Erosion occurs when rocks and soil break down or wash away. Communities and companies worldwide battle erosion daily—from roads collapsing to soil disappearing from farm fields to mudslides endangering mountain villages.
To prevent erosion, we must first understand what it is and how it happens. In this article, we’ll explore the types of soil erosion and how they impact our communities and environment.
Soil erosion is the process by which earthen particles wear down, break apart, wash away, or blow away. Water, ice, wind, and human activity—among other factors—can all cause erosion. Erosion leaves behind everything from small rills in your backyard to the Grand Canyon. You might also hear people call erosion weathering, although the two aren’t identical.
Weathering is large-scale erosion in which big rocks break down into little rocks called aggregates. It can happen because of temperature changes and chemicals like acid rain. As the weathered rock fragments grow smaller and smaller, they eventually become tiny sediment particles that wash away in the rain or blow away in the breeze. This sedimentary erosion is what most people think of when they hear the phrase “soil erosion.”
Weathering and erosion can occur in two ways: physical and chemical.
Physical erosion is when the rock or soil particle’s shape changes due to water, wind, or grinding with other particles. For example, the wind wears away tiny particles of rock from a mountain’s summit, gradually shortening it.
Chemical erosion breaks rocks and soil down by changing their chemical composition. For example, rainwater chemically reacts with limestone, creating carbonic acid that breaks the limestone apart.
Multiple types of soil erosion occur. And once you know what type of erosion you’re dealing with, you’ll have a better idea of how to control it. The types of soil erosion are typically named for either their cause or their effect. Let’s walk through some of the most common types.
Water erosion is the most common type of erosion. Rain, snowmelt, and bodies of water such as rivers, lakes, streams, and oceans all cause water erosion. Of more than 17,700 instances of erosion in Minnesota, water was responsible for 82%.1
The amount of water and its intensity impact how much soil or rock it erodes. For example, a torrential downpour causes more water erosion than a light shower. Crashing ocean waves erode more shoreline than a gently lapping lake. And waterfalls cascading downhill at high velocity cause more water erosion than streams on level ground.
Next, let’s break down the four main types of water erosion.
Splash erosion refers to how raindrops splash when they hit soil or rocks. These splashes may be tiny, but so are soil particles. Silt and clay particles are both less than one millimeter wide. So, even small raindrops can detach some of these tiny particles.
Foliage and structures impact splash erosion. For example, raindrops may land on tree leaves that funnel them to drip in a certain area. That area will experience more erosion, while other areas experience less. Or, if you’ve ever seen a divot in the ground under a building’s eaves, you’ve seen splash erosion in action.
Also called inter-rill erosion, sheet erosion occurs when water flows in a solid sheet over the surface of a road or other area. It primarily affects the topmost soil layer, washing away particles that came loose due to splash erosion, vehicle traffic, or other activities.
When sheets of water start to consolidate into narrow, shallow streams, those streams—called rills—put greater stress on the soil or rock, eroding more than sheets of water do. This process leaves behind ditches and washed-out areas. An analysis of over 70 erosion control studies found that sheet and rill erosion were responsible for the most unpaved road failures.2
Gullies are giant rills that can be over 150 yards deep. Lots of water rushes through gullies at once, making gully erosion one of the most intense types of water erosion. It’s common after especially heavy rains. For instance, many canyons in the American Southwest result from years of gully erosion during flash floods.
Wind erosion most impacts areas where winds are powerful—such as when gale-force winds erode mountaintops—or areas where soil has become extremely dry and dusty. Whenever you see a breeze kick up dust on a dirt road or parking lot, that’s wind erosion.
Landslides and mudflows are examples of gravity erosion. When wind and water weaken soil or rocks, they eventually can’t support their own weight. Gravity takes over, and the area collapses. Gravity erosion happens most often on steep slopes. Sliding or gravity-related soil loss accounted for 17% of the eroded sites on Minnesota’s roads.3
Abrasion is a type of erosion in which loosened rock and soil particles scrape against particles still attached to the parent rock or soil. As this physical contact occurs, additional fragments are scraped off.
Attrition is when those loose pieces bang against each other while drifting on water or wind currents; as they do so, they break each other into ever smaller pieces.
Soil erosion causes a wide range of natural disasters, such as floods, landslides, and mudslides. It can also shift the foundations of homes, buildings, and bridges.
Erosion also creates a vicious cycle: water washes away soil, decreasing plant life and the remaining soil’s resistance to erosion. Then, the next time it rains, even more soil washes away. America’s Appalachian Mountains are struggling with this very thing due to uncontrolled erosion from surface mines.
Over time, uncontrolled erosion can lead to agricultural failure. One of the best-known examples is the Dust Bowl of the 1930s. Decades of poor agricultural practices dating back to World War I depleted natural prairie grasses in the midwestern U.S. Tilling removed topsoil, and droughts dried out the soil’s exposed underlayers. The cropland became infertile, and the wind kicked up dust storms that swallowed whole towns, killing animals and people from Nebraska to Texas.
Today, one of soil erosion’s most common—and dangerous—impacts is road failure.
On the last weekend of March 2024, heavy rains caused a slope beneath California’s Highway 1 to collapse into the Pacific Ocean. Because another section of road collapsed a few weeks before, around 1,600 people were stranded between the two eroded areas. Transportation officials slowly began opening one lane of the road—the only lane that was left intact—for just two hours a day.4 However, with more storms set to roll in within the week and the road still unstable, authorities issued an evacuation warning for residents.5
Roads becoming impassable due to erosion is not a new issue—nor is it isolated in California. In Tanzania, farmers have historically lost more than half their cotton, rice, and seeds in accidents due to poor road conditions, and in 2019, up to 40% of the nation’s agricultural harvest couldn’t move to market primarily because of poor roads.6
Unpaved roads are especially susceptible to erosion, which is a serious concern given that a 2019 study estimated that more than 70% of the world’s roads are dirt or gravel (up to 90% in developing countries).7 Even in the U.S., 30% of roads are unpaved.8
Repairing road erosion is a financial burden for townships, counties, and states. In some areas, it’s almost impossible for the budget to keep up with the amount of work. So, we’re taking a deeper look at the causes of road erosion and, most importantly, how to prevent it.
Areas surrounding roads also suffer from erosion. In the 1980s, Minnesota measured erosion along more than 115,000 miles of road in all 87 counties. The state estimated that each county had lost some 1.3 million square feet of soil to erosion along its roads since their construction.9
Sediment erodes from road surfaces and makes its way into bodies of water, altering their flow and pH balance. These changes can weaken or kill aquatic life. The U.S. National Marine Fisheries Service found that watersheds with more than 3 miles of road per square mile did “not properly function” as salmon habitats; “properly functioning” watersheds had two miles of road per square mile or less.10
With so much at stake, it’s imperative to prevent soil erosion—especially on and around our roadways. In our next article, we’ll examine more causes behind erosion and share seven tips to help you keep your roads safe and stable for years to come.
