Headward erosion occurs when the source of a stream channel is lengthened by erosion of the rock and soil in its basin. This type of erosion occurs in the opposite direction of the flow of the stream.
Headward erosion causes the source of the stream to become larger, increasing the size of the drainage basin. When water is added to the source of a stream, its edges gradually erode, which widens and lengthens the basin in the opposite direction of the stream's flow.
The erosion becomes more extensive as the source grows. As the water causes the source to become larger, it may result in the joining of two different water sources and a steeper slope on which the streams flow downward from the source.