For a river to be able to flow into the sea there must be a slope towards the sea. The sea level is the ultimate limit to which rivers deepen their valleys. The base level of river erosion has been defined as an imaginary extension of sea level beneath the land surface.
The long profile of a river begins at sea-level and rises inland. Irregularity is the chief characteristic feature of the profile of a youthful river. This is according to the slopes and undulations of the initial surface.
In due course of time, provided there is no upliftment or subsidence of the land surface or change of climate, the long profile of a river valley would be gradually modified until it becomes nearly a smooth curve, gently concave to the sky, practically flat at the mouth, and steepening towards the source.
When a river develops such a profile, it is said to have graded. The term ‘graded’, even though hypothetical, implies that the river is everywhere provided with just the right velocities for the eventual transportation of stream load supplied to it.
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A graded stream represents such a condition when the load transported by the stream and the landscape through which it flows become mutually adjusted or balanced. Thus, a state of dynamic equilibrium among erosion, transported load, deposition and the stream’s capacity is reached. So “a graded stream is one in which, over a period of years slope is delicately adjusted to provide with available discharge and with channel characteristics, just the velocity required for transportation of the load supplied from the drainage basin”. 1
Local base level refers to the level of the main river at the particular point where a tributary enters into it. Remember that the local base level is applicable in case of a particular tributary. If there is no interruption of any sort, the graded tributaries join the main stream tangentially or nearly so.
If this is not so, then it is a clear indication that the cycle of erosion has been interrupted by any reason whatsoever. When a river drains into a lake, its level acts as a local base level for it. Later on, when the lake is partly drained and its area reduced, the lake is finally replaced by a broad lacustrine plain through which the river flows.
The point where the lower graded profile intersects the upper one is called a knick-point. Another expression for this term is rejuvenation head. It refers to the break of slope in the long profile of a stream resulting from the fall of base level. Knick-points migrate upstream as erosion progresses.
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In some streams the concave longitudinal profile is broken by steps that have a gentle upstream gradient and a steeper downstream gradient. Such breaks known as knick points are marked by rapids and waterfalls.