Gullies are significant landforms resulting from the concentrated power of flowing water, creating profound incisions in the Earth’s surface. These channels demonstrate the destructive force of erosion, fundamentally altering the landscape and contributing substantially to soil loss. Understanding these features is important for land management, particularly in agricultural and semi-arid regions where soil stability is vulnerable. While their formation is a natural process, human activities often accelerate their development, making them a primary concern in the study of land degradation.
Defining Gully Structures
A gully is a distinct, carved channel resulting from the removal of soil along a drainage line by surface runoff. These erosional features are characterized by steep or vertical walls and a defined headscarp, which is the abrupt upper limit of the channel. Gullies are defined as being cut more than 30 centimeters (about one foot) deep into the ground, distinguishing them from smaller features. In agricultural contexts, a true gully cannot be removed or smoothed out by normal tillage or plowing equipment. The water flow within them is ephemeral or intermittent, meaning they only carry water during and immediately following intense rainfall events or snowmelt.
The Mechanism of Gully Formation
Gully development begins when concentrated surface runoff gains enough velocity and volume to detach and transport soil particles. This process often starts with sheet erosion, which then concentrates into small channels called rills, leading to a high-velocity flow. Once the channel reaches a threshold depth and width, the erosive power of the water intensifies, initiating the main processes of gully growth.
Headward Erosion
The most dramatic growth occurs through headward erosion, where the gully actively cuts backward (upslope) into the land at the headscarp. The force of the falling water at this sharp drop-off undermines the soil, causing blocks of earth to collapse into the channel and be carried away.
Piping and Lateral Erosion
A second mechanism is piping, which involves subsurface erosion where water flows through naturally occurring soil tunnels. When the roof of one of these tunnels collapses, a new section of the gully channel is instantly formed. Gullies also advance laterally through bank erosion and mass movement, as the steep sidewalls become saturated and collapse inward.
Influencing Factors
The rate of formation is heavily influenced by the soil type; cohesive soils tend to form deep, narrow, V-shaped gullies, while non-cohesive soils are more prone to forming wider, U-shaped channels. Slope steepness is another factor, as a greater gradient increases the velocity of the runoff, accelerating the detachment and transport of soil.
Differentiating Gullies from Rills and Ravines
Gullies exist within a hierarchy of erosional channels, distinguished primarily by their size and permanence. The smallest features are rills, which are shallow, finger-like channels that can be entirely erased by standard agricultural practices like plowing. If left unchecked, rills deepen and widen, eventually transitioning into a gully. A gully is a more permanent feature that requires major earthmoving or engineered structures to stabilize and repair. Ravines are the next step up in this scale, representing erosional features that are significantly larger, deeper, and wider than gullies.
Environmental Consequences of Gully Erosion
The formation and growth of gullies have wide-ranging negative impacts on the landscape and water resources. Gully erosion directly leads to the degradation and loss of arable land, as the incised channels dissect properties and remove productive topsoil. Extensively gullied land becomes difficult to manage, hindering access for machinery and reducing the total area available for cultivation. Gullies serve as efficient conduits that transfer large volumes of sediment and runoff from uplands to downstream waterways. This increased sediment load contributes to the silting of rivers, reservoirs, and irrigation systems, reducing water storage capacity and degrading water quality by increasing turbidity and carrying pollutants.