water erosion

The breakdown of solid rock into smaller particles and its removal by water. As weathering, erosion is a natural geological process, but more rapid soil erosion results from poor land-use practices, leading to the loss of fertile topsoil and to the silting of dams, lakes, rivers and harbours. There are three classes of erosion by water: (a) splash erosion occurs when raindrops strike bare soil, causing it to splash, as mud, to flow into spaces in the soil and to turn the upper layer of soil into a structureless, compacted mass that dries with a hard, largely impermeable crust; (b) surface flow occurs when soil is removed with surface run-off during heavy rain; (c) channelised flow occurs when a flowing mixture of water and soil cuts a channel, which is then deepened by further scouring. A minor erosion channel is called a rill, a larger channel a gully.