A stream can be defined as any flowing body of water in a clearly defined channel. Streams may increase the size of their valleys by the erosion of the soil and rock surrounding their channels, either by widening their valleys or by headward erosion. In the process of headward erosion, the stream valley at the uppermost part of the stream channel is worn away, and the stream channel is lengthened in the upstream direction. Because the sides of the uppermost part of the stream valley are often steeper than the sides of the valley further downstream, the lengthening of the stream channel usually proceeds faster than the process of valley widening. Rates of headward erosion and channel lengthening vary between different streams, because some streams will have steeper valleys, resulting in faster water flow rates and faster erosion. In some cases, this results in a phenomenon called stream piracy, in which part of the drainage of one stream is captured by another, faster-eroding stream.
A stream that has lost part of its drainage is termed beheaded. Stream piracy is also called stream capture or river capture.
See also Drainage basins and drainage patterns; Stream valleys, channels, and floodplains