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{{Short description|Landform created by running water and/or mass movement eroding sharply into soil}}
{{About|the landform|other uses}}
[[File:Gully in the Kharkov region.jpg|thumb|A gully in [[Kharkiv oblast]], [[Ukraine]].]]
[[File:SomaliaLuftaufnahme1993.jpg|thumb|Gullied landscape in [[Somalia]].]]
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A '''gully''' is a [[landform]] created by [[Runoff (hydrology)|running water]], [[mass movement (geology)|mass movement]], or both, which [[erosion|erodes]] [[soil]] to a sharp angle, typically on a hillside or in river [[floodplain]]s or [[Fluvial terrace|terraces]].<ref>{{Cite web |date=2023-10-21 |title=Definition of GULLY |url=https://www.merriam-webster.com/dictionary/gully |access-date=2023-10-22 |website=www.merriam-webster.com |language=en}}</ref> 

Gullies resemble large [[ditch]]es or small [[valley]]s, but are metres to tens of metres in depth and width, are characterized by a distinct 'headscarp' or '[[headwall]]' and progress by [[Headward erosion|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 gradients) in [[Farmland (farming)|farmland]], and they can develop rapidly in rangelands from existing natural erosion forms subject to vegetative cover removal and livestock activity.<ref>{{Cite journal |last=Breeze |first=Andrew |date=January 1999 |title=A Celtic Etymology for Scots Gully "Large Knife" |url=http://www.tandfonline.com/doi/abs/10.1080/08957699909598048 |journal=ANQ: A Quarterly Journal of Short Articles, Notes and Reviews |language=en |volume=12 |issue=2 |pages=5–6 |doi=10.1080/08957699909598048 |issn=0895-769X|url-access=subscription }}</ref>

==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''. The term may be connected to the name of a type of knife used at the time, a ''gully-knife.''{{citation needed|date=March 2015}}

Water erosion is more likely to occur on steep terrain because of erosive pressures, splashes, scour, and transport. Slope characteristics, such as slope length and amounts proportionate to slope length, affect soil erosion. Relief and soil erosion are positively correlated in southeast Nigeria.<ref>{{Cite web |title=gully {{!}} Etymology, origin and meaning of gully by etymonline |url=https://www.etymonline.com/word/gully |access-date=2023-10-22 |website=www.etymonline.com |language=en}}</ref> There are three types of topography: mountains, cuesta landscapes, and plains and lowlands. While highlands with stable lithology avoid gullying yet allow for vigorous runoff, uplands with friable sandstones are more prone to erosion.{{cn|date=October 2023}}

== 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]], [[over-grazing]], or other means. Gullies in rangelands can be initiated by concentrated water flow down tracks worn by livestock or vehicle tracks. The flowing water easily carries the eroded [[soil]] after being dislodged from the ground, typically when [[rainfall]] falls during short, intense storms such as [[thunderstorms]].

A gully may grow in length through [[Headward erosion|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]]. Gully erosion may also advance laterally through similar methods, including mass movement, acting on the gully walls (banks), and the development of 'branches' (a type of [[tributary]]).

Gullies reduce the productivity of farmlands where they incise into the land and produce [[sediment]] that may choke downstream [[waterbodies]] 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]], in the prevention of gully erosion, and [[soil science|the]]  in remediation and rehabilitation of gullied landscapes.  The total soil loss from gully formation and subsequent downstream [[river]] [[sedimentation]] can be substantial, especially from unstable soil materials prone to [[dispersion (soil)|dispersion]].

When water is directed over exposed ground, gully erosion removes soil near drainage lines. This may result in divided properties, loss of arable land, diminished amenities, and decreased property values. Additionally, it can lead to sedimentation, discoloration of the water supply, and creating a haven for rodents.<ref name=":0">{{Cite web |last=Department of Jobs |first=Precincts and Regions |date=2020-06-04 |title=Gully erosion - Agriculture |url=https://agriculture.vic.gov.au/farm-management/soil/erosion/gully-erosion |access-date=2023-10-22 |website=Agriculture Victoria |language=en-AU}}</ref>

Water rushing over exposed, naked soil creates gullies and ridges that erode rock and soil. When water rushes across exposed terrain, it erodes or pushes dirt away, creating rills. Gravity causes rift erosion on a downward slope, with steeper slopes generating greater water flow. Sandier terrains are more commonly affected by rills most prevalent during the rainier months. Gullies develop when a rill is neglected for an extended time, thickening and expanding as soil erosion persists.<ref>{{Cite journal |last1=Nosko |first1=Radovan |last2=Maliariková |first2=Marcela |last3=Brziak |first3=Adam |last4=Kubáň |first4=Martin |date=2019-09-01 |title=Formation of Gully Erosion in the Myjava Region |journal=Slovak Journal of Civil Engineering |language=en |volume=27 |issue=3 |pages=63–72 |doi=10.2478/sjce-2019-0023 |bibcode=2019SJCE...27c..63N |s2cid=203849080 |issn=1338-3973|doi-access=free }}</ref>

The factors influencing gully erosion were investigated in Zaria, Kaduna state, Nigeria, utilizing SRTM data, soil samples, rainfall data, and satellite imagery. The findings indicated that the factors that had the biggest effects on gully erosion were slope (56%) and rainfall (26%), land cover (12%), and soil (6%). The investigation concluded that each particular component significantly influenced soil loss.<ref>{{Cite web |title=Journal of Research in Forestry, Wildlife and Environment |url=https://www.ajol.info/index.php/jrfwe |access-date=2023-10-22 |website=www.ajol.info}}</ref>

=== Effects of gullies ===
The loss of fertile farmland due to gully erosion is a severe environmental problem that lowers crop quality and may cause famine and food shortages. It also causes the soil to lose organic content, which has an impact on plant viability. As items washed from fields end up in rivers, streams, or vacant land, erosion also contaminates the ecosystem. Because of increased population expansion and increasing land demand, erosion also threatens the natural ecosystem, encroaching on natural forests. Important assets including homes, power poles, and water pipelines may also be destroyed.<ref>{{Cite web |date=2020-04-29 |title=Gully Erosion, Definition, Causes, Effects And Prevention - Jotscroll |url=https://www.jotscroll.com/forums/3/posts/159/gully-erosion-definition-causes-effects-and-prevention.html |access-date=2023-10-23 |website=www.jotscroll.com |language=en-US}}</ref>

=== Prevention of gullies ===
Effective land management techniques can prevent gullies. These techniques include keeping vegetation along drainage lines, using more water, classifying drainage lines as distinct land classes, stabilizing erosion, preventing vermin, distributing runoff evenly, keeping soil organic matter levels high, and avoiding over-cultivation. These tactics guarantee uniform rates of penetration and robust plant coverage.<ref name=":0" />

One serious environmental problem endangering sustainable development is gully erosion. Gullying prevention and control methods are dispersed and lacking, and they have low success and efficacy rates.<ref>{{Cite journal |last1=Frankl |first1=Amaury |last2=Nyssen |first2=Jan |last3=Vanmaercke |first3=Matthias |last4=Poesen |first4=Jean |date=January 2021 |title=Gully prevention and control: Techniques, failures and effectiveness |url=https://onlinelibrary.wiley.com/doi/10.1002/esp.5033 |journal=Earth Surface Processes and Landforms |language=en |volume=46 |issue=1 |pages=220–238 |doi=10.1002/esp.5033 |bibcode=2021ESPL...46..220F |hdl=1854/LU-8684568 |s2cid=228807598 |issn=0197-9337|hdl-access=free }}</ref> This review attempts to make a valuable contribution to effective gully prevention and management techniques by combining information from previous research. It is possible to stop the creation of gullies by changing how land is used, conserving water and soil, or implementing specific actions in areas with concentrated flow.<ref>{{Cite web |date=2020-04-29 |title=Gully Erosion, Definition, Causes, Effects And Prevention - Jotscroll |url=https://www.jotscroll.com/forums/3/posts/159/gully-erosion-definition-causes-effects-and-prevention.html |access-date=2023-10-23 |website=www.jotscroll.com |language=en-US}}</ref> Plant leftovers and other vegetation barriers can prevent erosion, although their usefulness is limited. The biophysical environment, terrain, climate, and geomorphology are examples of external elements that affect gully prevention and control.<ref>{{Cite journal |last1=Frankl |first1=Amaury |last2=Nyssen |first2=Jan |last3=Vanmaercke |first3=Matthias |last4=Poesen |first4=Jean |date=January 2021 |title=Gully prevention and control: Techniques, failures and effectiveness |url=https://onlinelibrary.wiley.com/doi/10.1002/esp.5033 |journal=Earth Surface Processes and Landforms |language=en |volume=46 |issue=1 |pages=220–238 |doi=10.1002/esp.5033 |bibcode=2021ESPL...46..220F |hdl=1854/LU-8684568 |s2cid=228807598 |issn=0197-9337|hdl-access=free }}</ref>

==== Stabilising gullies ====

Stabilizing gullies entails altering water flow to lessen scouring, sediment buildup, and revegetation. Water can be securely moved from the natural level to the gully floor using a variety of structures, including drop structures, pipe structures, grass chutes, and rock chutes. Structural modifications can be required along steep gully floors. Vegetation can reestablish itself thanks to sediments deposited over flatter gradients. Until the restoration is finished, damaged areas should be walled off.<ref>{{Cite web |last=Department of Jobs |first=Precincts and Regions |date=2020-06-04 |title=Gully erosion - Agriculture |url=https://agriculture.vic.gov.au/farm-management/soil/erosion/gully-erosion |access-date=2023-10-23 |website=Agriculture Victoria |language=en-AU}}</ref>

==== Gully remediation in Eastern Nigeria ====
Eastern Nigeria's people and ecology are seriously threatened by gully erosion. A research project focused on 370 families and nine risk regions evaluated the region's gully erosion issues.<ref>{{Cite journal |last1=Shit |first1=Pravat Kumar |last2=Paira |first2=Rumpa |last3=Bhunia |first3=GouriSankar |last4=Maiti |first4=Ramkrishna |date=2015-05-13 |title=Modeling of potential gully erosion hazard using geo-spatial technology at Garbheta block, West Bengal in India |journal=Modeling Earth Systems and Environment |language=en |volume=1 |issue=1–2 |pages=2 |doi=10.1007/s40808-015-0001-x |bibcode=2015MESE....1....2S |s2cid=130635566 |issn=2363-6211|doi-access=free }}</ref> The greatest perceived problem, according to the results, was biodiversity loss. In contrast, damage to properties, roads, and walkways was ranked as the least important issue. This implies a notable variation in the average evaluations across impacted individuals, underscoring the necessity for long-term repair approaches. Reducing soil loss, raising public knowledge of environmental issues, passing environmental legislation, and giving residents funds to strengthen their coping mechanisms are all advised by the study.<ref>{{Cite journal |last1=Igwe |first1=P. U. |last2=Ajadike |first2=J. C. |last3=Ogbu |first3=S. O. |date=2023-07-05 |title=Assessment of Gully Erosion Problems for its Remediation in Eastern Nigeria |url=https://cepajournal.com/index.php/jems/article/view/117 |journal=Journal of Environmental Management and Safety |language=en |volume=14 |issue=2 |pages=17 |issn=2141-1824}}</ref>

In Agulu-Nanka, Southeast Nigeria, a study examined the geoenvironmental causes driving gully erosion. It focuses on catchment management for gully erosion and geotechnical analysis.<ref>{{Cite journal |last=Nwankwoala |first=Hycienth O |date=2019-05-28 |title=Gully Erosion and Landslides in Southeastern Nigeria: Causes, Consequences and Control Measures |url=https://irispublishers.com/gjes/fulltext/gully-erosion-and-landslides-in-southeastern-nigeria-causes-consequences-and-control-measures.ID.000541.php |journal=Global Journal of Engineering Sciences |volume=2 |issue=4 |doi=10.33552/GJES.2019.02.000541|s2cid=198419282 |doi-access=free }}</ref> Through fieldwork, data was gathered utilizing GIS and GPS methods. According to the study, gully erosion occurs throughout, with Nanka/Oko having the highest concentration. The gully characteristic map shows variations in length and depth, emphasizing the necessity of considering gully vulnerability and giving erosion hazards immediate attention.<ref>{{Cite web |url=https://bibliotekanauki.pl/articles/1031662 |access-date=2023-10-23 |website=bibliotekanauki.pl}}</ref>

== Artificial gullies ==
Gullies can be formed or enlarged by several human activities.

Artificial gullies are formed during [[hydraulic mining]] when jets or streams of water are projected onto soft [[alluvial deposits]] to extract [[gold]] or [[tin ore]]. The remains of such mining methods are very visible landform features in old [[gold mining|goldfield]]s such as in [[California]] and northern Spain. The [[badland]]s at [[Las Medulas]], for example, was created during the Roman period by [[hushing]] or [[hydraulic mining]] of the gold-rich alluvium with water supplied by numerous aqueducts tapping nearby rivers.<ref>{{Cite journal |last1=Razavi-Termeh |first1=Seyed Vahid |last2=Sadeghi-Niaraki |first2=Abolghasem |last3=Choi |first3=Soo-Mi |date=2020-01-01 |title=Gully erosion susceptibility mapping using artificial intelligence and statistical models |journal=Geomatics, Natural Hazards and Risk |language=en |volume=11 |issue=1 |pages=821–844 |doi=10.1080/19475705.2020.1753824 |bibcode=2020GNHR...11..821R |s2cid=219070139 |issn=1947-5705|doi-access=free }}</ref> Each [[Aqueduct (watercourse)|aqueduct]] produced large gullies below by erosion of the soft deposits. The effluvium was carefully washed with smaller streams of water to extract the [[Gold nugget|nuggets]] and gold dust.{{citation needed|date=October 2011}}

== Termination of gullies ==
Gully initiation results from localized erosion by surface runoff, often focusing on areas where forest cover has been removed for agricultural purposes, uneven compaction of surface soils by foot and wheeled traffic, and poorly designed road culverts and gutters.<ref>{{Cite journal |last1=Raji |first1=Saheed Adekunle |last2=Akintuyi |first2=Akinlabi O. |last3=Wunude |first3=Emmanuel O. |last4=Fashoto |first4=Busayo |date=2023-07-01 |title=A machine learning-based spatial statistical method for modelling different phases of gully development in South-Eastern Nigeria |url=https://www.sciencedirect.com/science/article/pii/S1574954123001309 |journal=Ecological Informatics |volume=75 |pages=102101 |doi=10.1016/j.ecoinf.2023.102101 |bibcode=2023EcInf..7502101R |s2cid=258089564 |issn=1574-9541|url-access=subscription }}</ref> Termination of gully processes requires water-resource management, soil conservation, and community migration. Gully erosion is localized in the Coastal Plain Sands, Nanka Sands, and Nsukka Sandstone of the Anambra-Imo basin region. The most affected deposits are unconsolidated or poorly consolidated and have short dispersion times. Public education is essential for a sustainable termination strategy, and collaboration between the government, donors, the private sector, and rural people is crucial.<ref>{{Cite web |title=Osondu Resources and Information|url=http://www.osondu.com/abec/erosionindex.htm |access-date=2023-10-22 |website=www.osondu.com|archive-url=https://web.archive.org/web/20150219023906/www.osondu.com/abec/erosionindex.htm |archive-date=19 February 2015 |url-status=usurped}}</ref>

==On Mars==
{{Main|Gullies on Mars}}

Gullies are widespread at mid-to-high latitudes on the surface of [[Mars]] 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 [[Water on Mars|liquid water on Mars]] in the recent geological past, probably resulting from the slight melting of snowpacks on the surface<ref>Christensen, PR (2003). "Formation of recent martian gullies through melting of extensive water-rich snow deposits.". Nature 422 (6927): 45–8. {{doi|10.1038/nature01436}}. {{PMID|12594459}}.</ref> or ice in the shallow subsurface<ref>Costard, F., Forget, F., Mangold, N. & Peulvast, J. P. Formation of recent martian debris flows by melting of near-surface ground ice at high obliquity. Science 295, 110–113</ref> 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.<ref>Malin M. C. and Edgett K. S. (2000) Science, 288, 2330–2335.</ref>

==Gallery==
<gallery mode=packed heights="200px">
File:A gully (Budanova Gora) 1.jpg|A gully in [[Saratov Oblast]], Russia
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]], Brazil
File:Montane Savannah Landscape in West Java.png|Gully formed from volcanic sedimentation in [[West Java]], Indonesia
File:A gully erosion site.jpg|A Gully formed by flood in Isuaniocha community of [[Anambra State]], Nigeria
</gallery>

==See also==
{{div col}}
* {{annotated link|Arroyo (watercourse)}}
* {{annotated link|Badlands}}
* {{annotated link|Coulee}}
* {{annotated link|Couloir}} – a narrow gully with a steep gradient in a mountainous terrain
* {{annotated link|Dale (landform)}}
* {{annotated link|Draw (terrain)}}
* {{annotated link|Gulch}}
* {{annotated link|Lavaka}}
* {{annotated link|Linn (geography)}} – gully in Scotland or Northern England in rock
* {{annotated link|Ravine}}
* {{annotated link|Rill}} – a shallow channel cut into soil by erosion from flowing water
* {{annotated link|Sinkhole}}
* [[Terrace Crossing]] - a geographical zone between the sedimentation (downstream) part and the erosion (upstream) part of a river
* {{annotated link|Wadi}}
{{div col end}}

==References==
* ''Oxford English Dictionary''{{Full citation needed|date=November 2023}}
{{Reflist}}

== External links ==
* {{Commons category inline|Erosion channels|Gully}}

{{Rivers, streams and springs}}
{{Interwikiconflict}}
{{Authority control}}

[[Category:Canyons and gorges]]
[[Category:Environmental soil science]]
[[Category:Erosion landforms]]
[[Category:Fluvial landforms]]
[[Category:Slope landforms]]
[[Category:Soil erosion]]
[[Category:Soil landforms]]
[[Category:Valleys]]