As a river system approaches maturity, and as the drainage of the area becomes more complete, it will increase the number of its branches. Those branches which were not at all represented in the youthful stages of the system, and are opened out along lines of yielding rocks, are called subsequent, and all streams will develop more or fewer of such branches as they advance to maturity.

The foregoing classification of streams does not involve categories which are entirely exclusive of one another. Any stream, whatever its mode of origin, may become antecedent through diastrophic movements. Most superimposed streams are also consequent, but by no means are all consequent streams superimposed. The Columbia and Snake are both superimposed and antecedent.