Rainfall and surface runoff and water being
splashed by the impact of a single
raindrop If
the soil is saturated, or if the rainfall rate is
greater than the rate at which water can infiltrate into the soil,
surface runoff occurs. If the runoff has sufficient
flow energy, it will
transport loosened soil particles (
sediment) down the slope.
Rainfall, and the
surface runoff which may result from rainfall, produce four main types of soil erosion:
splash erosion,
sheet erosion,
rill erosion, and
gully erosion. Splash erosion is generally seen as the first and least severe stage in the soil erosion process, which is followed by
sheet erosion, then rill erosion and finally gully erosion (the most severe of the four). In
splash erosion, the
impact of a falling raindrop creates a small crater in the soil, ejecting soil particles. The distance the ejected soil particles travel can be as much as 0.6 m (two feet) vertically and 1.5 m (five feet) horizontally on level ground and in the absence of wind.
Sheet erosion is the transport of loosened soil particles by overland flow.
Gully erosion occurs when
runoff water accumulates and rapidly flows in narrow channels during or immediately after heavy rains or melting snow, removing soil to a considerable depth. Another cause of
gully erosion is
grazing, which often results in ground compaction. Because the soil is exposed, it loses the ability to absorb excess water, and erosion can develop in susceptible areas.
Rivers and streams 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, creating
head cuts and steep banks. 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, in a process known as
traction.
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.
Thermal erosion is the result of
melting and weakening
permafrost due to moving water. 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. 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 segment of the Beaufort Sea shoreline averaged per year from 1955 to 2002.
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.
Wind erosion , a rock formation in the
Altiplano,
Bolivia, sculpted by wind erosion 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. Wind erosion is of two primary varieties:
deflation, where the wind picks up and carries away loose 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 (e.g.
Saharan dust). Saltation is responsible for the majority (50–70%) of wind erosion, followed by suspension (30–40%), and then surface creep (5–25%).
Silty soils (e.g.
loess) tend to be the most affected by wind erosion; silt particles are relatively easily detached and carried away. Wind erosion is much more severe in arid areas and during times of drought. For example, in the
Great Plains, it is estimated that soil loss due to wind erosion can be as much as 6100 times greater in drought years than in wet years.
Mass movement Mass movement is the downward and outward movement of rock and sediments on a sloped surface, mainly due to the force of
gravity. 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. 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
downslope 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.
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.
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 in diameter by wind along the soil surface.
Tillage erosion Tillage erosion occurs in
cultivated fields due to the movement of soil by
tillage. There is growing evidence that tillage erosion is a major soil erosion process in agricultural land, surpassing water and wind erosion in many fields all around the world, especially on sloping and hilly lands. Tillage erosion results in
soil degradation, which can lead to significant reduction in
crop yield and, therefore, economic losses for the farm. == Factors affecting soil erosion ==