Alluvial fan

Alluvial Fan

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Alluvial fan

An alluvial fan is a fan

Fan (implement)

A hand-held fan is an implement used to induce an airflow for the purpose of cooling or refreshing oneself. Any broad, flat surface waved back-and-forth will create a small airflow and therefore can be considered a rudimentary fan....
-shaped deposit

Deposition (geology)

Deposition is the Geology process by which material is added to a landform or land mass. Fluids such as wind and water, as well as sediment gravity flows, Transportation previously Erosion sediment, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of sediment....
formed where a fast flowing stream

Stream

A stream is a body of water less than 60 feet wide with a current , confined within a stream bed and stream banks. Depending on its locale or certain characteristics, a stream may be referred to as brook, beck, Burn , creek, crick, kill, lick , rill, river syke, bayou, rivu...
flattens, slows, and spreads typically at the exit of a canyon

Canyon

A canyon, or gorge, is a deep valley between cliffs often carved from the landscape by a river. Most canyons were formed by a process of long-time erosion from a plateau level....
onto a flatter plain

Plain

In geography, a plain is an area of landscape with relatively high relief, as well as flat. Prairies and steppes are types of plains, and the archetype for a plain is often thought of as a grassland, but plains in their natural state may also be covered in shrublands, woodland and forest, or vegetation may be absent in the case of sandy or...
. A convergence of neighboring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial fan.

g to the flow

Flow

Flow may refer to:In science and technology:*Dataflow, computing term related to the flow of messages between software components*Environmental flow, the amount of water necessary in a watercourse to maintain a healthy ecosystem...
as stream gradient decreases, coarse-grained solid

Solid

A solid object is in the states of matter characterized by resistance to deformation and changes of volume. In other words, it has high values both of Young's modulus and of shear modulus; this contrasts e.g....
material carried by the water

Water

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An alluvial fan is a fan

Fan (implement)

Deposition (geology)

Stream

Canyon

Plain

Formation

Owing to the flow

Flow

Solid

Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
is dropped. As this reduces the capacity of the channel, the channel will change direction over time, gradually building up a slightly mounded or shallow conical fan shape. The deposits are usually poorly-sorted. This fan shape can also be explained with a thermodynamic justification: the system of sediment

Sediment

Sediment is any particulate matter that can be sediment transport by fluid dynamics, and which eventually is deposited.Sediments are most often transported by water transported by wind and glaciers....
introduced at the apex of the fan will tend to a state which minimizes the sum of the transport energy

Energy

In physics, energy is a scalar physical quantity that describes the amount of Work_ that can be performed by a force. Energy is an attribute of objects and systems that is subject to a conservation law....
involved in moving the sediment and the gravitational potential of material in the cone. There will be iso-transport energy lines forming concentric arc

Arc (geometry)

In geometry, an arc is a closed set segment of a differentiable curve in the two-dimensional manifold; for example, a circular arc is a segment of the circumference of a circle....
s about the discharge point at the apex of the fan. Thus the material will tend to be deposited equally about these lines, forming the characteristic cone shape.

In arid climates

Alluvial fans are often found in desert

Désert

?D?sert? is ?milie Simon's debut single, released in October 2002. The song was a huge success both critically and commercially in her homeland....
areas subject to periodic flash flood

Flash flood

A flash flood is a rapid flooding of geomorphic low-lying areas - washes, rivers and streams. It is caused by heavy rain associated with a thunderstorm, hurricane, or tropical storm....
s from nearby thunderstorms in local hill

Hill

A hill is a landform that extends above the surrounding terrain, in a limited area. Hills often have a distinct Summit , although in areas with Escarpment a hill may refer to a particular section of scarp slope without a well-defined summit ....
s. They are common around the margins of the sedimentary basin

Sedimentary basin

The term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification....
s of the Basin and Range

Basin and Range

The Basin and Range Province is a large geologic province which includes parts of the southwestern United States and northwestern Mexico, typified by basin and range topography....
province of southwestern North America

North America

North America is the northern continent of the Americas, situated in the Earth's northern hemisphere and almost totally in the western hemisphere....
. The typical watercourse

Watercourse

A watercourse is any flowing body of water. These include rivers, streams, brooks, anabranches et cetera....
in an arid

Arid

A region is said to be arid when it is characterized by a severe lack of available water, to the extent of hindering or even preventing the Individual growth and Morphogenesis of plant and animal life....
climate

Climate

Climate encompasses the temperatures, humidity, atmospheric pressure, winds, rainfall, atmospheric particle count and numerous other Meteorology elements in a given region over long periods of time, as opposed to the term weather, which refers to current activity of these same elements....
has a large, funnel-shaped basin at the top, leading to a narrow defile

Defile (geography)

Defile is a geographic term for a narrow pass or gorge between mountains or hills. It has its origins as a military description of a pass through which troops can march only in a narrow column or with a narrow front....
, which opens out into an alluvial fan at the bottom. Multiple braided streams are usually present and active during water flows.

Phreatophytes are plant

Plant

Plants are Life organisms belonging to the Kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae....
s that are often concentrated at the base of alluvial fans, which have long tap root

Root

In vascular plants, the root is the organ of a plant body that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial root or aerating ....
s (30–50 feet) to reach water. The water at this level is derived from water that has seeped through the fan and hit an impermeable layer that funneled the water to the base of the fan where it is concentrated and sometimes forms springs

Spring (hydrosphere)

A spring is a point where groundwater flows out from the ground, and is thus where the aquifer surface meets the ground surface.Dependent upon the constancy of the water source , a spring may be ephemeral or Perennial stream ....
and seeps

Seep (hydrology)

A seep is a wet place where a liquid, usually groundwater, has oozed from the ground to the surface. Seeps are usually not flowing, with the liquid sourced only from underground....
if the water is close enough to the surface. These stands of shrub

Shrub

A shrub or bush is a horticulture rather than strictly Botany category of woody plant, distinguished from a tree by its multiple stems and lower height, usually less than 5-6 m tall....
s cling onto the soil

Soil

Soil is the naturally occurring, unconsolidated or loose covering on the Earth's surface. Soil is composed of particles of broken rock that have been altered by chemical and environmental processes including weathering and erosion....
at their bases and over time

Time

Time is a component of the measurement used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects....
wind

WIND

The Global Geospace Science WIND satellite is a NASA science spacecraft launched at 04:31:00 EST on November 1, 1994 from launch pad 17B at Cape Canaveral Air Force Station in Merritt_Island%2C_Florida, Florida aboard a McDonnell Douglas Delta II 7925-10 rocket....
action often blows away sand

Sand

Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.As the term is used by geologists, sand particles range in diameter from 0.0625 to 2 millimeters....
around the bushes which form islands of habitat

Habitat (ecology)

A habitat is an ecological or Natural_environment area that is inhabited by a particular animal or plant species. It is the natural environment in which an organism lives, or the physical environment that surrounds a species population....
for many animal

Animal

Animals are a major group of multicellular, eukaryotic organisms of the Kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life....
s.

In humid climates

Alluvial fans also develop in wetter climates. In Nepal

Geography of Nepal

Sandwiched between two Asian giants--China and India--Nepal traditionally has been characterized as "a yam caught between two rocks." Noted for its majestic Himalayas, which in Sanskrit means "the abode of snow", Nepal is very mountainous and hilly....
the Koshi River

Koshi River

The Kosi River, called Koshi in Nepal , is a transboundary river between Nepal and India, and is one of the largest tributaries of the Ganges....
has built a megafan covering some 150,000 km2 below its exit from Himalayan foothills

Mahabharat Lekh

The Mahabharat Lekh or Range is a major east-west mountain range generally 1,500 to 3,000 meters high across Nepal. East of Nepal it extends through Sikkim and Geography of Bhutan but is difficult to differentiate from other Himalayan ranges in India's Arunachal Pradesh....
onto the nearly level plains the river traverses into India

Geography of India

The geography of India comprises most of the Indian subcontinent and is situated on the Indian Plate in the northerly portion of the Indo-Australian Plate....
before joining the Ganges. Along the upper Koshi tributaries, tectonic forces elevate the Himalayas

Himalayas

The Himalaya Range or Himalayas for short , meaning "abode of snow" ), is a mountain range in Asia, separating the Indian subcontinent from the Tibetan Plateau....
several millimeters annually. Uplift is approximately in equilibrium with erosion, so the river annually carries some 100 million cubic meters of sediment as it exits the mountains. Deposition of this magnitude over millions of years is more than sufficient to account for the megafan. .

In North America, streams flowing into California's Central Valley have deposited smaller but still extensive alluvial fans. That of the Kings River flowing out of the Sierra Nevada creates a low divide, turning the south end of the San Joaquin Valley

San Joaquin Valley

The San Joaquin Valley refers to the area of the California Central Valley of California that lies south of the Sacramento River Delta in Stockton, California....
into an Endorheic basin without a connection to the ocean.

Flood hazards

Alluvial fans are subject to flooding and can be even more dangerous than the upstream canyons that feed them. Their slightly convex perpendicular surfaces cause water to spread widely until there is no zone of refuge. If the gradient is steep, active transport of materials down the fan creates a moving substrate that is inhospitable to travel on foot or wheels. But as the gradient diminishes downslope, water comes down from above faster than it can flow away downstream, and may pond to hazardous depths.

In the case of the Koshi River, the huge sediment load and megafan's slightly convex transverse surface conspire against engineering efforts to contain peak flows inside manmade embankments. In August 2008 high monsoon flows breached the embankment, diverting most of the river into an unprotected ancient channel and across surrounding lands with high population density. Over a million people were rendered homeless and thousands of hectares of crops were destroyed. The Koshi is known as the Sorrow of Bihar for contributing disproportionately to India's death tolls in flooding that exceed all countries' except Bangladesh

Geography of Bangladesh

Bangladesh is a low-lying, riverine country located in South Asia with a largely marshy jungle coastline of 710 kilometers on the northern littoral of the Bay of Bengal....
.

Gallery

Image:Gigantic Alluvial Fan Being Uplift by New Fault.jpg|Fault scarp

Fault scarp

A fault scarp is the Topography expression of Fault attributed to the displacement of the land surface by movement along the fault. It can be caused by differential erosion along an old inactive geologic fault with hard & weak rock, or by a movement on an active fault....
cuts alluvial fan, Death Valley

Death Valley

Death Valley is a desert located in the southwestern United States. It is the lowest, driest, and hottest location in North America. Badwater, a depression located within Death Valley, is the specific location of the lowest elevation in North America at 85.5 meter below sea level....
Image:Mini Alluvial Fan Imprinted with Footprints.jpg|Mini alluvial fan, Death Valley

Death Valley

Death Valley is a desert located in the southwestern United States. It is the lowest, driest, and hottest location in North America. Badwater, a depression located within Death Valley, is the specific location of the lowest elevation in North America at 85.5 meter below sea level....
Image:Alluvial fn in Iran.jpg|Alluvial Fan in Southern Iran. Image from NASA's Terra satellite

Alluvial fan

Formation

In arid climates

In humid climates

Flood hazards

Gallery

See also

External links