Erosion is a geological process where earthen materials are detached and subsequently moved from one location to another. This movement is accomplished by natural forces acting as agents of transport, such as water, wind, ice, or gravity. Erosion must be distinguished from weathering, which is the process of breaking down rock and soil material in place. Weathering prepares the material, while erosion is the act of carrying those fragments away. Water and wind are the two most pervasive and impactful natural agents responsible for shaping the planet.
Water Erosion: Mechanisms and Forms
Water is the most widespread agent of erosion, affecting landscapes through a sequential process that begins with the impact of a single drop of rain. The first stage, known as splash erosion, occurs when raindrops strike bare soil at high velocity. This impact dislodges soil particles and breaks down the soil structure, making the particles vulnerable to transport.
When rainfall intensity exceeds the soil’s infiltration capacity, water flows across the surface as a thin, uniform sheet, leading to sheet erosion. This form removes topsoil in a relatively even layer, but it is responsible for significant losses of fertile soil and nutrients. As the sheet flow continues down a slope, the water concentrates into small, temporary channels called rills.
Rill erosion creates shallow, finger-like grooves in the soil that are typically less than a foot deep and can be smoothed out by farming practices. If these channels are left unchecked, the concentrated flow gains momentum and cutting power, transforming into the most advanced stage: gully erosion. Gullies are deep, wide channels that cannot be easily repaired by tillage and represent a severe loss of usable land.
Wind Erosion: Processes of Land Degradation
Wind is a powerful erosive force, particularly in arid, semi-arid, or agricultural regions that lack sufficient vegetation cover to anchor the soil. The movement of sediment by wind is categorized into three distinct mechanical processes based on particle size. The finest particles, less than 0.1 millimeters in diameter, are lifted high into the atmosphere and carried over long distances in a process called suspension. These suspended particles are responsible for large, visible dust storms.
The dominant mode of sediment transport in most wind erosion events is saltation, accounting for approximately 75% of the total material moved. Saltation involves medium-sized particles, between 0.1 and 0.5 millimeters, that are too heavy for continuous suspension but light enough to be lifted briefly. These particles travel in a hopping motion, impacting the ground and dislodging other particles to continue the erosive chain reaction.
The largest soil grains, ranging from 0.5 to 2 millimeters, are too heavy to be lifted, so they move by surface creep. In this process, the larger particles roll or slide along the ground after being pushed by the impact of the saltating grains. This rolling action causes these heavier aggregates to slowly migrate across the surface.
Stabilizing Land Against Erosion
Managing erosion requires strategies that reduce the kinetic energy of water and wind while increasing the cohesion of the soil. Establishing vegetative cover is one of the most effective controls. Plant root systems firmly anchor the soil, while the foliage reduces the impact energy of raindrops and the velocity of wind near the surface. Planting cover crops and using hydroseeding are common techniques to establish vegetation on disturbed sites.
Structural practices manage water flow on sloped terrain. Terracing is a classic method that creates level steps to slow runoff and reduce its erosive power. To combat wind erosion, physical barriers such as windbreaks or shelterbelts (rows of trees or shrubs) are planted perpendicular to the prevailing wind direction. These barriers reduce wind speed, causing the sediment load to drop out of the air and protecting the land downwind.