Drainage

1. Erosion is the wearing away of surface material by moving forces such as water, wind, and ice.
Erosion carries rock fragments, soil, and dissolved materials from one place to another. This is
distinct from weathering which does not involve movement.
Water erosion is the most important force for erosion. Rainfall can cause erosion when it
splashes onto the ground, or when it collects and flows into tiny rivulets and streams. Rivers
erode the land they travel through, forming canyons and shaping coastlines over time.
Wind
Wind is a natural erosion agent and can displace sand, soil and other debris. It can also sculpt
rock formations, such as the ventifacts that form in deserts.
The erosion power of wind is greatest where surfaces are bare and dry, as in arid climates.
Wind erosion is often caused by agricultural practices that remove vegetation and expose the
soil surface, as well as land clearing for development and oil and gas drilling.
When winds reach a certain threshold velocity, they start to erode and transport the particles
they carry. The amount of material moved is related to the particle size and adhesion, as well as
the surface condition and the velocity of the wind.
Once the wind carries particles far enough, they drop back to the ground as sediment or re-
enter the air as dust. Some of this dust can be carried many kilometers, even across oceans.
Wind erosion is often the cause of dust storms, which reduce plant productivity and cost the
economy through high cleaning bills, absenteeism and transport delays, sand-blasted crops,
and lower retail sales. It can also reduce the water holding capacity of the soil, which can lead to
reduced crop yields and increased salinity in groundwater supplies. It also increases the risk of
invasive species and changes in the biodiversity of an ecosystem.
Water
Erosion is the action of water and wind that breaks down rocks, carries away rock particles, and
deposits them at a new location. It can happen in both the mountains and at sea level, but is
more common in dry regions, such as deserts, where vegetation is sparse. Erosion is the most
common cause of land deterioration. It can lead to soil deterioration and degradation of crops,
and can even destroy civilizations.
Water erosion starts with raindrops that erode the surface of the ground as they hit and splash.
Over time this can add up to significant damage, especially when rainfall is heavy or occurs over
long periods of time. Rainwater erodes the surface of the earth, carving out valleys and shaping
mountains. As it flows downstream in rivers and streams, it erodes the bedrock, carrying away
sand, silt, clay, and organic matter. This sediment is deposited elsewhere, usually in lakes and
watercourses.

2. The severity of erosion depends on a number of factors, including slope, soil type and condition,
the nature of the underlying rock, the amount of plant cover, the intensity and duration of rainfall,
and temperature. The most important off-site effect of water erosion is the transport of sediment
into lakes, streams, rivers, and oceans, where it causes sedimentation of the bottom of lakes,
disrupts the ecosystems of streams and rivers, and contaminates drinking water.
Soil
Erosion begins when dirt is exposed to abrasive forces. Wind can cause erosion by blowing dirt
from a place where it has settled (eroding the topsoil), or by carrying soil particles across long
distances (surface drift). Erosion destroys the most fertile section of the ground, which contains
nutrients and organic matter that plants need to grow.
Water can also cause erosion. Flowing water can dislodge soil particles from their resting
places. Flowing water can carry these particles down slopes and into streams and rivers. This
kind of erosion is most common in arid and semi-arid regions. Water erosion is a major problem
for farmers, as it can reduce crop yields and delay or inhibit the emergence of seeds. Erosion
can also deposit sediments in downstream riverbeds, which can choke waterways and destroy
ecosystems.
The most important factor in controlling erosion is restoring or maintaining ground-covering
vegetation. Plants and crop residues that completely cover the ground can help prevent erosion
by intercepting raindrops before they reach the soil surface. The best way to do this is to plant
trees and grasses that are resistant to harsh weather conditions.
The destruction of natural plant cover and the use of modern farming methods that leave soil
bare are among the most significant causes of erosion. These activities can make erosion
happen faster and damage the ability of soil to hold moisture and nutrients, which in turn affects
crops.
Vegetation
The breakdown and movement of rock, sediment, soil, nutrients or pollutants is called erosion. It
may be caused by wind, water, ice or gravity. Erosion often occurs after weathering, but it differs
from weathering in that material is moved and dropped somewhere else (gravitational erosion),
whereas weathering happens in situ.
Plants, especially roots, can help hold soil in place. When the vegetation is removed, as in
deforestation or plowing for agriculture, erosion increases because there is no longer anything
to keep the soil in place.

3. Erosion can be accelerated by intense rains, river flooding or waves breaking on the beach.
Erosion can also be caused by glacial movement or by the sudden uplift of land from shifting
tectonic plates (gravitational uplift).
Thermal erosion describes how warm temperatures can cause permafrost to break apart along
rivers and coastlines, eroding and transporting soil, mud and other materials down a slope. This
type of erosion is also known as mass wasting, and it can reshape mountains and valleys,
devastating communities in its path.
The sculpting of rocks by winds is another type of erosion. The sand dunes of the Badain Jaran
section of the Gobi Desert are an example, as are the sculpted rocks of Arches National Park in
Utah. Wind erosion is also responsible for the infamous dust storms of the 1930s in North
America, known as the Dust Bowl.