How Does Soil Erosion Happen?

Factors Affecting Erosion

• Surface cover, topography, climate, water infiltration rate and the characteristics of the soil itself determine soil's vulnerability to erosion. Vegetative surface cover absorbs the force of falling water, slows runoff and deflects wind. Topography affects erosion because as slope length and gradient (steepness) increases, so does volume and speed of runoff, increasing damage. Topography also may deflect or channel wind, reducing or increasing its erosive potential. Climate factors include high-intensity rainstorms and strong or sustained winds. Rapid temperature cycles can cause freeze erosion: water in cracks and underground melts and refreezes rapidly, expanding with enough force to break rock and move soil.

Wind Erosion

• Wind moves the heaviest and biggest soil particles by lifting them from the soil surface in a process called saltation; when they return, their impact dislodges other particles. Smaller and lighter particles are moved by rolling, called soil creep, and suspension carries the finest dislodged particles away in what appears to be a fine dust. The Dust Bowl of the 1930s illustrates how the wind erosion process called deflation (because it lowers the land surface) removes the soil's fine particles, leaving only coarser, heavier and less fertile elements behind. When extreme droughts, high temperatures and unsuitable farming practices left acres of soil dry, loosened and bare, high winds stripped millions of acres of topsoil from the Great Plains, carrying some of it as far as New York City. Wind also wears away surfaces through abrasion, a process similar to sandblasting.

Water Erosion

• When raindrops hit the Earth, the force of their impact detaches particles from the soil aggregate. This initial stage of water erosion is called splash erosion. Sheet erosion occurs next, when the splashed soil becomes suspended in a thin layer of runoff crossing bare land. As sheet volume increases, the runoff becomes concentrated by the terrain, scouring out shallow rills that carrying soil downhill. Rills too deep to cross with wheeled equipment are named gullies. In streams and rivers, flowing water pulls soil loose from streambeds and banks, carrying it downstream until the water moves too slowly to keep the sediment suspended.

Erosion Control

• Preventing fertile topsoil loss and water-quality degradation caused by moving soil and any toxic chemicals it contains requires a two-fold approach: protecting the soil from the erosive forces of wind and water, and capturing any eroded soil before it leaves its original site. Farmers fight erosion by using proper tilling techniques and crop-rotation schedules. They contour land to reduce runoff and leave crop remnants in fields as surface cover. Trees serve as windbreaks. Catchment structures reduce the amount of sediment, fertilizer and manure that enter a watershed. Construction schedules, techniques and barriers can limit bare soil exposure to erosion at building sites.