<P> Human activities have increased by 10--40 times the rate at which erosion is occurring globally . Excessive (or accelerated) erosion causes both "on - site" and "off - site" problems . On - site impacts include decreases in agricultural productivity and (on natural landscapes) ecological collapse, both because of loss of the nutrient - rich upper soil layers . In some cases, the eventual end result is desertification . Off - site effects include sedimentation of waterways and eutrophication of water bodies, as well as sediment - related damage to roads and houses . Water and wind erosion are the two primary causes of land degradation; combined, they are responsible for about 84% of the global extent of degraded land, making excessive erosion one of the most significant environmental problems worldwide . </P> <P> Intensive agriculture, deforestation, roads, anthropogenic climate change and urban sprawl are amongst the most significant human activities in regard to their effect on stimulating erosion . However, there are many prevention and remediation practices that can curtail or limit erosion of vulnerable soils . </P> <P> Rainfall, and the surface runoff which may result from rainfall, produces four main types of soil erosion: splash erosion, sheet erosion, rill erosion, and gully erosion . Splash erosion is generally seen as the first and least severe stage in the soil erosion process, which is followed by sheet erosion, then rill erosion and finally gully erosion (the most severe of the four). </P> <P> In splash erosion, the impact of a falling raindrop creates a small crater in the soil, ejecting soil particles . The distance these soil particles travel can be as much as 0.6 m (two feet) vertically and 1.5 m (five feet) horizontally on level ground . </P>

What are the four types of soil erosion