242 E. C. ANDREWS. 



This is a necessity from the laws which govern streams. 

 And the only method open thus to the gravity stream is 

 the adjustment of its cross-sections so as to secure a flow 

 of even strength. It is not that the stream aims at exca- 

 vating below the general channel bottom and aims at 

 steepening the channel walls in " constrictions," but merely 

 that it is forced to express an increase of strength by 

 locally enlarging its cross-section, because the time factor 

 for both weaker and stronger stream action along the same 

 channel is constant. 



(i) Basins and associated forms in channel constrictions. 



We assume here that the channel slope is negligible and 

 that the iucrease of velocity is caused by relative smallness 

 of cross-section and not by additional volume of stream 

 material from side sources. [The case of a channel con- 

 fluence forming a constriction is dealt with later]. 



In Fig. 3 the curve A O is very pronounced the corrasion 

 being powerful in the direction A O and dying away toward 

 O. Therefore, as time progresses (the stream volume 

 remaining constant) the basin head must recede by corras- 

 ion until such time as the stream has no strength to move 

 its mass as a unit over the point A. When such a stage 

 arrives, the recession of A ceases until the point B is also 

 lowered. 



When the channel cross-section is so much enlarged that 

 there is no local increase of stream velocity at a basin 

 head, the cessation of the formation of that basin head has 

 arrived. At such a stage also the lower end of the basin 

 may be already partly silted up, owing to the general 

 enlargement of the whole cross-section of the channel 

 upstream, thus reducing the stream velocity still further 

 down stream along the reduced slope. 



In the case of a very large stream, therefore, whose 

 velocity has been increased by passage of a lengthy channel 



