256 E. C. ANDREWS. 



will the stream energy experience a less percentage of loss 

 by corrasive action in proportion as the downstream slope 

 of the obstacle is increased. In the case of the sides of the 

 obstacle the stream follows the lines of least resistance, 

 and will corrade with varying degrees of strength accord- 

 ing as these side slopes are inclined to, or away from, the 

 stream motion. Nevertheless, although the stream may 

 not cause much loss to a very steep downstream face by 

 actual abrasion, yet it may strongly corrade it by sapping, 

 by plucking, and by quarrying actions. 



Briefly, the obstacle by supposition is small as compared 

 with tlie stream volume. It does not, therefore, appreci- 

 ably affect the velocity. The stream thus passes over the 

 obstacle while local pressure, as flow and falling actions, 

 tends to bend the stream toward the downstream aspect 

 of the obstacle. Eddies and sapping action may thus be 

 produced. The end result is a net loss to the energy of 

 the stream. 



Obstacles large as compared ivith the stream. — In the 

 case of small obstacles the surface of the stream is not 

 altered perceptibly by the presence of the obstacle and 

 the passage of the same by the stream is attended by a 

 slight net loss only of stream energy. In the case of the 

 large obstacle or mass, the surface of the stream is 

 altogether changed. The stream now is either impounded 

 while rising over the mass or has its surface slope consider- 

 ably lowered. We will neglect the case where the stream 

 is impounded with practical loss of local energy and con- 

 sider the case of a very large mass presenting but little 

 obstruction to the stream flow on its upstream aspect and 

 presenting a marked declivity on its downstream side. 



In such case, stream motion is not seriously checked, 

 and the basal layers accomplish strong corrasion of the 

 upper portions of the obstacle. But on passing over the 



