Erosion

Erosion is the movement of soil, rock, or dissolved materials across the Earth's surface by forces like wind or flowing water, which then deposit these materials elsewhere. This sets it apart from weathering, where materials break down but stay put. Erosion can be physical, involving the removal of solid fragments, or chemical, where materials are dissolved away. The transported sediment or dissolved matter may travel only a few millimeters or thousands of kilometers.

The main forces driving erosion include rainfall, river currents, coastal waves, glacial scraping and plucking, floods, wind, groundwater, and mass movements like landslides. How fast erosion happens depends on the landscape, with steep slopes typically eroding fastest. Climate also plays a role, as factors like rainfall amount, storm frequency, wind speed, wave energy, and temperature (especially for ice-related processes) can speed up or slow down erosion. There are feedback loops too—for instance, the amount of sediment already carried by a river or glacier can affect how much more it erodes. After transport, the eroded material is deposited at a new location.

Although erosion occurs naturally, human actions have boosted global soil erosion rates by 10 to 40 times. In the Appalachian Mountains, intensive farming has caused erosion up to 100 times the natural rate. This excessive erosion creates both on-site and off-site problems. On-site, it strips away nutrient-rich topsoil, reducing farm productivity and potentially causing ecological collapse or desertification. Off-site, it leads to sediment clogging waterways, eutrophication of water bodies, and damage to roads and buildings. Water and wind erosion together account for about 84% of global land degradation, making unchecked erosion a major environmental issue.

Key human activities that worsen erosion include intensive agriculture, deforestation, road building, climate change, and urban sprawl. However, many prevention and remediation methods can limit erosion of vulnerable soils.

**Physical processes**

**Rainfall and surface runoff** Rainfall and the runoff it creates cause four main types of soil erosion: splash, sheet, rill, and gully erosion. Splash erosion is the mildest stage, where raindrop impacts throw soil particles up to 0.6 meters high and 1.5 meters sideways on flat ground. When the ground is saturated or rain falls faster than it can soak in, surface runoff occurs. If the runoff is strong enough, it carries loosened soil downhill—this is sheet erosion. Rill erosion forms small, temporary flow paths that both supply and transport sediment, especially on disturbed uplands where water erosion is greatest. Rill flow is only a few centimeters deep but can be quite steep, behaving differently from deeper river channels. Gully erosion happens when runoff gathers and cuts narrow channels during heavy rain or snowmelt, removing soil to significant depth. A gully is defined by a cross-section of at least one square foot, meaning it can’t be erased by normal tilling. Extreme gully erosion can create badlands, which form on easily eroded bedrock in erosion-friendly climates, especially where vegetation is limited.

**Rivers and streams** Stream erosion occurs as water flows along a linear path, cutting downward to deepen valleys and headward to extend them into hillsides, creating steep banks and headcuts. Early on, erosion is mostly vertical, producing V-shaped valleys with steep gradients. Once a base level is reached, erosion shifts to lateral widening, creating a flat valley floor and a narrow floodplain. The stream gradient becomes nearly flat, and sediment deposition becomes important as the river meanders. Floods cause the most erosion, as faster, larger water flows carry heavier sediment loads.