226 E. C. ANDREWS. 



Let the excavation A O B express such variable stream 

 action. [Fig. 3 (b)]. The question may now be put : — 

 what is the measure of the extent of this vertical corrasion; 

 in other words — what is its downward limit ? 



As has been pointed out already no stream can mechani- 

 cally corrade its channel base at points where such base is 

 occupied by stagnant debris. If the stream in its passage 

 should override debris without moving it, then it is not 

 corrading its channel base and sides. Pdr example, if the 

 channel base is cumbered with debris 1,000 feet in depth 

 and the stream can move a thickness of 999 feet only of 

 the debris at any spot, it is not corrading its channel 

 structures at that point. At such a point it is simply 

 readjusting its grade. In other words, the stream utilises 

 the channel bottom as a bridge for the transportation of 

 debris toward the main base level, and at spots where it 

 cannot move its load as a whole over this bridge of live 

 rock it actually protects it. [Fig. 2 (a)]. 



The depth, therefore, of such initial basin A O B in Fig. 

 3 (b) will be increased until the time arrives that the stream 

 PP' can only just maintain its general character as a 

 stream over the excavation while moving the mass of 

 stream and rock material filling the excavation of A O B 

 as a whole. Let such greatest depth be Z O. Now until 

 the point B be lowered, the stream we are considering 

 cannot cut below O because it has no ability to transport 

 its load at depths greater than Z O and a stream which is 

 unable to get at the channel structures proper cannot 

 corrade them. 



Briefly then, the stream P P' receives an acceleration of 

 velocity at A and excavates the basin A O B below the 

 associated base levels. At O the limit of its vertical 

 measure of strength is taxed in the transportation of its 

 load along the slope A O B. The stream is therefore capable 



