A gully is a

landform A landform is a natural or anthropogenic land feature on the solid surface of the Earth or other planetary body. Landforms together make up a given terrain, and their arrangement in the landscape is known as topography. Landforms include hills, ...

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, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...

or other relatively erodible material, typically on a hillside or in river floodplains or terraces. Gullies resemble large ditches 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' 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 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 gully may grow in length by means of headward (i.e. upstream) erosion at a knick point. This erosion can result from interflow and soil piping ( internal erosion) as well as

surface runoff Surface runoff (also known as overland flow) is the flow of water occurring on the ground surface when excess rainwater, stormwater, meltwater, or other sources, can no longer sufficiently rapidly infiltrate in the soil. This can occur when t ...

. 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, sand ...

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 Geomorphology (from Ancient Greek: , ', "earth"; , ', "form"; and , ', "study") is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or ...

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 ...

, 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 ...

sedimentation 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 variatio ...

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 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 ...

or tin ore. 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 badlands at Las Medulas for example, were created during the Roman period by hushing 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 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 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 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,

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-ei ...

. 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 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 Gui ...

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