A gully is a landform created by running water, mass movement, or commonly a combination of both eroding sharply into

soil Soil, also commonly referred to as earth or dirt Dirt is an unclean matter, especially when in contact with a person's clothes, skin, or possessions. In such cases, they are said to become dirty. Common types of dirt include: * Debri ...

or other relatively erodible material, typically on a hillside or in river floodplains or terraces. Gullies resemble large

ditch A ditch is a small to moderate divot created to channel water. A ditch can be used for drainage, to drain water from low-lying areas, alongside roadways or fields, or to channel water from a more distant source for plant irrigation. Ditches ar ...

es or small

valley A valley is an elongated low area often running between hills or mountains, which will typically contain a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams ove ...

s, but are metres to tens of metres in depth and width and are characterised by a distinct 'headscarp' or '

headwall In physical geography and geology the headwall of a glacial cirque is its highest cliff. The term has been more broadly used to describe similar geomorphic features of non-glacial origin consisting of a concave depression with convergent slopes ty ...

' and progress by headward (i.e. upstream) erosion. Gullies are commonly related to intermittent or ephemeral water flow usually associated with localised intense or protracted rainfall events, or snowmelt. Gullies can be formed and accelerated by cultivation practices on hillslopes (often gentle gradient) in

farmland Agricultural land is typically land ''devoted to'' agriculture, the systematic and controlled use of other forms of lifeparticularly the rearing of livestock and production of cropsto produce food for humans. It is generally synonymous with bo ...

, and they can develop rapidly in rangelands from existing natural erosion forms subject to vegetative cover removal and livestock activity.

Etymology

The earliest known usage of the term is from 1657. It originates from the French word ''goulet'', a diminutive form of ''goule'' which means ''throat''. It is possible that the term is connected to the name of a type of knife used at the time, a ''gully-knife.''

Formation and consequences

Gully erosion can progress through a variety and combination of processes. The erosion processes include incision and bank erosion by water flow, mass movement of saturated or unsaturated bank or wall material, groundwater seepage - sapping the overlying material, collapse of soil pipes or tunnels in dispersive soils, or a combination of these to a greater or lesser degree. Hillsides are more prone to gully erosion when they are cleared of vegetation cover, through

deforestation Deforestation or forest clearance is the removal of a forest or stand of trees from land that is then converted to non-forest use. Deforestation can involve conversion of forest land to farms, ranches, or urban use. The most concentrated ...

over-grazing Overgrazing occurs when plants are exposed to intensive grazing for extended periods of time, or without sufficient recovery periods. It can be caused by either livestock in poorly managed agricultural applications, game reserves, or nature rese ...

or other means. Gullies in rangelands can be initiated by concentrated water flow down tracks worn by livestock or vehicle tracks. The eroded

is easily carried by the flowing water after being dislodged from the ground, normally when

rainfall Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...

falls during short, intense storms such as during

thunderstorms A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are somet ...

. A gully may grow in length by means of headward (i.e. upstream) erosion at a

knick point In geomorphology, a knickpoint or nickpoint is part of a river or channel where there is a sharp change in channel slope, such as a waterfall or lake. Knickpoints reflect different conditions and processes on the river, often caused by previous ...

. This erosion can result from

interflow In hydrology, interflow is the lateral movement of water in the unsaturated zone, or vadose zone, that first returns to the surface or enters a stream prior to becoming groundwater. Interflow is sometimes used interchangeably with throughflow; howe ...

and soil piping (

internal erosion Internal erosion is the formation of voids within a soil caused by the removal of material by seepage. It is the second most common cause of failure in levees and one of the leading causes of failures in earth dams, responsible for about half of ...

) as well as surface runoff. Gully erosion may also advance laterally by similar methods, including mass movement, acting on the gully walls (banks) and by developing 'branches' (a type of

tributary A tributary, or affluent, is a stream or river that flows into a larger stream or main stem (or parent) river or a lake. A tributary does not flow directly into a sea or ocean. Tributaries and the main stem river drain the surrounding drai ...

). Gullies reduce the productivity of farmlands where they incise into the land, and produce

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...

that may choke downstream

waterbodies A body of water or waterbody (often spelled water body) is any significant accumulation of water on the surface of Earth or another planet. The term most often refers to oceans, seas, and lakes, but it includes smaller pools of water such as ...

, and reduce water quality within the drainage system and lake or coastal system. Because of this, much effort is invested into the study of gullies within the scope of geomorphology and

soil science Soil science is the study of soil as a natural resource on the surface of the Earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils; and these properties in relation to th ...

, in the prevention of gully erosion, and in remediation and rehabilitation of gullied landscapes. The total soil loss from gully formation and subsequent downstream

river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of w ...

sedimentation Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the ...

can be substantial, especially from unstable soil materials prone to

dispersion Dispersion may refer to: Economics and finance * Dispersion (finance), a measure for the statistical distribution of portfolio returns * Price dispersion, a variation in prices across sellers of the same item *Wage dispersion, the amount of variat ...

Artificial gullies

Gullies can be formed or enlarged by a number of human activities. Artificial gullies are formed during hydraulic mining when jets or streams of water are projected onto soft

alluvial deposits Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluv ...

to extract

gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile me ...

tin ore Cassiterite is a tin oxide mineral, SnO2. It is generally opaque, but it is translucent in thin crystals. Its luster and multiple crystal faces produce a desirable gem. Cassiterite was the chief tin ore throughout ancient history and remains ...

. The remains of such mining methods are very visible landform features in old goldfields such as in

California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...

and northern Spain. The

badland Badlands are a type of dry terrain where softer sedimentary rocks and clay-rich soils have been extensively eroded."Badlands" in ''Chambers's Encyclopædia''. London: George Newnes, 1961, Vol. 2, p. 47. They are characterized by steep slopes, m ...

s at Las Medulas for example, were created during the Roman period by

hushing Hushing is an ancient and historic mining method using a flood or torrent of water to reveal mineral veins. The method was applied in several ways, both in prospecting for ores, and for their exploitation. Mineral veins are often hidden below ...

or hydraulic mining of the gold-rich alluvium with water supplied by numerous aqueducts tapping nearby rivers. Each aqueduct produced large gullies below by erosion of the soft deposits. The effluvium was carefully washed with smaller streams of water to extract the

nuggets Nuggets may refer to: Music * ''Nuggets'' (series), a series of compilation albums by Elektra Records, continued by Rhino * '' Nuggets: Original Artyfacts from the First Psychedelic Era, 1965–1968'' * '' Nuggets II: Original Artyfacts from ...

and gold dust.

On Mars

Gullies are widespread at mid- to high latitudes on the surface of

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...

, and are some of the youngest features observed on that planet, probably forming within the last few 100,000 years. There, they are one of the best lines of evidence for the presence of

liquid water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a s ...

on Mars in the recent geological past, probably resulting from the slight melting of snowpacks on the surface or ice in the shallow subsurface on the warmest days of the Martian year. Flow as springs from deeper seated liquid water

aquifers An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...

in the deeper subsurface is also a possible explanation for the formation of some Martian gullies.Malin M. C. and Edgett K. S. (2000) Science, 288, 2330–2335.

Gallery

File:A gully (Budanova Gora) 1.jpg, A gully in

Saratov Oblast Saratov Oblast (russian: Сара́товская о́бласть, ''Saratovskaya oblast'') is a federal subject of Russia (an oblast), located in the Volga Federal District. Its administrative center is the city of Saratov. As of the 2010 Cen ...

Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-eig ...

. File:A gully (Budanova Gora) 3.jpg, Inside the gully (to the left) in Saratov Oblast, Russia. File:Voçoroca (23 12 24S - 48 47 59W) - REFON 3.JPG, ''Voçoroca'' (Portuguese for gully) in Avaré,

Brasil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area an ...

File:Montane Savannah Landscape in West Java.png, Gully formed from volcanic sedimentation in

West Java West Java ( id, Jawa Barat, su, ᮏᮝ ᮊᮥᮜᮧᮔ᮪, romanized ''Jawa Kulon'') is a province of Indonesia on the western part of the island of Java, with its provincial capital in Bandung. West Java is bordered by the province of Banten ...

Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guine ...

References

* ''Oxford English Dictionary''

External links

{{Authority control Environmental soil science Slope landforms Fluvial landforms Erosion landforms Canyons and gorges Soil erosion Valleys Soil landforms