A key function that streams perform is the movement of sediment from high in the watershed downslope to the depositional zone of the watershed. Streams naturally work to achieve a balance between the sediment entering a reach of stream and the sediment exiting the reach. Some erosion and deposition occur within the reach, as in a meandering stream, where erosion occurs on the outside of bends and deposition occurs on the inside of the bends. Yet, overall, the amount of sediment material moving through a reach in a natural, undisturbed stream is steady.
The amount of sediment and the size of the sediment particles that can be transported in a stream are directly related to the gradient of the stream channel and to the amount of water flowing in the stream channel at a particular time. As the flow of a stream is constantly changing, the amount and size of sediment particles that can be moved by the stream also changes. Streams work to maintain a balance, or equilibrium, between the stream flow and the transported sediment. Because this balance is frequently being reestablished, streams and the sediment they transport work to maintain dynamic equilibrium.
The relationship between stream flow velocity and the size of sediment particles which can be moved by the stream is shown in this graphic. Near the headwaters of the stream, where the slope of the stream is steep and the flow is rapid, relatively large particles can be moved by the stream. As the slope of the stream becomes more shallow, the velocity of the stream lessens and particles which can be moved progressively become smaller
Left: Adapted from Foreman, Richard T. T., 1995. Land Mosaics. Cambridge University Press.