CORRASION BY GRAVITY STREAMS. 219 



and variable motion increase in violence the actual strength 

 of the stream increases greatly in spite of this loss of energy 

 by internal work. 



Stream velocity in channels. — (See Fig. 2 a.) — Let N'" O 

 N'" be a stream channel, O a point on the thalweg, and let 

 N'N', N"N", N'"N"', and PP' represent varying surface levels 

 of the stream. Let the channel section be symmetrically 

 arranged about a vertical plane OL. For the first three 

 levels the stream is confined to the channel proper while 

 for the last mentioned level the stream overflows the banks 

 of its channel proper. Since the channel is assumed to be 

 developed by corrasion the stream material has a get away 

 and therefore the problem is not one of hydrostatics. From 

 considerations of pressure and friction of channel sides it 

 will be seen that the greatest stream velocity lies above 

 the thalweg and at a point not in contact with the sides. 

 But when friction is zero the greatest velocity is at O and 

 the stream suffers gradual diminution of velocity in a direc- 

 tion from O to L. 



Now if we confine our attention to the action of a stream 

 when in contact with the channel sides we shall find that, 

 all other things being equal, the greatest velocity of that 

 portion of the stream which is in actual contact with its 

 channel structures lies at O and the velocity is progres- 

 sively weakened from O to N"'. Thus at O it may be very 

 great, while almost negligible at BB'. Again, the higher 

 the stream rises above L the greater becomes the velocity 

 at O. This is a very important point when considering 

 corrasion and it applies to all gravity streams alike. 



Corrasion by Streams.— Preliminary. — If a rock frag- 

 ment be struck by, or be dragged over, another rock frag- 

 ment, a mutual loss is sustained. If one be more resistant 

 than the other, then it suffers less from the impact than 

 does the less resistant one. Earth material thus corrades 



