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channelisation — English
Water that flows unchannelled over the slopes on the Earth’s surface and join at the lowest point to form a stream or river. Rain water that does not infiltrate the soil and substrate (underlying rocks) flows downslope over the surface as unchannelled surface flow, or sheet flow (or overlandflow). Surface flow carries particles of topsoil and weathering debris along. At the base of the slope all the sheetflow plus its load of soil and weathering debris meet and become a channelled stream loaded with all the material delivered by the sheetflow. The volume and momentum of the stream plus its contents act as a very powerful erosion agent (see “erosion”) and carve a channel in which it can flow with a minimum of resistance. Channel flow performs erosion by means of a variety of processes such as abrasion, corrosion, attrition and dissolving (see “weathering”) and over thousands or millions of years a proper river channel is created. Please note that channelised flow is a form of overlandflow or surface flow, because the water flows on the surface, albeit restricted to a specific channel. (See “drainage basin”.)
