In adjusting to change in discharge and erodibility of it’s bed, a stream modifies it’s channel so that irregularities are minimized and the least energy is expended in the movement in of water and sediments along its course. Frequently this involves enlargement of the channel cross section by erosion or reductions of it’s dimensions through deposition.
The overall tendency is toward a smooth long profile – “A Profile of Equilibrium” in which all factors are in a state of balance Geologists have long used the term Grade, referring to the balanced stream. A Graded Stream is one in which the slopes has become so adjusted, under conditions of available discharge and prevailing channel characteristics, that the stream is just able to transport the sediment load available to it.
- If any of the controlling factors change, then the stream will adjust to absorb the change and restore an equilibrium position.
In actually, it is unlikely that a condition of perfect equilibrium is ever achieved in natural stream systems. A heavy rain may suddenly increase the discharge in one tributary, collapse of a bank may locally introduce an excess of sediments to the stream, or stream may encounter a suddenly a less eroded rock along its course.
- The graded stream is a system in equilibrium, its diagnostic characteristic is that any change in any one of the controlling factors will cause a displacement of the equilibrium in a direction that will tend to absorb the effect of change.
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