Base Level and Graded Stream

Longitudinal Profile:

Plot of the relative elevation of a stream bed from headwaters to mouth is called its “longitudinal profile”. The longitudinal profile of the stream is generally concave upward which is in accordance with the steady downstream decrease in slope.

Base Level:

The level which controls the depth of stream erosion is called a “base level”.
As the base level is the lower limit of the longitudinal profile, the stream can not cut below this level. There are two types of base-level: (i) Ultimate Base Level, and (ii) Local Base Level. The ultimate base level represents the lower level to which a stream can erode its valley. It is, therefore, the level at which the mouth of a stream enters a lake or the ocean. Resident rock beds, waterfalls, lakes, or artificial dam, which lied along a river course forsm the “local base level”. They act as limiting levels are temporary obstructions to the downstream encountered by a stream.

Any change in base level causes a stream to change its characteristics. Lowering of base level increases the stream’s gradient. As a result the velocities increase and down-cutting is accelerated. The erosion first starts from near the mouth and then works upstream until the stream profile is adjusted along its full length. Thus the bedrock channel is deepened and parts of the old valley floor are left as a terrace along the walls of the new valley. Such step-like features are called “river terraces.

A rise in base level reduces the stream’s sediment transporting capacity. As a result the stream deposits sediment thereby building up its channel Sometimes the capacity of a loaded stream is lowered to such an extent that heavy aggradations take place. Now the single river channel can no longer carry its load. In such circumstances the individual channel subdivides itself into a series of smaller channels. These channels are separated by many low islands which are the products of heavy deposition. Such a stream is called “braided stream”.

Profile of Equilibrium:

Streams have tendency to cut and remove the material from high gradient regions and deposit it in the low gradient areas. In due course of time when a uniform longitudinal slope is developed throughout the stream course, the process of erosion and deposition ceases. Such a slope is called the “profile of equilibrium” or “graded profile” of a stream. Thus a graded stream will not erode or deposit material but will simply transport it. As a matter of fact this stage is never reached due to many disturbances.