In physical geography, erosion is the displacement of solids (earth, mud, rock, and so forth) by the agent of wind or water, or movement in response to gravity. Erosion can be an entirely natural process, but it is, in many places, 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.
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. (World Bank 2001: China: Air, Land, and Water).