Stream Gradient

the difference in water-surface elevations measured at the source and at the mouth of a stream or at the ends of a section of a stream. The stream gradient of a section divided by the length of that section is called the slope. The gradient of a section of a stream is usually calculated for 1 km of the channel. This gradient amounts to several cm per km for typical streams on level ground and to several m per km for mountain streams.

(or stream slope). For a given stretch of a stream, the gradient is the ratio of the drop the stream undergoes and the length of the stretch. A stream gradient may be expressed in terms of the number of units of vertical distance per hundred units of horizontal distance or per thousand units of horizontal distance. In the former case the term “percent grade” is sometimes used.

The gradient of a stream generally decreases from the source to the mouth. In some cases, however, the gradient may vary in a different way as a result of the special nature of the topography or the rocks and soils through which the channel runs. Streams in mountain regions may undergo considerable drops over short distances; rapids and races may be observed in such cases. The gradients of individual stretches of a stream are determined from the water level during a period of a low, steady flow rate. The overall gradient of a stream is found by averaging the gradients of the individual stretches.

Occasionally the water surface of a stream may exhibit what may be called a transverse gradient, or transverse slope—that is, the surface from one side to the other may not be level. Such a gradient may be produced by various factors, including wind and the shape of the channel. At a bend, for example, the water surface may be higher at the convex bank.