6 BULLETIN 675, U. S. DEPARTMENT OF AGRICULTURE. 



of the farm land adjacent to the National Forests, it is evident that 

 the crops produced can not be marketed profitabl}'' except by feeding 

 to live stock. Even where suitable shipping facilities exist the farm 

 lands are of value chiefly in the production of winter feed for the 

 live stock handled on the National Forest ranges. Thus the deple- 

 tion of the range not only presupposes a decrease in the number of 

 stock grazed but tends to unbalance the agriculture of the locality. 

 As the forage production on range lands is decreased as a result 

 of erosion, the water available for irrigation purposes decreases. The 

 surface soil, containing as it does the decomposed vegetable matter, 

 is the chief absorbing and retaining agent of water. A series of 

 tests to determine the water-holding power of soils at different depths 

 was carried out by obtaining samples of a noneroded soil of lime- 

 stone origin in the spruce-fir type at 10,000 feet elevation, the results 

 of which were as follows : Water-holding capacity, from surface to a 

 depth of 6 inches, 56.4 per cent; 6 to 12 inches, 46 per cent; and 

 12 to 24 inches, 32.4 per cent. The percentage of organic matter con- 

 tained in the soil samples was 15.8, 11.3, and 6.8, respectively. Hence 

 the amount of water these soils retained against the force of gravity 

 is roughly in direct proportion to the amount of organic matter inter- 

 mingled with the soil particles. In the absence of this rich sponge- 

 iike soil surface, the water is readily carried away by gravity, and 

 the stream flow for irrigation purposes is extremely erratic and avail- 

 able only for a short time following rainstorms. Obviously, it is 

 often impossible for the farmer to avail himself of this flood water 

 for irrigation purposes for at least two reasons — the water may as- 

 sume torrential magnitude and carry with it so much sediment as not 

 to warrant its use for irrigation purposes, or owing to its unexpected 

 occurrence the farmer may not be able to make use of it. 



FACTORS DETERMINING THE AMOUNT OF ERRATIC RUN-OFF AND 



EROSION. 



The foregoing pages are intended to point out briefly the extent, 

 occurrence, and economic aspect of erratic run-off and erosion on 

 range lands similar in conditions to those studied. In order to be 

 able to offer rational recommendations as to methods of prevention 

 and control, however, it was necessary to study more in detail the 

 factors involved. This has been accomplished by the selection and 

 study of two areas where floods of unusual violence have originated 

 at various times. 



In the spring of 1912 two areas, designated as A and B, as similar 

 as possible in topographic, soil, and climatic conditions and vegeta- 

 tion, were selected for the study. (See fig. 1.) The areas are located 

 in the Manti National Forest at the head of Ephraim Canyon, on the 



