82 PHYSICAL GEOLOGY 



being the case, a slight upward current, formed by the deflection of 

 the water from the irregularities of the stream bed or side, will lift 



small particles and carry 

 them onward until they 



>- again fall to the bottom, 



or are caught up by an- 

 ^ • — *■* other current (Fig. 59). 



■^ /~{ ~\i ) The eddies and cross cur- 



rents of a river are espe- 



Fig. CQ. — Diagram showing how upward cur- • 11 rr * * 1 - 1 



rents are produced by irregularities on the bed Cially effective in this work 

 of a stream. during high water. In 



this way, sand and clay, 

 after many short journeys, are ultimately carried to the ocean. The 

 quantity of sediment carried by a stream depends upon its volume 

 and velocity and on the amount and nature of the accessible material. 



Factors Determining the Velocity of Streams. — The velocity 

 of a stream depends upon (1) the slope of its valley, (2) its volume of 

 water, (3) the amount of its load, and (4) the shape of its channel. It 

 is greatest in the middle of the stream, and some distance below the 

 surface. If the volume of a stream is increased eight times, its velocity 

 is doubled, since the velocity varies as the cube root of the volume ; if 

 the amount of the sediment is decreased, the velocity is increased ; 

 and, other things being equal, a stream following a straight channel 

 flows faster than one in a winding course, because it loses less energy in 

 friction with its sides. A stream which is ordinarily clear is often 

 muddy when swollen, both because of the greater run-off which enters 

 it, and because of the large amount of sediment which it is enabled to 

 tear from its bed and banks on account of its greater velocity. 



If the velocity of a stream is increased several times, its power 

 becomes almost incredible. It has been shown that a current moving 

 six inches a second will carry fine sand; one moving 12 inches a 

 second will carry gravel ; four feet a second, stones of about two 

 pounds weight; eight feet a second, stones of 128 pounds; 30 feet 

 a second, blocks of 320 tons ; if a stream can ordinarily move a pebble 

 of one ounce, it can move a stone of four pounds when doubled by a 

 flood. This fact is expressed in the law that the transporting power of 

 a stream varies as the sixth power of its velocity. Keeping the above 

 law in mind and remembering that a heavy object loses about one 

 third of its weight in water, it is easy to understand the cause of the 

 destructiveness of such floods as that which overwhelmed Johnstown, 



