezing and thawing

Cold weather causes water trapped in tiny rock cracks to freeze and expand, breaking the rock into several pieces. This can lead to gravity erosion on steep slopes. The scree which forms at the bottom of a steep mountainside is mostly formed from pieces of rock (soil) broken away by this means. It is a common engineering problem wherever rock cliffs are alongside roads, because morning thaws can drop hazardous rock pieces onto the road.^{[citation needed]}

In some places, water seeps into rocks during the daytime, then freezes at night. Ice expands, thus, creating a wedge in the rock. Over time, the repetition in the forming and melting of the ice causes fissures, which eventually breaks the rock down. This process is also called freeze-thaw weathering.^{[citation needed]}

Wind erosion

Árbol de Piedra, a rock formation in the Altiplano, Bolivia sculpted by wind erosion.

Main article: Aeolian processes

Wind erosion is a major geomorphological force, especially in arid and semi-arid regions. It is also a major source of land degradation, evaporation, desertification, harmful airborne dust, and crop damage—especially after being increased far above natural rates by human activities such as deforestation, urbanization, and agriculture.^[16][17]

Wind erosion is of two primary varieties: deflation, where the wind picks up and carries loose soil particles; and abrasion, where surfaces are worn down as they are struck by airborne particles carried by wind. Deflation is divided into three categories: (1) surface creep, where larger, heavier particles slide or roll along the ground; (2) saltation, where particles are lifted a short height into the air, and bounce and saltate across the surface of the soil; and (3) suspension, where very small and light particles are lifted into the air by the wind, and are often carried for long distances. Saltation is responsible for the majority (50-70%) of wind erosion, followed by suspension (30-40%), and then surface creep (5-25%).^[18][19]

Wind erosion is much more severe in arid areas, and during times of drought. For example, in the Great Plains, it is estimated that wind erosion soil loss can be as much as 6100 times greater in drought years, than in wet years.^[20]

Gravitational erosion

Wadi in Makhtesh Ramon, Israel, showing gravity collapse erosion on its banks.

Mass movement is the downward and outward movement of rock and sediments on a sloped surface, mainly due to the force of gravity.^[21][22]

Mass movement is an important part of the erosional process, and is often the first stage in the breakdown and transport of weathered materials in mountainous areas.^[23] It moves material from higher elevations to lower elevations where other eroding agents such as streams and glaciers can then pick up the material and move it to even lower elevations. Mass-movement processes are always occurring continuously on all slopes; some mass-movement processes act very slowly; others occur very suddenly, often with disastrous results. Any perceptible down-slope movement of rock or sediment is often referred to in general terms as a landslide. However, landslides can be classified in a much more detailed way that reflects the mechanisms responsible for the movement and the velocity at which the movement occurs. One of the visible topographical manifestations of a very slow form of such activity is a scree slope.^{[citation needed]}

Slumping happens on steep hillsides, occurring along distinct fracture zones, often within materials like clay that, once released, may move quite rapidly downhill. They will often show a spoon-shaped isostatic depression, in which the material has begun to slide downhill. In some cases, the slump is caused by water beneath the slope weakening it. In many cases it is simply the result of poor engineering along highways where it is a regular occurrence.^{[citation needed]}

Surface creep is the slow movement of soil and rock debris by gravity which is usually not perceptible except through extended observation. However, the term can also describe the rolling of dislodged soil particles 0.5 to 1.0 mm in diameter by wind along the soil surface.^{[citation needed]}

Exfoliation

Exfoliation is a type of erosion that occurs when a rock is rapidly heated up by the sun. This results in the expansion of the rock. When the temperature decreases again, the rock contracts, causing pieces of the rock to break off. Exfoliation occurs mainly in deserts due to the high temperatures during the day and cold temperatures at night.^[24]

Factors affecting erosion rates

Precipitation and wind speed

Climatic factors include the amount and intensity of precipitation, the average temperature, as well as the typical temperature range, seasonality, wind speed, and storm frequency. In general, given similar vegetation and ecosystems, areas with high-intensity precipitation, more frequent rainfall, more wind, or more storms are expected to have more erosion.^{[citation needed]}

Rainfall intensity is the primary determinant of erosivity, with higher intensity rainfall generally resulting in more erosion. The size and velocity of rain drops is also an important factor. Larger and higher-velocity rain drops have greater kinetic energy, and thus their impact will displace soil particles by larger distances than smaller, slower-moving rain drops.^[25]

Soil structure and composition

Erosional gully in unconsolidated Dead Sea (Israel) sediments along the southwestern shore. This gully was excavated by floods from the Judean Mountains in less than a year.

The composition, moisture, and compaction of soil are all major factors in determining the erosivity of rainfall. Sediments containing more clay tend to be more resistant to erosion than those with sand or silt, because the clay helps bind soil particles together.^[26] Soil containing high levels of organic materials are often more resistant to erosion, because the organic materials coagulate soil colloids and create a stronger, more stable soil structure.^[27] The amount of water present in the soil before the precipitation also plays an important role, because it sets limits on the amount of water that can be absorbed by the soil (and hence prevented from flowing on the surface as erosive runoff). Wet, saturated soils will not be able to absorb as much rain water, leading to higher levels of surface runoff and thus higher erosivity for a given volume of rainfall.^[27][28] Soil compaction also affects the permeability of the soil to water, and hence the amount of water that flows away as runoff. More compacted soils will have a larger amount of surface runoff than less compacted soils.^[27]

Vegetative cover

See also: Vegetation and slope stability

Vegetation acts as an interface between the atmosphere and the soil. It increases the permeability of the soil to rainwater, thus decreasing runoff. It shelters the soil from winds, which results in decreased wind erosion, as well as advantageous changes in microclimate. The roots of the plants bind the soil together, and interweave with other roots, forming a more solid mass that is less susceptible to both water and wind erosion. The removal of vegetation increases the rate of surface erosion.^[29]

Topography

The topography of the land determines the velocity at which surface runoff will flow, which in turn determines the erosivity of the runoff. Longer, steeper slopes (especially those without adequate vegetative cover) are more susceptible to very high rates of erosion during heavy rains than shorter, less steep slopes. Steeper terrain is also more prone to mudslides, landslides, and other forms of gravitational erosion processes.^[30][31][32]

Human activities that increase erosion rates

Agricultural practices

Tilled farmland such as this is very susceptible to erosion from rainfall, due to the destruction of vegetative cover and the loosening of the soil during plowing.

Unsustainable agricultural practices are the single greatest contributor to the global increase in erosion rates.^[33] The tillage of agricultural lands, which breaks up soil into finer particles, is one of the primary factors. The problem has been exacerbated in modern times, due to mechanized agricultural equipment that allows for deep plowing, which severely increases the amount of soil that is available for transport by water erosion. Others include mono-cropping, farming on steep slopes, pesticide and chemical fertilizer usage (which kill organisms that bind soil together), row-cropping, and the use of surface irrigation.^[34][35] Tillage also increases wind erosion rates, by dehydrating the soil and breaking it up into smaller particles that can be picked up by the wind. Exacerbating this is the fact that most of the trees are generally removed from agricultural fields, allowing winds to have long, open runs to travel over at higher speeds.^[36] Heavy grazing reduces vegetative cover and causes severe soil compaction, both of which increase erosion rates.^[37]

Deforestation

In this clearcut, almost all of the vegetation has been stripped from surface of steep slopes, in an area with very heavy rains. Severe erosion occurs in cases such as this, causing stream sedimentation and the loss of nutrient rich topsoil.

In an undisturbed forest, the mineral soil is protected by a layer of leaf litter and an humus that cover the forest floor. These two layers form a protective mat over the soil that absorbs the impact of rain drops. They are porous and highly permeable to rainfall, and allow rainwater to slow percolate into the soil below, instead of flowing over the surface as runoff.^[38] The roots of the trees and plants^[39] hold together soil particles, preventing them from being washed away.^[38] The vegetative cover acts to reduce the velocity of the raindrops that strike the foliage and stems before hitting the ground, reducing their kinetic energy.^[40] However it is the forest floor, more than the canopy, that prevents surface erosion. The terminal velocity of rain drops is reached in about 8 meters. Because forest canopies are usually higher than this, rain drops can often regain terminal velocity even after striking the canopy. However, the intact forest floor, with its layers of leaf litter and organic matter, is still able to absorb the impact of the rainfall.^[40][41]

Deforestation causes increased erosion rates due to exposure of mineral soil by removing the humus and litter layers from the soil surface, removing the vegetative cover that binds soil together, and causing heavy soil compaction from logging equipment. Once trees have been removed by fire or logging, infiltration rates become high and erosion low to the degree the forest floor remains intact. Severe fires can lead to significant further erosion if followed by heavy rainfall.^[42]

Globally one of the largest contributors to erosive soil loss in the year 2006 is the slash and burn treatment of tropical forests. In a number of regions of the earth, entire sectors of a country have been rendered unproductive. For example, on the Madagascar high central plateau, comprising approximately ten percent of that country's land area, virtually the entire landscape is sterile of vegetation, with gully erosive furrows typically in excess of 50 meters deep and one kilometer wide. Shifting cultivation is a farming system which sometimes incorporates the slash and burn method in some regions of the world. This degrades the soil and causes the soil to become less and less fertile.^{[citation needed]}

Roads and urbanization

Urbanization has major effects on erosion processes—first by denuding the land of vegetative cover, altering drainage patterns, and compacting the soil during construction; and next by covering the land in an impermeable layer of asphalt or concrete that increases the amount of surface runoff and increases surface wind speeds.^[43] Much of the sediment carried in runoff from urban areas (especially roads) is highly contaminated with fuel, oil, and other chemicals.^[44] This increased runoff, in addition to eroding and degrading the land that it flows over, also causes major disruption to surrounding watersheds by altering the volume and rate of water that flows through them, and filling them with chemically polluted sedimentation. The increased flow of water through local waterways also causes a large increase in the rate of bank erosion.^[45]

Climate change

Main article: Land degradation

The warmer atmospheric temperatures observed over the past decades are expected to lead to a more vigorous hydrological cycle, including more extreme rainfall events.^[46] The rise in sea levels that has occurred as a result of climate change has also greatly increased coastal erosion rates.^[47][48]

Studies on soil erosion suggest that increased rainfall amounts and intensities will lead to greater rates of erosion. Thus, if rainfall amounts and intensities increase in many parts of the world as expected, erosion will also increase, unless amelioration measures are taken. Soil erosion rates are expected to change in response to changes in climate for a variety of reasons. The most direct is the change in the erosive power of rainfall. Other reasons include: a) changes in plant canopy caused by shifts in plant biomass production associated with moisture regime; b) changes in litter cover on the ground caused by changes in both plant residue decomposition rates driven by temperature and moisture dependent soil microbial activity as well as plant biomass production rates; c) changes in soil moisture due to shifting precipitation regimes and evapo-transpiration rates, which changes infiltration and runoff ratios; d) soil erodibility changes due to decrease in soil organic matter concentrations in soils that lead to a soil structure that is more susceptible to erosion and increased runoff due to increased soil surface sealing and crusting; e) a shift of winter precipitation from non-erosive snow to erosive rainfall due to increasing winter temperatures; f) melting of permafrost, which induces an erodible soil state from a previously non-erodible one; and g) shifts in land use made necessary to accommodate new climatic regimes.^{[citation needed]}

Studies by Pruski and Nearing indicated that, other factors such as land use not considered, we can expect approximately a 1.7% change in soil erosion for each 1% change in total precipitation under climate change.^[49]

Global environmental effects

World map indicating areas that are vulnerable to high rates of water erosion.

During the 17th and 18th centuries, Easter Island experienced severe erosion due to deforestation and unsustainable agricultural practices. The resulting loss of topsoil ultimately led to ecological collapse, causing mass starvation and the complete disintegration of the Easter Island civilization.^[50][51]

Due to the severity of its ecological effects, and the scale on which it is occurring, erosion constitutes one of the most significant global environmental problems we face today.^[2]

Land degradation

Water and wind erosion are now the two primary causes of land degradation; combined, they are responsible for 84% of degraded acreage.^[1]

Each year, about 75 billion tons of soil is eroded from the land—a rate that is about 13-40 times as fast as the natural rate of erosion.^[52] Approximately 40% of the world's agricultural land is seriously degraded.^[53] According to the UN, an area of fertile soil the size of Ukraine is lost every year because of drought, deforestation and climate change.^[54] In Africa, if current trends of soil degradation continue, the continent might be able to feed just 25% of its population by 2025, according to UNU's Ghana-based Institute for Natural Resources in Africa.^[55]

The loss of soil fertility due to erosion is further problematic because the response is often to apply chemical fertilizers, which leads to further water and soil pollution, rather than to allow the land to regenerate.^[56]

Sedimentation of aquatic ecosystems

Soil erosion (especially from agricultural activity) is considered to be the leading global cause of diffuse water pollution, due to the effects of the excess sediments flowing into the world's waterways. The sediments themselves act as pollutants, as well as being carriers for other pollutants, such as attached pesticide molecules or heavy metals.^[57]

The effect of increased sediments loads on aquatic ecosystems can be catastrophic. Silt can smother the spawning beds of fish, by filling in the space between gravel on the stream bed. It also reduces their food supply, and causes major respiratory issues for them as sediment enters their gills. The biodiversity of aquatic plant and algal life is reduced, and invertebrates are also unable to survive and reproduce. While the sedimentation event itself might be relatively short-lived, the ecological disruption caused by the mass die off often persists long into the future.^[58]

One of the most serious and long-running water erosion problems worldwide is in the People's Republic of China, on the middle reaches of the Yellow River and the upper reaches of the Yangtze River. From the Yellow River, over 1.6 billion tons of sediment flows into the ocean each year. The sediment originates primarily from water erosion in the Loess Plateau region of the northwest.^{[citation needed]}

Airborne dust pollution

Soil particles picked up during wind erosion are a major source of air pollution, in the form of airborne particulates -- "dust". These airborne soil particles are often contaminated with toxic chemicals such as pesticides or petroleum fuels, posing ecological and public health hazards when they later land, or are inhaled/ingested.^{[59][60][61][62]}

Dust from erosion acts to suppress rainfall and changes the sky color from blue to white, which leads to an increase in red sunsets. Over 50% of the African dust that reaches the United States affects Florida.^[63] Dust events have been linked to a decline in the health of coral reefs across the Caribbean and Florida, primarily since the 1970s.^[64] Similar dust plumes originate in the Gobi desert, which combined with pollutants, spread large distances downwind, or eastward, into North America.^[65]

Tectonic effects

This section requires expansion.

Main article: Erosion and tectonics

The removal by erosion of large amounts of rock from a particular region, and its deposition elsewhere, can result in a lightening of the load on the lower crust and mantle. This can cause tectonic or isostatic uplift in the region.^[66][67]

Monitoring, measuring, and modeling erosion

Terracing is an ancient technique that can significantly slow the rate of water erosion on cultivated slopes.

See also: Erosion prediction

This section requires expansion.

Monitoring and modeling of erosion processes can help us better understand the causes, make predictions, and plan how to implement preventative and restorative strategies. However, the complexity of erosion processes and the number of areas that must be studied to understand and model them (e.g. climatology, hydrology, geology, chemistry, physics, etc.) makes accurate modelling quite challenging.^[68][69] Erosion models are also non-linear, which makes them difficult to work with numerically, and makes it difficult or impossible to scale up to making predictions about large areas from data collected by sampling smaller plots.^[70]

The most commonly used model for predicting soil loss from water erosion is the Universal Soil Loss Equation (USLE), which estimates the average annual soil loss as^[71]:

where R is the rainfall erosivity factor, K is the soil erodibility factor, L and S are topographic factors representing length and slope, and C and P are cropping management factors.

Erosion is measured and further understood using tools such as the micro-erosion meter (MEM) and the traversing micro-erosion meter (TMEM). The MEM has proved helpful in measuring bedrock erosion in various ecosystems around the world. It can measure both terrestrial and oceanic erosion. On the other hand, the TMEM can be used to track the expanding and contracting of volatile rock formations and can give a reading of how quickly a rock formation is deteriorating.^{[citation needed]}

Prevention and remediation

See also: Erosion control

A windbreak (the row of trees) planted next to an agricultural field, acting as a shield against strong winds. This reduces the effects of wind erosion, and provides many other benefits.

The most effective known method for erosion prevention is to increase vegetative cover on the land, which helps prevent both wind and water erosion.^[72] Terracing is an extremely effective means of erosion control, which has been practiced for thousands of years by people all over the world.^[73] Windbreaks (also called shelterbelts) are rows of trees and shrubs that are planted along the edges of agricultural fields, to shield the fields against winds.^[74] In addition to significantly reducing wind erosion, windbreaks provide many other benefits such as improved microclimates for crops (which are sheltered from the dehydrating and otherwise damaging effects of wind), habitat for beneficial bird species,^[75] carbon sequestration,^[76] and aesthetic improvements to the agricultural landscape.^[77][78] Traditional planting methods, such as mixed-cropping (instead of monocropping) and crop rotation have also been shown to significantly reduce erosion rates.^[79][80]

See also

• Badland
• Biorhexistasy
• Bridge scour
• Cellular confinement
• Coastal sediment supply
• Denudation
• Food security
• Geomorphology
• Groundwater sapping
• Highly erodible land
• Lessivage
• Marine terrace
• Riparian strips
• River anticlines
• Sediment transport
• Sphericity scale
• TERON (Tillage erosion)
• Vegetation and slope stability
• Vetiver System

Notes

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (April 2012)

1. ^ ^{a b} Blanco, Humberto & Lal, Rattan (2010). "Soil and water conservation". Principles of Soil Conservation and Management. Springer. p. 2. ISBN 978-90-481-8529-0. http://books.google.com/books?id=Wj3690PbDY0C&pg=PA2. 
2. ^ ^{a b} Toy, Terrence J. et al (2002). Soil Erosion: Processes, Predicition, Measurement, and Control. John Wiley & Sons. p. 1. ISBN 978-0-471-38369-7. http://books.google.com/books?id=7YBaKZ-28j0C&pg=PA1. 
3. ^ Montgomery, David (October 2, 2008). Dirt: The Erosion of Civilizations (1st ed.). University of California Press. p. ^{[page needed]}. ISBN 978-0-520-25806-8. http://books.google.com/books?id=HSu8r15-nnoC. 
4. ^ Julien, Pierre Y. (2010). Erosion and Sedimentation. Cambridge University Press. p. 1. ISBN 978-0-521-53737-7. http://books.google.com/books?id=Gv72uiVmWEYC&pg=PA1. 
5. ^ Toy, Terrence J. et al (2002). Soil Erosion: Processes, Predicition, Measurement, and Control. John Wiley & Sons. pp. 60–61. ISBN 978-0-471-38369-7. http://books.google.com/books?id=7YBaKZ-28j0C&pg=PA60. 
6. ^ Zachar, Dušan (1982). "Classification of soil erosion". Soil Erosion. Vol. 10. Elsevier. p. 48. ISBN 978-0-444-99725-8. http://books.google.com/books?id=o8ny2dUkpM8C&pg=PA48. 
7. ^ ^{a b} Food and Agriculture Organization (1965). "Types of erosion damage". Soil Erosion by Water: Some Measures for Its Control on Cultivated Lands. United Nations. pp. 23–25. ISBN 978-92-5-100474-6. http://books.google.com/books?id=6KeL3ix6ZqQC&pg=PA23. 
8. ^ Poeson, Jean et al. (2007). "Gully erosion in Europe". In Boardman, John & Poeson, Jean. Soil Erosion in Europe. John Wiley & Sons. pp. 516–519. ISBN 978-0-470-85911-7. http://books.google.com/books?id=vvOFRskFunwC&pg=PA516. 
9. ^ Poeson, Jean et al. (2002). "Gully erosion in dryland environments". In Bull, Louise J. & Kirby, M.J.. Dryland Rivers: Hydrology and Geomorphology of Semi-Arid Channels. John Wiley & Sons. ISBN 978-0-471-49123-1. http://books.google.com/books?id=qjHoYZXQee0C&pg=PA229. 
10. ^ Borah, Deva K. et al (2008). "Watershed sediment yield". In Garcia, Marcelo H.. Sedimentation Engineering: Processes, Measurements, Modeling, and Practice. ASCE Publishing. p. 828. ISBN 978-0-7844-0814-8. http://books.google.com/books?id=1AsypwBUa_wC&pg=PA828. 
11. ^ Nancy D. Gordon (2004-06-01). "Erosion and Scour". Stream hydrology: an introduction for ecologists. ISBN 978-0-470-84357-4. http://books.google.com/?id=_PJHw-hSKGgC&pg=PA113 
12. ^ "Thermal Erosion". NSIDC Glossary. National Snow and Ice Data Center. Archived from the original on 2010-11-18. http://nsidc.org/cgi-bin/words/word.pl?thermal%20erosion. Retrieved 21 December 2009. 
13. ^ Costard, F.; Dupeyrat, L.; Gautier, E.; Carey-Gailhardis, E. (2003). "Fluvial thermal erosion investigations along a rapidly eroding river bank: application to the Lena River (central Siberia)". Earth Surface Processes and Landforms 28 (12): 1349. Bibcode 2003ESPL...28.1349C. doi:10.1002/esp.592. 
14. ^ Jones, B.M.; Hinkel, K.M., Arp, C.D. and Eisner, W.R. (2008). "Modern Erosion Rates and Loss of Coastal Features and Sites, Beaufort Sea Coastline, Alaska". Arctic (Arctic Institute of North America) 61 (4): 361–372. http://digitization.ucalgary.ca/arctic/index.php/arctic/article/view/44/115. ^{[dead link]}
15. ^ See, for example: Alt, David (2001). Glacial Lake Missoula & its Humongous Floods. Mountain Press. ISBN 978-0-87842-415-3. http://books.google.com/books?id=s4y3c8fxeEwC. 
16. ^ Zheng, Xiaojing & Huang, Ning (2009). Mechanics of Wind-Blown Sand Movements. Springer. pp. 7–8. ISBN 978-3-540-88253-4. http://books.google.com/books?id=R6kYrbA3XSAC&pg=PA7. 
17. ^ Cornelis, Wim S. (2006). "Hydroclimatology of wind erosion in arid and semi-arid environments". In D'Odorico, Paolo & Porporato, Amilcare. Dryland Ecohydrology. Springer. p. 141. ISBN 978-1-4020-4261-4. http://books.google.com/books?id=rUsUPZbFHK8C&pg=PA141. 
18. ^ Blanco, Humberto & Lal, Rattan (2010). "Wind erosion". Principles of Soil Conservation and Management. Springer. pp. 56–57. ISBN 978-90-481-8529-0. http://books.google.com/books?id=Wj3690PbDY0C&pg=PA57. 
19. ^ Balba, A. Monem (1995). "Desertification: Wind erosion". Management of Problem Soils in Arid Ecosystems. CRC Press. p. 214. ISBN 978-0-87371-811-0. http://books.google.com/books?id=uS62XNzDZDsC&pg=PA214. 
20. ^ Wiggs, Giles F.S. (2011). "Geomorphological hazards in drylands". In Thomas, David S.G.. Arid Zone Geomorphology: Process, Form and Change in Drylands. John Wiley & Sons. p. 588. ISBN 978-0-470-71076-0. http://books.google.com/books?id=swz4rh4KaLYC&pg=PA588. 
21. ^ Van Beek, Rens (2008). "Hillside processes: mass wasting, slope stability, and erosion". In Norris, Joanne E. et al. Slope Stability and Erosion Control: Ecotechnological Solutions. Springer. ISBN 978-1-4020-6675-7. http://books.google.com/books?id=YWPcffxM_A0C&pg=PA17. 
22. ^ Gray, Donald H. & Sotir, Robbin B. (1996). "Surficial erosion and mass movement". Biotechnical and Soil Bioengineering Slope Stabilization: A Practical Guide for Erosion Control. John Wiley & Sons. p. 20. ISBN 978-0-471-04978-4. http://books.google.com/books?id=kCbp6IvFHrAC&pg=20. 
23. ^ Nichols, Gary (2009). Sedimentology and Stratigraphy. John Wiley & Sons. p. 93. ISBN 978-1-4051-9379-5. http://books.google.com/books?id=zl4L7WqXvogC&pg=PA93. 
24. ^ See: Glennie, K.W. (1970). "Desert erosion and deflation". Desert Sedimentary Environments, Volume 14. Elsevier. ISBN 978-0-444-40850-1. http://books.google.com/books?id=VvAdX7hhfCYC&pg=PA15. 
25. ^ Blanco, Humberto & Lal, Rattan (2010). "Water erosion". Principles of Soil Conservation and Management. Springer. pp. 29–31. ISBN 978-90-481-8529-0. http://books.google.com/books?id=Wj3690PbDY0C&pg=PA29. 
26. ^ Mirsal, Ibrahim A. (2008). "Soil degradation". Soil Pollution: Origin, Monitoring & Remediation. Springer. p. 100. ISBN 978-3-540-70775-2. http://books.google.com/books?id=Lr5bMTxom0IC&pg=PA100. 
27. ^ ^{a b c} Blanco, Humberto & Lal, Rattan (2010). "Water erosion". Principles of Soil Conservation and Management. Springer. p. 29. ISBN 978-90-481-8529-0. http://books.google.com/books?id=Wj3690PbDY0C&pg=PA29. 
28. ^ Torri, D. (1996). "Slope, aspect and surface storage". In Agassi, Menachem. Soil Erosion, Conservation, and Rehabilitation. CRC Press. p. 95. ISBN 978-0-8247-8984-8. http://books.google.com/books?id=-AqdSMDSUIgC&pg=PA95. 
29. ^ Styczen, M.E. & Morgan, R.P.C. (1995). "Engineering properties of vegetation". In Morgan, R.P.C. & Rickson, R. Jane. Slope Stabilization and Erosion Control: A Bioengineering Approach. Taylor & Francis. ISBN 978-0-419-15630-7. http://books.google.com/books?id=3jXg9pyfikQC&pg=4. 
30. ^ Whisenant, Steve G. (2008). "Terrestrial systems". In Perrow Michael R. & Davy, Anthony J.. Handbook of Ecological Restoration: Principles of Restoration. Cambridge University Press. p. 89. ISBN 978-0-521-04983-2. http://books.google.com/books?id=moJHjZ9qW_8C&pg=PA89. 
31. ^ Blanco, Humberto & Lal, Rattan (2010). "Water erosion". Principles of Soil Conservation and Management. Springer. pp. 28–30. ISBN 978-90-481-8529-0. http://books.google.com/books?id=Wj3690PbDY0C&pg=PA28. 
32. ^ Wainwright, John & Brazier, Richard E. (2011). "Slope systems". In Thomas, David S.G.. Arid Zone Geomorphology: Process, Form and Change in Drylands. John Wiley & Sons. ISBN 978-0-470-71076-0. http://books.google.com/books?id=swz4rh4KaLYC&pg=PA209. 
33. ^ Committee on 21st Century Systems Agriculture (2010). Toward Sustainable Agricultural Systems in the 21st Century. National Academies Press. ISBN 978-0-309-14896-2. http://books.google.com/books?id=wdm4qMW1azgC&pg=PT88. 
34. ^ Blanco, Humberto & Lal, Rattan (2010). "Tillage erosion". Principles of Soil Conservation and Management. Springer. ISBN 978-90-481-8529-0. http://books.google.com/books?id=Wj3690PbDY0C&pg=PA109. 
35. ^ Lobb, D.A. (2009). "Soil movement by tillage and other agricultural activities". In Jorgenson, Sven E.. Applications in Ecological Engineering. Academic Press. ISBN 978-0-444-53448-4. http://books.google.com/books?id=aRKO6ZazC8UC&pg=PA247. 
36. ^ Whitford, Walter G. (2002). "Wind and water processes". Ecology of Desert Systems. Academic Press. p. 65. ISBN 978-0-12-747261-4. http://books.google.com/books?id=OZ4hZbXS8IcC&pg=PA65. 
37. ^ Imeson, Anton (2012). "Human impact on degradation processes". Desertification, Land Degradation and Sustainability. John Wiley & Sons. p. 165. ISBN 978-1-119-97776-6. http://books.google.com/books?id=BjJQY-i7kNsC&pg=PA165. 
38. ^ ^{a b} Sands, Roger (2005). "The environmental value of forests". Forestry in a Global Context. CABI. pp. 74–75. ISBN 978-0-85199-089-7. http://books.google.com/books?id=UO1DAI60IQEC&pg=PA74. 
39. ^ The mycelia of forest fungi also play a major role in binding soil particles together.
40. ^ ^{a b} Goudie, Andrew (2000). "The human impact on the soil". The Human Impact on the Natural Environment. MIT Press. pp. 188. ISBN 978-0-262-57138-8. http://books.google.com/books?id=r8l-DMj3XTgC&pg=PA188. 
41. ^ Stuart, Gordon W. & Edwards, Pamela J. (2006). "Concepts about forests and water". Northern Journal of Applied Forestry 23 (1). http://treesearch.fs.fed.us/pubs/14744. 
42. ^ Goudie, Andrew (2000). "The human impact on the soil". The Human Impact on the Natural Environment. MIT Press. pp. 196–197. ISBN 978-0-262-57138-8. http://books.google.com/books?id=r8l-DMj3XTgC&pg=PA196. 
43. ^ Nîr, Dov (1983). Man, a Geomorphological Agent: An Introduction to Anthropic Geomorphology. Springer. pp. 121–122. ISBN 978-90-277-1401-5. http://books.google.com/books?id=3HSCQvZ7U2kC&pg=PA121. 
44. ^ Randhir, Timothy O. (2007). Watershed Management: Issues and Approaches. IWA Publishing. p. 56. ISBN 978-1-84339-109-8. http://books.google.com/books?id=PNBlSPdu0JAC&pg=PA56. 
45. ^ James, William (1995). "Channel and habitat change downstream of urbanization". In Herricks, Edwin E. & Jenkins, Jackie R.. Stormwater Runoff and Receiving Systems: Impact, Monitoring, and Assessment. CRC Press. p. 105. ISBN 978-1-56670-159-4. http://books.google.com/books?id=X0xt9HZbyToC&pg=PA105. 
46. ^ Intergovernmental Panel on Climate Change (IPCC) (1995). "Second Assessment Synthesis of Scientific-Technical Information relevant to interpreting Article 2 of the UN Framework Convention on Climate Change". p. 5. http://www.ipcc.ch/pdf/climate-changes-1995/2nd-assessment-synthesis.pdf. 
47. ^ Bicknell, Jane et al, ed. (2009). Adapting Cities to Climate Change: Understanding and Addressing the Development Challenges. Earthscan. p. 114. ISBN 978-1-84407-745-8. http://books.google.com/books?id=77Kmhw6sVOMC&pg=PA114. 
48. ^ For an overview of other human activities that have increased coastal erosion rates, see: Goudie, Andrew (2000). "Accelerated coastal erosion". The Human Impact on the Natural Environment. MIT Press. p. 311. ISBN 978-0-262-57138-8. http://books.google.com/books?id=r8l-DMj3XTgC&pg=PA311. 
49. ^ Pruski, F. F.; Nearing, M. A. (2002). "Runoff and soil loss responses to changes in precipitation: a computer simulation study". Journal of Soil and Water Conservation 57 (1): 7–16. http://www.jswconline.org/content/57/1/7.abstract. 
50. ^ Dangerfield, Whitney (April 1, 2007). "The Mystery of Easter Island". Smithsonian Magazine. http://www.smithsonianmag.com/people-places/The_Mystery_of_Easter_Island.html. 
51. ^ Montgomery, David (October 2, 2008). "Islands in time". Dirt: The Erosion of Civilizations (1st ed.). University of California Press. ISBN 978-0-520-25806-8. http://books.google.com/books?id=HSu8r15-nnoC&pg=PA217. 
52. ^ Zuazo, Victor H.D. & Pleguezuelo, Carmen R.R. (2009). "Soil-erosion and runoff prevention by plant covers: a review". In Lichtfouse, Eric et al.. Sustainable agriculture. Springer. p. 785. ISBN 978-90-481-2665-1. http://books.google.com/books?id=7cP-2jIIO2wC&pg=PA785. 
53. ^ Sample, Ian (August 30, 2007). "Global food crisis looms as climate change and population growth strip fertile land". The Guardian. http://www.guardian.co.uk/environment/2007/aug/31/climatechange.food. 
54. ^ Smith, Kate & Edwards, Rob (March 8, 2008). "2008: The year of global food crisis". The Herald (Scotland). http://www.sundayherald.com/news/heraldnews/display.var.2104849.0.2008_the_year_of_global_food_crisis.php. 
55. ^ Africa may be able to feed only 25% of its population by 2025
56. ^ Potter, Kenneth W. et al (2004). "Impacts of agriculture on aquatic ecosystems in the humid United States". In DeFries, Ruth S. et al. Ecosystems And Land Use Change. American Geophysical Union. p. 34. ISBN 978-0-87590-418-4. http://books.google.com/books?id=CVN5X57pjnAC&pg=PA34. 
57. ^ Da Cunha, L.V. (1991). "Sustainable development of water resources". In Bau, João. Integrated Approaches to Water Pollution Problems: Proceedings of the International Symposium (SISIPPA) (Lisbon, Portugal 19–23 June 1989). Taylor & Francis. pp. 12–13. ISBN 978-1-85166-659-1. http://books.google.com/books?id=79ztZNhaNvMC&pg=PA12. 
58. ^ Merrington, Graham (2002). "Soil erosion". Agricultural Pollution: Environmental Problems and Practical Solutions. Taylor & Francis. pp. 77–78. ISBN 978-0-419-21390-1. http://books.google.com/books?id=ITFYBiQ_-VAC&pg=PA77. 
59. ^ Majewski, Michael S. & Capel, Paul D. (1996). Pesticides in the Atmosphere: Distribution, Trends, and Governing Factors. CRC Press. p. 121. ISBN 978-1-57504-004-2. http://books.google.com/books?id=T9pRfLqXajQC&pg=PA121. 
60. ^ Science Daily (1999-07-14). "African Dust Called A Major Factor Affecting Southeast U.S. Air Quality". http://www.sciencedaily.com/releases/1999/07/990714073433.htm. Retrieved 2007-06-10. 
61. ^ Nowell, Lisa H. et al (1999). Pesticides in Stream Sediment and Aquatic Biota: Distribution, Trends, and Governing Factors. CRC Press. p. 199. ISBN 978-1-56670-469-4. http://books.google.com/books?id=UzpDTEO3ZVcC&pg=PA199. 
62. ^ Shao, Yaping (2008). "Wind-erosion and wind-erosion research". Physics and Modelling of Wind Erosion. Springer. p. 3. ISBN 978-1-4020-8894-0. http://books.google.com/books?id=XSwwVeraxjcC&pg=PA3. 
63. ^ Science Daily (2001-06-15). "Microbes And The Dust They Ride In On Pose Potential Health Risks". http://www.sciencedaily.com/releases/2001/06/010615071508.htm. Retrieved 2007-06-10. 
64. ^ U. S. Geological Survey (2006). "Coral Mortality and African Dust". http://coastal.er.usgs.gov/african_dust/. Retrieved 2007-06-10. 
65. ^ James K. B. Bishop, Russ E. Davis, and Jeffrey T. Sherman (2002). "Robotic Observations of Dust Storm Enhancement of Carbon Biomass in the North Pacific". Science 298. pp. 817–821. Archived from the original on 2010-11-18. http://www-ocean.lbl.gov/people/bishop/bishoppubs/paparobots.html. Retrieved 2009-06-20. 
66. ^ Nichols, Gary (2009). Sedimentology and Stratigraphy (2nd ed.). John Wiley & Sons. p. 99. ISBN 978-1-4051-9379-5. http://books.google.com/books?id=zl4L7WqXvogC&pg=PA99. 
67. ^ Burbank, Douglas W. & Anderson, Robert S. (2011). "Tectonic and surface uplift rates". Tectonic Geomorphology. John Wiley & Sons. pp. 270–271. ISBN 978-1-4443-4504-9. http://books.google.com/books?id=83FuAvtSwE4C&pg=PT270. 
68. ^ Blanco, Humberto & Lal, Rattan (2010). "Modeling water and wind erosion". Principles of Soil Conservation and Management. Springer. ISBN 978-90-481-8529-0. http://books.google.com/books?id=Wj3690PbDY0C&pg=PA81. 
69. ^ See also: Shai, Yaping (2008). Physics and Modelling of Wind Erosion. Springer. ISBN 978-1-4020-8894-0. http://books.google.com/books?id=XSwwVeraxjcC.  and Harmon, Russell S. & Doe, William W. (2001), Landscape Erosion and Evolution Modeling, Springer, ISBN 978-0-306-46718-9, http://books.google.com/books?id=RltaPlIHlrAC 
70. ^ Brazier, R.E. et al (2011). "Scaling soil erosion models in space and time". In Morgan, Royston P.C. & Nearing, Mark. Handbook of Erosion Modelling. John Wiley & Sons. p. 100. ISBN 978-1-4051-9010-7. http://books.google.com/books?id=pSO4X3XbhJIC&pg=PA100. 
71. ^ Ward, Andrew D. & Trimble, Stanley W. (2004). "Soil conservation and sediment budgets". Environmental Hydrology. CRC Press. p. 259. ISBN 978-1-56670-616-2. http://books.google.com/books?id=yANwmTjf588C&pg=PA259. 
72. ^ Connor, David J. et al (2011). Crop Ecology: Productivity and Management in Agricultural Systems. Cambridge University Press. p. 351. ISBN 978-0-521-74403-4. http://books.google.com/books?id=O2eh7vyvuscC&pg=PA351. 
73. ^ For an interesting archaeological/historical survey of terracing systems, see Treacy, John M. & Denevan, William M. (1998). "The creation of cultivable land through terracing". In Miller, Naomi A.. The Archaeology of Garden and Field. University of Pennsylvania Press. ISBN 978-0-8122-1641-7. http://books.google.com/books?id=MARsWXbqFCsC&pg=PA91. 
74. ^ Forman, Richard T.T. (1995). "Windbreaks, hedgerows, and woodland corridors". Land Mosaics: The Ecology of Landscapes and Regions. Cambridge University Press. ISBN 978-0-521-47980-6. http://books.google.com/books?id=sSRNU_5P5nwC&pg=PA177. 
75. ^ Johnson, R.J. et al (2011). "Global perspectives on birds in agricultural landscapes". In Campbell, W. Bruce & Ortiz, Silvia Lopez. Integrating Agriculture, Conservation and Ecotourism: Examples from the Field. Springer. p. 76. ISBN 978-94-007-1308-6. http://books.google.com/books?id=85KuKnayVMEC&pg=PA76. 
76. ^ Udawatta, Ranjith P. & Shibu, Jose (2011). "Carbon sequestration potential of agroforestry practices in temperate North America". In Kumar, B. Mohan & Nair, P.K.R.. Carbon Sequestration Potential of Agroforestry Systems: Opportunities and Challenges. Springer. pp. 35–36. ISBN 978-94-007-1629-2. http://books.google.com/books?id=oDzvVdD_dHwC&pg=PA35. 
77. ^ Blanco, Humberto & Lal, Rattan (2010). "Wind erosion". Principles of Soil Conservation and Management. Springer. p. 69. ISBN 978-90-481-8529-0. http://books.google.com/books?id=Wj3690PbDY0C&pg=PA69. 
78. ^ Nair, P.K.R. (1993). An Introduction to Agroforestry. Springer. pp. 333–338. ISBN 978-0-7923-2135-4. http://books.google.com/books?id=CkVSeRpmIx8C&pg=PA333. 
79. ^ Lal, Rattan (1995). Tillage Systems in the Tropics: Management Options and Sustainability Implications, Issue 71. Food and Agriculture Organization of the United Nations. pp. 157–160. ISBN 978-92-5-103776-8. http://books.google.com/books?id=Cxxj2VczLG0C&pg=PA157. 
80. ^ See also: Gajri, P.R. et al (2002). Tillage for sustainable cropping. Psychology Press. ISBN 978-1-56022-903-2. http://books.google.com/books?id=i_acg2gACb8C.  and Uri, Noel D. (1999). Conservation Tillage in United States Agriculture. Psychology Press. ISBN 978-1-56022-884-4. http://books.google.com/books?id=2uPYFG3XLoEC. 

Further reading

• Boardman, John; Poesen, Jean (2006). Soil erosion in Europe. Wiley. ISBN 978-0-470-85910-0. 
• Montgomery, David R. (2007) Soil erosion and agricultural sustainability PNAS 104: 13268-13272.
• Brown, Jason; Drake, Simon (2009). Classic Erosion. Wiley. 
• Vanoni, Vito A., ed. "The nature of sedimentation problems". Sedimentation Engineering. ASCE Publications. ISBN 978-0-7844-0823-0. http://books.google.com/books?id=TxGTDYz_FnwC&pg=PA1. 

External links

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• The Soil Erosion Site
• International Erosion Control Association
• USDA National Soil Erosion Laboratory
• The Soil and Water Conservation Society
• International Soil Conservation Organization
• Bioerosion website at The College of Wooster
• Pulawy Erosion Research Center
• Southwest Watershed Research Center

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Dansk (Danish)
n. - erosion, nedbrydning, underminering

Nederlands (Dutch)
erosie, verwering

Français (French)
n. - érosion, corrosion, effritement

Deutsch (German)
n. - Erosion, Auswaschung

Ελληνική (Greek)
n. - διάβρωση, αποσάθρωση, φθορά

Italiano (Italian)
erosione

Português (Portuguese)
n. - erosão

Русский (Russian)
эрозия, разъедание

Español (Spanish)
n. - erosión, abrasión

Svenska (Swedish)
n. - erosion, erodering

中文（简体）(Chinese (Simplified))
腐蚀, 侵蚀, 冲蚀

中文（繁體）(Chinese (Traditional))
n. - 腐蝕, 侵蝕, 沖蝕

한국어 (Korean)
n. - 부식

日本語 (Japanese)
n. - 浸食, 腐食

العربيه (Arabic)
‏(الاسم) تعريه, تآكل‏

עברית (Hebrew)
n. - ‮סחף, שחיקה, עירצון, ארוזיה‬

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