i 



BYBEE-MALOTT : THE FLOOD OF 1913 



137 



PART XL— OBSERVATIONS 



Damage to Soil 



Soil Washing and Erosion. When the water begins to flow 

 across the flood plain, sand silt and debris are deposited, their 

 position being determined largely by the velocity of the current. 

 In many places where the current is strongest the top soil is removed 

 and in some places great holes are cut. 



The amount of cutting depends upon the velocity of the cur- 

 rent, the kind of soil, and the amount and nature of the sediment 

 carried in suspension. I. C. Russell, in 'Rivers of North America,' 

 gives the following table on the transporting power of a stream: 



TABLE No. 3. 



Velocity of Current. Size of Material Moved. 



3 inches per second Fine clay and silt. 



6 inches per second Fine sand. 



12 inches per second Pebbles }4 inch in diameter. 



2 feet per second Pebbles 1 inch in diameter. 



2.82 feet per second Pebbles 2 inches in diameter. 



3.46 feet per second Pebbles 3 inches in diameter. 



4 feet per second. . . .'. Pebbles 4 inches in diameter. 



4.47 feet per second Pebbles 5 inches in diameter. 



4.90 feet per second Pebbles 6 inches in diameter. 



5.29 feet per second Pebbles 7 inches in diameter. 



5.65 feet per second Pebbles 8 inches in diameter. 



6 feet per second Pebbles 9 inches in diameter. 



Russell says concerning the above table: Tt must be under- 

 stood that the currents referred to in this table are bottom currents, 

 and in general may be taken at about half the central surface cur- 

 rent.' A study of the table shows that the transporting power 

 increases in a greater ratio than the increase in velocity. 



Le Conte, in his 'Elements of Geology,' shows that the trans- 

 porting power of a current varies as the sixth power of the velocity. 

 Thus, under this law it will be seen that by doubling the velocity 

 of a current the transporting power will be increased sixty-four 

 times. That is, if a stream having a given velocity will carry a 

 pebble weighing two ounces, it will carry a pebble weighing 64 

 ounces if its velocity is doubled. This law applies only to material 

 held in suspension. Larger materials may be rolled along on the 

 bottom of the stream bed. 



3—1424 



