Gully erosion is the removal of soil by concentrated water flow that carves deep, steep-sided channels into the landscape, typically too large to be smoothed over by normal tillage. Unlike the thin, shallow rills that form after a heavy rain and can be plowed away, gullies persist and grow, sometimes reaching several meters deep. They eat into farmland, fragment fields, choke waterways with sediment, and are far more expensive to fix than to prevent. Understanding how gullies start, what accelerates them, and what actually works to stop them matters for anyone managing land, from smallholder farmers to regional planners.
How a Gully Begins
A gully does not appear out of nowhere. It starts where flowing water concentrates enough force to break through the soil surface and begin cutting downward. This typically happens in low points on a slope, along plow furrows, at the edges of roads, or wherever runoff converges. The key factor is a tipping point: when the erosive power of flowing water exceeds the soil’s ability to resist it. Research has shown that where this threshold is crossed depends heavily on soil surface conditions, and the critical shear strength of saturated soil is a reliable predictor of where rills and gullies will start forming.1Earth Surface Processes and Landforms. Soil erosion resistance effects on rill and gully initiation points and dimensions
Topography plays a central role. Gullies tend to initiate where the combination of slope steepness and upslope drainage area is large enough to generate dangerous flow velocities. A systematic review across global datasets confirmed that this relationship holds worldwide, though the exact threshold shifts with local conditions like vegetation, soil type, and rainfall patterns.2Earth-Science Reviews. Revisiting slope-area thresholds for gully initiation: a systematic review and meta-analysis In Mediterranean croplands, for instance, the threshold for gully initiation fluctuates with seasonal rainfall and how much plant cover is present at any given time.3Land Degradation & Development. Rainfall and Vegetation Effects on Temporal Variation of Topographic Thresholds for Gully Initiation in Mediterranean Cropland and Olive Groves
Once It Starts, It Grows Fast
What makes gully erosion especially destructive is the way gullies expand. The most common growth mechanism is headcut retreat: water pours over the lip of the gully head, plunges to the bottom, and scours a plunge pool at the base. This undermines the headwall, which collapses, and the gully extends upslope. Modeling work on headcut retreat has shown that the rate of advance depends on the volume of water flowing in, the height of the headcut, the upstream slope, and the soil’s erodibility.4Journal of Geophysical Research: Earth Surface. Development of gullies on the landscape: A model of headcut retreat resulting from plunge pool erosion A tall headcut in loose soil with a big drainage area can retreat several meters in a single storm season.
Gullies also widen over time. Their sidewalls slump when saturated, and each new storm carries the collapsed material downstream, exposing fresh soil. This feedback loop means a small gully can become a sprawling ravine within a few decades if nothing is done. Some gullies stabilize naturally once they reach bedrock or a resistant layer, but in deep soils, there is often no natural brake.
Human Activities That Trigger Gully Erosion
Nature can create gullies on its own, but human land use dramatically accelerates the process. Three categories of disturbance stand out.
Deforestation is one of the strongest drivers. Clearing forest strips away the root networks and canopy cover that protect soil. A study in western Iran using radioactive tracers to measure erosion found that converting forested hillslopes to vineyards increased soil loss roughly fivefold, from about 5 to 6 tonnes per hectare per year under forest to about 26 to 33 tonnes per hectare per year under vines.5PubMed Central. Understanding deforestation impacts on soil erosion rates using (137)Cs, (239+240)Pu, and (210)Pb(ex) and soil physicochemical properties in western Iran The vineyard topsoil also lost roughly half its organic matter and percolation stability, making it even more vulnerable to future gullying.
Overgrazing and poor agricultural practices on sloping land are another common trigger. When livestock compact the soil surface and remove plant cover, infiltration drops and overland flow increases. Mountain pastures and forests subjected to heavy grazing develop gullies where the degraded surface can no longer absorb rainfall.6CATENA. Soil degradation and overland flow as causes of gully erosion on mountain pastures and in forests
Roads and other infrastructure concentrate runoff in ways the natural landscape was never designed to handle. Research in the Pacific Northwest found that drainage from ridgetop roads effectively extended the channel network, triggering both gullying and landslides downslope.7Water Resources Research. Road surface drainage, channel initiation, and slope instability Unpaved roads, construction sites, and poorly designed drainage ditches are among the most common artificial starting points for new gullies worldwide.
What Gullies Do to Farmland
The on-site damage from gully erosion goes well beyond the soil carried away. A gully physically removes cultivable land, but it also creates buffer zones on either side that farmers cannot safely plow or plant. And crops growing near an active gully tend to yield less because the soil is drier, thinner, and poorer in nutrients. A detailed field study in southern Ethiopia quantified all three effects together and found that gully-affected fields suffered an estimated total crop yield reduction of roughly 55 to 59 percent. About 40 percent of that loss came from land physically swallowed by the gully or rendered unusable, while the remaining 60 percent came from reduced productivity on the land that was still technically farmable.8International Soil and Water Conservation Research. Quantifying the impacts of gully erosion on farmers’ land productivity: The case of Shafe catchment, southern Ethiopia
These are not isolated cases. In arid and semi-arid regions of North Africa, satellite and machine-learning analysis showed that about 20 percent of cultivated land was lost between 2000 and 2020, with gully erosion as a major contributor, hitting southern areas hardest.9Natural Hazards. Enhancing gully erosion susceptibility prediction and agricultural land loss assessment through machine learning and remote sensing techniques In the Ethiopian highlands, a study tracking 22 gullies over three decades found they had collectively displaced roughly 340,000 tonnes of soil and converted about 10 hectares of agricultural land into unproductive wasteland, with an average erosion rate of around 62 tonnes per hectare per year.10N/A. Investigating long year gully erosion and its impacts on soil loss, land competition and crop yield reduction, north-west Ethoipia
For smallholder farming communities, this kind of land loss is devastating. A field that shrinks by half in productivity can push a household from subsistence into food insecurity within a few years, especially when there is no new land to move onto.
Damage Beyond the Field
Gully erosion does not just hurt the land it cuts through. The sediment it produces travels downstream, filling reservoirs, clogging irrigation channels, degrading water quality, and smothering aquatic habitats. In Mediterranean regions, where stored surface water is critical for agriculture, researchers have measured the contribution of gullies to reservoir siltation. In some small catchments, gully and channel erosion was the dominant source of sediment filling reservoirs, outpacing the contribution from sheet and rill erosion on hillslopes.11Land Degradation & Development. Relative Contribution of Rill/Interrill and Gully/Channel Erosion to Small Reservoir Siltation in Mediterranean Environments A reservoir that fills with sediment loses storage capacity, which means less water for irrigation and drinking, and expensive dredging or even dam replacement.
Gully sediment also carries nutrients, pesticides, and organic carbon into waterways. Research on steep loess slopes found that sediment is the primary vehicle for organic carbon loss during erosion events, with sediment-bound organic carbon accounting for the vast majority of total organic carbon mobilized by runoff.12Wiley Online Library. Inhibiting Effects of Vegetation Coverage on Runoff, Sediment, and Organic Carbon Loss on Steep Loess Gully‐Slopes This means gully erosion strips carbon from upland soils and deposits it in lowland waterways, contributing to both soil degradation on the hillslope and eutrophication or pollution downstream.
The Role of Extreme Rainfall and Climate Change
Gully erosion is closely tied to the intensity of rainfall events, not just total rainfall volume. A single extreme storm can do more gully damage than an entire average rainy season. Monitoring on China’s Loess Plateau, one of the most gully-eroded landscapes on Earth, found that extreme rainfall events of 150 millimeters or more accounted for roughly 70 to 75 percent of measured gully head erosion.13Land Degradation & Development. Active Gully Head Erosion Rates Characteristics on the Loess Plateau: InSAR‐Based Calculation and Response to Extreme Rainfall The rest of the rainfall in a given period contributed comparatively little to gully advance.
This finding has uncomfortable implications for climate change. Climate models broadly project increases in the frequency and intensity of extreme precipitation events across much of the world, even in regions where total annual rainfall may decrease. If gully erosion is disproportionately driven by the biggest storms, then a future with more extreme events means a future with faster gully growth, even without any change in land use. Regions already prone to gullying, like the Sahel, the Ethiopian highlands, parts of South America, and the Loess Plateau, face compounding risk as both land pressure and rainfall extremes increase together.
Grassed Waterways and Vegetative Approaches
One of the most effective and relatively low-cost strategies for preventing gully formation in agricultural landscapes is the grassed waterway. This is a broad, shallow, grass-lined channel installed along the natural drainage path of a field, designed to carry runoff without allowing it to scour a gully. The grass slows the water, reduces its erosive power, traps sediment, and reinforces the soil with roots. A paired-watershed study measured runoff reductions of 10 to 90 percent and sediment delivery reductions of 77 to 97 percent in fields with grassed waterways compared to fields without them.14PubMed. Effectiveness of grassed waterways in reducing runoff and sediment delivery from agricultural watersheds The grass sward itself was not damaged by the sediment deposited on it, suggesting the approach is self-sustaining over time.
The hydraulic effects of vegetation in these channels are well documented: plants increase roughness, slow flow velocity, and change sediment transport behavior, all of which reduce the likelihood of new gully initiation.15IOP Conference Series: Earth and Environmental Science. Study on Hydraulic Properties and Sediment Transport Law of Grassed Waterway The same principle applies on steep slopes, where increasing vegetation coverage can reduce organic carbon loss by two-thirds to nearly 100 percent, mainly by controlling the sediment that carries the carbon.12Wiley Online Library. Inhibiting Effects of Vegetation Coverage on Runoff, Sediment, and Organic Carbon Loss on Steep Loess Gully‐Slopes
Vegetative solutions work best as prevention. Once a gully is actively expanding, planting grass alone is rarely enough because the concentrated flow washes out seedlings before they can establish. In those cases, structural measures are usually needed first to stabilize the gully before vegetation can take hold.
Check Dams and Structural Controls
Check dams are small barriers built across a gully channel to slow water flow, trap sediment, and reduce the energy of runoff. They are among the oldest and most widely used structural interventions for gully stabilization. On China’s Loess Plateau, check dams have been deployed since the 1950s and have significantly altered local hydrology, raising groundwater tables and creating expanding sedimentary areas behind each dam that indirectly reduce further gully erosion.16Water. Physics-Based Simulation of Hydrologic Response and Sediment Transport in a Hilly-Gully Catchment with a Check Dam System on the Loess Plateau, China
Where you place a check dam matters. Experimental work comparing check dam locations within a stream system found that dams placed further downstream were more effective at trapping fine sediment than those placed upstream. The proportion of clay and silt captured decreased moving from downstream to upstream positions. The physical construction also affects performance: dams built with angular, broken rock trapped more fine sediment than those built with rounded stones.17PubMed. Evaluation of the effect of porous check dam location on fine sediment retention (a case study) For land managers designing a series of check dams, this suggests that if the primary goal is to keep fine sediment out of downstream water bodies, prioritizing the lower reaches of the gully system yields the best results.
Check dams come in many forms: gabion baskets filled with rock, loose stone barriers, log structures, and even sandbag walls. The best material depends on what is available locally and how permanent the installation needs to be. In many developing regions, check dams built from locally gathered stone are the most practical option. They do require maintenance, however. A check dam that fills with sediment and is not managed can fail catastrophically during a large storm, releasing a surge of stored sediment and water that may cause more damage than the original gully.
Prevention Versus Rehabilitation
One of the clearest findings in the gully erosion literature is that prevention is far cheaper than cure. An economic analysis of gully management in the Kilimanjaro region of Tanzania compared the cost-effectiveness of installing soil conservation measures before gullies form versus rehabilitating existing gullies. The benefit-cost ratios told a stark story: for maize fields, installing preventive conservation measures yielded a ratio of about 6 to 1, meaning every dollar invested returned roughly six dollars in avoided losses. Rehabilitating an existing gully returned only about 2 to 1. For sunflower fields the contrast was even sharper, with prevention returning about 1.76 to 1 while rehabilitation dropped to just 0.72 to 1, meaning the costs of fixing the gully actually exceeded the benefits. Three of the individual gullies studied could not be rehabilitated at a net economic gain at all.18Remote Sensing of Environment. On-Site Cost of Gully Erosion and Benefit-Cost of Rehabilitation vs. Establishment of Conservation Measures in the Kilimanjaro Region, Tanzania
The practical takeaway is direct. If you can see the early signs of concentrated flow on a slope, such as small rills appearing after storms, bare soil in drainage paths, or minor headcuts forming at field edges, intervening at that stage with grassed waterways, contour plowing, or small rock barriers is far more effective and affordable than waiting until a mature gully has developed. Once a gully is deep and actively expanding, the cost of stabilization climbs sharply and success rates drop.
Monitoring Gullies With Modern Tools
Tracking gully erosion used to mean driving stakes into the ground and measuring how much they were exposed after each storm. Today, drones equipped with lidar and photogrammetry sensors offer a much faster and more detailed picture. Repeated drone flights over the same area can produce elevation models that show exactly where soil has been lost and where it has been deposited, down to centimeter-scale resolution.19Remote Sensing. Estimating Gully Erosion Induced by Heavy Rainfall Events Using Stereoscopic Imagery and UAV LiDAR
The challenge is uncertainty. Small changes in elevation between two drone surveys can easily be swallowed by measurement noise, making it hard to distinguish real erosion from instrument error. Researchers have developed methods using fuzzy logic to estimate the uncertainty in these elevation-difference maps based on factors like how dense the lidar point cloud is and how rough the terrain surface is. One study found that such methods could account for over 96 percent of the raw uncertainty in the data, dramatically improving the reliability of erosion estimates from drone surveys.20CATENA. Improving the application of UAV-LiDAR for erosion monitoring through accounting for uncertainty in DEM of difference For land managers, this means that routine drone monitoring is becoming a practical tool for catching new gully heads early, before they grow into expensive problems.
Satellite-based radar monitoring is also being used at larger scales. On the Loess Plateau, interferometric synthetic aperture radar has been applied to measure gully headcut retreat rates across broad areas without anyone setting foot on the ground.13Land Degradation & Development. Active Gully Head Erosion Rates Characteristics on the Loess Plateau: InSAR‐Based Calculation and Response to Extreme Rainfall These techniques work best for monitoring known gully systems over time, and they complement rather than replace on-the-ground assessments for new or rapidly changing sites.
Where Gully Erosion Is Worst Around the World
Gully erosion is a global problem, but some regions are especially hard hit. The Ethiopian highlands are one of the most studied hotspots, where steep terrain, intense seasonal rains, population pressure on agricultural land, and thin volcanic soils create ideal conditions for gully formation. Parts of sub-Saharan Africa, particularly the Sahel and East Africa, face similar combinations of vulnerability. China’s Loess Plateau is another iconic gully landscape, where wind-deposited silt soils tens of meters thick are extremely erodible once vegetation is disturbed. Mediterranean regions around North Africa and southern Europe deal with gullying in both croplands and rangelands, amplified by seasonal drought-flood cycles. And across South America, rapid deforestation in areas like the Brazilian cerrado and the Argentine Pampas has triggered widespread gully development in recent decades.
What these regions share is a combination of erodible soils, intense or concentrated rainfall, steep or undulating terrain, and human land use that removes the protective vegetation cover. Any one of those factors alone can usually be managed, but when they stack up, gullies become almost inevitable without deliberate intervention. The encouraging side of this pattern is that addressing even one factor, especially restoring vegetation cover or managing where runoff concentrates, can push a landscape back below the threshold where gullies initiate.