Erosion is the displacement of solids (soil, mud, rock, and so forth) by the agents of wind, water, ice, or movement in response to gravity. Although the processes may be simultaneous, erosion is to be distinguished from weathering, which is the decomposition of rock. Erosion is an important natural process, but in many places it is increased by human activities. Some of those activities include deforestation, overgrazing and road or trail building. Likewise, humans have sought to limit erosion by terrace-building and tree planting.
A certain amount of erosion is natural and in fact is fact healthy for the ecosystem. For example, gravels continually move downstream in watercourses. Too much erosion, however, can cause problems, clogging streams with gravel, filling reservoirs with sediment, reducing soil fertility and water quality.
What causes erosion to be severe in some areas and minor elsewhere? It is a combination of many factors, including the amount and intensity of precipitation, the texture of the soil, the steepness of the slope, and ground cover (from vegetation, rocks, etc.).
The first three factors do not change much. In general, given the same kind of vegetative cover, you expect areas with high-intensity precipitation, sandy or silty soils, and steep slopes to be the most erosive. Soils with a lot of clay that receive less intense precipitation and are on gentle slopes tend to erode less.
The factor that is most subject to change is the amount and type of ground cover. When fires burn an area or when vegetation is removed as part of timber operations, building a house or a road, the susceptibility of the soil to erosion is greatly increased.
Roads are especially likely to cause increased rates of erosion because, in addition to removing ground cover, they can significantly change drainage patterns. A road that has a lot of rock and one that is "hydrologically invisible" (that gets the water off the road as quickly as possible, mimicking natural drainage patterns) has the best chance of not causing increased erosion.
One of the most serious and long-running water erosion problems on the planet is in China, on the middle reaches of the Yellow River and the upper reaches of the Yangtze River. From the Yellow River, over 1.6 billion tons of sediment flow each year into the ocean. The sediment originates primarily from water erosion in the Loess Plateau region of northwest China.