454 A NATIONAL PLAN FOR AMERICAN FORESTRY 



greater average flow on peak days, and nearly 36 percent greater flow 

 in May; (3) an increase in the flow during the period April to June, 

 inclusive, from 66 percent of the total annual flow to 73 percent; (4) a 

 decrease in the flow during the period July to September, inclusive, 

 from 13 percent of the yearly flow to only 9 percent; and (5) approxi- 

 mately 96.5 percent as much run-off from 88 percent as much precipi- 

 tation. These changes are of great moment. April to June flow is, 

 of course, chiefly the result of surface run-off from melting snow, 

 while July to September run-off results almost entirely from the slow 

 drainage of ground water. The effect of the fires appeared to increase 

 the spring flood flow, and to do this largely at the expense of ground 

 storage of water that would have fed the streams later in the year, 

 particularly during the summer period. 



EROSION 



Of equal or greater importance with effective regulation of stream 

 flow is control of erosion. A large part of the forested area of the 

 Columbia River Basin is made up of coarse, readily erosible granitic 

 soil. This soil is so loose that where plant cover is scarce or absent it 

 is readily swept off in sheet erosion. Rapid run-off then forms gullies 

 from a few inches to several feet deep. Loose soil on the edges of the 

 gullies soon crumbles, and within a year or two after being formed 

 many of the smaller gullies have so smoothed over as to be almost 

 unnoticeable. Such abnormal erosion is taking place over extensive 

 areas. 



How serious such erosion is on this granitic soil is shown by a Forest 

 Service survey of a part of the Boise River watershed in Idaho. 

 Approximately 62 percent of an area of nearly 350,000 acres had 

 suffered distinct sheet erosion and 10 of this 62 percent was also cut 

 by gullies of a type not readily obliterated by creeping soil. Deple- 

 tion of vegetation from past overgrazing and trampling by livestock 

 of soils inadequately protected by vegetation are important causes of 

 the erosion. Of the 190,991 acres in the grazed timbered area, sheet 

 erosion had taken place on 64 percent, and on 8 of this 64 percent 

 gully erosion also had taken place. Of nontimbered grazed areas 

 more than 78 percent had suffered sheet erosion and 15 percent 

 included in this had suffered gully erosion. Of the 52,817 acres of 

 timber and brush areas where the cover was too dense or the slope too 

 steep to permit grazing only 16 percent showed sheet erosion and but 2 

 of this 16 percent showed gully erosion. 



In many places crowns of old grass plants are found elevated 4 to 8 

 inches above the surrounding surface, from which erosion has removed 

 all fine dark soil, exposing a raw, inert stratum of unconsolidated 

 gravelly sand that offers no real resistance to water erosion. On those 

 areas protected by a dense cover of timber, wheatgrass, and yarrow, 

 or of mountain brush a mellow black soil 6 to 18 inches deep still 

 remains and soil loss through normal erosion is probably offset by soil 

 formation. 



Studies by the Intermountain Forest and Range Experiment Sta- 

 tion in this locality showed that noneroded soils contain from 6 to 19 

 percent organic matter as compared with % to 4 percent in the raw 

 eroded soils. The average water-holding capacity of the soils of 

 greater humus content was 81 percent, as compared with only 44 per- 



