A NATIONAL PLAN" FOR AMERICAN FORESTRY 455 



cent for the eroded soils lacking in humus. On the latter soils there is 

 no measurable depth of litter, whereas there is 1 to 3 inches of litter 

 on lightly grazed noneroded soils of nontimbered areas and 1 to 6 

 inches on the soils of densely timbered areas. 



Some rather large areas of private forest within the Columbia 

 River basin have been heavily cut and burned, and this has caused 

 material thinning of the timber stand and some erosion ; the principal 

 cause of erosion, however, has been the decline of the herbaceous 

 undergrowth of the forest, largely from overgrazing but in part from 

 fires. Bunch grasses, which in good stands effectively control erosion 

 and build soil, have, over large areas, been almost replaced by downy 

 brome and other inferior annuals or perennials. This replacement is 

 especially marked on the several million acres of forest land in the 

 public domain. Forage production on the forested public-domain 

 areas and on intermingled private land has been found by Forest 

 Service observers to have been reduced in many instances by from 

 50 to 80 percent. Such conditions are adversely affecting the live- 

 stock industry as well as the watershed-protection values of the 

 forest. 



FLOODS 



Where only scant cover is present on the watersheds to interrupt 

 run-off, the combination of semi torrential rains, rapid snow melt, and 

 ready formation of gullies causes local floods on many of the smaller 

 tributaries. These local floods sometimes do considerable damage, 

 destroying improvements and depositing mudrock flows at the mouths 

 of the drainages, sometimes destroying good agricultural land. 



A cloudburst in 1932 on Loon Creek, on the Chain's National Forest 

 in Idaho, for example, caused heavy run-off to originate on a 1931 

 burn, resulting in a deposit of sand and debris in tributaries of the 

 creek so heavy as to destroy fishing in parts of this stream for at least 

 several years. A similar rain in 1932, on a 1931 burn on the water- 

 sheds of Richardson and Mann Creeks, on the Idaho National Forest, 

 caused a heavy flood that cut deep gullies, gutted stream channels to 

 bedrock, and swept debris down these canyons into the Salmon River. 

 The mud flow at the mouth of Mann Creek dammed the Salmon 

 River to a depth of 20 to 25 feet and for a length of 450 feet, causing 

 the formation of new temporary rapids. 



High water can be expected practically every spring in nearly all 

 important tributaries of the Columbia River as a result of the melting 

 of the large accumulations of snow in the mountains. This high water 

 in the main streams seldom assumes the proportions of destructive 

 floods, although usually it causes minor damage along the stream 

 courses. It does, however, move a considerable quantity of silt which 

 has collected in small tributaries down to the main river channels. 

 By 1930, 15 years after the construction of the Arrowrock Dam of the 

 Boise project, silting was estimated to have reduced the storage 

 capacity of the reservoir by 7,000 to 8,000 acre-feet. This amount of 

 storage space represents more than $100,000 of the original cost of the 

 dam. Officials of the water-users' organization estimated that by 

 1930 the sand deposit at the power dam had reduced its capacity by 

 25 percent, and considerable silting of canals and other irrigation 

 works has caused additional expense. 



168342 33 vol. 1 30 



