Water flow Steady state

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Water flow Steady state

Graded river

Grade and Steady state. In time, irregularities such as rapids and waterfalls will be eroded away by the river. Observations show that irregularities on alluvial channels disappear very rapidly. Rivers tend to attain a condition of equilibrium, which is called grade. The term was first introduced by G.K. Gilbert. an American geologist, but W.M. Davis enlarged on the theme to fit in with his theory of cyclic development. Davis regarded grade as a condition of balance between erosion and deposition, brought about by the ability of the river to adjust its capacity to the amount of work being done. He envisaged that the adjustments made by the stream in reaching and maintaining grade were principally adjustments in channel gradient. He linked the concept of grade with the attainment of a smooth concave upward profile.

Although most streams tend towards this general outline. most geomorphologists find it difficult to recognize a graded stream as Davis defined it, because even streams having irregular profiles can be shown to have a balance between erosion and deposition. The best way to regard grade is not as a two-dimensional phenomenon, but as one in three dimensions; grade is now taken to be as Gilbert first defined it - namely a condition of balance where the slope, width, depth and other channel characteristics are adjusted to the prevailing volume of water and the load it is carrying. This cannot be a short-term equilibrium, because erosion and deposition are taking place almost continuously, but viewed over a long term, such changes tend to cancel out. Thus, for example, a gravel bar that is deposited in the channel at low water will be removed in flood. This kind of oscillating balance has been called dynamic equilibrium, but using the nomenclature of an open system, such a river is in a steady state.

This state is self-regulatory; the river reacts to any change of environmental factors by adjusting itself to absorb the change and re-establish a new steady state. For example, if there was an increase in the volume of water supplied to a river which was already in a steady state, then this would result in a change in channel characteristics These would adjust to carry the new volume: erosion might change the depth, width and channel roughness.

In summary, then, the term 'steady state' can be applied to rivers, which have reached a state of self-regulation and maintain stable channel characteristics. They do so by adjusting their long profile, cross-sectional shape and channel roughness.

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