Cárcavas: what they are, formation and examples

Gulls are forms of soil erosion that occur when rainwater, instead of infiltrating into the ground or flowing gently over the surface, begins to concentrate in certain points and drain forcefully. This process drags soil particles with it, creating small grooves that, with time and the continuity of the water flow, become deep and well-defined channels. In other words, a gully is a type of eroded channel that is formed by the action of water on unprotected or degraded soils.

They usually appear on land with little vegetation, on slopes or hillsides, where the soil is directly exposed to precipitation. Vegetation typically acts as a natural barrier that slows down water and helps maintain soil structure. However, when the land has been deforested, cultivated without control or urbanized without planning, the loss of vegetation cover facilitates the beginning of the erosive process that leads to the formation of gullies.

Gullies not only represent a visual alteration of the landscape, but also imply important ecological and economic consequences. On an environmental level, they cause the loss of fertile soil, which affects agricultural productivity and the biodiversity of the place. In addition, they can alter the natural course of water currents, promote flooding and contribute to the sedimentation of rivers, reservoirs or irrigation canals. From a social and economic perspective, gullies can damage infrastructure, such as rural roads, homes or crops, and generate high costs for their rehabilitation or control.

An important characteristic of gullies is that once they begin to develop, they tend to advance and deepen if measures are not taken to stop the process. Water flowing through a gully can increase in volume and speed with each rain, further eroding the banks of the channel and causing it to extend in both depth and length. Therefore, in many rural areas, gully control is a key aspect of soil conservation plans.

The formation of gullies is a gradual but persistent process that develops in several stages driven mainly by the action of water and ground conditions that favor erosion. These are the stages of gully formation:

1. Start of erosion: It all starts with surface runoff. The water begins to form small grooves called microgrooves or microchannels, which are very narrow and shallow. Although they may seem harmless, these channels mark the beginning of the linear erosion process.
2. Furrow Development: With repeated rainfall and continuous runoff, these microfurrows enlarge and become more defined furrows. At this stage, the water gains speed and volume, which increases its capacity to drag soil particles. As erosion intensifies, the grooves become deeper and wider, marking the passage towards more severe erosion.
3. Formation of gullies themselves: When the grooves exceed a certain threshold in terms of depth and width, they become gullies. This point is critical, as gullies are not only more visible and destructive, but they are also more difficult to stop.
4. Retreat and expansion: A typical characteristic of gullies is their retreat towards the upper part of the slope. This means that, as the water continues to drain, the head of the gully (the highest part of the channel) moves uphill. This setback can lead to the gullies affecting more and more areas of land, damaging arable land, roads or nearby infrastructure.

The formation of gullies does not occur in isolation, but depends on a combination of natural and human factors:

• Slope of the land: Inclined areas favor the increase in the speed of the water, which increases its erosive capacity.
• Soil type: Sandy or highly erodible soils are more likely to be washed away by water. Soil compaction also influences, which reduces infiltration. Here you can learn about the Types of soils and their main characteristics.
• Vegetative cover: The absence of vegetation leaves the soil unprotected, facilitating the start of erosion.
• Land use: Activities such as intensive agriculture, overgrazing, deforestation or construction without conservation measures contribute to the formation of gullies.
• Rain intensity: Heavy, sudden rains generate greater runoff, especially in climates with intense seasonal rainfall.

We recommend you read these other Evidence Network articles on What is soil erosion: causes and consequences and Water erosion: definition, types, causes and consequences.

Gullies can be found in many parts of the world and under different geographical and climatic contexts. Although they usually appear in rural areas, they can also form in peri-urban or even urban areas if conditions allow it. Let’s see some examples:

• Gullies in agricultural areas: One of the most common examples occurs in poorly managed agricultural land. When a field is plowed on a slope without following contour lines, and trees or bushes that acted as natural barriers are also eliminated, rainwater flows strongly over the ground, initiating the formation of gullies.
• Gullies in Spain: In Spain, especially in the region of Aragon, Castilla-La Mancha and the southeast of the peninsula, there are gullies that have developed very visibly due to the combination of clay soils, steep slopes and scarce vegetation. These gullies have formed highly eroded landscapes known as “badlands”, a term that means “bad lands” due to their low productivity and difficult access.
• Urban or peri-urban gullies: Although less frequent, gullies can also form in urban environments, especially in neighborhoods built on slopes without adequate drainage planning.
• Gullies as a natural attraction or scientific study: In some places, gullies have become an object of study or even a tourist attraction, thanks to their geomorphological shapes. An interesting example is the Cárcavas Natural Park in Madrid, near the municipality of Patones.

If you want to read more articles similar to Cárcavas: what they are, training and examples, we recommend that you enter our Ecosystems category.