A gully is a relatively deep, steep-sided channel carved into soil or soft sediment by concentrated water flow, distinguished from smaller erosion features by a retreating headwall (called a head scarp) and active sidewall collapse. The term gets used loosely in everyday language to mean any ditch or trench in the landscape, but in geomorphology it has a more specific meaning tied to how the channel forms and grows. That distinction matters because gullies behave differently from rills, streams, and ravines, and managing them requires understanding the particular erosion processes at work.
What Makes a Gully a Gully
The core of the scientific definition rests on three features working together: a head scarp that actively retreats upslope, internal erosion driven by mass movement and sidewall collapse, and the transport of loosened soil material out of the channel void. A recent classification study argued that this combination of traits is what separates a gully from other incised landforms, rather than any single measurement of width or depth.1Wiley Online Library. What type of gully is that? The need for a classification of gullies In other words, a gully is not just a big ditch. It is a landform that grows itself: water pours over the head scarp, undermining the soil beneath, while the steep walls slump inward and feed more material downstream. That self-reinforcing erosion loop is the defining behavior.
Older definitions sometimes relied on simple depth cutoffs, often pegging a gully as any erosion channel deeper than about 30 centimeters (roughly one foot). That threshold is still used in some agricultural soil-loss handbooks, but geomorphologists increasingly see it as insufficient on its own. A channel can be 30 centimeters deep without having a retreating head scarp, and a gully in soft volcanic ash can start out shallower than that before rapidly deepening. The process matters more than the measurement.
How Gullies Differ from Rills
Rills are the fine, finger-like channels that appear on bare soil after a heavy rain. They form from sheet flow concentrating into threads and are shallow enough that ordinary tillage erases them. Ephemeral gullies sit in a gray zone: they are larger than rills but can still be smoothed over by a plow, only to reform in the same low spot during the next storm. Research using rare-earth-element tracers to track sediment movement found that ephemeral gullies generally range from about 20 centimeters deep, occasionally exceeding 50 centimeters, and that they reform repeatedly along the same flow paths.2Elsevier / Journal of Hydrology. Identifying interrill, rill, and ephemeral gully erosion evolution by using rare earth elements as tracers A permanent gully, by contrast, is too deep and wide for normal farm equipment to cross or fill. Once a gully reaches that stage, it is a fixture of the landscape unless major earthmoving or engineering intervenes.
The practical distinction is important for farmers. Rills are a sign of surface erosion that better tillage practices or cover crops can address. An ephemeral gully forming in the same swale every spring is a warning that concentrated flow is exceeding what the soil can handle. Left unchecked, that ephemeral gully can deepen past the plow layer and become permanent, at which point restoration becomes far more expensive.
Continuous and Discontinuous Gullies
Not all permanent gullies look the same. Some run in a single connected channel from headwall to outlet, while others appear as a chain of disconnected segments separated by undisturbed ground. These two forms tend to develop under different conditions. A study comparing the two types in South Africa found that continuous gullies were more common on gentle footslopes in saturated zones with large upslope drainage areas and highly erodible duplex soils, whereas discontinuous gullies tended to form on rolling slopes with smaller contributing areas and shallower, more stable soils.3Land Degradation & Development. Factors controlling gully development: Comparing continuous and discontinuous gullies Soil properties turned out to be the factor that most clearly separated the two types. The takeaway is that the same landscape can host very different gully forms depending on local soil depth, drainage, and how saturated the ground gets.
How Gullies Start
Gully formation is not random. It follows a threshold: erosion begins where the force of concentrated runoff exceeds the soil’s resistance. That force depends on how steep the slope is and how much drainage area feeds water to a given point. Researchers have long expressed this as a relationship between local slope and upslope area, where gullies initiate above a critical combination of the two.4Earth-Science Reviews. Revisiting slope-area thresholds for gully initiation: a systematic review and meta-analysis Early fieldwork across multiple datasets found that the exponent governing this relationship was remarkably consistent at around −0.4, meaning that as drainage area increases, the critical slope needed to trigger a gully drops in a predictable way.5Geomorphology. Geomorphic threshold conditions for ephemeral gully incision In plain terms: a steep hillside with a tiny catchment might never gully, while a gentler slope collecting runoff from a large area above it can incise rapidly.
Once a head scarp forms, it advances upslope through a process called headcut retreat. Water flowing over the lip of the scarp plunges into the base, scooping out a pool that undermines the overhang until it collapses. A modeling study showed that the rate of this retreat depends on discharge, the height of the headcut, the upstream slope, and the erodibility of the soil.6Journal of Geophysical Research: Earth Surface. Development of gullies on the landscape: A model of headcut retreat resulting from plunge pool erosion Tall headcuts in soft soil can retreat meters per year; short headcuts in clay-rich ground may barely budge.
The Role of Subsurface Piping
Not all gullies start from the surface. Water seeping through soil can dissolve or dislodge particles along underground pathways, gradually hollowing out tunnels called pipes. When those tunnels grow large enough, the roof collapses and a gully appears seemingly overnight. A dendrogeomorphological study (using tree-ring evidence to date erosion events) confirmed that gullies are complex systems shaped by subsurface piping as well as surface runoff.7Catena. Impact of piping on gully development in mid-altitude mountains under a temperate climate: A dendrogeomorphological approach The pipes develop parallel to the slope and progress until a sudden collapse opens the surface. This mechanism is especially common in dispersive soils, where sodium-rich clays break apart easily when wet, and in layered soils where a permeable horizon sits above an impermeable one. If you have ever seen a sinkhole appear in a field seemingly without warning, piping may have been the hidden driver.
What Triggers Gully Erosion
Land use change is the single most cited human trigger. Overgrazing strips vegetation cover, exposing soil to concentrated runoff. Poor road drainage funnels water into unchanneled hillslopes. Cultivation on steep slopes without terracing removes the root networks that hold soil in place. A global review concluded that inappropriate cultivation and irrigation systems, overgrazing, log-haulage tracks, road building, and urbanization are the most common triggers, and that land-use change is expected to have a greater impact on gully erosion than climate change.8CATENA. Gully erosion: Impacts, factors and control
That said, rainfall intensity still matters enormously. A global analysis of gully headcut retreat rates found that the single strongest predictor was what the researchers called “rainy day normal,” essentially the average amount of rain that falls on each day it rains. This factor alone explained close to half the variation in retreat rates across monitored gullies worldwide. The same study projected that because climate change is expected to increase rainfall intensity in most regions, gully erosion will become more intense and widespread in coming decades.9Earth-Science Reviews. How fast do gully headcuts retreat? Year-to-year variability was also extreme: retreat rates measured over fewer than five years could be off by more than 100 percent from the long-term average, which is a caution against drawing conclusions from short monitoring windows.
How Much Sediment Gullies Produce
Gullies punch above their weight when it comes to sediment. They occupy a small fraction of any catchment’s area but can dominate its sediment budget. In a small catchment in southern China, gully erosion was estimated to account for over half of total sediment output during one monitoring period.10Geomorphology. Soil erosion changes during the last 30 years and contributions of gully erosion to sediment yield in a small catchment, southern China In a larger Tanzanian catchment, gully erosion contributed a more modest share, roughly 1.6 percent of the long-term total sediment yield, reflecting how the proportion varies with gully density, soil type, and the relative contribution of other erosion sources.11Physics and Chemistry of the Earth, Parts A/B/C. Estimating gully erosion contribution to large catchment sediment yield rate in Tanzania The wide range between these two figures underscores a recurring theme in gully science: local conditions, especially soil erodibility and land cover, drive outcomes far more than any universal rule.
Beyond sediment, gullies export soil organic carbon, nutrients, and sometimes pollutants into downstream waterways. A gully chewing through fertile topsoil strips carbon that took centuries to accumulate and delivers it to rivers and reservoirs, where it can degrade water quality and reduce storage capacity. For farmers, the loss is twofold: the land immediately around the gully becomes unusable, and the broader field loses its most productive soil layer through headcut migration and sidewall collapse.
Stopping and Repairing Gullies
The standard engineering response is to install check dams, small barriers spaced along the gully floor that slow water, trap sediment, and reduce the energy available for further erosion. In a semiarid catchment in southeastern Spain, check dams successfully encouraged sedimentation upstream of each structure, though researchers also noted that the dams caused erosion immediately downstream and that most structures filled completely after just a few torrential rainstorms.12CATENA. Effectiveness and geomorphological impacts of check dams for soil erosion control in a semiarid Mediterranean catchment: El Cárcavo (Murcia, Spain) That finding highlights a limitation: check dams buy time and redistribute sediment, but they do not address the underlying runoff problem.
Combining structural barriers with revegetation appears to work better. In active gully systems in the Andean mountains, a study found that flow barriers paired with afforestation trapped roughly 77 to 86 percent of eroded material before it could reach the river network.13CATENA. Check dams and afforestation reducing sediment mobilization in active gully systems in the Andean mountains Trees and shrubs stabilize the gully walls with their roots and reduce surface runoff through interception and transpiration, tackling both the symptom and one of the causes. On agricultural land, grassed waterways, diversion terraces, and controlled drainage can prevent ephemeral gullies from deepening into permanent ones, but these interventions work best when applied early. Once a gully has a well-developed head scarp several meters tall, bioengineering alone is rarely enough.
Socioeconomic Damage
Gully erosion is not just a soil-science problem. In southeastern Nigeria, active gullies have swallowed roads, destroyed buildings, and displaced communities. Research from Enugu State documented severe effects on land and property values, with gully-affected areas experiencing land degradation, infrastructure loss, and broad socioeconomic disruption.14Journal of Environmental Management and Safety. THE EFFECT OF GULLY EROSION ON LAND AND PROPERTY VALUES: EVIDENCE FROM ENUGU STATE, NIGERIA In parts of West Africa, gullies tens of meters deep cut through entire neighborhoods, and residents must choose between risky proximity and abandoning property they cannot sell. The intersection of erodible soils, intense seasonal rainfall, and rapid unplanned urbanization makes these regions especially vulnerable, but the pattern repeats in any setting where infrastructure expands into gully-prone terrain without adequate drainage design.
Regional Names and Cultural Contexts
Gully-like features go by many names depending on where you are. In Madagascar, large gullies carved into the laterite highlands are called “lavaka,” a Malagasy word meaning “hole.” These features can be enormous, sometimes hundreds of meters across, and have long been attributed to deforestation and farming. However, a study using sediment dating at Lac Alaotra found that many lavaka predate significant human presence, challenging the assumption that they are purely an artifact of land mismanagement.15PubMed Central. Gullying and landscape evolution: Lavaka in Lac Alaotra, Madagascar shed light on rates of change and non-anthropogenic controls Natural factors like weathering depth, groundwater seepage, and geological structure appear to play a larger role than previously assumed. The finding is a useful reminder that while human activity accelerates gully erosion almost everywhere it occurs, gullies are also a natural part of landscape evolution in many environments.
In Australia, the term “gully” carries a different everyday meaning entirely. Australians commonly use “gully” to describe any narrow, vegetated valley or drainage line in the bush, regardless of whether active erosion is present. That colloquial sense overlaps only loosely with the geomorphological definition and can cause confusion when scientists and land managers talk past each other.
Gullies as Habitat
For all the damage they cause, gullies and the drainage lines associated with them can serve as important ecological niches. In the dry box-ironbark forests of southern Australia, shallow gullies support moister, more sheltered microclimates than the surrounding ridges and slopes. A bird survey found that species richness was about a third higher in gullies than on adjacent ridges, and total bird densities were nearly twice as great. Some species occurred exclusively in one habitat or the other, even when gully and ridge sites were less than two kilometers apart.16Biological Conservation. The conservation value of mesic gullies in dry forest landscapes: avian assemblages in the box–ironbark ecosystem of southern Australia The implication for land managers is that stabilizing or filling gullies in dry landscapes needs to account for the habitat they provide, not just the erosion they represent.
Gullies on Mars
The word “gully” has traveled well beyond Earth. When high-resolution images from Mars orbiters revealed steep-sided channels on crater walls around the year 2000, scientists called them gullies and debated whether they were proof of recent liquid water on the Martian surface. A comprehensive review of Martian gully research noted that proposed formation mechanisms range from liquid-water seepage and snowmelt to dry granular flows lubricated by carbon dioxide, with comparison to terrestrial analogs favoring debris flows from surface melting as the closest match.17Geological Society, London, Special Publications. Martian gullies: a comprehensive review of observations, mechanisms and insights from Earth analogues
A separate, stranger class of Martian gully exists on sand dunes. These “linear dune gullies” consist of parallel, often sinuous channels with raised edges and pit-shaped endings. Laboratory experiments under Martian atmospheric pressure showed that blocks of frozen carbon dioxide sliding and burrowing downslope can reproduce these features: on steep slopes the ice blocks slide and cut shallow, straight channels, while on gentler slopes they burrow into the sand and migrate slowly by explosive gas-driven sediment transport.18Geophysical Research Letters. Sliding and Burrowing Blocks of CO2 Create Sinuous “Linear Dune Gullies” on Martian Dunes by Explosive Sublimation‐Induced Particle Transport These features share the visual vocabulary of terrestrial gullies, with channels, levees, and headward growth, but form through a mechanism that has no analog on Earth. The Martian example stretches the definition of “gully” about as far as it can go, illustrating how the term has become a broad morphological label in planetary science even as geomorphologists back home continue tightening its meaning for terrestrial landscapes.