34 



E. C. ANDREWS 



ACTION OF FLOODS ALONG MATURELY DEVELOPED STREAM CHANNELS 



The maturely developed stream channel is composed of reaches 

 of alluvial flats, alternating with graded rocky slopes. These alluvial 

 stretclies may be regarded as fleeting baselevels. In periods of 

 ordinary water the stream accomplishes very little cutting or trans- 

 porting, its energy being absorbed in establishing a perfect adjust- 

 ment of channel grade, which has been temporarily upset by a freshet 



Fig. 9. — The Arthur River at its entrance to Milford Sound. Note the double 

 cliff slope of Sheerdown Hill to the right. See also in this connection Fig. 11. 



or flood. Consider now the effect of a great flood on this graded 

 channel. Along certain points of the channel — notably where con- 

 vergence of streams occurs — deep holes are scooped out below the 

 temporary baselevels. In many cases, along the lower stream 

 courses, basins are formed whose floors lie considerably below their 

 grand baselevel (generally the sea-surface). Every physiographer 

 is aware of such facts. Excavation of banks also keeps pace with 

 this scooping action. Nevertheless, on the other side of the stream 

 aggradation is proceeding hand in hand with this corrasion. The 



