58 



PHYSIOGRAPHY 



caught by minor upward currents (Fig. 44 and carried up in 

 spite of gravity. It is chiefly by means of these minor upward 

 currents in the main current that sediment is kept in suspension. The 

 particles of sediment suspended in a stream are dropped and picked 

 up again repeatedly, and the long journey of any particle is made up 

 of many short ones. Particles of mud carried from Dakota to the 

 Gulf of Mexico ordinarily make many stops along the route. 



Amount of load. The amount of sediment a stream carries 

 depends on (1) its swiftness, (2) its volume, and (3) the amount and 

 kind of sediment which it can get. Swift and large streams can 

 carry a heavier load than slow and small ones. The effect of velocity 

 on the carrying power of streams may be seen in most creeks and 



Fig. 45. Tools with which a river works. They can be moved only when 

 the stream is in flood. Bighorn Mountains. 



rivers which are wider in some places than in others. Where the 

 channel is narrow, the current is swift, and here, in many cases, all 

 fine material has been swept away, leaving only pebbles and larger 

 stones (bowlders) in the channel (Fig. 45). Where the channel is 

 wider, on the other hand, the bottom of the same stream may be 

 covered with sand or mud. By narrowing the channel of the Mis- 

 sissippi by making jetties near its mouth, in 1875, James B. Eads 



