324 A NATIONAL PLAN FOR AMERICAN FORESTRY 



the Utah State Land Board, have shown that the 75-foot or deeper 

 channel cutting and the enormous amounts of waste debris deposited 

 by these recent floods were far in excess of any earlier flood action in 

 that locality since the geologic Lake Bonneville ceased to exist some 

 30,000 or more years ago. 



After the floods of 1930 the governor's special flood commission 

 established the fact that the heavily silt laden flood waters had 

 collected chiefly on small areas of private land at the heads of the 

 drainages where the vegetative cover had been destroyed or seriously 

 depleted by overgrazing, fire, and to some extent by logging. These 

 areas are badly gullied and the surface soil has been stripped away 

 through sheet erosion. Slopes, too steep for grazing, that at inter- 

 mediate elevations make up the greater part of the mountain face, 

 bear a substantial brush or forest cover. No gullies originated on 

 these densely vegetated slopes, where the thick litter cover and the 

 large humus content in the surface soil permitted effective penetration 

 of water and restrained the surface flow sufficiently to prevent undue 

 soil or water losses. 



There are in the West large areas of coarse readily erosible granitic 

 soil. This is especially true of the Boise River drainage of south- 

 western Idaho where the underlying granite easily disintegrates, 

 crumbling to coarse sand and fine gravel which combined make up 

 80 percent or more of the total soil mass. These soils are so extremely 

 loose that where the plant cover becomes scarce the soil is readily 

 swept off as sheet erosion. In the subalpine forest zone 4 to 8 inches 

 of the black surface soil has been removed, and at lower elevations 

 practically all the surface soil has been lost. Heavy run-off quickly 

 causes the formation of 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 appear to be healing when actually they are not. 



Past overgrazing on practically all of the open areas within the 

 forest and on some timbered areas of the Boise River watershed is 

 known to have greatly reduced the protecting vegetative cover and 

 trampling of these inadequately protected soils has often set in motion 

 a downhill movement of the soil which is greatly accelerated by run- 

 off from torrential rains. 



Of nearly 350,000 acres examined by the Forest Service, more than 

 80 percent is in timber and heavy brush, which occupies all the north 

 and parts of the south slopes. Fifty-six percent of the timber or 

 brush areas subject to grazing has suffered sheet erosion, and 9 per- 

 cent additional has suffered gully erosion as the result of overgrazing. 

 Of the 53,000 acres of timber and brush areas too dense or on too steep 

 slopes to permit grazing, only 14 percent had suffered sheet erosion 

 and but 2 percent additional light gully erosion. 



In contrast to these heavily grazed areas on the Boise watershed, 

 only about 5 to 10 percent of similar locations and exposures on the 

 game preserve on the nearby Payette drainage, subjected for many 

 years only to light grazing, have lost the top layer of black soil. 

 Beneath 1 to 3 inches of litter in open areas and 1 to 6 inches in the 

 dense timber, the preserved soils contain 4 to 11 percent organic 

 matter. This compares with 1 to 2 percent on the raw eroded soils, 

 on which there is no measurable depth of litter. 



