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The study of water flow in a channel is known as the science of Hydraulics. When water flows in a stream it is subject to two basic forces, gravity and friction. Gravity exerts an impelling force, which puts pressure on the confining walls of the channel; a small part of the gravitational force is aimed downstream causing flow. Opposing the downstream flow is the force of frictional resistance between the water and the bed of the channel.

The water flow is not steady and uniform, but in all but the most sluggish streams, is affected by turbulence. This takes the form of a variety of chaotic movements and eddy systems. Turbulence in streams is extremely important because it creates upward motion in the flow, which lifts and supports fine particles of sediment. If laminar flow were to prevail in streams, whereby the water flowed in parallel horizontal layers; then any sediment would remain on the bed. But laminar flow is rare in natural streams, although it is common in groundwater movement. The effect of friction ensures that the velocity distribution of water flow in a river is not even; water closest to the banks moves only slowly, whereas that near the centre moves fastest. The highest velocity is usually located in midstream about one-third of the distance down from the surface to the bed.

Transport. A stream carries material in three ways: the dissolved load is composed of soluble materials transported invisibly in the form of chemical ions; the suspended load consists of clay and silt held in the water by the upward elements in the turbulent motion; larger materials move as bed load close to the channel floor by saltation, rolling, and sliding. The percentage contributions to the total load vary widely with the nature of the river and the season at which observations are taken.

The load carried by a stream increases with increased discharge and velocity. We refer to stream capacity to denote the largest amount of debris a stream can transport and to stream competence in relation to the diameter of the largest particle than can be moved. The longest velocity at which grains of a given size move is said to be the critical erosion velocity. High velocities are required to move gravel. A wide area, rather than a line, is used in the graph to define erosion velocity, because the value of erosion velocity varies with the depth and temperature of the water and with the density of the grain.

Cycle of River erosion. In the geographical cycle, the uplifted land is transformed by erosion and weathering. This transformation can be represented according to a geographical cycle whose various stages are known youth, mature and old age. The streams have an important hand in maintaining this transformation. Among all the sources, the work of river is the greatest and therefore its erosion is considered to be Normal. The erosional cycle, which sets in by a stream, is known as Normal Cycle of Erosion.

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